Arcata City Council
June 3, 2026

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4. Case Studies
Across California coastal municipalities use different types of effluent discharge practices. This section presents case studies of existing practices that may inform the City’s options to adapt to SLR, climate change and regulation advancement over time. Wastewater discharge requirements vary depending on the location of the discharge and are generally summarized below:

Surface Water Discharge - Discharge into rivers, lakes, and streams Must comply with the Regional Board Water Quality Control Plan (Basin Plan) and Federal NPDES permits which sets limits on pollutants.

Enclosed Bays and Estuaries Discharge - Discharge into enclosed bays and estuaries The Enclosed Bays and Estuaries Policy (EBEP) was adopted by the State Water Board and regulates discharge into enclosed bays and estuaries. The EBEP includes provision requiring enhancement of the Bay to be able to discharge.

Ocean Discharge - Discharge into the ocean
Must meet the California Ocean Plan standards, which include treatment and monitoring requirements. NPDES permits specific to ocean discharges are required.

Land Discharge - Discharge onto land
Must comply with Waste Discharge Requirements (WDRs) to prevent contamination of groundwater and surface water. Must also comply with Title 22 recycling requirements. Effluent is applied at agronomic rates, without percolation into groundwater.

Groundwater Discharge - Discharge directly or indirectly into groundwater Must comply with the Regional Board Water Quality Control Plan (Basin Plan). Must meet standards to protect groundwater quality, often involving treatment and monitoring. WDRs or other specific groundwater discharge permits are required. Groundwater discharge may be through percolation ponds, irrigation above agronomic rates with percolation into groundwater, or through groundwater injection.

Wastewater discharge for select coastal California systems are discussed below. These systems were selected to provide examples of discharge strategies to inform the development of alternatives for the AWTF future discharge.

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4.1 McKinleyville CSD
Permitted discharge: surface water, land application, and groundwater McKinleyville CSD (MCSD) operates a wastewater management facility (WWMF) that treats residential and commercial wastewater. With a population of approximately 16,500 residents in the MCSD the average dry weather flow of 1.37 MGD and a peak wet weather flow of 3.08 MGD.

4.1.1 System Description
The WWMF provides advanced secondary treatment and consists of the following treatment train:
– Headworks
– In-Basin Extended Aeration System
– Secondary Clarifiers
– Chlorine Disinfection
MCSD employs surface water discharge and land application for effluent disposal. The effluent discharge location is based upon the time of year and hydrologic conditions in the Mad River. From October 1 through May 14 when river flow is above 200 cubic feet per second (cfs), MCSD chlorinates/dechlorinates and discharges to the Mad River. From May 15 through September 30, or when the Mad River flow drops below 200 cfs, MCSD chlorinates and conveys the effluent through polishing wetlands and then irrigates land and grows fodder for organic dairy operations.

Treated recycled water has been used successfully for irrigation with the effluent produced by the WWMF since the 1970s. The irrigated areas used by MCSD are located approximately 5 miles north of the AWTF, near the mouth of the Mad River. (NCRWQCB, 2024).

Figure 10 MCSD Recycled Water Land Application Area Just North of Hammond Bridge.

The MCSD’s irrigation areas are utilized as follows:
– Treated water is conveyed to stormwater treatment wetlands and used to sustain vegetation and the adjacent forested area during dry months. Prior to the onset of the wet season and potential for storm water overflows from the wetland, conveyance of treated water to this particular use area is ceased and the wetland is allowed to dry through evapotranspiration and infiltration.

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– Treated water is conveyed to the Lower and Upper Fischer Ranch and Pialorsi Ranch and used for agricultural irrigation. The Lower and Upper Fischer Ranch and Pialorsi fields are leased to local ranchers who utilize the property for fodder crop production (e.g., hay and/or silage). – There can be periods during the winter season when the Mad River falls below 200 CFS and an alternative means of effluent disposal is required. During such periods, the only available alternative to Mad River discharge is to discharge water to the Upper Fischer Ranch flood cells. There are 11 floodable trench cells where treated water is allowed to pond and infiltrate.

The Upper Fischer Ranch flood cells can be used for winter disposal. Cells are rotated into and out of service depending on soil permeability and vegetation demands. Winter disposal to these cells is possible due to the following attributes:

1. Soils permeability and infiltration are high and can accommodate application rates of more than 1.5 inches per hour.

2. Higher quality recycled water being produced by the WWMF and applied to reuse areas since November 2017 exhibit nitrate concentrations less than 10 mg-N/L.

3. Existing monitoring wells surrounding this area can be used to monitor groundwater quality if and when the flood cells are used for winter season disposal.

4. Direction of groundwater flow is in a southwest direction, away from residential homes and towards the Mad River and Pacific Ocean.

5. Besides MCSD’s monitoring wells, there are no other wells located between the flood cells and the Mad River or Pacific Ocean.

Table 33 Existing MCSD Recycled Water Irrigation Areas (acres) Recycled Water Use Area Approximate
Irrigation
Area (acres)
Application
Method
Type of Use Public Access
Hiller Storm Water Treatment
Wetland and Forested Area 25 Spray Wetland Irrigation Restricted Lower Fischer Ranch 45 Spray Fodder Restricted
Upper Fischer Ranch 36 Flood & Spray Fodder Restricted
Pialorsi Ranch – West 35 Spray Fodder Restricted
Pialorsi Ranch – East 54 Spray Fodder Restricted

4.1.2 Considerations for AWTF Planning
MCSD is the closest treatment system to the City of Arcata. Use of the Mad River for discharge is an alternative considered by the City. In addition, recycled water irrigation for summer disposal is also considered further in this study for the City of Arcata. It should be noted that the McKinleyville effluent flows are lower than Arcata’s, and Arcata would require more land for recycling than MCSD. A system similar to McKinleyville’s may be feasible for the City of Arcata.

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4.2 City of Fortuna
Permitted discharge: surface water and groundwater via percolation ponds (currently under revision) The City of Fortuna Wastewater Treatment Plant (WWTP) treats wastewater from 12,300 residential, commercial, and institutional users in the City of Fortuna and the Rohnerville-Campton Heights area, with an average dry weather flow of 1.5 MGD and a peak wet weather flow of 7.0 MGD.

4.2.1 System Description
The WWTP utilizes the following treatment train:
– Headworks
– Primary Clarifiers
– Return Activated Sludge Aeration Basins
– Secondary Clarifiers
– Disinfection with Chlorine Gas and Ammonia
Effluent is discharged to one of two discharge points, based upon the time of year. From October 1 through May 14, treated effluent is discharged to Strongs Creek, a tributary to the Eel River. From May 15 to September 30, treated effluent is discharged into percolation ponds adjacent to the Eel River. Historically, the percolation ponds were considered a discharge to groundwater. RWQCB staff have found that the percolation ponds are below the mean high-water mark of the Eel River and water has been observed surfacing from groundwater into the percolation ponds.

For these reasons, the percolation ponds are now considered hydrologically connected to the Eel River and are now subject to requirements associated with surface water discharge as defined in the Final Effluent Limitations for Strongs Creek (NCRWQCB, 2017).

The City evaluated land discharge to agricultural areas in the Eel River floodplain and found that discharge flows would exceed the available land area using agronomic water application rates and that the resulting discharge to groundwater would result in potential degradation as limited dilution occurs in addition to the effluent eventually migrating back to the Eel River through the groundwater aquifer. To address the issues with the percolation basins, the City is pursuing an Amendment to the Basin Plan for a Lower Eel River Exception to Seasonal Discharge Prohibition. The exception would allow the City to discharge to Strongs Creek year-round. The amendment is currently part of the RWQCB Triannual review process for consideration.

4.2.2 Considerations for AWTF
Similar to Arcata, Fortuna has been evaluating long-term solutions for effluent treatment and disposal. Fortuna has not been able to identify available land for recycled water use at agronomic rates and is pursuing a Fortuna- Specific yearround discharge to the Eel River. The City of Arcata may consider a similar approach to discharge year-round to the Mad River which would require pursuit of an exception to the Basin Plan in coordination with treatment plant improvements.

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4.3 City of Rio Dell
Permitted discharge: surface water, land irrigation, and groundwater The City of Rio Dell WWTP treats wastewater from approximately 3,900 residential and commercial users within the city, with an average dry weather flow of 0.4 MGD and a peak wet weather flow of 2.5 MGD.

4.3.1 System Description
The WWTP utilizes the following treatment train:
– Headworks
– Aero-Mod Secondary Treatment
– Solids Stabilization System
– Chlorine Disinfection and Dechlorination
Effluent is discharged to one of two discharge points, based upon the time of year. From October 1 through May 14, treated effluent is discharged to the Eel River. From May 15 to September 30, treated effluent is discharged to a 23- acre irrigation area used to grow hay and alfalfa. The hay grass and alfalfa are harvested as fodder for beef cattle. (NCRWQCB, 2017)
Figure 11 City of Rio Dell Irrigation Area West of Highway 101 Adjacent to Eel River (Source, City of Rio Dell)

4.3.2 Considerations for AWTF
Similar to McKinleyville, Rio Dell discharges to surface water in the winter and land in the summer at rates higher than agronomic demand resulting in groundwater discharge as well. The City of Rio Dell’s population and associated wastewater flows are much smaller than Arcata’s, resulting in less land needed for summer recycled water use. Rio Dell provides another local example of wastewater disposal, however the variation in flows from Arcata make this system less comparable to what Arcata would need for disposal.

GHD | Rural Community Assistance Corp. & City of Arcata | 12616645 | Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study 48

4.4 Mendocino City CSD
Permitted discharge: ocean and land application
The Mendocino City Community Services District (MCCSD) operates a wastewater collection, treatment, and disposal facility that serves a population of approximately 4,000 people, including 1,000 full-time residents and many visitors and tourists to Mendocino City, Russian Gulch State Park, and Headlands State Park. The Facility treats domestic and commercial wastewater and has an average dry weather design treatment capacity of 0.3 MGD and a peak daily wet weather treatment capacity of 1.0 MGD.

4.4.1 System Description
The facility utilizes the following treatment train:
– Solids Grinding at Headworks with Comminutors
– Extended Aeration Activated Sludge
– Secondary Clarification
– Tertiary Filtration
– Chlorination and Dechlorination
The WWTP Discharges to two locations, the Pacific Ocean via an approximately 1000-foot outfall pipe, and to the Mendocino High School (MHS) to irrigate the school’s athletic fields during the summer months via a 55,000-gallon storage tank (NCRWQCB, 2020).

4.4.2 Considerations for AWTF
The MCCSD system provides an example of an ocean outfall system that also incorporates some recycled water use to offset potable needs. The ocean outfall has been in place since the 1970s. The MCCSD system flows are much less than Arcata. Unlike Arcata, the MCCSD residents currently obtain potable water from individual groundwater wells, as opposed to a robust wholesale water provider (HBMWD) and the need for recycled water is much higher.

The MCCSD system provides an example of incorporating some water reuse while also operating a reliable disposal system that is not dependent on a specific recycled water volume use.

GHD | Rural Community Assistance Corp. & City of Arcata | 12616645 | Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study 49

4.5 Monterey One Regional WWTP, Ocean Outfall and
Groundwater Enhancement
Permitted discharge: ocean, land application, and groundwater The Monterey One Regional WWTP serves approximately 250,000 people within the Castroville, Del Rey Oaks, Marina, Monterey, Pacific Grove, Salinas, Sand City and Seaside wastewater districts. The Monterey One WWTP was first put into operation in February 1990, with a treatment capacity of 29.6 MGD with an average of 17 MGD.

4.5.1 System Description
The WWTP treats influent to secondary treatment levels for ocean outfall with a treatment train consisting of:
– Headworks
– Primary Clarification
– Trickling Filters
– Bioflocculation Basins
– Secondary Clarification
Treated effluent is discharged to the ocean just outside of the south end of Monterey Bay (Figure ). The discharge pipeline is 60 inches in diameter and runs 2 miles underground from the WWTP to the coast, then 2 miles out into the ocean. The end of pipe is closed, with the last 1000 feet of pipe having ports to disperse effluent. Discharge depth is approximately 100 feet below the ocean surface.

Figure 12 Monterey One Treatment Plant and Ocean Outfall (Source: Monterey One) The receiving water is part of the Monterey Bay National Marine Sanctuary, designated as such on September 15, 1992. The purpose of the National Marine Sanctuaries Program is to protect areas of the marine environment which possess conservation, recreational, ecological, historical, research, educational, or aesthetic qualities of special national significance. Construction of the pipeline began in 1981 under an Ocean Outfall Pipeline Lease Agreement

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between the districts presented previously and the California State Lands Commission. The lease agreement extends through November 2030.

In addition to the ocean outfall, Monterey One also utilizes a groundwater injection well and recycled water irrigation for effluent discharge. Secondary treated effluent from the WWTP is pumped to an Advanced Treatment Plant where it is put through an additional four-step Advanced Water Treatment (AWT) purification process of Ozone (O3) PreTreatment, Membrane Filtration (MF), Reverse Osmosis (RO), and Oxidation with Ultraviolet Light (UV) and Hydrogen Peroxide (H2O2). The treated water is injected into the seaside groundwater basin to reduce diversions from the Carmel River System by up to 3,500 acre-feet per year (AFY). The Seaside groundwater basin is utilized for drinking water supply for local water districts. (NCRWQCB, 2023)

4.5.2 Considerations for AWTF
The Monterey One system provides another example of an ocean outfall system that also incorporates water reuse to fulfill needs in the area. The plant produces recycled water to offset local agricultural needs. The system also injects tertiary treated water into the groundwater to prevent saltwater intrusion into the local aquifer. Arcata has a robust wholesale water provider (HBMWD) and thus there is very little demand for recycled water. In addition, saltwater intrusion is not a known issue and there is not a need for groundwater injection as a mitigation measure.

4.6 Gualala CSD
Permitted discharge: land application and groundwater
The Gualala CSD (GSCD) WWTP is located on Highway 1 approximately 100 miles north of San Francisco. The WWTP treats wastewater from the Towns of Gualala and Sea Ranch, with an average daily dry weather flow of 0.13 MGD and a peak wet weather flow of 0.27 MGD. Each service connection to the Gualala side of the collection system includes a Septic Tank Effluent Pumping process for primary treatment, which uses the septic tank to pretreat (primary treatment) the wastewater before it is conveyed to the WWTP.

4.6.1 System Description
The treatment train consists of:
– Septic Tank Effluent Pump System
– Aeration
– Primary Clarification
– Travelling Bridge and Fine Screen Filter
– Chlorine Disinfection
The WWTP meets tertiary treatments levels for municipal irrigation and groundwater recharge. The effluent is disinfected with chlorine before discharge to four effluent storage ponds with a 28.4 million gallon capacity. The ponds deliver water to an 80-acre golf course for irrigation. The ponds have been shown to not have sufficient capacity to handle extreme precipitation events, so a temporary allowance for a 2 MG percolation pond has been granted to GCSD. The pond has been estimated to percolate 600,000 gallons per day. In recent years, the pond has been used infrequently during the winter months without any measurable impact to groundwater quality (NCRWQCB, 2022).

4.6.2 Considerations for AWTF
The Gualala system is a fully land disposal system including storage of both treated and untreated effluent and reuse on a golf course. As discussed above, the system capacity is exceeded during high precipitation events, which may become more frequent with climate change. The Gualala system shows the drawbacks of full reliance on a land based recycled water system, which has less flexibility to address climate changes.

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4.7 Tolowa Dee-Ni’ Nation
Permitted discharge: groundwater
The Tolowa Dee-Ni’ Nation Smith River Rancheria WWTP is located on Highway 101 and Ocean View Drive approximately one mile south of the Oregon/California border. The WWTP treats the Smith River Rancheria wastewater, with an average daily dry weather flow of 2,000 gpd and a design capacity 58,000 gpd.

4.7.1 System Description
The treatment train consists of:
– Headworks
– Membrane Biological Reactor
The effluent is discharged to an approximately 20-acre leach field which percolates to groundwater in the Smith River basin. The Rancheria is required to submit a quarterly report detailing groundwater depth, elevation, flow direction and water quality parameters (NCRWQCB, 2009).

4.7.2 Considerations for AWTF
The Tolowa Dee-Ni’ Nation treats significantly lower flows than Arcata. While it is an innovative approach, the City’s flows and volume are so much greater than the Tolowa Dee-Ni’ Nation system, it is not a feasible example of a system that Arcata could employ.

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4.8 City of Healdsburg
Permitted discharge: surface water and land application The City of Healdsburg WWTP treats wastewater from the City of Healdsburg, with an average dry weather flow of 1.4 MGD and a peak wet weather flow of 4.0 MGD.

4.8.1 System Description
The WWTP utilizes the following treatment train:
– Headworks
– Aerobic, Anoxic, and Pre-Anoxic basins
– Microfiltration through a membrane bioreactor
– Return activated sludge pumping from the MBR back to the aeration basin – UV Disinfection
During wet periods, effluent is discharged to Basalt Pond which is physically connected to the Russian River. During dry periods, water is discharged through an extensive recycled water irrigation system which serves approximately 1,200 acres of vineyards and two fill stations. The fill stations provide trucked water for construction uses (primarily soil compaction and dust control), non-dairy livestock drinking water, and landscape and vineyard irrigation at agronomic demand. Irrigation occurs primarily during spring, summer, and fall and may occur during dry periods in the winter (NCRWQCB, 2022).

4.8.2 Considerations for AWTF
Similar to McKinleyville and Rio Dell discharges, the Healdsburg system provides an example of surface water discharge in the winter and land disposal in the summer. Effluent flows are similar to the City of Arcata. The biggest difference is the demand for recycled water in the area due to lack of local water supply availability. The City of Arcata does not have agricultural systems needing water like Healdsburg does. Agricultural operations in the Arcata vicinity have access to HBMWD’s water system or pump from groundwater.

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4.9 Pelican Bay State Prison
Permitted discharge: land and groundwater via rapid infiltration basins The Pelican Bay State Prison (PBSP), located near Crescent City, CA has an individual waste discharge permit from the Regional Board. The PBSP WWTP was constructed in 1989 and treats 0.75 to 1.25 mgd of wastewater

4.9.1 System Description
The advanced WWTP employs a number of treatment processes to meet the discharge. The treatment process train consists of:
– Raw wastewater screening
– Grinding and influent pumping
– Secondary treatment with an extended aeration activated sludge process designed for total nitrogen removal – Advanced treatment which combines phosphorus removal with wastewater filtration – Disinfection by chlorination
– Effluent pumping
– Dechlorination
– Final effluent polishing through constructed wetlands – Disposal by land application.

Treated effluent from the PBSP WWTP is routed for final polishing of the effluent to a constructed wetland and then dechlorinated. Effluent is eventually applied to rapid infiltration basins. The PBSP operates well below its design capacity due to a reduced inmate population, therefore effluent applied to the rapid infiltration basins generally evaporate before it can infiltrate.

4.9.2 Considerations for AWTF
The PBSP WWTP relies on full land discharge with infiltration to groundwater. The large land requirements for a system even including percolation into groundwater without a second discharge point such as surface water limits the feasibility of system like PBSP’s for Arcata.

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4.10 Decentralized Systems

4.10.1 Small Scale Reuse Systems
Decentralized wastewater treatment involves the treatment of wastewater closer to the source. A method known as sewer scalping, or sewer mining, is an innovative decentralized wastewater treatment approach that extracts a portion of wastewater from a sewer line, treats it locally, and typically reuses the clean water for non-potable purposes like irrigation, cooling, or industrial use. Often these types of projects are part of a larger water recycling program in areas that have issues with reliable water supplies. Two small scale examples are presented below.

Demonstration Project Anaheim City Hall:

Permitted discharge: land
A membrane system was designed to pump wastewater from a local sewer line, treat the wastewater for reuse, and return the waste activated sludge and screenings to the sewer line. The design net output of the system is ~100,000 gallons per day base on an intake capacity of 150,000 gallons per day. The recycled water is used for irrigation and toilet flushing within the requirements of California’s Title 22 Regulations. At this site, the membrane plant automation design allows for periodic operator attention only when equipment maintenance or chemical refilling is required.

Trinidad Rancheria Membrane System:

Permitted discharge: groundwater
The Trinidad Rancheria installed a membrane bioreactor treatment system in 2002 to serve the newly constructed casino. Approximately 60% of the treated wastewater is recycled back into the casino and used for toilet flushing. The remaining treated wastewater is dispersed back into the environment by means of a dispersal field (leachfield) located just south of the Tribal Office. The recycled system was a key element in reducing the need for potable water from the City of Trinidad’s surface water system.

4.10.2 Composting Toilets
The potential for use of composting toilets has been brought up by the public during community meetings. These types of systems are typically used in remote locations, at state and national parks, at educational or demonstration sites, or for recreational vehicles. These systems are often still maintained by an oversite agency.

Santa Cruz County Composting Toilets Pilot Program:

Permitted discharge: not applicable
The County of Santa Cruz implemented a pilot composting toilet program, which was expanded in 2024. The pilot program was managed by GiveLove, a California-based nonprofit with extensive experience in Container-Based Sanitation systems. The program was implemented to address concerns related to the State’s strict septic laws, and as a backup system to be implemented in the case of a natural disaster. The water-free waste system is held in sealable containers lined with biodegradable bags that catch the waste and suppress odors with sawdust. The containers are eventually collected and transported to a centralized location, from which point they will be collected and sent to a wastewater treatment facility to be used as fertilizer.

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4.10.3 Feasibility of Decentralized Systems
Decentralized systems can be feasibly implemented in Arcata. Typically, the development of small scale reuse system and the justification for the costs of treatment are driven by lack of available potable water supply. The City of Arcata has access to a cost effective reliable potable water source, reducing the need for local recycled water. Thus, further evaluation of small scale reuse system is not included in this study.

A composting toilet program could be implemented by the City of Arcata. Issues to be addressed include oversight staffing to ensure no negative public health impacts, how to dispose of used containers, risk and insurance for the receiving facilities. A composting toilet program would not replace the need for a centralized collection, treatment, and disposal system in the City. The City can conduct further studies on the development and implementation of a composting toilet program as a secondary alternative for specific use cases. Further evaluation of composting toilets is not included in this study.

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5. Adaptation Pathways and Options
Generally, adaptation strategies for SLR can be described in three categories: Protect, Accommodate and Retreat (Figure 13). In practice, adaptation often requires a combination of these approaches. For example, Phase I of the AWTF Improvement Project and the proposed Levee Augmentation Project combine protection and accommodation strategies. For this feasibility study, adaptation options are outlined for the collection system, treatment system, and the discharge/enhancement marshes. Within these sections, the options are characterized by the type of strategy, potential timeline, and goal.

Figure 13 Adaptation Strategies
In acknowledgement that some adaptation is already occurring and will contribute to addressing SLR risks, this report uses three timelines: near-term (2025-2055), mid-term (2055-2075), and end of the century (2075-2105). The nearterm planning horizon includes Phase I of the Arcata Wastewater Treatment Plant Improvement Project, which has an estimated design life through 2055. This feasibility study focuses on the mid-term and long-term horizons, including adaptation options that may already be in the planning phase as well as new concepts for addressing mid- and longterm risks.

Sequencing adaptation options based on the timing and magnitude of flooding exceeding acceptable thresholds is a common practice for planning for SLR and is often called “adaptation pathway” planning. The following descriptions of adaptation options are thus outlined in a pathway approach based on these timelines and relevant projected triggers.

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5.1 Coastal Infrastructure and Wetland Resilience Case
Study Projects to Support Adaptation
Adaptation strategies to address future sea level rise often need to address the resilience of wetland habitats and the protection of infrastructure in a mutually beneficial approach. These strategies are often labelled "nature-based solutions" and consist of living levees, dynamic berms, creating marsh transition zones, or restoring coastal habitats that can absorb flood waters and protect infrastructure. Nature-based solutions are generally constructed using natural materials that provide ecological benefits and are intended to enhance the resilience of ecosystems and infrastructure to flooding and erosion.

There are several examples of California nature-based adaptation projects intending to both provide resilience for wetland habitat and flood protection needs for critical infrastructure. These include the Natural Shoreline Infrastructure (NSI) in Humboldt Bay for Intertidal Coastal Marsh Restoration and Transportation Corridor Protection, the Oro Loma Living Levee Project on San Francisco Bay, and the Bayshore Bikeway Resiliency Project in South San Diego Bay.

5.1.1 Natural Shoreline Infrastructure Project in Humboldt Bay The NSI Project is in progress along a vulnerable segment of Humboldt Bay shoreline adjacent to the Highway 101 transportation corridor between Eureka and Arcata. Transportation infrastructure, utilities, businesses, low-income residential areas, and wildlife areas are protected by this shoreline segment and a recently completed adaptation plan (Humboldt County 2021) identified the substantial risks to critical resources from continued shoreline erosion and coastal flooding (Figure 14).

Figure 14 Location of NSI Project Shoreline on North Humboldt Bay The NSI Project aims to reduce flooding to Highway 101 and adjacent areas by implementing a Living Shoreline through the restoration of historical salt marsh in combination with the completed elevated rail prism constructed by the Humboldt Bay Trail South project. The salt marsh restoration will serve to attenuate wind wave overtopping of the railroad prism, reducing flooding of the 101 transportation corridor and future erosion to the railroad prism, preserving the flood protection provided by the prism for future flood events. Additionally, the sediment trapping properties of the proposed salt marsh are intended to maintain pace with SLR thereby providing continued benefit into the future (Figure 15). The proposed project is intended to provide significant flood risk protection and sea-level rise resiliency to at least the year 2050, and moderate risk protection to approximately 2 feet of sea-level around year 2070 (GHD, NHE and USFWS, 2022).

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Figure 15 Conceptual representation of wind wave dissipation on the rail prism pre-project (top), at the marsh edge following project implementation (middle) and with sea level rise (bottom).

Permitting for the NSI Project is an ongoing effort, with coordination with the following permits, agencies and policies:
– Wetlands “No Net Loss” Policy
– Federal Water Pollution Control Act, A.K.A. Clean Water Act (CWA) – Endangered Species Act (ESA)
– Migratory Bird Treaty Act of 1918
– National Environmental Policy Act (NEPA)
– California Environmental Quality Act (CEQA)
– California Coastal Act
– California Fish and Wildlife Section 1600 Permit
– California Endangered Species Act (CESA)
– California Porter-Cologne Act
– California State Lands Commission Lease or Permit / Harbor District Shoreline Development Permit – Humboldt County Permits
– City of Eureka Permits

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5.1.2 Oro Loma Living Laboratory
The Oro Loma Living Laboratory is designed to study the concept of a horizontal levee. Instead of a traditional levee to protect against storm surges, a horizontal levee uses vegetation on a gentle slope to break waves, a form of natural shoreline infrastructure. The ecosystems that live on horizontal levees can thrive while helping to further process wastewater from treatment plants.

The Oro Loma Living Laboratory consists of a 1.4-acre experimental habitat slope on San Francisco Bay. The levee is 480 feet long and 150 feet wide with a 5 foot drop from crest to toe with a 30:1 slope. 100,000 to 300,000 GPD of wastewater that has undergone through secondary treatment is injected at the top and dispersed through 18 vertical cells, each one with a different combination of soil and plant habitat. The wastewater passes through the sub-layers of the cells, each one equipped with sampling wells at the top, bottom, and two thirds of the slope. The research team studies the idea that the sub-surface filtering processes will support native plants and purify the water enough so that one day this kind of system can be directly connected to the edge of the Bay. A new phase of research is underway for the levee. Funding from the State Water Resources Control Board recently enabled the reconfiguration of a portion of the existing site to test more efficient designs and treatment of reverse osmosis concentrate (ROC). The results of research, led by UC Berkeley and funded by Valley Water, will help optimize future designs (replicability, treatment capacity, and cost-effectiveness) for other projects in the region and beyond (San Francisco Estuary Partnership, 2022).

Figure 16 Oro Loma Living Laboratory

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5.1.3 Bayshore Bikeway Resiliency Project in South San Diego Bay The Bayside neighborhood in Imperial Beach is one of the City's most vulnerable segments to coastal flooding. The Bikeway and the residential community are situated on south San Diego Bay behind former salt ponds (Pond 10 and 10A) where wetland restoration is occurring. Under existing conditions, the region is prone to flooding during existing extreme tides and projected sea level rise will drastically increase the risk of tidal (non-storm) flooding at this location with projected 3.5 feet of increase sea levels impacting large portions of SR75 and the neighborhood.

The project design incorporates a variety of nature-based features to address SLR. These include: an earthen living levee and ecotone slope that elevates the Class 1 Bikeway to increase flood protection for the elementary school and residents; a multi-purpose detention basin to improve flood retention and control stormwater discharge to the Bay; and the restoration and enhancement of existing tidal marsh with the implementation of the living levee and a bridge to allow tidal flushing.

Figure 17 Bayshore Bikeway Resiliency Project in South San Diego Bay

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5.2 Adaptation Feasibility Approach
OPC 2024 sea level rise guidance advises that local governments consider the risks associated with various Sea Level Scenarios and determine their tolerance for, or aversion to, those risks (OPC, 2024). Timelines and types of projects are identified and characterized as low, medium-high, and extreme risk aversion. For low-risk averse projects, characterized as short-term and lower consequence, it is recommended that the Intermediate Scenario be applied. For medium-high risk averse projects, characterized as long lifespan (2075 and beyond) and higher consequences such as residential and commercial development, the Intermediate-High Scenarios is recommended. Critical infrastructure, including wastewater treatment facilities, are considered extreme risk aversion projects and characterized by little to no adaptive capacity, would be irreversibly destroyed or significantly costly to relocate/repair and would have considerable public health, public safety, or environmental impacts. For extreme risk aversion projects, the High Scenario is recommended. However, OPC acknowledges that limited situations to allow designing and constructing to the High Scenario may be feasible and therefore an adaptation pathways approach is recommended, in which smaller amounts of SLR is incorporated into initial project design while also developing options to address higher future SLR amounts.

Risk aversion is also included in standard engineering design through the use of design criteria that consider extreme low likelihood events and additional factors of safety, such as freeboard, to accommodate additional uncertainty. The OPC Intermediate SLR scenario is described as a reasonable estimate of the upper bound of most likely SLR in 2105 (OPC, 2024). This scenario is utilized for the initial evaluation of the most likely year when an adaptation strategy may no longer meet reference flood design criteria and the evaluation of adaptation pathways. The OPC Intermediate-High and High scenarios are then referenced as context for understanding the earlier failure to meet design criteria should sea levels rise at faster rates. For example, an adaptation strategy that maintains performance for the 1% (1-in-100) annual chance water level of 14 feet and includes 1 foot of freeboard (design elevation 15 feet) would meet design criteria under the Intermediate SLR scenario until approximately 2105. Under the Intermediate-High scenario, the strategy would meet design criteria until 2085 and would exhibit a 50% (1-in-2) annual chance of flooding in 2105.

Under the High scenario, the strategy would meet design criteria until 2075 and would be exposed to flooding approximately six times per year (MMMW) in 2105.

Consistent with Section 3.4 Reference Flood Design Criteria, adaptation strategies were largely focused on their resilience to the 1% annual chance water level (100-year recurrence) event which is the design criteria typically used for wastewater lift stations and treatment facilities.

Figure 18 Still Water Datums and OPC Intermediate SLR Projection (Upper Bound of Most Likely Range of SLR by 2100).

Near-term Mid-term Long-term
Phase I AWTF
Improvements, Levee
Maintenance
Feasibility Planning

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5.3 Collection System

5.3.1 Near-Term Adaptation Strategies

5.3.1.1 I & I Reduction
Table 34 I & I Reduction Strategy
Strategy Continue I&I reduction inspections and repairs Adaptation Types(s) Accommodation
Goal Upgrade and repair collection system assets to improve their resilience and reduce extreme flows to the AWTF
Inflow and Infiltration (I&I) through aged, cracked of damaged collection piping will contribute to high peak flow rates at the AWTF and potential SSO’s. These high flows will combine with SLR related flooding and infiltration, potentially overwhelming the conveyance capacity of the system. Currently the City has an ongoing mitigation effort to reduce I&I to the collection system, with the goal of reducing peak wet weather flows at the AWTF. The City is continually identifying deficient parts of the collection system such as tree root damage, aged pipes and gravity lines that are leaking or compromised at joints. In 2018, the City completed the Infiltration and Inflow Project which repaired and replaced the following components as presented in Figure 19:
– 41,325 linear feet of cured in place pipelining installed – 29 manholes replaced, newly installed, or rehabilitated – 1,200 linear feet of sewer main replaced
– 7,500 linear feet of sewer lateral pipe replaced
– 500 lateral cleanouts installed
– 620 service lateral connections
In addition to the 2018 I&I Reduction Project, the City implements a Sewer Lateral Replacement Program which requires inspection of sewer laterals for buildings and homes over 25 years when the property is sold, when a major remodel ($30,000 and up) is planned, or for projects involving two or more new drainage fixture units. The program is intended to identify aging and deficient laterals which increase I&I to the system. Many sewer collection pipes in low elevation areas near the bay are currently exposed to high year-round to seasonal groundwater. As groundwater levels increase near the Bay with increases in SLR, infiltration could increase if pipeline deficiencies are not addressed. The next I&I reduction project with large scale lining and pipe replacement is scheduled to begin in 2026.

Continuing I&I reduction efforts will help the AWTF accommodate SLR by reducing peak flows to the facility in the future.

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Figure 19 2018 I&I Reduction Project. Orange = Replaced Piping, Purple = Pipe Rehab via Lining.

5.3.1.1.1 I&I Reduction Feasibility
The City has successfully completed and will continue to implement I&I reduction projects. Sewer pipelines once lined and repaired are sealed underground systems that are resilient to the presence of groundwater. In addition to continuing the efforts presented above, efforts for sealing and rehabilitating manholes in low lying regions of the City should be prioritized for preventing inflow from overtopped manhole lids. These practices can be used in the long-term as well to keep the collection system adapted to SLR.

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5.3.2 Mid-Term and Long-Term Adaptation Strategies

5.3.2.1 Elevate Vulnerable Lift Station Components
Table 35 Elevate Vulnerable Lift Stations Strategy
Strategy Elevate vulnerable lift stations
Adaptation Types(s) Accommodation
Goal Maintain function of First Street Lift Station by elevating electrical equipment and/or the structure
By 2075, the First Street Lift Station building is expected to be exposed to flooding multiple times per year and the backup power supply and electrical equipment will no longer meet reference freeboard criteria. The Meadowbrook and Samoa Lift Stations all exhibit elevations above the 1-in-100 annual chance water level through 2105, but do not meet freeboard criteria at the end of the century.

The First Street station is the most vulnerable and would be prioritized for structural improvements to maintain its function. The First Street Lift Station Improvement Project is currently in the design phase with anticipated construction in summer 2026. Elevating electrical equipment and/or the structure’s first floor would enable all the Lift Stations to endure through the end of the century.

5.3.2.1.1 Lift Station Adaptation Feasibility
Elevating Lift station components to mitigate disruptive and damaging flooding is a relatively low cost and simple measure for adapting the collection system to SLR. Vulnerabilities to lift stations can be addressed as part of scheduled maintenance projects in the future. This strategy will not be analysed further in this report, but will be included within the City’s capital improvement programming.

5.3.2.2 Reroute Collection System for Treatment Plant Relocation Table 36 Reroute Collection System for Treatment Plant Relocation Strategy Strategy Evaluate the feasibility of rerouting the collection system Adaptation Types(s) Retreat/Accommodation
Goal Redesign and reroute the collection system to accommodate the relocation of the AWTF treatment plant.

Currently, the Collection system is designed to route flow to the AWTF, which is the lowest elevation point of the system. If the treatment plant is relocated, all inflow through the collection system will need to be rerouted to the new location. This would likely require new pumping facilities, along with upgrades and/or relocations of sewer lift stations and forcemains.

If the relocation of the treatment plant is considered feasible, a comprehensive evaluation of how to feasibly reroute the collection system to the new location would also need to be undertaken. The evaluation would include potential requirements for pumping and electricity usage. Alternately, a new pump station and force main could be installed to move effluent from where it currently flow to the new treatment site location.

5.3.2.2.1 Rerouting the Collection System Feasibility
Rerouting the collection system to predominately gravity flow to a new treatment plant location would require most connections, laterals and gravity lines to be assessed and adapted to gravity flow to the new location. This would require excavation of streets and roads throughout the City. All pump stations would need to be reconfigured to pump to the new location, likely requiring replacement of pumps, resizing of forcemains and overhauling the City’s collection system operation procedures. Assessment of the gravity system in detail was outside of the scope of this study and would require further analysis to determine feasibility. For further assessment of modifying the collection system for

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SLR adaptation in this study, it was assumed that a new pump station and pipeline from the existing AWTF location would be implemented to transfer influent to the new treatment facility location. This would require protection of access to the existing site and protection of a portion of the existing footprint for the new pump station.

5.4 Treatment System

5.4.1 Implementation of Nature Based Protection Strategies As described in the Coastal Commission’s Draft Nature-Based Adaptation Strategies Guidance (CCC 2025), traditional shoreline protective devices, often considered “hard” or “grey” armoring techniques, can “interfere with sediment transport, interrupt natural bluff erosion and beach formation processes, and redirect wave action…which can lead to negative effects like habitat loss and decreased coastal access and recreation space.” Under the right conditions and when thoughtfully designed, nature-based adaptation strategies can provide protection and build resilience to coastal hazards, while providing additional environmental benefits. As presented in Section 5.1, there are local agencies in California trialing the use of “soft” or nature-based only shoreline protection projects. Living shoreline projects are most beneficial when leveraged to help attenuate wave overtopping of infrastructure and where they can provide opportunity for habitat restoration or creation.

Nature-based solutions could include vegetative planting, dune creation, dune restoration, beach nourishment, and living shoreline creation. Designs typically include geometry, elevation, and vegetative components. Geometry typically aims to create less-steep slopes to dissipate wave energy and provide suitable conditions to support vegetative growth. Crest elevations are designed based on water levels and the effects of wind and waves. Vegetation can improve stability of the substrate to provide erosion protection. These systems are considered dynamic and are expected to adjust following construction. Special considerations with respect to the type of solution and characteristics of the local processes are required to implement nature-based solutions so they maintain their function over time, remaining geomorphically stable.

Historically, dunes have not been located along the interior Arcata Bay, and no beaches are present. Vegetation alone would not provide protection from flooding. Therefore, these strategies are not considered appropriate for protection at the AWTF site. A living shoreline is considered below as Arcata Bay has had successful salt marsh restoration projects occur. These projects have been implemented to prevent erosion caused by wind waves and currents.

The 100-year wind wave event is estimated to produce waves just over 3 feet in height with a peak period of 3 seconds (Section 3.2), which has not caused significant overtopping, damage or erosion to the currently vulnerable shoreline protection structures presented in Section 3.5.5. When considering a living shoreline as a protection strategy, they are most beneficial when leveraged to help attenuate wave overtopping of infrastructure. Therefore, with the site’s minor wind wave exposure and no history of damaging overtopping and erosion at the project site, it is anticipated that a living shoreline would provide limited protection for critical infrastructure. The main driver of flooding at the AWTF is still water flooding; therefore, flood protection strategies need to protect and/or elevate existing structures to prevent still water overtopping driven by sea level rise. Instead of a stand-alone strategy, living shoreline components could be used to augment levee improvements to reduce infrequent wave overtopping potentially extending the lifespan of improvements. Living shorelines could also provide secondary benefits which could be used to provide enhancements to meet the requirements of the EBEP or other oversight regulations.

The existing AWTF and Enhancement Marsh levees were built on historical mudflat beyond the extent of historical salt marsh habitat (Figure 20). Therefore the living shoreline would be converting historic mudflat to salt marsh. The feasibility of creating a geomorphically stable feature is a challenge on mudflats, as the location has not historically demonstrated favorable conditions to support salt marsh. Sediment or fill placed on the outside of the existing levees to create the required slope for the living shoreline would be difficult to stabilize without extensive subgrade preparation (excavating down several feet into the existing mudflat and replacing with engineered fill), leading to slumping, erosion and eventually failure of the slope.

The Natural Shoreline Infrastructure project is being implemented along a section of shoreline that was historically salt marsh, indicating that the existing geomorphic conditions are favorable for salt marsh habitat development. Even in a

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favorable location, a hybrid approach with a cobble or shingle beach fronting the salt marsh to stabilize the marsh will be necessary. The projects presented in Section 5.1 are generally located in areas which see significant exposure to waves and currents that drive shoreline erosion and overtopping.

Figure 20 1870 Salt Marsh Extents at the AWTF Location.

5.4.1.1 Feasibility of Nature Based Protection Strategies Implementing a stable nature-based only living shoreline at the AWTF and Enhancement Marshes would be difficult and would not provide effective flood protection to the facilities without elevating the existing structures as well. With the challenges presented above, implementing a successful nature-based only living shoreline strategy is considered infeasible and ineffective. Further analysis will be limited to presenting nature-based strategies as a hybrid component to hard infrastructure for levee maintenance and augmentation projects.

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5.4.2 Near-Term Adaptation Strategies

5.4.2.1 Phase 1 Improvements
Table 37 Phase 1 Improvements Strategy
Strategy Phase I Improvements elevating essential electrical equipment Adaptation Types(s) Accommodation
Goal Address vulnerability of electrical equipment to 2055 SLR threats As presented in Table 4 typical elevations of essential treatment facilities range from 10.0 to 14.4 feet. Phase I improvements elevate essential electrical equipment to 14 feet, achieving 2 feet of freeboard above the 1% annual chance water level through approximately 2055. Under the High SLR scenario, the freeboard is reduced to 1 foot through 2055.

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5.4.2.2 AWTF Levee Maintenance
Table 38 Minor AWTF Levee Improvements Strategy
Strategy Maintain Levees at 11.5 feet NAVD88
Adaptation Types(s) Accommodation
Goal Protect AWTF assets and access from to 2055 SLR threats Based on the vulnerabilities presented in Section 3.5.5, the AWTF headworks and generator building are at risk of nuisance flooding (less than 6 inches) during the current 100-year tidal flood event along the northeast side of the facility. This levee section is comprised of paved and dirt trails and is not exposed to wind waves. Therefore, this section would not benefit from living shoreline protection. Up to approximately 4,500 lineal feet of low-lying sections of levee would be elevated to 11.5 feet NAVD88 to protect the site through 2055 under the intermediate SLR Scenario.

This elevation was selected to match the existing typical crest elevation of the existing levees and provide consistent protection along the length of the levee system.

Figure 21 Nuisance Flooding over Bay Trail into NW Corner of AWTF at approximately 10-foot NAVD88 tide on January 3rd, 2026

5.4.2.2.1 AWTF Levee Maintenance Feasibility
Maintaining and elevating the low-lying levees protecting the AWTF would be a relatively simple and cost-effective adaptation strategy to protect the AWTF from flooding vulnerabilities. It is anticipated that maintenance actions will need to be implemented at the AWTF regardless of the long-term adaptation strategy selected for protection of the AWTF. Not maintaining the levees will result in more frequent overtopping events as SLR progresses, leading to erosion and eventually failure of the levees protecting the AWTF, which would result in partially treated wastewater discharged to Humboldt Bay.

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5.4.3 Mid-Term to Long-Term Adaptation Strategies

5.4.3.1 AWTF Levee Augmentation Project
Table 39 Levee Augmentation Project
Strategy Proposed Levee Augmentation Project
Adaptation Types(s) Protection
Goal Elevate the existing levee to 15ft to protect the central plant facilities Select locations of the perimeter levee around the AWTF are currently vulnerable to the 100-year tidal water level flood of 10.7 feet NAVD88. The City is currently working on the Levee Augmentation Project which proposes to elevate the perimeter levee around the core treatment facilities to 15 feet. The project is currently at 30% design. The proposed augmentation would provide enhanced flood protection for the AWTF core infrastructure, which includes:
– Headworks
– Primary Clarifiers
– Oxidation Ponds
– Treatment Wetlands
– Disinfection
– Corp Yard
The proposed Levee Augmentation Project would protect the AWTF to water surface elevation of 14 feet with 1 foot of freeboard. A levee with a crest elevation of 15 ft would meet or exceed the design criteria to approximately 2105.

Under the Intermediate-High and High scenarios, this duration is reduced to 2085 and 2075, respectively. The south and west levees are exposed to wind wave action, with a 100-year wind event producing waves of just over one foot.

A living shoreline could be implemented along the levees to help potentially extend the design life of the improvements. However, this will significantly increase the costs and disturbances of the project with limited flood protection benefits. Implementation of living shoreline as part of the protection strategy versus armoring alone for the levees will be considered in the alternatives analysis.

5.4.3.1.1 AWTF Levee Augmentation Feasibility
Based on the 30% design completed by SHN, the AWTF Levees can be elevated to 15 feet with hard armoring strategies such as concrete floodwalls, sheet piles and RSP. The proposed improvements are designed to stay within the existing footprint of the AWTF site and within previously disturbed areas, with no permanent conversions of wetlands or mudflat surrounding the AWTF. Implementation of a living shoreline along the new levee improvements would result in a permanent conversion of mudflats to another habitat type and gradient. Based on an initial concept of creating living shoreline along 4400 feet of levee that is exposed to wind wave action, with a 10:1 slope from 15 to 5 feet NAVD88, there would be approximately 7.7 acres of converted mudflat. With this conversion area, implementation of a living shoreline has greater disturbed area, higher costs and limited protection improvements than hard armoring within the existing, previously disturbed footprint alternative.

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5.4.3.2 Decentralized Treatment
Table 40 Identify opportunities for decentralized treatment Strategy Identify opportunities for decentralized treatment Adaptation Types(s) Accommodation
Goal Reduce flows to ensure AWTF effectiveness and scale Decentralized Treatment utilizes small-scale, dispersed treatment systems throughout a municipality to reduce the required capacity of a central treatment plant, and to reduce piping and pumping infrastructure to move wastewater through the system. These small-scale treatment systems could be used in conjunction with a central plant to pretreat influent, allowing for reduced treatment requirements at the plant. These smaller systems can also be used to redirect peak wet flows away from the central plant, often referred to as “scalping”, allowing the central plant to be designed to lower flow requirements. This is particularly useful in systems with large amounts of I&I. The “scalped” wastewater would be treated at a small package plant and used nearby. Typical uses would be irrigation or industrial reuse. The size of the system would be based on the proposed volume of wastewater to be treated. System could range in size from small package plants that could be sited in an area as small at 10,000 square feet or more conventional systems that could range in size from a half-acre to one acre.

Considerations for a decentralized system include treatment purpose (reuse or discharge), need, seasonality, reliability, capital and operations costs, site control and public health protection, and public acceptance. A decentralized treatment system for discharge only would require a new Regional Board permit and discharge point, increased operations costs to monitor and maintain multiple facilities, and would not include any beneficial reuse.

5.4.3.2.1 Decentralized Treatment Feasibility
Decentralized Treatment for discharge (irrigation) is often implemented in areas where potable water is scarce, especially in the summer. Currently, the City only irrigates parks in the summer months, which is when peak wastewater flows are at their lowest, which would not reduce the required peak wet weather treatment volume. The City obtains its water for irrigation from HBMWD, which is a reliable and cost-effective supply, making scalping for irrigation less financially feasible. Decentralized treatment is not considered a stand-alone treatment option but could be implemented to supplement other strategies. Based on the limited demand and utility of decentralized treatment for disposal of City effluent, further analysis of Decentralized Treatment will not be considered. If future opportunities for decentralized treatment emerge, the City should evaluate the potential projects on a case-by-case basis.

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5.4.3.3 Relocate AWTF
Table 41 Relocate AWTF
Strategy Relocate the Treatment Plant
Adaptation Types(s) Retreat
Goal To relocate the treatment facility to an area with an elevation of at least 14 ft.

Relocating the AWTF would require selecting a new location large enough to house the new treatment facilities, and at a location with an elevation that limits pump influent to a much higher elevation to reduce energy usage.

As an initial step, the area needed for a new wastewater treatment plant was evaluted. Five different potential technologies were evaluated, including Conventional Activated Sludge, Extended Aeration or Oxidation Ditch, Membrane Bioreactor, Lagoons, Ponds or Wetlands System, and Trickling Filter/Biofilters. Treatment site areas for a 6 mgd plant ranged from 7 acres to 148 acres, as shown in Table 42. For this study it was assumed that a minimum of approximately 25 acres would be needed for a new facility based on the range of treatment technologies evaluated.

Table 42 Planning Level Footprint Sizes for Wastewater Treatment Facilities Based on Technology Type Treatment Type Footprint Acres per MGD of Influent Typical Required Area for 6 MGD Treatment Plant (acres)
Conventional Activated Sludge 2 – 5 21
Extended Aeration or Oxidation Ditch 3 – 6 27
Membrane Bioreactor 0.5 – 2 7
Lagoons, Ponds or Wetlands System 10 – 40 148
Trickling Filter/Biofilters 1.5 – 3 13
In addition to the area needed for the treatment system, consideration was given to the hydraulics of the system, with the goal to maximize the benefits of gravity flow and reduce the need for pumping of influent to the AWTF. To achieve this, a preliminary analysis was conducted to assess areas in and adjacent to the City boundary ranging in elevation from 13 to 30 feet (Figure 22). Only undeveloped, industrial or agricultural parcels in close proximity to the City were considered for this analysis. It was assumed that sites with some areas exhibiting elevations of 13 feet could be raised for flood protection.

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Figure 22 AWTF Potential Relocation Areas

5.4.3.3.1 AWTF Relocation Feasibility
Relocating the AWTF would include site selection, land acquisition, planning, permitting, engineering design and construction. The site selection and land acquisition alone could take 5 to 10 years and would include an evaluation of appropriate parcels considering former use/ legacy pollutants, sensitive habitats, topography, land use designation, and other factors. This would be followed by coordination with existing owners and assumes enough acreage can be agreed upon without using eminent domain. If an agreeable land seller(s) can be found with enough land for a new treatment facility, the City would then conduct land appraisals, find funding for the new land, conduct permitting, environmental documents, and design, before finally constructing. It is anticipated that it would likely take a minimum of 20 years and as long as 30 years to site and construct a new treatment facility.

While relocation is feasible, the current AWTF would require some flood protection improvements to protect assets in the near-term to provide enough time to plan and construct a new facility.

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5.5 Discharge/ Enhancement Marshes
As presented in Section 2.3.6, the purpose of the Enhancement Marshes is to provide enhanced secondary treatment to effluent from the AWTF before discharge to comply with the EBEP. Based on recent RWQCB communications, the requirements to meet the conditions for EBEP definition of enhancement are found in the State Water Resources Control Board (SWRCB) Order No. 79-20 and are excerpted as follows:

Enhancement, as it is presently defined in a memo dated October 21, 1974 from Bill Dendy, a former Executive Officer of the State Board, to Dr. David Joseph, Executive Officer of the Regional Board, requires:
...(l) full uninterrupted protection of all beneficial uses which could be made of the receiving water body in the absence of all point source waste discharge along with (2) a demonstration by the applicant that the discharge, through the creation of new beneficial area or a fuller realization, enhances water quality for those beneficial uses which could be made of the receiving water in the absence of all point source waste discharges, In short, "enhancement" is interpreted in the memo to require not only to provide full protection of beneficial uses which the body is capable of supporting but also yield a positive water quality benefit."
As specifically applied to Humboldt Bay, the Board interprets the enhancement provision of the Bays and Estuaries Policy to require: (1) full secondary treatment, with disinfection and dechlorination, of sewage discharges; (2) compliance with any additional NPDES permit requirements issued by the Regional Board to protect beneficial uses; and (3) the fuller realization of existing beneficial uses or the creation of new beneficial uses either by or in conjunction with a wastewater treatment project. The latter requirement could conceivably be met by the creation of additional marshlands or wetlands, such as is proposed by Arcata.
….the Board is of the opinion, based upon the above findings, that there is a reasonable probability that they could do so through a wastewater treatment project or projects which provide consistent and reliable secondary treatment, comply with the Regional Board's NPDES requirements, and involve the creation of additional marshlands or wetlands or other enhancing factors.

Based on the excerpt above, the main requirement for enhancement is the fuller realization of existing beneficial uses or the creation of new beneficial uses to provide a positive water quality benefit to the bay. Water quality benefits at the City’s existing Enhancement Marshes are measured through monitoring of nutrients, dissolved oxygen and pH at the inlet and discharge of all three enhancement marshes three times per month. The monitoring consistently shows that the Enhancement Marshes increase the water quality before discharge to Humboldt Bay. Other beneficial uses of the marshes are described further below.

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5.5.1 Existing AWTF and Enhancement Marshes Ecological and Community Benefits
In addition to providing wastewater treatment and direct enhancement to the water quality of Humboldt Bay, the AWTF, Enhancement Marshes and surrounding area provide a multitude of ecological and community benefits. The benefits provided can be broken down into Freshwater Wetland, Brackish Wetland and Community Benefits. It is important to note that the benefits discussed below are closely intertied to create a thriving ecosystem, and breaking up the treatment, enhancement and recreational benefits will reduce the overall benefits provided by the system.

5.5.1.1 Freshwater Wetland Benefits
The AWTF Oxidation Ponds and Enhancement Marshes provide a unique freshwater habitat on the shore of Humboldt Bay. The freshwater wetlands provide habitat for amphibians such as Northern Red-legged frogs and Northern Pacific tree frogs, as well as many species of freshwater invertebrates such as Diving beetles and Scud. Additionally, many vegetation species such as reeds, cattails and duckweed that are used for secondary treatment of wastewater are also a favourable food source for the more that 300 migratory bird species which use the marshes as a stopping point during their spring and fall migrations. Mammals such as Virginia opossum, raccoon, grey fox, and river otters have been observed in and around the enhancement marshes, including a multigenerational river otter family, indicating that the marshes are a favourable habitat to rear young.

5.5.1.2 Brackish Wetland Benefits
Klopp Lake and Brackish Marsh experience a muted tide with tidal exchange limited with culverts and gates, creating a unique brackish wetland habitat sheltered from large tidal surge events. This sheltered habitat provides favourable conditions for a portion of Humboldt Bay fishes to complete their various lifecycle phases. Fish eating birds such as cormorant, pelicans, egrets, heron, kingfisher, and mergansers follow the fish as well as Humboldt Bay river otters.

5.5.1.3 Community Benefits
The Enhancement Marshes provide secondary benefits to the local community with 5 miles of trails and open space for recreation. The Arcata Marsh Interpretive Center provides educational opportunities for visitors with information on the natural treatment system benefits, that includes services provided to wildlife. Based on counts performed in 2025, the were over 11,500 visitors to the Arcata Marsh Interpretive Center, and an average daily usage of 416 pedestrians and 369 cyclists utilizing the trails.

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5.5.2 Near-Term Adaptation Strategies

5.5.2.1 Enhancement Marshes Protection Structures Maintenance Table 43 Enhancement Marshes Protection Structures Maintenance Strategies Strategy Elevate the sections of the berm around the enhanced marshes that are 10ft or under Adaptation Types(s) Protect/ Nature-Based Solution
Goal Protect and sustain remaining sensitive wetland habitats, continue to conform to EBEP, and provide extended resilience for access roads
While Phase I improvements are elevating electrical equipment and the Levee Augmentation Project would protect the treatment facility, there are currently no actions in progress to protect the Enhancement Marshes. The Enhancement Marshes provide enhanced treatment required under the City’s discharge permit and conformance with the EBEP.

The Enhancement Marshes are currently protected from tidal inundation and flooding by berms. Many of the berms also serve as access roads in and around the facility. The berms were originally constructed to retain wastewater during treatment, and not as flood protection structures.

Based on the risk assessment presented in Section 3.6.4, the Enhancement Marshes are at risk of flooding during the current 100-year tidal flood event. The critical flood paths for overtopping the Enhancement Marshes are along South I Street into the northwest corner of Hauser Marsh, along South I Street from the north into the Gearhart Marsh, and along the eastern edge of Allen Marsh (Figure 23). The most vulnerable location to overtopping is the northwest corner of Hauser Marsh, which is protected by South I Street. Based on the Intermediate OPC Scenario, in 2055, this section of South I Street is projected to overtop up to six times per year and have a 1-in-10 annual chance of potential failure. This vulnerability is moved up to 2045 and 2040 for the intermediate-high and high scenarios respectively. This location of I Street is currently vulnerable to a 10-year tidal flood event of 10.1 feet. Based on field observations during a 10.03-foot NAVD88 water level on January 3rd, 2026, South I St at Hauser Marsh was overtopped by approximately 6-8 inches with flow into Hauser Marsh (Figure 24). At the north end of South I Street, flow was also observed along I Street into Brackish Marsh (Figure 25).

Given the ecological value of the Enhancement Marshes, there is secondary ecological value in upgrading the lowlying sections of the levees. This will enable the Enhancement Marshes to continue functioning as a freshwater wetland ecosystem benefiting the species that currently rely on this ecosystem (e.g. red-legged frogs, several avian species) through 2055, with potential to protect through 2075. Additionally, it will allow the community to continue using the roads and trails throughout the Enhancement Marshes.

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Figure 23 Enhancement Marshes Levee Vulnerability

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Figure 24 Approximately 10-foot NAVD88 Water Level briefly overtopping South I St into Hauser Marsh on January 3rd, 2026.

Figure 25 Approximately 10-foot NAVD88 Water Level flowing down South I St into Brackish Marsh on January 3rd, 2026.

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5.5.2.1.1 Enhancement Marshes Protection Maintenance Feasibility Maintaining the low-lying levees protecting the Enhancement Marshes would be a relatively simple and cost-effective adaptation strategy to protect the enhancement marshes from flooding vulnerabilities. It is anticipated that maintenance actions will need to be implemented at the Enhancement Marshes regardless of the long-term adaptation strategy selected for protection of the AWTF. Not maintaining the levees will result in more frequent overtopping events as SLR progresses, leading to erosion and eventually failure of the levees protecting the Enhancement Marshes, which would result in partially treated wastewater discharged to Humboldt Bay.

5.5.3 Mid-Term to Long-Term Adaptation Strategies

5.5.3.1 Determine Adaptative Capacity of Enhancement Marshes to Saltwater Intrusion Table 44 Determine Adaptative Capacity of Enhancement Marshes to Saltwater Intrusion Strategy Monitor and study the adaptive capacity of the enhanced marshes to overtopping Adaptation Types(s) Accommodate/Nature-based solution
Goal Determine conditions if some of the marshes can persevere under future conditions.

As presented in Section 5.5.2.1, critical flood paths for overtopping the Enhancement Marshes are along South I Street into the northwest corner of Hauser Marsh, along South I Street from the north into Brackish Marsh, and along the eastern edge of Allen Marsh (Figure 23). Even if low areas are elevated to 11.5 feet as proposed in the earlier adaptation strategy, then it is anticipated that overtopping events would begin to occur semi-annually in the midcentury and become more frequent in late century.

The Enhancement Marshes can likely accommodate some increase in salinity from overtopping events while still maintaining enhancement treatment performance. However, the volume and duration of inflow from overtopping events that could be accommodated is not known. Furthermore, overtopping pathways demonstrate that waters will originate from different sources (e.g. Humboldt Bay, Butcher Slough, South I pond) that likely vary in salinity levels and other water quality aspects. Depending on the future conditions of these adjacent waterways, it is possible that some of these marsh areas could maintain their biodiversity and treatment functions even with mild to moderate overtopping.

Further research and studies would support an analysis to determine the saline inflow that the marshes can accommodate while still maintaining vegetation, benefiting species, and meeting discharge requirements. These studies could also help to determine if some of the marshes could naturally adapt to changing conditions.

5.5.3.1.1 Enhancement Marshes Adaptive Capacity Evaluation Feasibility The City is conceptualizing a study to determine the adaptive capacity of the Enhancement Marshes. The study as currently conceived as of the writing of this report would be a multi-year study conducted by the Arcata Marsh Research Institute (AMRI) with Cal Poly Sponsored Programs in a controlled wetland environment that would be designed to answer questions such as:
– What duration and magnitude of saltwater intrusion can a freshwater marsh recover? – What is the timeframe for recovery and what is the new normal after recovery? – Does the marsh function as a wastewater treatment unit in the presence of saltwater? – What plant species are best suited for periodic saltwater intrusion? While the outcomes of the study are uncertain, AMRI and Cal-Poly Humboldt have extensive experience studying the marshes and have the resources and expertise to execute the studies. This strategy is further considered in the evaluation of alternatives.

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5.5.3.2 Enhancement Marshes Levee Augmentation
Table 45 Enhancement Marshes Levee Augmentation
Strategy Elevate levees surrounding Enhancement Marshes to 15 ft Adaptation Types(s) Protect/Nature-based solution
Goal Protect Enhancement Marshes through approximately 2105 To protect the Enhancement Marshes in place through 2105, all of the berm complex would need to be elevated to 15 feet to provide the same level of protection as the Levee Augmentation Project would provide for the AWTF. Without protection of the Enhancement Marshes, the City would no longer meet the requirements of the EBEP and would require an alternative enhancement method, or development of a discharge outside of the bay.

5.5.3.2.1 Enhancement Marshes Levee Augmentation Feasibility The Enhancement Marsh levees can be elevated to 15 feet with hard armoring strategies such as concrete floodwalls, sheet piles and RSP. Improvements could likely be designed to stay within the existing footprint of the Enhancement Marshes, with minimal disturbance of wetlands and mudflat surrounding the marshes. Implementation of a living shoreline along with levee improvements could also be considered, but would likely require the permanent conversion of mudflats and may not be geomorphically stable. This strategy is considered further in the evaluation of alternatives.

5.5.3.3 Enhancement Marshes Relocation
Table 46 Relocate Enhancement Marshes
Strategy Relocate the Enhancement Marshes
Adaptation Types(s) Retreat
Goal To relocate the Enhancement Marshes to an area with an elevation of at least 14 ft.

To minimize impacts on pumping requirements from the AWTF to the Enhancement Marshes, potential locations for relocating the Enhancement Marshes should be as low as possible while being above SLR inundation water levels.

This would minimize the energy required for pumping to the Enhancement Marshes, while still being above the design level for the 1% annual chance flood zone.

5.5.3.3.1 Enhancement Marshes Relocation Feasibility
Relocating the Enhancement Marshes would follow the same analysis as relocating the AWTF. A preliminary analysis for relocating the AWTF was conducted to assess areas in and adjacent to the City boundary ranging in elevation from 13 to 30 feet (Figure 22). It was assumed that sites with some areas exhibiting elevations of 13 feet could be raised for flood protection. It is anticipated that it would likely take a minimum of 20 years to site and construct relocated enhancement marshes. It may also be infeasible to relocate the marshes and retain the benefits of the existing Enhancement Marshes, which have been in place for almost 50 years. Relocation of the Enhancement Marshes is not considered further in this report. However a new enhancement project is considered below.

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5.5.3.4 Alternative Enhancement for Bay Discharge
Table 47 Alternative Enhancement
Strategy Continue Bay discharge but without the existing Enhancement Marshes Adaptation Types(s) Retreat/Accommodation
Goal Meet EBEP requirements with new technology or other wetland facilities As presented under the vulnerability and risk assessment, the City’s Enhancement Marshes are vulnerable to sea level rise. The treatment function of the marshes could be replaced with mechanical systems. However, as discussed in Section 2.3.6 above the marshes also provide beneficial enhancement to the Bay which is a compliance requirement under the EBEP for discharge into Humboldt Bay.

In 1979, the State Water Board held a fact-finding hearing on the application of the EBEP to waste discharges to Humboldt Bay. Following the hearing, the SWRCB adopted Order WQ 79-20. As specifically applied to Humboldt Bay, the SWRCB interpreted the enhancement provision of the Bays and Estuaries Policy to require: (1) full secondary treatment, with disinfection and dechlorination, of sewage discharges; (2) compliance with any additional NPDES permit requirements issued by the Regional Board to protect beneficial uses; and (3) the fuller realization of existing beneficial uses or the creation of new beneficial uses either by or in conjunction with a wastewater treatment project.

The SWRCB’s EBEP requires phasing out municipal and industrial wastewater discharges to bays such as Humboldt Bay unless they enhance water quality. Per the EBEP, discharges must remove persistent pollutants to the maximum extent practicable, prevent untreated bypasses, minimize pollutant concentrations through effective treatment and dilution, and protect beneficial uses, including fisheries, recreation, and wildlife habitat.

5.5.3.4.1 Enhancement Criteria
Clarity has not been provided detailing what types of projects could be considered to provide fuller realization of existing beneficial uses or the creation of new beneficial uses either by or in conjunction with a wastewater treatment project. On February 19th, 2026, the RWQCB issued Resolution R1-2026-0005 for Project Criteria for an Exception to the Enclosed Bays and Estuaries Policy and the Prohibition of Waste Discharges to Humboldt Bay applicable specifically to the City of Eureka wastewater treatment facility. If Arcata were to pursue a new enhancement project, a similar resolution to the City of Eureka exception would be required. The resolution provided further information on the criteria for a potential exception to the EBEP. A summary of the classes of projects that may create or further enhance, support or protection Beneficial uses is presented below.

Climate Adaptation and Resilience
• Relocation or protection of vulnerable infrastructure (including wastewater infrastructure) • Reengineering of drainage and stormwater conveyance infrastructure or restoration of streams and other waterways draining to the Bay
• Improving or relocating infrastructure, protecting in-place, managed retreat of infrastructure and/or implementing nature-based solutions or hybrid approaches • Utilizing nature-based solutions such as living shorelines, horizontal levees, eco-tone slopes, or wetland or marsh habitat enhancement can attenuate wave energy, reduce erosion, improve water quality, and enhance beneficial use support
Habitat Restoration and Creation
• Create and restore wetlands to improve Humboldt Bay water quality, increase resilience, and support estuarine and aquatic habitats
• Create wetlands and other features at the end of the treatment processes to provide effluent polishing and enhance beneficial uses while also adding or enhancing wetland habitat at the end of the treatment process for polishing of effluent

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• Restore natural hydrologic features such as stream corridors, groundwater recharge areas, floodplains, and wetlands
• Re-establish historic salt marsh areas, or create new salt marsh areas, would allow for the fuller realization of Beneficial Uses
Removal of Legacy Pollutants Impacting the Bay
• Remove legacy pollutants (pollutants outside the responsibility of the City) impacting the Bay Disadvantaged Communities and Public Health
• Implement measures to provide access to clean water and public health, safety and welfare for disadvantaged communities (include, but are not limited to, mobile home parks and similar residential uses) • Sewer unsewered areas around Humboldt Bay. Sewer projects will need to demonstrate effectiveness and pollutant removal that would be otherwise discharged without expanding sewer service to unsewered areas

5.5.3.4.1 Alternative Enhancement for Bay Discharge Feasibility Many of the classes of enhancement projects included in the RWQCB’s Draft Resolution could be used to meet the exception criteria of the EBEP. However, the scope, quantity and extent of these projects was not defined in the resolution. Further discussion with the RWQCB will be required to determine what would be accepted as to meet exemption project requirements. This alternative is considered feasible and further discussed in this report.

5.5.3.5 Ocean Discharge
Strategy Develop an alternative ocean discharge/outfall Adaptation Types(s) Retreat/Accommodation
Goal Establish a new ocean discharge location that would not be subject to EBEP standards Throughout California, there are many coastal municipalities that utilize ocean outfalls for effluent disposal. Ocean outfalls consist of three components:

1. Onshore pumping

2. Transmission pipeline

3. Diffuser
The onshore pumping system would be designed to move treated effluent from the treatment plant through a transmission pipeline to the outfall diffuser. The diffuser is the final segment of the pipeline which has ports evenly spaced along the pipeline to discharge the effluent and promote mixing with the surrounding ocean. The transmission pipeline would require routing around the Bay or across the Arcata Bottoms following existing roads, directionally drilling under Humboldt Bay, or a combination of the two. As the City is a significant distance from the Ocean, regardless of route, the energy costs and carbon emissions would increase with this option. Based on current land use zoning, state and federally protected lands and waters, site access and existing outfall infrastructure, a new outfall on the Samoa Peninsula or near Mad River Beach are considered to be the most feasible locations for a new outfall. The existing Redwood Marine Terminal II Outfall on Samoa Peninsula is also a potential discharge point.

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5.5.3.5.1 Regulatory Requirements
Ocean outfalls in California are regulated by the California Coastal Commission, US Army Corps of Engineers, Regional Water Quality Control Board and US Fish and Wildlife department, as well as County and State land use requirements. A preliminary list of expected permits required for a new Ocean Outfall is presented in Table 48 below:

Table 48 Permits Required for Ocean Outfall
Agency Permit
California Coastal
Commission
Coastal Development Permit
US Army Corps of
Engineers
Clean Water Act Section 404 Permit
Regional Water
Quality Control
Board
Clean Water Act Section 401 Certification- shoreline construction as regulatory waters NPDES (Includes Cooperation with California Department of Fish and Wildlife Marine Unit and California Coastal Commission monitoring and reporting requirements) US Fish and
Wildlife Service
Marine Mammal Protection Act Incidental Harassment Authorization (hydroacoustic impacts during construction)
National Marine Fisheries Service consultation (Endangered Species Act Section 7) and Essential Fish Habitat consultation (Magnuson–Stevens Fishery Conservation and Management Act) County County Use Permit (required if terrestrial development occurs) State States Land Commission Lease
Additionally, the Coastal Act stipulates limitations on new outfalls that are not for coastal dependent uses. From discussions with Melissa Kraemer of the Coastal Commission, the following guidance was presented for development of new outfalls in California:

“ …for example, for a coastal-dependent facility like a live fish holding facility or a desalination plant, the Coastal Act permits new ocean intake and outfall lines. But for other non-coastal dependent uses, including “incidental public service purposes,” it’s less clear that brand new outfalls can be permitted (the language is ambiguous). On the other hand, even if an interpretation is made that the Coastal Act disallows new outfall lines for non-coastal dependent uses, there are other provisions under the Coastal Act that would allow the permitting of such new development if on balance permitting the outfall line would be more protective of coastal resources (e.g., water quality) than not permitting it. So, for example, if the City of Arcata needed a new ocean outfall line to discharge its treated wastewater to the ocean in lieu of discharging it to Humboldt Bay in violation of [the RWQCB’s] water quality standards, the Commission may be able to approve a new ocean outfall line if doing so would be more protective of water quality than not approving the line. However, we’d want to see alternatives evaluated (e.g., could they use the existing Samoa [Redwood Marine Terminal II] outfall line instead) that would avoid the need for a new outfall line.” - Melissa Kraemer, North Coast District Manager, California Coastal Commission Based on the guidance presented above, the City will first consider the use of the existing Redwood Marine Terminal Outfall before pursuing a brand new outfall, as well as collect and present evidence that an ocean outfall is more protective of local water quality than other discharge alternatives.

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5.5.3.5.2 Transmission Pipeline Feasibility
A pipeline from the AWTF to the outfall would need to be constructed for an ocean discharge. As previously mentioned, an outfall on the southern end of Samoa Peninsula or near Mad River Beach are considered the most feasible options. Potential alignments and sensitive areas are shown in Figure 26. The sensitive areas to be avoided for a new transmission pipeline are listed below:
– Coast directly west of the City is US Fish and Wildlife Humboldt Bay Wildlife Refuge – Samoa Marine Conservation area directly offshore
– Wildlife Refuge and Marine Conservation Area
– Environmentally Sensitive Habitat (ESHA)
– Independently owned conservation areas such as Friends of the Dunes, and the community of Manila.

To the north, ocean outfall opportunities are limited by Mad River. However, there is a potential corridor for an outfall between the northern edge of the protected areas and the southern bank of the Mad River. To transmit effluent to the peninsula, the pipeline could wrap around the Bay generally following State Route 255 to the outfall, or could be directionally drilled underneath the Bay to a point on the peninsula and then be routed to the outfall. Either alignment would be a minimum of approximately 7.5 miles. Similarly, from the AWTF to the Mad River Beach discharge location would also be approximately 7.5 miles, either following existing county roads or directionally drilling segments of the alignment. To serve the discharge pipeline, a discharge pump station at the AWTF facility would be required, as well as a potential booster pump station near the midpoint of the pipeline. Extensive design and hydraulic modelling would be required to select the alignment, pipe size and pumping requirements for the transfer pipeline. While a new transmission line from the AWTF to an ocean outfall location is a large and complex project, it was determined permitting, design and construction of the pipeline would be a feasible project and will be further analysed.

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Figure 26 Potential Pipelines and Oceans Outfall Locations Adjacent to the City of Arcata

5.5.3.5.3 Redwood Marine Terminal II (RMTII) Outfall Feasibility GHD has previously contacted Humboldt Bay Harbor, Recreation, and Conservation District (Harbor District) regarding the condition and capacity of the RMTII existing outfall pipe. It was determined that upgrades to port diffusers and other improvements would be required to increase the likelihood of compliance with the California Ocean Plan. The Harbor District indicated the outfall capacity of RMTII is approximately 30 million gallons per day (MGD). Current uses and planned commitments to the RMTII outfall sum approximately 13 MGD, which leaves 17 MGD of uncommitted capacity. The peak instantaneous discharge flow of the AWTF has been measured as 16.5 MGD, with a peak daily flow of 7 MGD, indicating that the RMTII outfall has sufficient capacity for the City’s effluent. Future growth of the City may require development of a storage facility for effluent attenuation during peak flow events.

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The City’s use of the RMTII outfall could limit and constrain future coastal dependent industrial uses on the Samoa Peninsula, potentially conflicting with existing economic and land use development planning underway by the Economic Development Division of Humboldt County and the Harbor District. Further coordination with the Harbor District and other stakeholders should be conducted to assess the long-term feasibility and impacts to the peninsula future growth from the City utilizing RMTII for discharge.

On August 13th, 2025, the City of Arcata, GHD, RCAC and the Harbor District met to further discuss potential for the City to utilize RMTII for discharge. The Harbor District confirmed the capacity, condition, and current planned future uses of the outfall could allow the City to use RMTII. The Harbor District was open to further discussing and planning of the concept and this alternative is considered further in this report.

5.5.3.5.4 New Outfall Feasibility
Two potential new ocean outfall locations were considered. First, on the southern end of the Samoa Peninsula south of RMTII, or just south of the Mad River near Mad River Beach. Either alternative would require similar costs for design engineering, permitting, and construction. If the treatment plant was relocated to the northern side of the City, the Mad River Beach outfall would be a shorter alignment, but extends into remote agricultural land without significant industrial infrastructure in place to support the construction and operation of the outfall. Additionally, access to this location is impacted by SLR and fluvial flooding of the Mad River.

The Samoa Peninsula outfall alternative would be located in a previously developed industrial area south of the existing RMTII Outfall. New development on the Peninsula such as the Humboldt Bay Heavy Lift Terminal is planned for the future, in addition to the existing communities of Samoa, Manila and Fairhaven. It is assumed that access to the Peninsula will be maintained in the future around the impacts of SLR.

Based on the guidance from the Coastal Commission, permitting a new outfall would be a significant effort with an assessment of the impacts to prove the Outfall is the least harmful to water quality in the area. A new outfall will also require extensive design and engineering of the pumping station(s), transmission pipeline, and diffuser, along with dilution modeling of the effluent entering the ocean. Typically, a new ocean outfall is expected to extend two to five miles offshore to meet dilution requirements and minimize impacts to coastal resources. With the challenges presented in this section and the previously present favorable response from the Harbor District regarding the City utilizing RMTII, further analysis of a new ocean outfall will not be pursued.

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5.5.3.6 Land Application and Reuse
Table 49 Land Application and Reuse
Strategy Establish a land-based discharge, surface-water discharge, or a combination of these options Adaptation Types(s) Retreat/Accommodation
Goal Establish a new discharge that would not be subject to EBEP standards Throughout California, many municipalities utilize secondary treated wastewater effluent for agricultural irrigation. The effluent is applied to agricultural lands to meet the agronomic demand of the crop, or how much water the crop can uptake at a given time. This rate varies throughout the year based on many factors, with the primary drivers being rainfall, infiltration to groundwater, and the growth stage of the crop. Typically, crops can uptake more water during the spring and summer due to high growth rates and high temperatures leading to more evaporation. During the fall and winter, agronomic water demand is lower due to more rainfall reducing the need for irrigation, lower temperatures reducing evaporation, and lower plant growth rates due to less sunlight. Due to the heavy winter precipitation, cool summers and high groundwater, land application of effluent on the North Coast requires significantly more area per volume of effluent than other regions of California.

5.5.3.6.1 Water Balance Model
To assess the feasibility of land application of effluent from the AWTF, a preliminary water balance was conducted based on existing AWTF effluent flow data, local climate data, and local soil property infiltration data to estimate the required land application area for disposal.

WWTP Effluent Flow Data
Effluent flow data was provided by the City of Arcata to GHD on July 16, 2024. Daily flow data was provided in Million Gallons per Day (MGD) from January 1, 2019 through March 31, 2024. The data was analysed to obtain average monthly and daily flows. Future growth and loading to the AWTF was not included in this analysis.

Precipitation and Evapotranspiration Data
Precipitation data used for the water balance was acquired from the National Oceanic and Atmospheric Administration (NOAA) precipitation data servers for yearly, monthly and water year monthly precipitation. This data was used to calculate monthly 100-year precipitation, average monthly precipitation, and monthly evaporation. This was then used to create a water balance to determine the storage and irrigation requirements for disposing of the AWTF effluent.

Standard pan evaporation data was obtained from the Western Regional Climate Center for Ferndale, California from 1963-1973. Data locations closer to the AWTF were incomplete or not available. The pan evaporation data was used to calculate evapotranspiration for estimating the irrigation potential of the Land Application areas used to dispose of effluent.

Water Balance Model
A volumetric water balance model was created using Excel’s GRG Nonlinear Solver to estimate monthly irrigation and storage requirements for disposing of the treated effluent. The model utilized the data presented above with evapotranspiration and groundwater infiltration calculations to determine storage volumes and required irrigation area for eight different irrigation scenarios. Irrigation area was based on a standard model of maximum crop water uptake for flood irrigated alfalfa. The model assumed that irrigation would only occur when monthly evapotranspiration exceeded monthly effective rainfall, and a storage pond would be required to store water during periods without irrigation. The model runs were various combinations of using 100-year monthly precipitation, average monthly precipitation, year-round irrigation, summer only irrigation, and with and without infiltration to groundwater to estimate land areas, and included a 10% allowance for setbacks, berms, ancillary equipment, and access. Table 50 below presents the results of the model runs.

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Table 50 Water Balance Scenario Runs
Run Scenario Irrigation
Period
Irrigation Area
(Acres)
Pond
Area
(Acres)
Total Land
Area needed
(Pond and
Irrigation
Acres)
Total Land Area
needed with
assumed 10%
buffer for human
contact exclusion
zone (Acres)
1 agronomic demand at 100-yr monthly
precipitation
year
round 1709 218 1927 2120
2 agronomic demand at average monthly
precipitation
year
round 866 131 997 1097
3 agronomic demand at 100-yr monthly
precipitation with percolation to
groundwater assumption
year
round 1629 222 1851 2036
4 agronomic demand at average monthly
precipitation with percolation to
groundwater assumption
year
round 834 138 972 1069
5 agronomic demand at 100-yr monthly
precipitation
summer
only 482 50 532 585
6 agronomic demand at average monthly
precipitation
summer
only 285 35 320 352
7 agronomic demand at 100-yr monthly
precipitation with percolation to
groundwater assumption
summer
only 460 50 510 560
8 agronomic demand at average monthly
precipitation with percolation to
groundwater assumption
summer
only 274 35 309 340
As presented in Table 50, year-round land application of effluent requires approximately four times more land area than summer only application, as plant uptake and evaporation of water is lower during the winter months.

Additionally, for permitting purposes the land application system must be designed to meet regulations for the 100- year precipitation event. Percolation into groundwater can be a point of contention for many municipalities if it is found to be hydraulically connected to surface water. Scenarios 1 and 5 were chosen for the preliminary application area analysis presented below.

5.5.3.6.2 Preliminary Application Area Analysis
The year round and summer only application representative total required areas are presented in Figure 27. As shown, the year-round application area of 2,120 would require a significant portion of the Arcata Bottoms, much of which is expected to be inundated by SLR in the next 50 -100 years. Implementing year round land application would include site analysis, land owner outreach, groundwater modeling, irrigation modeling, acquiring land use agreements, purchasing land, then constructing and maintaining a distribution system to the fields. The summer only irrigation has the same constraints as the year round discharge, but at roughly a quarter of the size. If treated wastewater is used for irrigation, the land would likely lose organic certification for the period of irrigation and up to several years after, based on the organic certification programs in Humboldt County.

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Figure 27 Summer and Year-Round Required Irrigation Area Reference Scale.

To assess the feasibility of land application of discharge, agricultural lands in the nearby vicinity were identified as shown in Figure 28. Parcels over 30 acres that are above 13 feet and below 30 feet NAVD88 were considered to be the most viable option for discharge, as this reduces the number of land use agreements and irrigation distribution laterals while excluding parcels that would be tidally inundated by the end of the century, or would require pumping to significantly higher elevation than AWTF.

Several of the parcels presented in Figure 28 would be required for summer only land application. Depending on the relocation of the treatment plant, parcels at the south or north end of the City could be used. Regardless of the parcels selected, land use agreements for irrigation, coordination with the RWQCB, and infrastructure for each property would need to be developed. Year-round irrigation would require the majority of the available agricultural land from the edge of the tidal inundation zone to the City limits for effluent disposal. With the variety of stakeholders, owners and agricultural uses throughout the Arcata Bottoms, securing sufficient acreage for yearround application of effluent would be difficult. For summer only application, a secondary winter discharge, such and surface water discharge, would be required.

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Figure 28 Nearby Agricultural Parcels Over 30 Acres Outside of the 100-yr SLR Flood Zone.

5.5.3.6.1 Land Application Discharge Feasibility
There is currently very little demand for additional sources of irrigation water in the Arcata area. The large area required for year-round land discharge, 2120 acres, would occupy most of the Arcata Bottoms outside of the 100-year tidal flood zone. Due to the large land requirement and minimal need for irrigation supply, year-round surface water discharge was considered infeasible.

Summer only discharge would require about a fourth of the land at 590 acres. However, it would still require negotiation for lease or purchase of multiple parcels, significant permitting, as well as installing pump stations and many distribution lines. Additionally, irrigation water is currently sourced from individual groundwater wells throughout the bottoms. These wells are currently sufficient for irrigation needs, making it unlikely that there will be sufficient demand for irrigation water from the City. Finally, many farms in the bottoms are certified organic. Utilizing effluent for irrigation would not meet the organic certification requirements. Successful implementation of summertime land application is considered to be infeasible.

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5.5.3.7 Surface Water Discharge
Table 51 Surface Water Discharge
Strategy Establish surface-water discharge
Adaptation Types(s) Retreat
Goal Establish a new discharge location
Discharge of treated wastewater to surface waters such as rivers, streams and ponds is utilized for effluent disposal throughout California. In Humboldt County, the cities of Rio Dell, Fortuna and McKinleyville utilize surface water discharge to the Eel and Mad River. Discharge to the rivers if allowed in the North Coast Basin must meet the following conditions:
• Discharge is limited to October 1 through May 14
• The discharge of secondary treated wastewater shall be adjusted at least once daily to avoid exceeding, to the extent practicable, one percent of the most recent daily flow measurement of the water body. • In no case shall the total volume of secondary treated wastewater discharged in a calendar month exceed one percent of the total volume of the water body in the same calendar month For Arcata, the surface water bodies of Mad River, Jacoby Creek, Gannon Slough and McDaniel Slough were considered for discharge. Based on effluent flow data from the City, a peak daily effluent discharge of 7 MGD or 10.8 cfs is expected. Flow volume in Jacoby Creek, Gannon Slough and McDaniel Slough fluctuate significantly in response to precipitation events, occurring in steep watersheds with short time of concentrations, indicating that meeting the one percent daily volume of flow requirement would be challenging during dry periods of the winter.

Jacoby Creek, Gannon Slough and McDaniel Slough also drain into Humboldt Bay, which adds permitting complexity to meet the requirements of the EBEP. Therefore, preliminary analysis for surface water discharge was focused on the Mad River.

Areas of interest for discharge to the Mad River are presented in Figure 29. The State Board has classified the mouth of the Mad River as an enclosed estuary and therefore prohibits discharge under the EBEP. The extent of the estuary is defined as: “Estuarine waters will generally be considered to extend from a bay or the open ocean to a point where there is no significant mixing of fresh and saltwater.” No formal documentation of the extent of the Mad River estuary was found, thus it was assumed to be at some point downstream of the Hammond Bridge. East of Arcata, HBMWD operates six Ranney wells for drinking water from the Mad River. Discharge would need to be located downstream of the wells to avoid drinking water contamination. Discharge of the AWTF effluent would need to occur upstream of the estuary and downstream of the HBMWD intake reach.

Year-round surface water discharge was considered, but would require an amendment to the Water Quality Control Plan for the North Coast Basin. The City of Fortuna is pursuing an amendment, but it would only cover their specific discharge to the Eel River. To obtain an amendment to the Basin Plan is likely a 10 to 15 year effort if it would even be considered or approved.

Similar to an ocean outfall, a pipeline to deliver effluent to the discharge point would be required. From the existing AWTF to the Mad River discharge reach would be approximately 6 to 7 miles depending on the final discharge point, either following existing county roads or directionally drilling segments of the alignment. If the AWTF facility was relocated to the north side of the City, the pipeline would be significantly shorter. To serve the discharge pipeline, a discharge pump station at the AWTF facility would be required, as well as a potential booster pump station near the midpoint of the pipeline. The pipeline would also transfer effluent to the land application areas during the dry months.

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5.5.3.7.1 Surface Water Discharge Feasibility
Year-round surface water discharge was considered infeasible as it is not currently allowed in the Mad River.

Discharge to the Mad River would be limited to the winter months. The discharge would need to be located upstream of the Mad River estuary, downstream of the HBMWD intake region to avoid contamination of drinking water and coordinated with the MCSD discharge to maintain water quality regulatory compliance for both discharge points.

Additionally, as presented above, a Mad River discharge would need to be paired with a land application area for dry weather discharge. The land application area was deemed to be infeasible due to the 600-acre required area, complex land use agreements with local farmers, and lack of irrigation demand. Therefore, the pursuing a surface water discharge to the Mad River would not be a feasible discharge method.

Figure 29 Mad River Surface Water Discharge Areas of Limitations

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5.5.3.8 Groundwater Injection
Table 52 Groundwater Injection
Strategy Groundwater Injection for Discharge
Adaptation Types(s) Retreat
Goal Establish a new discharge that would not be subject to EBEP standards Injection wells are used to transport fluid from above to below ground into a porous targeted geologic formation.

Injected fluids have consisted of potable water, non-potable water, municipal wastewater, industrial/oil wastewater, and other chemically mixed fluids. Typically, injection wells are constructed for the following purposes:
• Aquifer Storage and Recovery
• Indirect Potable Reuse
• Prevention of Seawater Intrusion
• Subsidence Mitigation
• Oil Extraction
• Waste Discharge
• Carbon Dioxide Sequestering
• Stormwater Water Quality Compliance
Each type of injection well has its own benefits and regulations. The EPA classifies a Class I injection well as a well used to inject hazardous and non-hazardous wastes into deep, confined rock formations (EPA, 2022). Class I wells are typically drilled thousands of feet below the lowermost underground source of drinking water to prevent future contamination (EPA, 2022).

There are approximately 240 operating wastewater disposal wells in the United States, all of which are in Florida, where the underlying geology of the Gulf Coast is favorable.

Injection-based waste wells in California have traditionally been dominated by the oil and gas industries, which use injection wells, classified as Class II injection wells, to increase oil production or return unusable wastewater back to its source. California has approximately 55,000 Class II injection wells regulated by the Underground Injection Control Program under the California Department of Conservation.

The AWTF is located in a complex geologic setting, approximately 30 miles north of the Mendocino Triple Junction. The Mendocino Triple Junction is where three (3) crustal plates, the Gorda, North American, and Pacific plates intersect. There are just over 100 other triple junctions located across the world. The junction areas experience high seismic activity between three active tectonic plates. At the Mendocino Junction, the northeast-southwest directed compression associated with collision of the Gorda and North American tectonic plates dominates the region.

The Gorda plate is actively subducting beneath North America plate north of Cape Mendocino along the southern portion of the Cascadia Subduction Zone (CSZ) (Carver, 1987).

The AWTF is located in the Mad River Valley groundwater basin, which is categorized as a very low-priority groundwater basin by DWR’s sustainable groundwater management program.

5.5.3.8.1 Regulatory Requirements
A Class I Injection Well would need to meet the requirements of the Department of Toxic Substances Control (DTSC), DWR, North Coast RWQCB, and the EPA. There are currently no Class I municipal wastewater discharge wells in California. A municipal wastewater discharge well would require a hazardous waste facilities permit and would be regulated by the Toxic Injection Well Control Act of 1985. The application would need to satisfy Resource Conservation and Recovery Act Hazardous Waste Permit Figure 30 Class I
Injection Well (EPA)

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(RCRA) requirements and provide clear evidence to DTSC, DWR, and NCWQCB that there is no risk of contamination of underground sources of drinking water. The level of treatment of wastewater required is relatively uncertain and is dependent on hydrogeologic conditions, aquifer quality characteristics, and regulatory discretion; however, it is likely to be a minimum of tertiary treatment.

Provided the initial geologic and engineering studies support the favorable construction a municipal Class I injection well, a summary of the major requirements is as follows:
• DTSC hazardous waste facilities permit (RCRA) that includes the following:
– A finding by DTSC that wastes cannot be disposed of in an alternative way, with public notice and opportunity for comment.
– The injection well is located a minimum of ½ miles away from any other drinking water source. – Groundwater monitoring plan for the target aquifer.
– Injection zone horizontal and vertical extent limitations and hydrogeologic findings that there is no potential for migration of hazardous waste to other underground water sources. – A hydrogeological assessment report, submitted and approved by DTSC. Major components of the hydrogeological assessment report are as follows:

A. Characterization of hazardous waste properties.

B. Characterization of surface water bodies within one mile of the project.

C. Structural geology analysis within three miles of the facility that is influential to groundwater flow or movement relative to discharge wastes.

D. Characterization of target aquifer and vertical confining layers, including physical and chemical characteristics and hydraulic pressures.

E. Design specifications of the injection well and monitoring wells.

F. Operation and Maintenance Plan for injection facilities.

G. Contingency Plans for well failures and shutdowns to prevent migration of contaminations. • Compliance with regional waste discharge requirements and water quality control plans of the NCRWQCB. • Development of a maximum allowable surface injection pressure of the target aquifer. Estimate of the lifespan of the injection well based on the maximum allowable pressure.

5.5.3.8.2 Groundwater Protection Wells
Injection projects that protect and improve groundwater quality are generally in service of a beneficial use as defined by the Sustainable Groundwater Management Act (SGMA). In California this is generally in the form of seawater intrusion protection, where treated wastewater is injected along the coastline to form a barrier that protects inland sources of potable water from seawater intrusion.

Requirements will vary depending on target aquifer characteristics and recycled water characteristics, as the treated wastewater will have to improve the groundwater quality. For example, if the recycled water has concentrations of nitrates and the target aquifer does not then a nutrient management plan may be required.

If the target aquifer is hydraulicly connected to underground sources of groundwater (aquifers that could be used as domestic or municipal water source), then injected wastewater would likely need to satisfy the requirements for Indirect Potable Reuse. At a minimum wastewater would likely need to be treated to tertiary levels.

To implement this type of well project, a hydrogeological assessment report would be required to show that injected water will flow towards the ocean and not replenish a potential underground source of water. Additionally, previous projects have required a Recycled Water Master Plan, a Salt and Nutrient Management Plan, a Pilot Test of Recycled Water Injection, and a Tracer Study of injected Water to be completed for permit approval:

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5.5.3.8.3 Indirect Potable Reuse Wells
Indirect potable reuse projects use advanced treated water to recharge aquifers that are currently or could potentially be used as an underground water source. For regulation purposes this would be classified as a groundwater replenishment reuse project and would require that injected recycled water undergo advanced treatment following the regulations set forth in Title 22, Division 4, Chapter 3 Article 5.2 of the California Code of Regulations.

In addition to treatment requirements, groundwater monitoring requirements for Title 22 would include a minimum of two monitoring wells downgradient of the injection well. An indirect potable reuse groundwater recharge project through direct injection would require full advanced treatment (reverse osmosis and advanced oxidation treatment) that meet Title 22 standards. This includes the following:
o Monitoring the effectiveness of the treatment process o Maintaining a wastewater source control program
o Maintaining diluent water requirements
o Maintaining adequate injected water response retention time o Maintaining a record of the recycled municipal wastewater contribution of injected water, which is the fraction of allowable treated wastewater to diluent water, as determined by the Division of Drinking Water (DDW)
o Maintaining an operation optimization plan
Additionally, treatment would need to demonstrate adequate protection for a minimum of the following:
o Pathogenic microorganism control
o Nitrogen compounds control
o Regulated contaminants and physical characteristics control o Dilution water requirements
o Total organic carbon requirements

5.5.3.8.4 Groundwater Injection Feasibility
The WWTP is located in the Mad River Valley groundwater basin. Potable water in the region is provided by the Humboldt Bay Municipal Water District, a wholesale water district extracting water from the Mad River Basin just north of the City.

While the project’s location is along Humboldt Bay, seawater intrusion in the area is not viewed as a major hazard to potable water sources. Thus, there is little benefit a seawater intrusion barrier that would provide protection of local water sources. Additionally, as potable water in the region is provided by HBMWD there is no need for indirect potable reuse in the region.

As presented under the regulatory analysis, to permit a new wastewater effluent injection well, a finding by DTSC that wastes cannot be disposed of in an alternative way must be made. This will be a difficult finding since the City already disposes of effluent in alternate ways.

Based on the lack of seawater intrusion and demand for indirect potable water reuse, groundwater injection is not recommended as a feasible alternative.

5.6 Consolidation
Consolidation of municipal wastewater districts around Humboldt Bay was first explored in the 1970s with the formation of the Humboldt Bay Wastewater Authority (HBWA) Joint Powers Agency between the Cities of Arcata and Eureka, County of Humboldt, and Humboldt and McKinleyville Community Services Districts to finance, construct, operate and maintain a regional wastewater treatment works. The regional plan consisted of a treatment plant on the

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Samoa Peninsula, an interceptor system with pump stations to convey wastewater to the plant, including a transbay crossing, and effluent disposal through an ocean outfall.

Design of the HBWA project was completed in September 1976, and in October HBWA received State and Federal Clean Water Grants for construction, totalling 87.5% of eligible project costs. The remaining 12.5% and all ineligible project costs were to be funded locally by the sale of revenue bonds. Two lawsuits filed by a group of Humboldt citizens, however, blocked the sale of revenue bonds for 2.5 years, thereby delaying construction of most of the regional project. When the design of the HBWA project was completed in 1976, the project was estimated to cost approximately $37 million for construction plus $9 million for all other costs, including engineering, right-of-way acquisition, financing and administration. During the next few years these costs rose to approximately $50 million and $13 million, respectively. During this period, HBWA constructed a portion of the regional project - a pump station and length of interceptor to convey McKinleyville's waste to Arcata - at a cost of approximately $1.5 million. (State Water Resources Control Board, 1979)
In 1979, the board held a factfinding meeting to assess the balance of evidence presented for the HBWA project and found that while HBWA was a cost-effective alternative for wastewater treatment and disposal in the region, due to significant public opposition to the project, its timely implementation was unlikely. The most controversial issue that surfaced was the cost and energy consumption of operating the ocean outfall. Additionally, evidence presented by the City of Arcata indicated that they could demonstrate that the bay disposal of secondarily treated and disinfected effluent would enhance the quality of the receiving waters. This evidence indicated that the discharge of secondary, disinfected and dechlorinated effluent would adequately protect the bacterial quality of the Bay. Further, the Enhancement Marshes were shown to result in the fuller realization of existing beneficial uses and, hence, a positive water quality benefit for the Bay. Based on these findings, the board allowed the development of the AWTF, and the regional HBWA project was not fully realized.

5.6.1 City of Eureka Ocean Outfall Consolidation
Currently, the City of Eureka discharges to the mouth of Humboldt Bay. Originally, this discharge point was permitted because it was shown that discharge of the outgoing tide resulted it no significant retention of effluent in the Bay.

However, a 2014 study found that more of the effluent remained in the Bay than originally thought, resulting in the RWQCB ordering the City of Eureka to evaluate discharge alternatives. The City of Eureka is currently completing preliminary studies to assess an ocean outfall. On July 31, 2025, the City of Eureka met with the AWTF Feasibility Study team to discuss Eureka’s discharge investigations and opportunities for consolidation. The City of Eureka staff indicated that pursuing a joint outfall could potentially be considered but would require significant coordination of design and permitting. Due to the location of the two systems no opportunities for joint infrastructure other than an ocean outfall were identified. There were few benefits to consolidation identified, and considering that the City of Arcata could pursue an ocean outfall using the existing RMTII outfall, consolidation with the City of Eureka is not considered further for this study.

5.6.2 McKinleyville Consolidation
As presented previously, prior to the development of McKinleyville’s wastewater treatment facility, a length of interceptor forcemain and pump station were constructed to transfer raw wastewater from McKinleyville to Arcata for treatment. The interceptor crossed Mad River at Hammond Bridge and was routed across the bottoms to the AWTF.

After HBWA dissolved, McKinleyville constructed their own treatment facility and the interceptor was abandoned. The exact condition and alignment of the interceptor is unknown, but the interceptor could be assessed as a potential alternative for either a joint surface water discharge point, or transfer of untreated influent from the Arcata to the McKinleyville treatment facility. Currently, McKinleyville’s infrastructure is working at or near capacity, indicating consolidation would require expansion and upgrades to their system. The MCSD Treatment Facility is also approximately 30 feet higher in elevation than the AWTF, which would significantly increase cost and energy demands associated with pumping. As no opportunities for shared infrastructure were identified, and no significant benefits for consolidation, this alternative was not considered further in this study.

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5.7 Alternative Development and Prioritization
As previously presented, there are multiple strategies for adapting the Arcata wastewater system to SLR. These strategies can be implemented over various timelines. This section presents a review of the input received during public outreach, criteria developed to evaluate alternatives, a summary of alternatives, and the final priority alternatives to be further analysed.

5.7.1 Public Outreach and Input

5.7.1.1 November 2024 Public Meeting
On November 14, 2024, the City of Arcata hosted an in-person workshop at the D Street Neighborhood Center. The meeting was attended by 31 residents and facilitated by RCAC, GHD and the City of Arcata. The workshop had the following goals:
– Inform the community on the multiple efforts the City is leading to address Sea Level Risk and Hazard Mitigation as it relates to the wastewater treatment facility and the treatment marshes. – Discuss and solicit input from the community on the following topics:
• Wastewater disposal infrastructure alternatives
• Importance of wastewater discharge continuing to have a beneficial reuse in the environment • Priorities and wastewater system considerations
The workshop consisted of a presentation, supporting posters, and facilitated breakout sessions after the presentation to solicit input directly from the community. The presentation discussed the following projects and had accompanying posters located throughout the room:
• Local Coastal Program
• Arcata Sea Level Rise Risk Assessment
• Wastewater Treatment Facility Levee Augmentation Project • Wastewater Treatment Facility Long-Range Feasibility Project (This project) There were three breakout tables that each tackled the following topics and would rotate after 10 minutes. The topics were facilitated by City or GHD staff and a notetaker was assigned to each table to document the input received. – Wastewater Disposal Infrastructure
• Goal: Receive public feedback on alternatives mentioned in the presentation and discuss potential concerns to address in the study.
• Key Takeaways:
– There is strong community attachment to the AMWS, with support for enhancing and protecting these areas, particularly through levee raising and wetland creation. – Land application of recycled water was seen as a possible solution for reducing effluent discharge into the bay, but concerns about groundwater levels and available land were noted. – There is significant concern about SLR and its impact on marshlands and surrounding areas, with calls for measured retreat and improved protection strategies. – Decentralized treatment systems and global examples of similar efforts were discussed as potential avenues for further exploration.
– The community values the ancillary benefits of wastewater management, such as tourism, wildlife habitat, and local identity, and suggests that these should guide future planning decisions.

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– Wastewater Effluent in the Environment
• Goal: Community to identify the importance of wastewater discharge and that it continues to have a beneficial reuse in the environment. Additionally, to identify new ideas, and weigh the benefits and costs of beneficial reuse.
• Key Takeaways:
– Sustainability and Environmental Protection were central themes, with strong support for using marshlands, eco-levees, and agricultural reuse.
– Decentralized Systems and diversified discharge methods were favored to minimize risks and improve flexibility.
– There were significant concerns about the environmental impact of ocean outfalls, groundwater injection, and the byproducts of land application.
– Participants showed an interest in innovative technologies like composting toilets and small-scale, decentralized solutions, but emphasized the need for careful consideration of location and environmental suitability for each method.
– Decision Making Criteria (Values) and Wastewater System Considerations • Goal: Identify community priorities and weigh criteria that will be used against each alternative under evaluation.
• Key Takeaways: See Section 5.7.2 below for further discussion of criteria ranking.

Additional information on the input gathered at the November 14, 2024 public meeting can be found in in Appendix F.

5.7.1.2 August 28, 2025 Public Meeting
On August 28, 2025, the City of Arcata hosted an in-person workshop at the D Street Neighborhood Center. The meeting was attended by 34 residents and facilitated by RCAC, GHD and the City of Arcata.

The workshop had the following goals:
• Inform the community on the Arcata Wastewater Treatment Feasibility Study background and updates. • Present decision-making criteria and values developed at the November 2024 workshop. • Discuss and solicit input from the community on adaptation strategies through envisioning future wastewater retreat and protection scenarios.

The workshop consisted of a presentation, supporting posters, and facilitated breakout sessions after the presentation to solicit input directly from the community. The presentation and supporting posters discussed the Wastewater Treatment Facility Long-range Feasibility Study including the following topics:
• Vulnerability and Risk Assessment of Wastewater Infrastructure to Sea Level Rise and Flooding • Wastewater Discharge Case Studies
• Adaptation Strategies
• Wastewater Facility and Discharge Alternatives Prioritization The breakout session were focused on both protection and retreat adaption strategies. Key takeaways are summarized below.

Protection Option Common Discussion Points
• Incorporate protection of other low-lying areas such as South G Street, 255 / 101 between Arcata and Eureka, and agricultural land
• Cost analysis should include phasing options, energy considerations, and equity impacts of protecting private property
• Maintenance of the ecological and other benefits of the marsh is integral to Arcata’s culture and values • Phased approach that continues assessing land options and technology advancements for future relocation

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Retreat Option Common Discussion Points
• Shared sense of maintaining the community benefits of the existing enhancement marshes with no net loss of ecological or recreational benefits
• Consider land banking for a new wastewater treatment facility and careful attention to the future site characteristics and aesthetics
• Incorporating future population growth and climate change in project planning and design • Conduct regional collaboration that involves local/state government, tribal and regulatory partners. • Need attention to cost-effectiveness leveraging project phasing, grants, and potential consolidation opportunities to reduce impact on rate payers.

Additional information on the input gathered at the August 25, 2025 public meeting can be found in in Appendix F.

5.7.1.3 Other Public Engagement Events
In addition to the two public meetings specifically run for this project, a presentation of this project was given to a forum at Cal Poly Humboldt in December 2025 and to the City Council on January 21st, 2026. Additionally, this document was posted on the City’s website with a notice sent to attendees of the project meetings asking for feedback.

5.7.2 Criteria for Alternative Prioritization
Criteria for assessing alternatives were separated into non-negotiable or flexible (negotiable) criteria. These criteria were presented to the community at a public meeting in November 2024. Non-negotiable criteria must be met by any adaptation strategy. Negotiable criteria have inherent flexibility and are not required for the City to operate the wastewater system. All strategies would be designed to meet the non-negotiable criteria, while flexible criteria are useful for helping determine which strategies are more important to the City and community. Ability to meet criteria was ranked on a high, medium, or low basis.

The Non-negotiable criteria and ranking information are as follows:
• Meets Regulatory Requirement: While all strategies could eventually meet regulatory criteria, those that more readily meet regulatory were ranked higher.
• Constructability: This criterion is ranked higher for alternatives that may be easier to design, permit and construct.
• Operability: This criterion is ranked higher for strategies that keep facilities co-located, require less technical expertise, and have better stability.
• Flexibility of system for future treatment concerns: This criterion is ranked higher for strategies that could be adapted more easily in the future.
• Resource efficiency and minimal environmental impact: For strategies that have a lesser impact on the environment and maximize the efficiency of existing resources received a higher ranking. • Cost efficient: The City desires to have the lowest cost strategy, with the least impact on rate payers. Full cost estimates were not developed prior to alternative selection, the relative size of strategies was used to understand relative cost.

Table 53 below presents the flexible criteria which was presented to the community. Based on the responses from the 17 participants, the use of natural treatment systems, costs to users and maintaining and expending secondary benefits such as recreation and wildlife habitat were ranked as the most important criteria.

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Table 53 Flexible Criteria Ranking
Negotiable (flexible) Criteria Relative importance to
the Community
Ranking Description
Use natural systems as part of the
treatment process
1 Strategies that encompass greater use of natural
systems were ranked higher
Costs 2 As the community considered cost an important factor, strategies that may have a lower cost were ranked
higher
Incorporate new beneficial reuse/ ancillary
benefits
3 Strategies that added in beneficial reuse were ranked higher
Maintain existing beneficial reuse/ ancillary
benefits
4 Strategies that maintained existing beneficial uses
were ranked higher
Proactive climate change readiness 5 Strategies that resulted in more proactive climate readiness were ranked higher
Stay within existing footprint of the AWTF 6 Strategies that kept facilities within the existing AWTF footprint were ranked higher.

5.7.3 Summary of Adaptation Strategies
The strategies presented previously were divided by area of the system (collection, treatment, and discharge), and identified by adaptation approach (protect, accommodate, and retreat) and implementation timeline. Table 54 through Table 56, below present the complete list of adaptation strategies which were assessed. The tables include a brief description of the strategy and if the strategy is considered further in the report or not.

Table 54 Summary of SLR Adaptation Strategies for the Collection System Collection System Strategy
Type
Description Study Evaluation
Consideration
Near-term Adaptation
Strategies
I & I Reduction Protect/
Accommodate
Replacement and lining of sewer pipelines to
reduce potential infiltration and sealing of
manholes to reduce potential inflow.

City is currently
implementing projects. Will
not be further evaluated.

Mid-term and Long-Term
Adaptation Strategies
Elevate Electrical
Equipment
Accommodate Raise the height of critical electrical equipment to above 15’ NAVD 88 elevation.

To be implemented as part
of larger Capital
Improvement Project
planning when maintaining
and upgrading facilities.

Will not be further
evaluated.

Continued Collection
System Upgrades
Protect/
Accommodate
Replacement and lining of sewer pipelines to
reduce potential infiltration and sealing of
manholes to reduce potential inflow.

Reroute Collection System
for Treatment Plant
Relocation
Retreat If a new treatment facility location is selected, options for re-routing the collection system
should be evaluated. For this study, only a new
pump station and pipeline to move effluent from
the AWTF to a new site was considered.

New pump station and raw
effluent pipeline. Will be
further evaluated.

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Table 55 Summary of SLR Adaptation Strategies for the Treatment System Treatment System Strategy
Type
Description Study Evaluation
Consideration
Near-term
Phase 1 Electrical
Upgrades
Accommodate Raise the height of critical electrical equipment to above 15’ NAVD88 elevation.

Currently being
implemented at the AWTF.

Will not be further
evaluated.

Minor Levee
Maintenance
Accommodate Identify and protect specific vulnerable locations of the levee surrounding the AWTF and raise levee
system to a minimum of 11.5’ NAVD88 which could
include both grey and green infrastructure
components
May be required for flood
protection to allow time for
mid to long-term strategy
implementation. Will be
further evaluated.

Mid-term to Long-Term
Levee Augmentation
Project
Protect Protect the AWTF with a new levee system raised to 15’ NAVD88 which could include both grey and
green infrastructure components
Provides end of century
protection to the AWTF.

Will be further evaluated.

Relocate AWTF Retreat Retreat the AWTF to a new location not susceptible to coastal flooding
Provides long term, resilient
treatment capacity. Will be
further evaluated.

Decentralized
Treatment (Not stand
Alone)
Retreat/
Accommodate
Small water recycling systems, residential greywater
reuse, composting toilets, and other focused
wastewater reuse that can be incorporated into the
City’s future planning documents.

Stand alone project that
does not resolve the larger
need for wastewater
system adaptation. Will not
be further evaluated.

Table 56 Summary of SLR Adaptation Strategies for Discharge and the Enhancement Marshes Discharge/
Enhancement Marshes
Strategy Type Description Study Evaluation
Consideration
Near-term Adaptation
Strategies
Enhancement Marshes
Protection Structures
Maintenance
Accommodate Identify and protect specific vulnerable locations of the levee surrounding the Enhancement Marshes
and raise levee system to a minimum of 11.5’
NAVD88 which could include both grey and green
infrastructure components. Discharge would
continue to be into Humboldt Bay.

Provides near-term
resiliency. Will be further
evaluated.

Mid-term to Long-Term
Adaptation Strategies
Determine Adaptative
Capacity of
Enhancement Marshes
to Saltwater Intrusion
Accommodate An adaptive management approach would be
taken with the Enhancement Marshes and the
effectiveness of the system under increasing
coastal flooding studied. This alternative would
include minor Levee Maintenance at critical low
spots around the Marsh. Discharge would continue
to be into Humboldt Bay.

Provides near-term to midterm resiliency. Will be
further evaluated.

Enhancement Marshes
Levee Augmentation
Protect Protect the Enhancement Marshes with a new
levee system raised to 15’ NAVD88 which could
include both grey and green infrastructure
components. Discharge would continue to be into
Humboldt Bay.

Provides end of century
protection to the
Enhancement Marshes.

Will be further evaluated.

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Discharge/
Enhancement Marshes
Strategy Type Description Study Evaluation
Consideration
Enhancement Marshes
Relocation
Retreat The Enhancement Marshes would be retreated
and a new levee not installed. Discharge would
continue to be into Humboldt Bay.

Enhancement Marsh
relocation would have
similar considerations as a
new enhancement project
and is not evaluated
separately. Will not be
further evaluated.

Alternative
Enhancement for Bay
Discharge
Retreat A new enhancement project would be developed,
that may include wetlands similar to the existing
Enhancement Marshes or another project that
would provide fuller realization of existing
beneficial uses or the creation of new beneficial
uses.

Provides mid-term to longterm resiliency. Will be
further evaluated.

Ocean Discharge – RMT
II
Retreat A new connection to the HBHRCD outfall would be permitted for an Ocean Outfall.

Provides long-term
resiliency. Will be further
evaluated.

Ocean Discharge – New
Outfall
Retreat Development of a new ocean outfall north of the Mad River or on the Samoa Peninsula would be
developed.

As an existing outfall could
be used, further study of a
new outfall is not
recommended. Will not be
further evaluated.

Consolidation with City
of Eureka
Accommodate Potential for pursuing a combined new ocean outfall, which would require significant coordination
of design and permitting.

High value synergies not
identified and Arcata can
pursue an ocean discharge
with HBHRCD. Will not be
further evaluated.

Consolidation with
MCSD
Accommodate No specific consolidation projects identified with MCSD.

High value synergies not
identified. Will not be
further evaluated.

Surface Water
Discharge
Retreat Year round surface water discharge was not
feasible. Wintertime surface water discharge to the
Mad River was considered, but may conflict with
MCSD’s existing surface water discharge. It would
also require land disposal of effluent in the
summertime.

Determined to be
Infeasible due to
regulations and potential
MCSD conflicts. Will not
be further evaluated.

Land Application Retreat
Year-round land application was determined to be
infeasible. Summertime land application of effluent
would require 590 acres of land.

Determined to be
Infeasible due lack of
demand for irrigation water,
need to coordinate with
multiple land owners, and
operational complexities.

Will not be further
evaluated.

Groundwater Injection
and Indirect Potable
Reuse
Retreat There is not a need for groundwater injection to recharge potable aquifers or to prevent seawater
intrusion resulting from over pumping of
groundwater, and thus groundwater injection would
be for disposal only, which is not permitting in
California.

Determined to be
Infeasible, not permittable
and no demand. Will not
be further evaluated.

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5.7.4 Priority Alternatives
Combinations of the feasible strategies form eight alternatives, consisting of a treatment strategy and a discharge strategy as shown in Table 57.

Table 57 Summary of Alternatives
Treatment Humboldt Bay Discharge Ocean Discharge
Determine Adaptative
Capacity of
Enhancement Marshes
to Saltwater Intrusion/
Enhancement Marshes
Protection Structures
Maintenance
Enhancement Marshes
Levee Augmentation
Alternative
Enhancement for Bay
Discharge/
Enhancement Marshes
Protection Structures
Maintenance
Ocean Discharge –
RMT II/ / Enhancement
Marshes Protection
Structures
Maintenance
AWTF Levee
Augmentation Project
Alternative 1 * Alternative 2 Alternative 3 Alternative 4 Relocate AWTF/
Levee Maintenance/
Reroute Collection
System
Alternative 5 Alternative 6 Alternative 7 * Alternative 8 * * Identified as a Priority Alternative
Alternatives were evaluated qualitatively based on how well they met the criteria presented in Section 5.7.2 and the City’s desire to have a range of alternatives further investigated. Three alternatives were preliminarily identified as priority. However the components of all eight feasible alternatives were further evaluated below and in the separate environmental analysis. Thus, a full rating and ranking of the alternatives was not completed. The primary reasons for prioritization are included in the descriptions of the priority alternatives below. • Alternative 1: Protect Existing Facility. Protects the facility at the current location by augmenting the existing levee around the facility and maintaining enhancement marsh levees while monitoring for enhancement efficacy. This alternative was selected as it had the highest constructability based on the maximizing use of the existing system and maintained the greatest benefits of the existing enhancement marshes. • Alternative 7: Retreat AWTF and Enhancement Marshes. Relocates the facility to a new location, develops new Enhancement Marshes, and maintains a Humboldt Bay treated water discharge. This alternative was selected as it represented full retreat of the existing facilities while maintaining the existing Bay discharge and the types of enhancements that the current marshes bring to the area. • Alternative 8: Retreat AWTF and Develop Ocean Water Discharge. Relocates the facility to a new location and discharges treated wastewater to the Pacific Ocean. This alternative was selected as it also represents full retreat but includes removal of the Bay discharge with an ocean outfall.

The adaptation strategies included in all eight feasible alternatives presented below, including a description and concept design, and Class V capital cost estimate, and changes in O&M costs.

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6. Alternatives Analysis
The sections below present a description of each alternative and an opinion of probable cost based on preliminary analysis. As no final sites for retreat options have been selected, assumptions were made regarding transmission piping and pumping.

The cost estimates presented below are Class 5 estimates. These types of estimates are used for initial screening and feasibility with a design level between 0 to 2%. There is a wide accuracy range for Class 5 estimates, and a range of - 30% to +50% was used in this study.

6.1 AWTF Levee Augmentation
AWTF Levee Augmentation is a potential component of Alternative 1, Alternative 2, Alternative 3, and Alternative 4.

6.1.1 Levee Description
The perimeter levee around the AWTF exhibits elevations between 9.5 and 12 feet which are vulnerable to overtopping during extreme tidal events in the next 10 to 50 years. The Levee Augmentation Project proposes to elevate and fortify approximate 7,600 linear feet of perimeter levee around the core treatment facilities to 15 feet NAVD88. The proposed augmentation would provide enhanced flood protection for the AWTF core infrastructure, which includes the headworks, primary clarifiers, treatment wetlands, disinfection, and corporation yard.

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Figure 31 Levee Augmentation Project with RSP
The Levee Augmentation project is currently at 30% Design review between SHN and the City of Arcata. It is assumed that all new improvements will not displace existing trails. The features described below would be designed to provide flood protection for a water surface elevation of 14 feet with 1 foot of freeboard. A levee with a crest elevation of 15 feet would meet flood design protection to the 2105 intermediate scenario 100-year flood event. Under the Intermediate-High and High scenarios, design protection is reduced to 2085 and 2075, respectively.

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Project features included in the 30% design submittal are:
• Constructing a 162 LF concrete flood wall along the edge of the trail at the northern edge of the facility. This method would reduce potential construction complications to underground pipes from sheet piles, while maintaining the existing trail footprint.

Figure 32 Typical Concrete floodwall for AWTF
• Installing a 20 LF floodgate at the facility entrance to prevent flooding down the driveway into the facility. As proposed in the City’s 2025 Local Coastal Plan update, a future project will look at raising the road to access the facility and protect the residential and industrial areas around south G Street.

Figure 33 Typical Floodgate Section

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• Augmenting 4400 LF of existing armored levee that is exposed to potential wind waves with a sheet pile wall with shoreline protection to prevent overtopping and reduce wave impacts to shoreline. New RSP would be placed on top of the existing RSP and would not extend into mudflat or marshes surrounding the facility (Figure 34).
• The current shoreline protection design is proposed to use RSP as presented above. To provide the same level of protection with a living shoreline, new development would extend to the mudflat surrounding the facility at approximately a 10:1 slope. This would disturb and convert approximately 7.7 acres of existing mudflat.

Figure 34 Typical RSP and Sheet Pile Section
• Install 3000 LF sheet pile wall along existing earthen levees that are not subject to wave-wind action requiring additional protection to prevent still water overtopping along sheltered northeast side of AWTF (Figure 35). No trails will be displaced by the sheet pile wall. The wall will be exterior to trails, providing protection to the trails.

The sheet piles will extend approximately two to five feet above the existing levee crest.

Figure 35 Typical Sheet pile and trail section

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6.1.2 Living Shoreline Description
Approximately 4400 LF of the AWTF levee exhibits southwest exposure to wind waves (Figure 36). This stretch of levee could benefit from implementing a living shoreline. The living shoreline would transition from the levee crest elevation at approximately 15 feet to approximately 5 feet at the toe of the levee. With an assumed 10:1 slope down to the mudflat, the shoreline would extend up to 100 feet out from the existing levee toe, converting approximately 7.7 acres of existing wetland and mudflats surrounding the marshes.

Figure 36 Levee Augmentation Project with Living Shoreline

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6.1.3 Project Costs
Based on the 30% Design completed by SHN, the AWTF Levee Augmentation Project is estimated to cost between $20,400,000 and $24,500,000 (Appendix B).

Based on the 50% Design completed for the NSI project which has similar location and wave exposure to the AWTF levees, it was estimated that implementing a living shoreline would cost approximately $2,500 per linear foot of shoreline with a crest elevation 11.5 feet. As the Levee Augmentation is proposing to elevate the structures to 15 feet, this unit cost was increased by 30% to account for the extra fill material to match the increased crest elevation, increasing the unit cost to $3,250 per LF. With the 4400 LF of shoreline exposed to wave action at the AWTF, it is expected that adding living shoreline to the levee augmentation would cost an additional $14,300,000, for a total project cost of ranging from $34,700,000 to $38,800,000.

There are no additional O&M costs associated with the levee repair. Annual costs would be similar to the existing system. There may be additional O&M costs for a living shoreline, especially in the first few years after installation. As these costs are very dependent on the final design and vegetation, an estimate of O&M costs for the living shoreline cannot be made, and no costs are included at this time.

6.2 AWTF Relocation
AWTF relocation is a potential component of Alternative 5, Alternative 6, Alternative 7, and Alternative 8.

6.2.1 Description
For this alternative component, the AWTF will be retreated to a new location outside of the 2105 flood hazard zone as presented in Section 5.4.3.3. The City’s influent would re-routed to the new location from the collection system and would then need to be pumped to the eventual discharge location. With relocation of the AWTF, the existing site would require restoration. The details of this have not been evaluated yet, but it is assumed that portions of the site would be graded/filled to increase the tidal extent while preventing any increased off-site flooding vulnerabilities to the South G Street community.

6.2.1.1 AWTF Levee Maintenance
As presented in Section 3.5.5, select locations of the AWTF levees exhibit crest elevations of less than 10.0 feet NAVD88 and are vulnerable to nuisance flooding at the current 10-year tidal flood event of 10.1 feet. These low-lying sections of levee are likely caused by settlement of the fill prism over the last 70 years from earthquake events and natural movement of the soft soils that are typically found in proximity of Humboldt Bay. It is expected that it will take at least 30 years to successfully relocate the AWTF. Raising these low-lying sections of levee is recommended to provide near term flood protection to the facilities.

The AWTF headworks and generator building along the northeast side of the facility are at risk of nuisance flooding (less than 6 inches) during the current 100-year tidal flood event. This levee section is comprised of paved and dirt trails and is not exposed to wind waves. Therefore this section would not benefit from living shoreline protection. Up to 4,500 LF of low-lying sections would be maintained and elevated to 11.5 feet NAVD88 to protect the site for 20-30 years as the new facility is designed, permitted and constructed. This elevation was selected to match the typical crest elevation of the existing levees.

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Figure 37 Existing AWTF Crest Elevations.

Table 58 Quantities for AWTF Levees to be raised.

Existing Levee Crest
Elevation
Lineal feet to be modified Proposed feet raised above existing elevation

9.5 - 10 ft 117 2.5
10 - 11 ft 1,945 1.5
11 - 12 ft 2,454 0.5
TOTAL 4,516

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6.2.1.2 Influent Transfer Pump Station and Pipeline
As presented in Section 5.3.2.2, the collection system will need to be rerouted to deliver influent to the new facility.

There are potential opportunities to intercept flows higher up in the system to divert to the new site, but for the purposes of this study it was assumed that a new pump station at the current site and transfer pipeline to the new location would be needed. Conceptual routing of the new transfer pipeline estimated that it would be approximately 3.7 miles in length using the most direct alignment, northbound along South G Street and Alliance Road, and westbound along Spear Avenue and Upper Bay Road to the approximate center of the retreat study area identified in Section

5.4.3.3 (Figure 38). Pending a focused evaluation of the existing AWTF’s influent flow records, growth forecasts, and conducting actual pump/pipe hydraulic analyses, it is anticipated that a 24-inch or larger diameter forcemain will be required between the new lift station and the new AWTF.

Figure 38 Conceptual Design for Influent Transfer Pump Station and Pipeline to New AWTF Location

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6.2.1.3 Treatment Overview
It was assumed that the new AWTF would need to provide tertiary treated wastewater to meet future discharge regulations. The following is a brief description of the three levels of treatment for tertiary treatment:
 Primary treatment - This is an initial process or series of processes that physically remove large solids, generally by screening, grit removal, and gravity settling. This step also protects the equipment within the treatment plant by removing oversized materials, like trash, rags, and rocks, from affecting the performance of the treatment process.  Secondary treatment - This is generally a biological process that converts organics in the wastewater into a form that can be removed by settling or consumed by microorganisms contained within the treatment process that can then be removed by settling. Typical types of secondary treatment processes include facultative or aerated ponds, suspended activated sludge, or attached growth processes such as trickling filters. Essentially the process creates an environment where a concentration of different microorganisms can grow, using the wastewater as a food source, thereby purifying the wastewater. Secondary treatment is typically the minimum treatment level required for most municipal wastewater systems.

Advanced secondary treatment includes modifications to the biological process such that nutrients (mainly nitrogen and possibly phosphorus) are removed.  Tertiary treatment - This is typically a filtration step, where the solids in the secondary treated wastewater are further separated from the wastewater by passing through a fine filter. The filter media can be sand, textile, or a membrane. Tertiary treatment is not always necessary or required for all applications.

Additional process considerations beyond the treatment levels include:
 Disinfection - This is a final step in the treatment of the liquid portion of the wastewater, and it is used to remove bacteria and other harmful organisms prior to discharge (disposal). The most common disinfection method mixes some form of chlorine with wastewater for at least 30 to 60 minutes. Improvements in technology have made use of UV light, another cost effective and popular method of disinfection. It is expected that UV disinfection will be utilized for the new AWTF facility.
 Solids treatment - Depending on the type of treatment processes used, various types of sludge are formed and removed from the treatment process. These require additional processing to stabilize the solids to a level that makes them acceptable for disposal or reuse as compost. It is expected that a similar to existing solids processing system will be utilized for the new AWTF.

For the new treatment facility, two treatment technologies were assessed: Conventional Activated Sludge (CAS) and Moving Bed Biofilm Reactor (MBBR). These two treatment technologies were selected for preliminary evaluation based on treatment effectiveness, relative cost efficiency, small footprint and successful implementation in the local area.

6.2.1.4 Conventional Activated Sludge (CAS)
A conventional activated sludge oxidation ditch is a biological wastewater treatment process that uses a continuousloop channel (ditch) with mechanical aerators to provide mixing and oxygen for microbial growth. Wastewater circulates around the ditch, allowing microorganisms to oxidize organic matter and, in many designs, nitrify ammonia.

The mixed liquor then flows to a secondary clarifier, where solids settle and are either returned to the ditch (RAS) to maintain biomass or wasted (WAS) to control sludge age. The process is valued for its operational stability, long detention time, and simple control, making it well suited for small to medium-sized treatment plants. Further preliminary sizing calculations for the CAS System can be found in Appendix E.

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6.2.1.5 Moving-Bed Biofilm Reactor (MBBR)
In the past 20 years, the MBBR has been established as a simple yet robust, flexible, and compact technology for wastewater treatment. MBBR technology has demonstrated success with BOD removal, ammonia oxidation, and nitrogen removal applications in a variety of different treatment configurations designed to meet a wide range of effluent quality standards, including stringent nutrient limits. MBBRs use specially designed plastic carrier elements for biofilm attachment held in suspension throughout the reactor by turbulent energy imparted by aeration, liquid recirculation, or mechanical mixing energy. In most applications, the reactor is filled between one-third and twothirds full of carriers. Perforated plates or sieves located on the effluent end of the reactor allow treated water to pass through to the next treatment step but retain the media inside the reactor. Further preliminary sizing calculations for the MBBR System can be found in Appendix D.

6.2.2 Project Costs

6.2.2.1 AWTF Levee Maintenance
Based on the analysis presented above (up to 4,500 LF of low-lying sections maintained and elevated to 11.5 feet), it was estimated that the AWTF Levee Maintenance would have a total project cost ranging from $3,400,000 to $7,200,000 (Appendix B).

There are no additional O&M costs associated with the levee maintenance or living shoreline. Annual costs would be similar to the existing system.

6.2.2.2 Transfer Pipeline + Pump Station
Based on the analysis presented above, it was estimated that the Transfer Pipeline and Pump Station would have a total project cost ranging from $29,100,000 to $62,300,000 (Appendix B).

6.2.2.3 New AWTF
Based on the analysis presented above, it was estimated that the CAS AWTF option would range from $76,000,000 to $162,900,000, and the MBBR option would range from $80,800,000 to $173,100,000 (Appendix B). these estimates include decommissioning costs for the existing AWTF site.

6.2.2.4 Total Cost
Based on the project components presented above, the AWTF Relocation Total Project Costs are presented in Table 59.

Table 59 AWTF Relocation Costs
Project Component Lower Bound Cost Upper Bound Cost
AWTF Levee Maintenance $3,400,000 $7,200,000
Transfer Pipeline and Pump Station $29,100,000 $62,300,000 New AWTF, MBBR $80,800,000 $173,100,000
New AWTF, CAS $76,000,000 $162,900,000
Total, MBBR $113,300,000 $242,600,000
Total, CAS $108,500,000 $232,400,000

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6.2.2.5 Operations and Maintenance Costs
It is assumed that the new wastewater treatment plant would have similar O&M costs to the existing AWTF.

Operational costs of the new system would include pumping from the existing AWTF to the new location. Pending a engineering design and a detailed hydraulic evaluation, the following preliminary evaluation was completed to provide a rough estimate of pumping costs.

Hazen Williams formula for headloss and the waterpower equation were used to estimate the energy required for pumping. It was assumed that approximately 20,000 LF of 24” HDPE pipe would be required to transfer the influent to the new location, with an approximately 15 feet of elevation gain to reach the new headworks. An additional 30% of headloss was added to account for minor losses and design uncertainties. It was assumed that the average design flow of 2.3 MGD for dry weather would occur from May through October, and the average wet weather design flow of 5 MGD would occur October – April. This analysis assumed a constant pumping rate, with no consideration for wetwell or equalization basin holding capacity. A total yearly energy usage of approximately 240,000 kWh was estimated. With an average energy cost of $0.30/kWh, this would result in a yearly pumping cost of approximately $72,000 (Appendix C). Due to the hydraulic complexity of operating high and low flow pumping system with a pumping distance approximately 4 miles in length, this analysis should be considered a rough estimate to be revisited during design of the system.

In addition to electricity costs, the City would be responsible for the new pump station. Annual maintenance costs, including future replacement for the new pump station was estimated at 1% of the equipment costs, or $130,000 annually. This results in a total estimated annual increase in O&M costs of $210,000.

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6.3 Enhancement Marshes Levee Maintenance and
Adaptive Management
Enhancement Marshes levee Maintenance and Adaptive Management is a potential component of Alternative 2 and Alternative 6.As presented in Section 3.5.5, select locations of the Enhancement Marshes levees exhibit crest elevations of less than 10.5 feet NAVD88 and are vulnerable to the current 100-year tidal flood event of 10.7 feet.

These low-lying sections of levee are likely caused by settlement of the fill prism over the last 50 years from earthquake events and natural movement of the soft soils that are typically found in proximity of Humboldt Bay.

Regardless of the long-term adaption strategy selected for these facilities, it is recommended that these low-lying sections of levee are addressed to provide near term flood protection to the facilities.

6.3.1 Levee Maintenance Description
From analysis of the existing crest elevation of the enhancement marshes levees, the typical crest elevation of the levees is between 11 and 12 feet NAVD88 (Table 60). Low lying sections of the Enhancement Marsh levees would be elevated to an elevation of approximately 11.5 feet NAVD88, which will provide protection to the 2055 100-year flood event, and 2075 25-year flood event (OPC Intermediate Scenario). In addition to levee improvements, the City will consider beginning studies to determine the adaptive capacity of the Enhancement Marshes to saltwater intrusion from limited overtopping during high water level events. Additional protection against wind waves with living shorelines and marsh restoration along exposed levees segments could be added but would increase the footprint and result in a conversion of habitat beyond the existing footprint of the levees out into the bay.

Table 60 Enhancement Marshes Levee Crest Elevation
Existing Crest Elevation Linear Feet
7 - 8 ft 380
8 - 9 ft 316
9 - 10 ft 187
10 - 11 ft 1,070
11 - 12 ft 3,186
12 - 13 ft 809
13 - 14 ft 78
14+ ft 516
Total 6,542
Maintenance of Enhancement Marshes levees would need to elevate approximately 5,100 LF of levee to 11.5 ft NAVD88 (Figure 39). The levees can generally be classified by the type of access route along the crest; paved roads with a top width of approximately 20 feet (South I Street), gravel access roads with a top width of approximately 12 feet, and gravel trails with a top width of approximately 10 feet. Of the 5,100 LF to be maintained, 3,100 LF would be elevated less than 1 foot, which could be completed with minor grading and re surfacing with little to no change in the footprint of the levees. The remaining 2,000 LF would be elevated one to four feet and would require that the overall width of the levee be expanded inward into the treatment area. The levees have an existing 2:1 (H:V) side slope which will be maintained. For every 1-foot increase in height, there will be a corresponding 4-foot increase (2 feet per side) in width. The reduction in treatment area of the marshes is anticipated to be approximately 0.5 acres of a total 27 acres, or a 2% reduction in treatment area. It is expected that this reduction in area would not adversely affect operations (Table 61). Enhancement Marshes protection structure maintenance is a potential component of all alternatives.

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A specific geotechnical investigation was outside of the scope of this project, but from previous work completed in the area high groundwater and either soft and/or liquefiable soil is expected to underlie the existing levees, meaning that settlement of the levees is to be expected over time. To account the impact of settlement, the levees will need to be overbuilt by approximately 1 foot to settle to the target crest elevation over time. The existing top 6 inches of subgrade on the levee faces will need to be prepared and compacted, and approximately 2 feet of subgrade at the toe of the levees will need to be excavated and replaced with suitable fill. Additionally, geogrid could be needed on all disturbed subgrade to further stabilize the material (Figure 40).

6.3.2 Living Shoreline Description
Approximately 3,100 LF of the Enhancement Marshes levee with southwest exposure are exposed to wind waves.

This stretch of levee could benefit from implementing a living shoreline. The living shoreline would transition from the levee crest at approximately 11.5 feet to approximately 5 feet at the toe of the levee. With an assumed 10:1 slope down to the mudflat, the shoreline would extend 70 feet out from the existing levee toe, converting approximately 3.5 acres of existing wetland and mudflats surrounding the marshes.

6.3.3 Adaptive Management Description
In addition to elevating low-lying sections of the levees, the City will also investigate the impacts of infrequent saltwater intrusion into the enhancement marshes from extreme tide events. The marshes are a freshwater ecosystem, but it has been speculated that they could maintain effectiveness with some increase in salinity. The purpose of this study would be to identify a threshold of tidal inflows when the Enhancement Marshes no longer provide effective enhanced secondary treatment of wastewater.

The study is envisioned to be a multi-year project which could be conducted by the Arcata Marsh Research Institute with collaboration Cal Poly Sponsored Programs (graduate and academic research programs) in a controlled marsh pilot cell would be designed to answer questions such as:
 What duration and magnitude of saltwater intrusion can a freshwater marsh recover from?  What is the timeframe for recovery and what is the new normal after recovery?  Does the marsh function as a wastewater treatment unit in the presence of saltwater? With the threshold for effective treatment identified, the timeline and best course for long term adaptation of the enhancement marshes can be determined.

6.3.4 Project Costs
Based on the analysis presented above, it was estimated that the Enhancement Marsh Levee Maintenance would have a total project cost ranging from $4,700,000 to $10,000,000 (Appendix B). Based on the 50% Design completed for the NSI project which has similar location, wave exposure and elevation to the enhancement marsh levees, it was estimate that implementing a living shoreline would cost approximately $2,500 per linear foot of shoreline. With the 3100 LF of shoreline exposed to wave action at the Enhancement Marshes, it is expected that adding living shoreline to the Levee Maintenance would cost and additional $7,800,000, for a total project cost ranging from $12,500,000 to $17,800,000.

For the Adaptive Management Study, estimated costs were based on 2025/2026 salaries described in the Professional Services Agreement with Arcata Marsh Research Institute. Project hours are estimated based on the number of projects and hours described in the 2025/2026 Scope of Work and Compensation in the Professional Services Agreement with Arcata Marsh Research Institute. A modeling effort of the salinity impacts will also be required to support the pilot study. Based on the information presented, the pilot study is expected to range from $100,000 to $200,000, depending on staffing, material costs and modeling level of effort.

There are no additional O&M costs associated with the levee maintenance or living shoreline. Annual costs would be similar to the existing system.

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Figure 39 Enhancement Marsh Levee elevations sand conceptual living shoreline.

Table 61 Enhancement Marshes Levee Maintenance Quantities Existing
Crest
Elevation
LF LF Paved
Roads (20 ft
width)
LF Trails
(gravel,
10ft width)
LF Dirt/Gravel
Access Road
(12 ft width)
Raise Levee ~ (ft) treatment area
reduction
(acres)
7 - 8 ft 380 155 225 0 4.5 0.14
8 - 9 ft 316 160 100 56 3.5 0.09
9 - 10 ft 187 20 167 0 2.5 0.03
10 - 11 ft 1070 400 620 50 1.5 0.10
11 - 12 ft 3186 800 686 1700 0.5 0.15
Total 5139 1535 1798 1806 NA 0.51

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Figure 40 Concept Design for Levee Maintenance

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6.4 Enhancement Marshes Levee Augmentation
Enhancement Marshes levee augmentation is a potential component of Alternative 2 and Alternative 6.

6.4.1 Description
As presented in Sections 3.5.5, 3.6.5, and 5.5.3.2, , the Enhancement Mashes are currently vulnerable to the 10-year,

10.1 foot tidal flood event. To provide protection to 2105 and to match the level of protection that the AWTF Levee Augmentation Project will provide to the treatment plant, the protection structures surrounding the Enhancement Marshes would be elevated to 15 feet NAVD88, providing protection to a 14-foot water level with one foot of freeboard.

Under the Intermediate-High and High scenarios, design protection is reduced to 2085 and 2075, respectively.

The Enhancement Marshes protection structures have the same seismic, geotechnical, engineering and environmental considerations as the AWTF Levee Augmentation Project. For the purposes of this study, it was assumed that the same combination of sheet piles, concrete floodwalls and shoreline protection could be implemented at the Enhancement Marshes.

Approximately 3,100 LF of the Enhancement Marshes levee with southwest exposure are impacted by wind waves.

This stretch of levee could benefit from implementing a living shoreline. The living shoreline would transition from the levee crest at approximately 15 feet to approximately 5 feet at the toe of the levee. With an assumed 10:1 slope down to the mudflat, the shoreline would extend 100 feet out from the existing levee toe, impacting approximately 7.2 acres of existing wetland and mudflats surrounding the marshes.

6.4.2 Project Costs
The cost of the Enhancement Marsh Levee Augmentation was based on the SHN 30% Design cost estimate for the AWTF Levee Augmentation Project. The total length for the AWTF levee’s is 7,600 LF, resulting a per LF unit cost of $1400. The total length of the Enhancement Marsh levee’s is 6500 LF. It is expected that the total project cost will range from $10,700,000 to $23,000,000 (Appendix B).

Based on the 50% Design completed for the NSI project which has similar location and wave exposure to the enhancement marsh levees, it was estimate that implementing a living shoreline would cost approximately $2,500 per linear foot of shoreline with a crest elevation 11.5 feet. As the Levee Augmentation is proposing to elevate the structures to 15 feet, this unit cost was increased by 30% to account for the extra fill material to match the increased crest elevation, increasing the unit cost to $3,250 per LF. With the 3100 LF of shoreline exposed to wave action at the Enhancement Marshes, it is expected that adding living shoreline to the levee augmentation would cost an additional $10,000,000, for a total project cost ranging from $20,700,000 to $33,000,000.

6.5 New Enhancement Project
Alternative enhancement methods are a potential component of Alternative 3 and Alternative 7.

6.5.1 Description
To continue discharging to Humboldt Bay if the Enhancement Marshes were decommissioned due to SLR flooding, the EBEP would be required a new enhancement project to provide fuller realization of existing beneficial uses or the creation of new beneficial uses either by or in conjunction with a wastewater treatment project. Criteria for a new enhancement project was presented in Section 5.5.3.3, based on recent guidance the RWQCB has been developing on enhancement criteria for the City of Eureka. The RWQCB identified four classes of projects: Climate Adaptation and Resilience; Habitat Restoration and Creation; Removal of Legacy Pollutants Impacting the Bay; and Disadvantaged Communities and Public Health.

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For analysis in this study, it was assumed that new enhancement marshes similar in size and function to the existing facility, but outside the flood zone, would replace the existing marshes. However, the final enhancement project could range from infrastructure protection corridors to wetlands creation to pollutant removal to water quality projects.

The existing Allen, Gearheart, and Hauser Marshes are 10.4 acres, 7.3 acres, and 9.2 acres is size respectively for a total of 26.9 acres of march area. The marsh complex also includes land for transfer piping and pumps to move water between the various marshes and then to the Bay discharge point. In addition to the enhancement marshes an additional 20% land area was incorporated for ancillary features and potential setbacks for a total estimated land area of 32 acres. In addition to onsite development, piping, and controls, a discharge pump station and piping from the AWTF to the enhancement location will be required.

Similar to the timeline discussion for a relocated wastewater treatment plant, maintenance on the existing Enhancement Marsh levees would need to be completed to protect the facility until a new project could be implemented. It is estimated that a new enhancement project would take between 15 to 30 years to plan, permit, design and construct.

6.5.2 Project Costs
Project costs for the enhancement project would be similar to ecosystem restoration projects, which can range significantly in cost per acre depending on existing site conditions such as topography, soil type and stability, project scale, design complexity, site goals, and other factors. A review of enhancement project costs along the California coast was conducted, including the Elk River Estuary and Tidal Enhancement Project in Eureka, Dutch Slough Tidal Marsh Restoration Project in Oakley California, and Salt River Ecosystem Restoration Project in Ferndale California, among others. Restoration costs excluding planning, design, and permitting ranged from $9,200 per acre to $156,000 per acre. For this study, an estimated unit cost of $100,000 per acre for restoration was selected. Additional considerations for yard piping and instrumentation and controls for flow control were included in the estimate. The total project cost is estimated to range from $33,700,000 to $72,300,000 (Appendix B).

In addition, the Enhancement Marshes should be maintained in the near term to allow time for the new enhancement project to be constructed. The estimated Enhancement Marsh Levee Maintenance was estimated to have a total project cost ranging from $4,700,000 to $10,000,000 (Appendix B). For a total cost of between $38,400,000 and $82,300,000.

Operational costs of the new system would be similar to the existing enhancement marshes, with the addition of pumping from the AWTF to the new enhancement location. Pending engineering design and a detailed hydraulic evaluation, the following preliminary evaluation was completed to provide a rough estimate of pumping costs.

Hazen Williams formula for headloss and the waterpower equation were used to estimate the energy required for pumping. It was assumed that approximately 20,000 LF of 24” HDPE pipe would be required to transfer the influent to the new location, with an approximately 15 feet of elevation gain. An additional 30% of headloss was added to account for minor losses and design uncertainties. It was assumed that the average design flow of 2.3 MGD for dry weather would occur from May through October, and the average wet weather design flow of 5 MGD would occur October – April. This analysis assumed a constant pumping rate, with no consideration for wetwell or equalization basin holding capacity. A total yearly energy usage of approximately 240,000 kWh was estimated. With an average energy cost of $0.30/kWh, this would result in a yearly pumping cost of approximately $70,000 (Appendix C). Due to the hydraulic complexity of operating high and low flow pumping system with a pumping distance approximately 4 miles in length, this analysis should be considered a rough estimate to be revisited during design of the system. In addition to electricity costs, the City would be responsible for the new pump station. Annual maintenance costs for the new pump station was estimated at 1% of the equipment costs, or $130,000 annually. The total estimated additional O&M costs for this component is $200,000 annually.

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6.6 RMTII Ocean Outfall
Ocean discharge is a potential component of Alternative 4 and Alternative 8.

6.6.1 Description
To utilize the existing RMTII outfall, an effluent pump station at the treatment plant site, a transfer pipeline to the outfall, and a connection to the existing outfall would be required. With a primary alignment along Highway 255, the estimated length of this route is 9.5 miles. For this alternative, it is assumed a same size forcemain (24-inch or larger) required from the new lift station to the new WWTP will be installed for the effluent forcemain.

The ocean outfall piping originates from the vacated Louisiana Pacific (Samoa) Pulp Mill on Vance Road and was the ultimate discharge component of a wastewater management system that serviced the company’s Redwood Marine Terminal II (RMT II) operational area. The site and outfall pipe is now owned by the Humboldt Bay Harbor, Recreation and Conservation District (HBHRCD). The 48” HDPE outfall pipeline is 1.5 miles in length and extends approximately

1.25 miles into the ocean. This Alternative would require installing a similar connection to the existing 48” outfall piping. Previous studies (by others) have indicated the total capacity of the outfall piping could be as high as 40 MGD.

Maintenance on the existing Enhancement Marsh levees would need to be completed to protect the facility until a new project could be implemented. It is estimated that a new connection to the existing RMTII ocean outfall could be completed within 20 to 30 years, allowing enough time to plan, permit, design and construct.

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Figure 41 Conceptual Design for Pipeline to RMTII
The following assumptions were made for the outfall alternative:
• Geotechnical conditions and native soil are suitable for pipeline construction for the majority of the pipeline alignment
• The Mad River Slough can be crossed on Highway 255 using trenchless methods, such as a jack and bore or horizontal direction drilling installation
• Caltrans/Humboldt County permitting can be obtained for installation of the pipeline • Based on ongoing efforts to replace existing transmission pipelines on the Samoa Peninsula, design, permitting and construction of the pipeline and connection to RMTII is expected to take 20 to 30 years to complete

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6.6.2 Project Costs
Based on the analysis presented above, the total project cost for the discharge pump station, pipeline and connection to RMTII is estimated to range from $58,700,000 to $125,900,000 and is further detailed in Appendix B. In addition, the Enhancement Marshes should be maintained in the near term to allow time for the new enhancement project to be constructed. The estimated Enhancement Marsh Levee Maintenance was estimated to have a total project cost ranging from $4,700,000 to $10,000,000 (Appendix B). For a total cost of between $63,400,000 and $135,900,000.

Operational costs of the new system would include pumping from the AWTF to RMTII. Pending engineering design and a detailed hydraulic evaluation, the following preliminary evaluation was completed to provide a rough estimate of pumping costs.

Hazen Williams formula for headloss and the waterpower equation were used to estimate the energy required for pumping. It was assumed that approximately 50,000 LF of 24” HDPE pipe would be required to transfer the influent to the new location, with an approximately 15 feet of elevation gain to reach the new headworks. An additional 50% of headloss was added to account for minor losses and design uncertainties. It was assumed that the average design flow of 2.3 MGD for dry weather would occur from May through October, and the average wet weather design flow of 5 MGD would occur October – April. This analysis assumed a constant pumping rate, with no consideration for wetwell or equalization basin holding capacity. A total yearly energy usage of approximately 486,000 kWh was estimated. With an average energy cost of $0.30/kWh, this would result in a yearly pumping cost of approximately $150,000 (Appendix C). Due to the hydraulic complexity of operating high and low flow pumping system with a pumping distance approximately 9.5 miles in length, this analysis should be considered a rough estimate to be revisited during design of the system.

In addition to electricity costs, the City would be responsible for the new pump station. Annual maintenance costs for the new pump station were estimated at 1% of the equipment costs, or $150,000 annually.

Lastly, there would be annual service fees to the Harbor District for use of their facilities. Annual service costs were estimated at $200,000. This is a placeholder estimate. Actual costs would depend on future rate negotiations between the City and the Harbor District, which have not been initiated. Total estimated annual operations and maintenance costs would be $500,000.

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6.7 Alternative Cost Summary
A summary of the estimated costs by alternative is shown in Table 62 including both Capital Costs and O&M costs.

Table 62 Summary of Alternative Costs
Treatment Humboldt Bay Discharge Ocean Discharge
Maintain and Adaptive
Management of the
Enhancement Marshes
Enhancement Marshes
Levee Augmentation
New Enhancement RMTII Ocean Discharge
Augment AWTF
Levees
Alternative 1
Capital Cost: $29,800,000
Additional Annual O&M
Cost: N/A
Alternative 2
Capital Cost:
$39,300,000
Additional Annual
O&M Cost: N/A
Alternative 3
Capital Cost:
$82,800,000
Additional Annual
O&M Cost: $200,000
Alternative 4
Capital Cost:
$122,100,000
Additional Annual O&M
Cost: $500,000
Retreat AWTF Alternative 5
Capital Cost:
$177,800,000
Additional Annual O&M
Cost: $200,000
Alternative 6
Capital Cost:
$187,300,000
Additional Annual
O&M Cost: $200,000
Alternative 7
Capital Cost:
$230,800,000
Additional Annual
O&M Cost: $400,000
Alternative 8
Capital Cost:
$270,100,000
Additional Annual O&M
Cost: $700,000

6.8 Timeline
Table 63 below presents the expected timeline for flood events to begin to impact treatment operations, based on the OPC intermediate SLR scenario. Flood level is presented on a scale of nuisance (temporary shallow flooding that does not directly impact treatment but requires varying degrees of cleanup), disruptive (flooding that disrupts operations but does not permanently damage assets) to damaging flooding.

The Enhancement Marshes, corporation yard and select manholes experienced nuisance flooding from the tide of record for Humboldt Bay of 10 feet NAVD88 on January 3rd, 2026. This included temporary overtopping causing saltwater intrusion into the enhancement marshes, corporation yard and several manholes. These events are expected to become more frequent in the next 30 years and begin to disrupt treatment by 2055. Continuing to floodproof manholes and the collection system, along with levee repairs at the Enhancement Marshes and AWTF will help address these issues.

By 2055, It is expected that water levels will begin to occasionally overtop into the oxidation ponds and treatment wetlands, disrupting operations. Water levels that cause extended overtopping (many hours of inflow), resulting damaged levees and treatment equipment are expected to by 2075. Damages are expected to result in extended periods without wastewater service and potential wastewater discharges. These impacts can be addressed with Levee Augmentation or Relocation of the AWTF and Enhancement Marshes.

By 2075, water levels are expected to regularly exceed the protection that Levee Maintenance Projects would provide.

At this point, the Enhancement Marshes and AWTF would need to have Levee Augmentation in place or be fully retreated from the existing location.

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By 2105, the 100-year flood event is expected to be approximately 14 feet, or the design event for Levee Augmentation Projects. The 1 foot of freeboard incorporated into the design allows for some flexibility to extend the life of these improvements, especially if coupled with Living Shoreline features to attenuate wind wave overtopping. The magnitude and extent of sea level rise beyond 2105 is uncertain, but it is expected that extreme flood event water levels will exceed the 15-foot crest elevation of the Levee Augmentation projects within a few decades of 2100, and the AWTF, Enhancement Marshes and other City infrastructure will need to be relocated from the low-lying areas at and around the existing site as implementation of further protection measures will be extremely difficult.

Table 63 Timeline for Disruption of Treatment Operations by Asset.

Asset
Water Level
Vulnerability
(ft, NAVD88)
Year
Vulnerable
(OPC
Intermediate
Scenario)
Year
Vulnerable
(OPC IntHigh
Scenario)
Year
Vulnerable
(OPC High
Scenario) Flood Level Consequence Adaptation Strategies Collection System
Manholes 10.1 2025 2025 2025 Nuisance Minor I&I reduction projects Lift stations 11.7 2075 2060 2055 Disruptive Major Floodproofing and elevation of critical components
AWTF
Corporation Yard 10.1 2025 2025 2025 Nuisance Minor AWTF Levee Repairs Headworks 10.7 – 11.1 2025 - 2055 2025 - 2045 2025 - 2040 Nuisance – Disruptive Minor AWTF Levee Repairs or Augmentation
Oxidation Ponds 11.1 – 11.7 2055 - 2075 2045 - 2060 2040- 2055 Disruptive – Damaging Major - Severe AWTF Levee Augmentation or Relocation
Treatment Wetlands 11.1 – 11.7 2055 - 2075 2045 - 2060 2040- 2055 Disruptive – Damaging Major - Severe AWTF Levee Augmentation or Relocation
Emergency Pond
Pump Station 11.9 2075 2060 2055 Disruptive Moderate AWTF Levee Augmentation or Relocation Electrical Equipment
for Essential
Facilities
14 2105+ 2085 2075 Damaging Catastrophic
AWTF Levee Augmentation or
Relocation
Discharge
Enhancement
Marshes

10.1 - 11.1 2025 - 2055 2025 - 2045 2025 - 2040 Nuisance - Damaging Major - Severe Enhancement Marsh Levee
Repairs, then Levee
Augmentation, New
Enhancement or Ocean
Discharge

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In addition to the timeline describing the need for adaptation strategies, the timeline for implementation of the strategies was also considered. Table 64 shows the estimated number of years needed to complete planning, design, permitting and construction for each adaptation. As many of the alternatives require securing new facility sites and/or long pipeline encroachments the implementation length is conservative and could be completed faster depending on availability of land and ease of permitting.

Table 64 Approximate Years Needed to Implement Adaptation Strategies.

Component Approximate Time
Needed to
Implement
Adaptation from
Planning to
Implementation
Approximate Year
for Mitigation
Effectiveness (OPC
Intermediate
Scenario)
Approximate Year
for Mitigation
Effectiveness
(OPC
Intermediate-High
Scenario)
Approximate
Year for
Mitigation
Effectiveness
(OPC High
Scenario)
Treatment Strategies
AWTF Levee Augmentation 6 Years 2105 2085 2075
AWTF Retreat 30 Years 2105 and beyond 2105 and beyond 2105 and beyond
Disposal Strategies
Adaptive Management of Marshes 6 Years 2055-2075 2045 - 2060 2040-2055 Marsh Levee Augmentation 10 Years 2105 2085 2075
New Enhancement Project 25 Years 2105 and beyond 2105 and beyond 2105 and beyond
HBHRCD Ocean Outfall 20 Years 2105 and beyond 2105 and beyond 2105 and beyond
Living Shorelines 10 Years N/A N/A N/A

6.9 Cost Benefit Analysis

6.9.1 Financial Parameters and Assumptions
Wastewater facilities are long‑lived assets with upfront capital costs, periodic upgrade and replacement costs, and decades of operations and maintenance costs. Because these projects span long timelines and can be initiated over different time periods, an analysis of life cycle costs was conducted to compare costs across years and account for inflation and discount rates. A summary of the factors used for this study are below:
• Inflation Rate: This rate is used to project how project costs will increase over time. The historical inflation rate have varied significantly, especially over the last 10 years. During covid inflation rates hit 9% compared to a more typical rate of 2% to 3%. For this study an inflation rate of 2.5% was used to project future capital and O&M costs.
• Discount Rate: This rate reflects the time value of money and addresses how future costs compare to presentday dollars. Using a discount rate converts all future costs into present value, letting you compare alternatives fairly. For this study a typical discount rate of 3% was used. • Salvage Value: To account for the value left in new assets at the end of planning analysis, a straight line salvage was used assuming equal value over the life of the asset. A salvage value was only assigned to treatment equipment that still has useful life at the end of the analysis. • Maintenance and Replacement Costs: For new equipment added to the wastewater system, an estimate of annual costs for maintenance and future replacement were incorporated into the analysis. The cost was estimated as 1% of the project equipment only costs. This applied to treatment systems and pump stations. • Planning Horizon: As the mitigation strategies would be implemented over a long period of time, the planning horizon was 80 years. For comparison purposes, a 40 year life cycle analysis was also run.

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• Future Improvements: Due to the length of the planning period, costs for future improvements were included for some alternatives. A cost of $20 Million was used as a placeholder for future Phase II facility improvements. Future treatment upgrades are assumed to be needed approximately every 20 years and were estimated at $40,000,000, which is half the estimated cost of a new treatment system.

6.9.2 Life Cycle Cost Analysis
Project costs include both capital costs and operational costs. Decommissioning of existing facilities was included in the costs of alternatives which consider retreating facilities. The parameters discussed above were used to develop an estimated lifecycle cost. Specific future improvements could not be fully detailed, as it would require decisions on technology, facility locations, the final combination of treatment and disposal strategies, and other factors. For this reason, a direct comparison of existing versus future operations costs was not conducted. It was assumed that treatment and disposal costs would be similar to existing costs except for major new equipment, primarily pump stations. Table 65 below presents the assumptions used to estimate the life cycle cost for each of the strategies.

Table 65 Life Cycle Cost Assumptions.

Capital
Costs
Year
Costs
Incurred
for
Analysis
New
Annual
O&M Costs
Years
incurred
Other Cost Assumptions

TREATMENT
Augment
AWTF Levees
~$22 Million 2031 N/A Similar
to Existing
System
N/A Near and mid-term treatment improvements
would be needed. Including Phase 2 treatment
upgrade estimated at $20,000,000 in 2040, and
future upgrades in 2060, 2080, and 2100
estimated at ~$40 million each. A salvage value
on future improvements was incorporated.

Retreat AWTF ~ $170.5
Million
2056 $202,800 2055 -
2105
Levee maintenance would need to occur at the
existing AWTF due to timing, Decommissioning
costs for the existing facility were included in the
capital costs. A future upgrade was included in
2076 and 2096 estimated at ~40 million each. A
salvage value on future improvements was
incorporated.

DISPOSAL
Maintain and
Adaptive
Management of
the
Enhancement
Marshes
~$7 Million 2031 N/A Similar
to Existing
System
N/A This alternative would require additional
adaptations within the 80-year planning horizon
that were not added to the costs. No salvage
value was included.

Enhancement
Marshes Levee
Augmentation
~$17 Million 2035 N/A Similar
to Existing
System
N/A No additional costs added to the analysis. No
salvage value was included.

New
Enhancement
~60 Million 2045 $200,000 2045-2105 Decommissioning costs for the existing facility were included in the capital cost. Levee
maintenance would need to occur at the
existing enhancement marshes. No salvage
value was included.

RMTII Ocean
Discharge
~99.6 Million 2045 $500,000 2045-2105 No additional costs added to the analysis. No salvage value was included.

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6.9.3 Summary of Costs and Benefits
The primary benefit of all the proposed alternatives is continued operation of the wastewater system under increasing pressure from SLR. The City currently experiences minimal emergency maintenance costs during flooding, but these costs are anticipated to increase over time. Damage cost estimates were not developed or incorporated into this analysis.

Table 66 shows the estimated life cycle costs for each of the adaptation strategies, rounded to the nearest $100,000. This is followed by the life cycle costs by Alternative in Table 67. The back up documentation on the life cycle costs can be found in Appendix G. Scenarios for both an 80-year and 40-year life cycle costs were run to enable a comparison over time.

Of the treatment strategies, augmentation of the levee has a lower life cycle cost. Construction of the levee augmentation project does not prevent the City from continuing to pursue retreat. Due to the long timeline anticipated to relocate the treatment facility, some levee maintenance to address vulnerable sections would be required for the retreat alternative.

Of the disposal strategies, maintenance and adaptive management of the enhancement marshes had the lowest life cycle cost followed by enhancement marsh levee augmentation, The adaptive approach would likely have a higher long-term cost over the 80-year period, as additional levee enhancement would need to be built prior to 2105. Of the new disposal options, a new enhancement project has the lower life cycle cost.

Table 66 Adaptation Strategy Life Cycle Costs
80-Year Present Worth Life Cycle
Costs
40-Year Present Worth Life
Cycle Costs
Treatment Strategies
Augment AWTF Levees $110,800,000 $49,400,000
Retreat AWTF $202,700,000 $79,700,000
Disposal Strategy
Maintain and Adaptive Management of the
Enhancement Marshes1 $7,100,000 $7,100,000
Enhancement Marshes Levee Augmentation $16,000,000 $16,000,000 New Enhancement $71,600,000 $65,500,000
RMTII Ocean Discharge $114,700,000 $80,900,000
Living Shoreline Strategy Addition
AWTF Levee Living Shoreline Additional Cost $13,600,000 $13,600,000 Enhancement Marshes Levee Living Shoreline
Additional Cost $9,600,000 $9,600,000
1 Adaptive management approach will require additional adaptations prior to 2105 that are not included in the 80-Year life cycle cost.

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Table 67 Adaptation Alternatives Life Cycle Costs
Alternative Strategy 80-year Present
Worth Life Cycle
Cost1
40- Year Present
Worth Life Cycle
Cost
1 AWTF Levee Augmentation and Adaptive Management of the Enhancement Marshes with Minor Levee Maintenance, Bay
Discharge1
$117,900,000 $56,500,000
2 AWTF Levee Augmentation and Enhancement Marsh Levee
Augmentation, Bay Discharge
$126,900,000 $65,500,000
3 AWTF Levee Augmentation and Retreat/ modify Enhancement Marshes, Bay Discharge
$182,400,000 $115,000,000
4 AWTF Levee Augmentation and Consolidation with Humboldt Bay Harbor Recreation and Conservation District, Ocean Outfall Discharge $225,500,000 $130,300,000
5 Retreat AWTF to a New Location and Adaptive Management of the Enhancement Marshes with Minor Levee Maintenance, Bay
Discharge1
$209,800,000 $86,800,000
6 Retreat AWTF to a New Location plus Protect the Enhancement Marshes, Bay Discharge
$218,800,000 $95,800,000
7 Retreat AWTF to New Location plus Retreat/ Modify Enhancement Marshes, Bay Discharge
$274,300,000 $145,300,000
8 Retreat AWTF to new location and Consolidation with Humboldt Bay Harbor Recreation and Conservation District, Ocean Outfall Discharge $317,400,000 $160,600,000
1 The Adaptive management approach will require additional improvements in approximately 25-50 years that are not included in the 80-year life cycle cost.

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6.10 Constraints Analysis
The constraints analysis of each alternative is presented in Table 68 below. The analysis considers the Protection Timeline, Adaptation Methodology, RWQCB Requirements, Constructability, Environmental Requirements, Energy Efficiency, and Secondary Benefits.

Table 68 Alternatives Constraints Analysis
Alternative Protection
Timeline
Adaptation
Methodology
RWQCB Requirements Engineering/ Constructability/ Geotechnical Environmental Requirements Energy Efficiency Secondary Benefits Alternative 1: Augment AWTF Levees and
Maintain and Adaptive Management of
the Enhancement Marshes
~2055 Protect/
Accommodate
Low
The City would continue with the
existing permitted treatment and
discharge systems.

Low
The City has completed preliminary design of the
AWTF levee. Standard engineering practices can be
used and the site accessed from disturbed areas. The
City already performs marsh maintenance.

Medium
Levee augmentation is likely to have moderate aesthetic impacts. Both AWTF levee augmentation and
Enhancement Marsh maintenance activities would have
varying degrees of biological impacts based on the design of the shoreline protection features.

Low
This option leverages the existing
system and add no new energy
demands.

High
Maintains benefits of the
existing Enhancement
Marshes, and allows for
nature based system to be
added to the AWTF levee
Alternative 2. Augment AWTF Levees and
Augment Enhancement Marshes’ Levees
~2105 Protect Low
The City would continue with the
existing permitted treatment and
discharge systems.

Low
The City has completed preliminary design of the
AWTF levee. Standard engineering practices can be
used and the site accessed from disturbed areas. The
City already performs marsh maintenance.

Medium
Levee augmentation is likely to have moderate aesthetic impacts and varying degrees of biological impacts based on the design of the shoreline protection features.

Low
This option leverages the existing
system and add no new energy
demands.

High
Maintains benefits of the
existing Enhancement
Marshes, and allows for
nature based system to be
added to the AWTF levee
Alternative 3. Augment AWTF Levees and
New Enhancement
~2105 Protect/ Retreat High
The City's permit would need to
be updated with an approved
new Enhancement project.

Medium
The City has completed preliminary design of the
AWTF levee. Standard engineering practices can be
used and the site accessed from disturbed areas. The
final location and benefits of a new enhancement
project are determined. Construction may be
complicated by land use, site conditions, and other
factors.

High
Levee augmentation is likely to have moderate aesthetic impacts and varying degrees of biological impacts based on the design of the shoreline protection features. New
Enhancement Marshes are anticipated to have medium to
high impacts to agriculture, air quality, energy, GHG,
biology, land use, and recreation
Medium
This option leverages the existing
treatment system, but includes
new pumping to and from the
new enhancement project.

High
Has the potential to create
new enhancement benefits
to the Bay.

Alternative 4: Augment AWTF Levees and
Ocean Discharge
~2105 Protect/ Retreat Medium
The City's permit would need to
be updated for ocean effluent
limits and discharge.

Medium
The City has completed preliminary design of the
AWTF levee. Standard engineering practices can be
used and the site accessed from disturbed areas. The
construction of the transmission main to the RMT II
outfall will include coordination with Caltrans, work in the coastal zone, and the need to avoid sensitive
resources in potential overland areas.

High
Levee augmentation is likely to have moderate aesthetic impacts and varying degrees of biological impacts based on the design of the shoreline protection features. Ocean
discharge is likely to have increased air quality, greenhouse gas emissions, and energy impacts however could provide water quality and biological benefits compared to bay
discharge.

Medium-High
This option leverages the existing
treatment system, but includes
new pumping to the ocean
outfall, located on the Samoa
Peninsula.

Low
Does not require Bay
enhancements.

Alternative 5. Retreat AWTF and Maintain
and Adaptive Management of the
Enhancement Marshes
~2055 Retreat/ Protect Low
The City's permit would need to
be updated for a new treatment
process, but the discharge
system would remain the same.

Medium
The final location of a new treatment facility is not
known. Construction may be complicated by land use,
site conditions, and other factors. The City already
performs marsh maintenance.

Medium
Retreat of the AWTF is anticipated to have impacts on all resource categories, the highest on agriculture, biology, and land use. Enhancement Marsh maintenance activities
would have varying degrees of biological impacts based on the design of the shoreline protection features.

Medium
This option leverages the existing
disposal system, but add new
energy demands to move effluent
to a relocated treatment facility.

Medium-High
Maintains benefits of the
existing Enhancement
Marshes.

Alternative 6. Retreat AWTF and Augment
Enhancement Marshes’ Levees
~2105 Protect/ Retreat Low
The City's permit would need to
be updated for a new treatment
process, but the discharge
system would remain the same.

Medium
The final location of a new treatment facility is not
known. Construction may be complicated by land use,
site conditions, and other factors. The City already
performs marsh maintenance.

Medium
Retreat of the AWTF is anticipated to have impacts on all resource categories. Enhancement Marsh augmentation
would have varying degrees of biological impacts based on the design of the shoreline protection features.

Medium
This option leverages the existing
disposal system, but add new
energy demands to move effluent
to a relocated treatment facility.

Medium-High
Maintains benefits of the
existing Enhancement
Marshes.

Alternative 7. Retreat AWTF and New
Enhancement
2105 and
beyond
Retreat Medium
The City's permit would need to
be updated for a new treatment
process, and the City would
need to negotiate with the
Regional Board on a new
enhancement Project
High
The final location of a new Treatment facility and
enhancement project are not determined. Construction
may be complicated by land use, site conditions, and
other factors.

High
Retreat of the AWTF is anticipated to have impacts on all resource categories. New Enhancement Marshes are
anticipated to have moderate to high impacts on all
resource categories, except aesthetics.

Medium-High
This option adds new energy
demands to move effluent to a
relocated treatment facility and to
and from a new enhancement
project.

Medium-High
Has the potential to create
new enhancement benefits
to the Bay.

Alternative 8. Retreat AWTF and Ocean
Discharge
2105 and
beyond
Retreat/
Accommodate
Medium
The City's permit would need to
be updated for a new treatment
process and updated for ocean
effluent limits and discharge.

Medium
The final location of a new treatment facility is not
known. Construction may be complicated by land use,
site conditions, and other factors. The construction of the transmission main to the RMT II outfall will include coordination with Caltrans, work in the coastal zone,
and the need to avoid sensitive resources in potential
overland areas.

High
Retreat of the AWTF is anticipated to have impacts on all resource categories. Ocean discharge is likely to have
increased air quality, greenhouse gas emissions, and
energy impacts however could provide water quality and
biological benefits compared to bay discharge.

High
This option adds new energy
demands to move effluent to a
relocated treatment facility and
for pumping to the ocean outfall,
located on the Samoa Peninsula.

Low
Does not require Bay
enhancements.

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7. Funding and Financing Plan

7.1 Community Funding Factors
Economic status, population, and other community characteristics can influence the City’s ability to obtain funding.

According to the 2020 US Census, the City has a population of 18,578 and a Median Household Income (MHI) of $47,000. The City’s MHI is approximately 60% of the statewide MHI, qualifying the City as economically disadvantaged under many funding programs. The City serves vulnerable populations, including the large student population.

As the Humboldt Bay area is recognized for its vulnerability to sea level rise, the City would likely qualify for programs that give funding preference for sea level rise resiliency projects. These types of projects could include protection, adaptation, and retreat strategies, with each funding program having its own emphases. With relatively clean air in comparison to the rest of California the City typically does not receive funding preference for projects that improve air quality.

The feasible alternatives did not include land application of effluent or groundwater discharge or injection. Thus, the Alternatives included in this study would not include water recycling funding programs or groundwater funding programs such as the Bureau of Reclamation’s Title XVI Program funding or SWRCB recycling and groundwater funding.

7.2 Funding Programs

7.2.1 State Water Resources Control Board (SWRCB), Clean Water State Revolving Fund (CWSRF)
The CWSRF is a program run by the State of California. It is funded by interest income and annual allocations of federal funds through the Clean Water Act as well as State proposition funding and other State funding streams in some years. The SWRCB puts out a CWSRF Intended Use Plan (IUP) annually which includes the details on principal forgiveness/ grant (grant/PF) eligibility and loans. The current CWSRF IUP (Fiscal Year 25-26) limits grant/PF eligibility to “small disadvantaged communities”, which is defined as communities with populations less than or equal to 20,000 and an MHI of less than 80% of the statewide MHI.

The City of Arcata qualifies as a small, disadvantaged community under the current CWSRF IUP. As available funding amounts vary year to year depending on the State and Federal budgets, each fiscal year the State sets maximum grant/PF amounts and caps on eligible costs. Due to limited grant/PF availability in FY25-26, the State Water Board further prioritized grant/PF funds. Projects that address violations of waste discharge requirements or National Pollutant Discharge Elimination System (NPDES) permits and projects that connect previously unsewered areas or join communities to regionalize wastewater treatment works are considered priority for grant/PF funding. Grant/PF construction funds are available to eligible applicants that serve Small DACs as described in Table 69 below.

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Table 69 Small DAC Construction Grant/ PF Eligibility Criteria (State of California, 2025) Project Type Percentage of Total
Eligible Project Cost
Maximum Grant/PF
Amount
Maximum Grant/PF Per
Residential Connection
Addresses a Violation of WDR or
NPDES Permit
100%
$50 million ($75
million if loan
component1)
$60,000
Septic-to-Sewer, Consolidation,
or Regionalization $175,0002
Secondary Priorities 50% $25 Million $30,000
1 If a project requires funding beyond the $50 million grant/PF, the project may receive an additional 50% grant/PF and loan split for the remaining cost (maximum grant of $75 million). If applicable, applicants can self-fund or co-fund rather than take out a loan. 2 The Deputy Director may approve up to $200,000 per residential connection for good cause.

As the City’s adaptation project drivers are not related to addressing violations, the sea level rise resiliency projects would fall under secondary priorities and in FY 25-26 be eligible of a maximum of 50% funding up to $25 Million. The IUP maximum grant/ PF amounts and percent eligibility can change from year to year and the City should check back closer to when a project may be implemented. For additional funds, the City could take on a loan from the SWRCB.

Loan terms are typically 20 years, but can be extended to up to 30 for small disadvantaged communities and the current interest rate is around 2%.

7.2.2 USDA Rural Development Funding Programs
USDA Rural Development’s primary program to support funding for wastewater systems is the Water and Waste Disposal Loan and Grant Program. However that program is limited to rural areas and towns with populations of 10,000 or less, and Arcata would not qualify.

The Community Facilities Direct Loan & Grant Program serve areas of up to 20,000 people for essential community facilities. Typical types of projects funded under this program are police and fire stations, health centers, libraries, or food banks. It may be that a relocated enhancement marsh that protects public recreation could qualify, but unlikely.

Projects serving 5,500 people or less and projects serving low-income communities having an MHI below 80% of the state nonmetropolitan MHI are prioritized. The City would be eligible for a maximum of 35% grant funding and the remainder would be a loan if a project was determined to be eligible. The City should contact the local USDA Rural Development Specialist to confirm funding eligibility and potential terms.

7.2.3 Environmental Protection Agency WIFIA
The Water Infrastructure Finance and Innovation Act of 2014 (WIFIA) established the WIFIA program, a federal credit program administered by EPA for eligible water and wastewater infrastructure projects. The WIFIA program accelerates investment in our nation’s water infrastructure by providing long-term, low-cost supplemental loans for regionally and nationally significant projects. Local government agencies are considered eligible applicants. The interest rate is equal or greater to the US Treasury rate of a similar maturity, with a maximum maturity term of 35 years. For a small community (less than 25,000 people per WIFIA guidelines), such as Arcata, the minimum funding amount is $5 Million, and small communities are eligible to received loans up to a maximum of 80% of project costs.

The WIFIA program has additional credit, repayment, and interest rate pathways that should be reviewed if the City decides to pursue this funding source.

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7.2.4 Ocean Protection Council Senate Bill 1 Sea Level Rise Adaptation Planning Grant Program – Track 2
Senate Bill 1 (Atkins, 2021), the Sea Level Rise Mitigation and Adaptation Act, was signed into law in 2021. Senate Bill 1 (SB 1) directs the Ocean Protection Council (OPC) to administer grants to local and regional governments to plan for SLR and implement adaptation projects to build resilience along the coast. The Track 2 funding would pick up where the City’s previous planning left off to be able to fund Nature-based and Green-Gray Hybrid Adaptation Projects/Feasibility Study/Design Plans.

Under the OPC program Green-Gray Hybrid SLR Adaptation Projects are those that incorporate natural coastal ecosystems with gray infrastructure to combine the values of wave attenuation and flood control of natural ecosystems with the benefits of engineered structures. The result is a mixed hybrid ecosystem in which the conservation and restoration of natural coastal ecosystems can extend the lifespan of gray infrastructure, while also supporting fisheries, regulating water quality, and sequestering carbon.

Project funding for Track 2 projects ranges from $1,500,000 - $10,000,000, with a maximum project timeline of 3 years. Proposals are accepted through a competitive process. There is a set aside for environmental justice communities, which under the OPC SB 1 program includes multiple definitions which Arcata could likely meet including population under 50,000, locally defined underserved communities, and being an economically disadvantaged community.

Funding for the program comes from multiple sources, including state tax revenue and bond proceeds. There is currently no set final funding round. This funding source should be explored for adaptive components of the City’s selected project.

7.2.5 California Bond funding
The State of California has a long established voter initiate process. Most recently voters approved Proposition 4:

Bonds for Safe Drinking Water, Wildfire Prevention, and Protecting Communities and Natural Lands From Climate Risks for $10 billion in funding. This funding will be released through multiple state agencies and programs, including funding for drought, flooding, and water supply and for sea level rise and coastal areas. The Prop 4 funding is further described below, but future State proposition funds or other new state programs could be used by the City for a portion of the sea level rise adaptations for the wastewater system.

7.2.6 Federal Appropriation
Funding through a federal appropriation is another mechanism available to the City of Arcata. This is typically achieved through Congressionally Directed Spending (CDS) via the US Senate or Community Project Funding (CPF) via the US House of Representatives. Individual members of Congress submit requests to the US House or Senate Appropriations Commission for inclusion in the final appropriations bills. Application deadlines vary, but are typically around the end of March each year. The City should reach out to their current representatives for the most up-to-date information on the application process, including deadlines, submission guidelines, and submittal procedures.

Projects must be eligible under a specific item in the relevant Appropriations bill. Funds must typically be spent within one federal fiscal year. Demonstration of local community support is a key factor in a successful application, including letter of support, data highlighting the project needs, and governmental support resolutions.

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7.3 Funding Strategy
Future wastewater costs represent major projects for the City. While funding options have been identified, the costs likely exceed potential grant funding and any loan costs would be passed onto the rate payers. Funding programs change and evolve and often the more work that has been completed towards implementation improves funding application competitiveness.

If the City moves toward retreat options, investment in the identification and analysis of site options that the City could purchase would be beneficial. Potential land banking for future retreat adaptation strategies would better position the City for future project implementation.

As was done with this study, it is recommended that the City continue to invest in planning for future sea level rise.

Effort expended in the near term, can have big results in the long run and position the City for future project implementation. Studies such as the Enhancement Marsh adaptive capacity analysis help position the City to better understand the lifespan of existing facilities.

8. Preliminary Project Report Findings
This report identified current and future flooding risks for City of Arcata Wastewater assets and developed adaptation strategies to address the vulnerabilities. From the full set of strategies considered, eight strategies were considered to be feasible and were evaluated in further detail. Preliminary costs estimates were developed and compared with a cost-benefit analysis, considering operational costs of each strategy.

The Arcata Wastewater Treatment Facility (AWTF), Enhancement Marshes, and supporting collection system face increasing exposure to tidal flooding, sea level rise (SLR), and storm-driven coastal hazards. By mid- and late-century, SLR will increasingly compromise levees, electrical equipment, treatment components, and enhancement marshes— ultimately threatening both treatment and effluent disposal reliability. Phase I upgrades (2023-2025) protect core treatment operations through ~2055, after which additional adaptation is required to maintain safe and reliable wastewater service.

8.1 Summary of Existing and Future Vulnerabilities
Key vulnerabilities identified include:
• 44 manholes currently subject to overtopping during current 100-yr tide events (~10.7 ft NAVD88), with exposure increasing to 50+ manholes by 2105.
• First Street Lift Station floods during major tides and loses design freeboard by 2075. • Enhancement Marsh levees (min. ~9.5 ft) and AWTF levees (<10 ft) are already overtopped during 10.1-ft tides and will experience multiple overtopping events in the next few years to decades. • By 2055, AWTF assets are anticipated to experience yearly nuisance flooding, with potential for minor impacts to operations. The enhancement marshes are expected to see overtopping a few times a year during king tide events, disrupting operations and potentially damaging levees. • By 2105, AWTF assets are anticipated to experience monthly to daily flooding, with potential for severe to catastrophic operational failures (electrical equipment, pump stations, levees). • Public access, habitat value, and EBEP compliance are all at risk if levees fail or marsh treatment capacity is lost.

These vulnerabilities drive the need for near-, mid-, and long-term adaptive investment.

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8.2 Summary of Existing Projects, Planning, and Next
Steps
The City’s immediate next steps focus on utilizing the Feasibility Study to inform policy direction and future project development. In spring of 2026, staff plan to present the Feasibility Study and detailed information on the three preferred alternatives to City Council, building on prior staff updates regarding project progress.

The goal of the spring Council presentation is to receive clarifying questions and policy direction, including guidance on which alternative—or alternatives—the Council would like staff to further evaluate. Based on Council direction, the City anticipates returning near the end of 2026 with recommendations supported by additional technical studies and environmental review.

The City is advancing multiple adaptation strategies to address sea level rise vulnerabilities across wastewater infrastructure and natural systems. These strategies focus on improving resilience through targeted capital improvements, operational flexibility, and applied research.

Key actions include testing and implementing accommodation strategies at lift stations, particularly through a capital improvement project at the First Street Lift Station. This project includes pump replacement and evaluation of equipment elevations to better accommodate future sea level rise conditions.

Within the wastewater treatment plant, the City is initiating a retrofit of the emergency pond pump station to enhance operational flexibility, allowing water to be moved more efficiently under changing conditions. This effort builds on completed Phase 1 improvements and will also address sea level rise vulnerability by evaluating the elevation of critical electrical equipment.

In parallel, the City is collaborating with Cal Poly partners at the Arcata Marsh Research Institute to study the adaptive capacity of enhancement marshes, including their response to periodic exposure to brackish or saline water. These studies will inform long-term adaptation planning and nature-based resilience strategies.

The City is advancing the shoreline‑wide sea level rise adaptation strategy through a recently awarded Ocean Protection Council grant. This effort will focus on further identifying critical triggers, threshold surface elevations, and anticipated timeframes to inform when design and implementation of adaptation measures will be needed. The planning will address areas along the Arcata shoreline outside of the wastewater treatment plant, supporting proactive, phased adaptation and ensuring infrastructure and shoreline assets are addressed in advance of increasing sea level rise impacts.

Together, these actions demonstrate the City’s ongoing commitment to proactive adaptation planning, timely implementation, and informed decision‑making in advance of increasing sea level rise impacts.

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8.3 Adaptation Alternatives Evaluated
Eight alternatives were developed by combining treatment location options discharge pathways.

Alternative 1 – Augment AWTF Levees + Maintain Marshes
• Protection lifespan: ~2040 – 2055 (based on enhancement marsh vulnerability and range of OPC SLR scenarios)
• Approach: Hard/soft levee improvements at AWTF; marsh maintenance. • Method: Protect/Accommodate
• Pros: Lowest capital cost; allows continued bay discharge. • Cons: Marsh vulnerability increases past 2055; not long-term resilient.

Alternative 2 – Augment Both AWTF & Marsh Levees
• Protection lifespan: ~2075 – 2105 (based on range of OPC SLR scenarios) • Approach: Raise all levees to ~15 ft NAVD88.
• Method: Protect
• Pros: Longest lifespan of “protect-in-place” options; maintains existing discharge approach. • Cons: Highest long-term O&M needs; significant permitting for levee upgrades.

Alternative 3 – AWTF Levee Augmentation + New Enhancement Marsh • Protection lifespan: ~2075 – 2105 (based on range of OPC SLR scenarios) • Method: Protect/Retreat
• Pros: Maintains AWTF onsite while shifting marsh functions to a less vulnerable location. • Cons: High capital cost; permitting complexity around EBEP compliance.

Alternative 4 – AWTF Levees + Ocean Discharge (via RMT II) • Protection lifespan: ~2075 – 2105 (based on range of OPC SLR scenarios) • Method: Protect/Retreat
• Pros: Removes EBEP constraints; ocean discharge offers regulatory stability. • Cons: Requires new pipeline and outfall agreements; high technical and permitting complexity.

Alternative 5 – Retreat AWTF + Maintain Marshes
• Protection lifespan: ~2045 – 2055 (based on enhancement marsh vulnerability and range of OPC SLR scenarios)
• Method: Retreat/Protect
• Pros: Moving the plant solves long-term SLR risk.
• Cons: Marsh vulnerability persists; requires large capital outlay in the future.

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Alternative 6 – Retreat AWTF + Augment Marsh Levees
• Protection lifespan: ~2017 – 2105 (based on enhancement marsh vulnerability and range of OPC SLR scenarios)
• Method: Protect/Retreat
• Pros: Long-term resilience for both treatment and marsh functions. • Cons: Still costly; requires dual large-scale projects.

Alternative 7 – Retreat AWTF + New Enhancement Marsh
• Protection lifespan: >2105 (for all OPC SLR scenarios) • Method: Retreat
• Pros: Highest resilience; new marsh ensures EBEP compliance. • Cons: One of the most expensive options.

Alternative 8 – Retreat AWTF + Ocean Discharge
• Protection lifespan: >2105 (for all OPC SLR scenarios) • Method: Retreat
• Pros: Ultimate long-term resilience; ocean discharge avoids future marsh SLR constraints. • Cons: Highest capital cost; requires outfall agreements and regulatory approvals.

8.4 Cost Comparison (Order-of-Magnitude, Class 5)
(Values rounded per report guidance; full ranges in document.) • Lowest capital cost: Alternative 1 (~$30M)
• Highest capital cost: Alternative 8 (~$270M)
• Lowest life-cycle cost (80-years): Alternative 1 (~$118M) • Highest life-cycle cost (80-years): Alternative 8 (~$317M) • Lowest life-cycle cost (40-years): Alternative 1 (~$57M) • Highest life-cycle cost (40-years): Alternative 8 (~$161M) Note: Life-cycle costs do not include anticipated future enhancement marsh levee augmentation or relocation after the adaptive management and maintenance approach in Alternatives 1 and 5.

8.5 Constraints Affecting Alternatives
The study identifies multiple constraints that influence feasibility and should be contemplated as the project moves forward:

Regulatory Constraints
• EBEP: Continued bay discharge requires demonstrable ecological enhancement; SLR threatens marsh viability.

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• CCC Coastal Development Permit (CDP): Requires evaluation of non-hard armoring options for SLR adaptation.
• Ocean Outfall Permitting: Complex, multi-agency coordination (USACE, CCC, State Lands, NOAA, etc.).

Physical / Engineering Constraints
• Existing levee elevations insufficient against projected SLR. • Mudflat substrate limits feasibility of nature-based solutions without extensive fill. • AWTF site is low-lying, subsiding, and surrounded by tidal wetlands. • Access roads (South G and South I Streets) flood early and often under SLR.

Operational Constraints
• Maintaining compliance during construction.
• Pumping requirements if AWTF is relocated inland.

Financial Constraints
• Large capital projects place pressure on ratepayers in a disadvantaged community. • Multiple competing infrastructure needs across the City.

Land & Siting Constraints
• Limited high-ground parcels for AWTF relocation.
• New marsh creation must be outside coastal flood zones yet hydrologically suitable. • Larger parcels, 10 acres or more, may be difficult to find. • Relocated AWTF should be sited close to the existing collection system to reduce pumping needs..

Community & Social Constraints
• The community values recreational and ecological benefits of the existing AWTF and Enhancement Marshes and preservation of some public benefits should be considered if a new marsh is developed. • Sustainability and environmental protection were central themes from public input, with strong support for using marshlands and eco-levees.
• Wastewater treatment systems can be controversial when constructed near existing development and consideration of impacts to neighboring properties must be given. • The community values the ancillary benefits of wastewater management, such as tourism, wildlife habitat, and local identity, and suggests that these should guide future planning decisions. • There were significant concerns about the environmental impact of ocean outfalls.

Environmental Constraints
• See Appendix H for environmental constraint considerations.

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8.6 Overarching Conclusion
Without proactive adaptation, AWTF faces escalating risks from SLR and flooding, threatening wastewater service reliability and environmental compliance. A phased approach combining near-term levee maintenance and asset floodproofing, with mid to long term levee augmentation, coupled with potential future relocation of treatment and discharge assets can address current flooding vulnerabilities while buying time to complete design, permitting and construction of long-term adaptation solutions for Arcata’s Wastewater assets. Without intervention, SLR will cause frequent flooding, levee erosion, loss of marsh treatment capacity, and potential catastrophic failure of critical infrastructure at AWTF by the end of the century.

A phased adaptation strategy can support the City’s long term goals as follows:
• Near-term (2025–2055): Levee maintenance & I&I reduction to maintain operations. • Mid-term (2055–2075): Levee augmentation or initiation of relocation planning. • Long-term (2075–2105+): Relocate AWTF and modify discharge strategy (new marsh or ocean outfall).

Ultimately, the City must balance cost, operational continuity, environmental compliance, and long-term resilience, with priority alternatives creating a clear and adaptable roadmap.

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9. References
ASCE. (2015). ASCE/SEI 24-14 Flood Resistant Design and Construction. Reston, Virginia: American Society of Civil Engineers.

Carver, G. (1987). Late Cenozoic Tectonics of the Eel River Basin Region, Coastal Northern California, In Schymiczek, H. and Suchsland, R. (Eds): Tectonics, Sedimentation and Evolution of the Eel River and Associated Coastal Basins of Northern California. San Joaquin Geological Society, 61-72.

City of Arcata. (2016). Title VIII Building Regulations Chapter 4, Flood Hazard Mitigation Standards. City of Arcata.

City of Arcata. (2022). 2022 Infiltration and Inflow Reduction, Technical Package. City of Arcata.

Department of Water Resources (DWR). (2004). California’s Groundwater Bulletin 118: Mad River Groundwater Basin, Mad River Lowland Subbasin. Mad River Groundwater Basin, Mad River Lowland Subbasin. DWR.

EPA, United States Environmental Protection Agency. (2023). Protecting Underground Sources of Drinking Water from Underground Injection. Retrieved from epa.gov/uic.

FEMA. (2021). Meeting the Criteria for Accrediting Levee Systems on Flood Insurance Rate Maps. Washington D.C.:

Federal Department of Homeland Security.

FEMA. (2024, December 17). Glossary. Retrieved from FEMA.gov: https://www.fema.gov/about/glossary/base-floodelevation-bfe GHD, NHE and USFWS. (2022). Natural Shoreline Infrastructure in Humboldt Bay for Intertidal Coastal Marsh Restoration and Transportation Corridor Protection. County of Humboldt.

Grace, R. (1978). Marine Outfall Systems, planning, design and construction. Prentice-Hall Inc.

NCRWQCB. (2009). Waste Discharge and Water Recycling Requirements For The Smith River Community Services Districit Municipal Wastewater Treatment Plant Humboldt County.

NCRWQCB. (2017). Waste Discharge and Water Recycling Requirements For The City Of Rio Dell Municipal Wastewater Treatment Plant Humboldt County.

NCRWQCB. (2017). Waste Discharge Requirements For The City Of Fortuna Municipal Wastewater Treatment Plant Humboldt County.

NCRWQCB. (2019). Waste Discharge Requirements For The City Of Arcata Municipal Wastewater Treatment Plant Humboldt County.

NCRWQCB. (2020). Waste Discharge Requirements and Water Recycling Requirements for the Mendocino City Community Services District Wastewater Treatment Plant Mendocino County.

NCRWQCB. (2022). Waste Discharge and Water Recycling Requirements For The City Of Healdsburg Municipal Wastewater Treatment Plant Sonoma County.

NCRWQCB. (2022). Waste Discharge and Water Recycling Requirements For The Gualala Community Services Districit Wastewater Treatment Facility Mendocino and Sonoma Counties .

NCRWQCB. (2023). Waste Discharge Requirements and Water Recycling Requirements for The Monterey One Water Regional Wastewater Treatment Plant And Advanced Water Purification Facility Discharge To The Pacific Ocean.

NCRWQCB. (2024). aste Discharge Requirements and Water Recycling Requirements for the McKinleyville Community Services District Wastewater Management Facility.

NCRWQCB. (2025). Resolution No. R1-2026-0005 Project Criteria for an Exception to the Enclosed Bays and Estuaries Policy Prohibiting Waste Discharges to Humboldt Bay. Santa Rosa CA: NCRWQCB.

NHE. (2015). Humboldt Bay: Sea Level Rise, Hydrodynamic Modeling, and Inundation Vulnerability Mapping.

McKinleyville, CA: State Coastal Conservancy and Coastal Ecosystems Institure of Northern California.

NHE. (2024). Existing Condition Coastal Flood Assessment for the City of Arcata Sea Level Rise. McKinleyville, CA:
for the City of Arcata.

OPC. (2024). State of California Sea-Level Rise Guidance 2024 Science and Policy Update. California Natural Resources Agency.

Patton, J. R., Williams, T. B., Anderson, J. K., Hemphill-Haley, M., Burgett, R. J., Weldon II, R., . . . Leroy, T. H. (2023). 20th to 21stCentury Relative Sea and Land LevelChanges in Northern California:Tectonic Land Level Changes and theirContribution to Sea-Level Rise, Humboldt BayRegion, Northern California. Tektonika.

RTCWG, R. C. (1992). Cape Mendocino Earthquakes. Retrieved from Cape Mendocino Earthquakes.

San Francisco Estuary Partnership. (2022). Oro Loma Living Laboratory. Retrieved from San Francisco Estuary Partnership.

State Water Resources Control Board. (1979). Water Quality Order 79-20.

United States Census Bureau. (2022). Arcata, Califonia. Retrieved from US Census Website.

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USACE. (2023). HEC-FDA Flood Damage Reduction Analysis. Davis, California: US Army Corps of Engineers Institute for Water Resources Hydrologic Engineering Center.

USDA. (2022). Conservation Practice Standard, Dike and Levee, Code 356. Washington D.C.: United States Department of Agriculture.

Winzler & Kelly. (1994). City of Eureka Storm Drain Master Plan. Eureka, CA: City of Eureka.

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Appendices
Appendix A Sea Level Rise Vulnerability and Capital Improvement Project Adaptation Plan Vulnerability & Risk Assessment
Appendix B Cost Estimates
Appendix C Preliminary Pumping Calculations
Appendix D Moving-Bed Biofilm Reactor Treatment Modeling Appendix E Conventional Activated Sludge Treatment Modeling Appendix F Public Outreach Summaries
Appendix G Life Cycle Cost Calculations
Appendix H Preliminary Environmental Memorandum

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Appendix A
Sea Level Rise Vulnerability
Assessment and Capital
Improvement Project
Adaptation Plan
Vulnerability & Risk
Assessment
(Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study Appendix)

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Appendix B
Cost Estimates
(Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study Appendix)

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Appendix C
Preliminary Pumping Calculations
(Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study Appendix)

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Appendix D
Moving-Bed Biofilm Reactor Treatment
Modeling
(Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study Appendix)

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Appendix E
Conventional Activated Sludge Treatment
Modeling
(Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study Appendix)

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Appendix F
Public Outreach Summaries
(Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study Appendix)

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Appendix G
Life Cycle Cost Calculations
(Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study Appendix)

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Appendix H
Preliminary Environmental Memorandum
(Arcata Wastewater Treatment Facility Sea Level Rise Adaptation Feasibility Study Appendix)

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ghd.com The Power of Commitment

ENERGY COMMITTEE
Two Vacancies—Live or Work in the Planning Area

ONE APPLICANT
Resident Alex Fukuchi

APPOINTMENT
Term ends
September 30, 2027
Term ends
September 30, 2028

FOREST MANAGEMENT COMMITTEE
One Vacancy—No Planning Area Requirement

THREE APPLICANTS
Eamon Engber
Henry (Hank) Harrison
Hugh Scanlon

APPOINTMENT
Term ends
March 31, 2029

WETLANDS AND CREEKS COMMITTEE
Two Vacancies—Live or Work in the Planning Area

TWO APPLICANTS
Resident Kanton Levine
Planning Area Julie Weeder

APPOINTMENT
Term ends
March 31, 2028
Term ends
March 31, 2029

CANDIDATE LIST
WEDNESDAY, JUNE 3, 2026

STAFF REPORT – CITY COUNCIL MEETING
TO: Honorable Mayor and City Council Members
FROM: Emily Sinkhorn, Director of Environmental Services PREPARER: Emily Sinkhorn, Director of Environmental Services DATE: June 3, 2026
TITLE: Hold a Public Hearing to Review the City of Arcata 2025 Updated Urban Water Management Plan and Water Shortage Contingency Plan.

RECOMMENDATION:

It is recommended that the Council:
1) Open the public hearing.
2) Receive a staff report and review any questions with staff. 3) Open public comment to receive public input on the 2025 Urban Water Management Plan and Water Shortage Contingency Plan.
4) Close public comment.
5) If applicable, provide any direction to staff to modify the 2025 Urban Water Management Plan and Water Shortage Contingency Plan.
6) Close the public hearing.

INTRODUCTION:

The Urban Water Management Planning Act defines an “urban water supplier” as a public or privately owned supplier providing water for municipal purposes either directly or indirectly to more than 3,000 customers or supplying more than 3,000 acre-feet of water per year. The Plan requires an urban water supplier to complete an Urban Water Management Plan and update the plan at least every five years. The State Legislature amended the Urban Water Management Planning Act in 2018, requiring suppliers to include a Water Shortage Contingency Plan with specific elements in the Urban Water Management Plan.

The City’s release of the draft 2025 Urban Water Management Plan and opportunities for public input have been publicly noticed and made available on the City website at https://www.cityofarcata.org/326/Drinking-Water.

BACKGROUND:

The California Legislature enacted Assembly Bill 797 (California Water Code [CWC] Section 10610, et. seq., known as the Urban Water Management Planning Act) in 1984. The Act defines an urban water supplier as a public or privately owned supplier providing water for municipal purposes either directly or indirectly to more than 3,000 customers or supplying more than 3,000 acre-feet of water per year. CWC section 10621 requires urban water suppliers to complete an Urban Water

Management Plan (UWMP) and update the plan at least every five years, on or before December 31, in years ending in five and zero.

The Water Conservation Act of 2009, otherwise known as SB X7-7, set a statewide goal of reducing urban water use by 20 percent by the year 2020. In 2010, each urban water supplier was required to calculate its baseline water use, in gallons per capita day (GPCD), establish an interim target for 2015, and define a 2020 target. In 2020, UWMP urban water suppliers were required to demonstrate compliance with the 2020 target.

In 2018, the State Legislature modified UWMP laws to require a Water Shortage Contingency Plan (WSCP) with specific elements. The WSCP is a document that provides a supplier with a structured action plan for dealing with drought or catastrophic water supply shortage. The WSPC includes six standard water shortage levels and appropriate response actions for each shortage level, with a corresponding estimate of the extent the action will address the gap between supplies and demands.

The California Department of Water Resources reviews and approves each plan to ensure conservation and efficient use of water.

DISCUSSION:

The Arcata City Council has approved an updated Urban Water Management Plans every five years, and most recently the 2020 UWMP was approved on June 16, 2021. The City’s five-year review is due to the state on July 1, 2026.

There are several revisions in the 2025 UWMP including updates to population estimates, the Water Shortage Contingency Plan, and descriptions of the Demand Management Measures implemented over the past five years. A required plan element from 2020 is the adoption of a Water Shortage Contingency Plan.

The source of drinking water for the City is water purchased from Humboldt Bay Municipal Water District (HBMWD). HBMWD draws source water from wells below the bed of the Mad River northeast of Arcata. The City has a Right to 1,095 million gallons per year and in 2025 used approximately 50% of this volume. The Humboldt Bay region is very fortunate to have a reliable water supply as even across multiple drought years, HBMWD’s Ruth Reservoir has filled to capacity.

Humboldt Bay Municipal Water District, the wholesale supplier, updated its Water Shortage Contingency Plan in 2020. Updates were made to the by the City to reflect the updates made in the wholesale supplier’s plan. The Water Shortage Contingency Plan provides guidance for declaring a water shortage independent of the wholesaler and guidance on water use restrictions and prohibitions, consumption reduction methods, and penalties and enforcement under each declared stage of action.

To estimate future water demand population estimates for the Arcata Service Area were updated in the 2025 UWMP based on the number of new water service connections and the persons per connection estimate utilized in the 2020 UWMP. The Service Area population for 2025 is reported as 21,185. The City of Arcata General Plan 2045 cites an anticipated population growth, to about 27,000 people, by the year 2045, which was used for water demand projections in the UWMP.

Relative to other parts of the state, Arcata has a low per capita water use. Actual daily per capita water use for the Arcata Service Area in 2020 was 84 gallons per capita per day (GPCD), well below

the 2020 target of 113 GPCD. 2025 daily per capita water use for the Service Area was 77 GPCD in 2025, demonstrating a continual decline in water use by Arcata residents and businesses.

California Water Code section 10631 requires distribution system water losses to be quantified and reported in the UWMP. Distribution system water losses are defined as the physical water losses from the water distribution system, including storage facilities, up to the point of customer consumption. Real water loss is reported for the 2025 calendar year, to identify potential areas where water may be recovered, and to project future water loss. The City is prioritizing infrastructure improvements to reduce water loss across the Arcata public water system and the Steel Waterline Replacement Project Phase I is actively under construction with a second phase planned in the near future. The City has also recently sought grant funding to implement water meter replacements across the City to upgrade older, unreliable water meters.

POLICY IMPLICATIONS:

The Urban Water Management Plan provides background data to support the City’s General Plan Policy Public Facilities (PF)1-1: To Manage the City’s potable water resources to ensure adequate quantities for community use, to promote water conservation, to maintain water quality, and not to deplete source supplies. The City’s Water Shortage Contingency Plan is incorporated into the Urban Water Management Plan and sets triggers for declaring a water shortage emergency including implementing stages of action to achieve targeted consumption reductions.

ENVIRONMENTAL REVIEW (CEQA):

Per California Water Code Section §10652 the preparation and adoption of Urban Water Management Plans are statutorily exempt from the California Environmental Quality Act (CEQA).

BUDGET/FISCAL IMPACT:

Without an approved Urban Water Management Plan, the City would not be eligible to receive grant or loan funding through the Department of Water Resources (DWR), the State Water Resources Control Board (SWRCB). The City has been fortunate to receive substantial grants and loans through DWR and SWRCB over the last five years to improve and repair essential infrastructure.

RECOMMENDED COUNCIL ACTION:
1) Open the public hearing.
2) Receive a staff report and review any questions with staff. 3) Open public comment to receive public input on the 2025 Urban Water Management Plan and Water Shortage Contingency Plan.
4) Close public comment.
5) If applicable, provide any direction to staff to modify the 2025 Urban Water Management Plan and Water Shortage Contingency Plan.
6) Close the public hearing.

ATTACHMENTS:

A. 2025 Urban Water Management Plan & Water Shortage Contingency Plan Draft – City of Arcata

City of Arcata
Urban Water Management Plan 2025
Completed: May 2026

DRAFT VERSION FOR PUBLIC REVIEW

DRAFT VERSION FOR PUBLIC REVIEW

1 Table of Contents
Chapter 1 Introduction and Overview ........................................................................................................... 6 Lay Description ......................................................................................................................................... 6 Chapter 2 Plan Preparation ........................................................................................................................... 7

2.1 Basis for Preparing an UWMP ............................................................................................................ 7

2.2 Regional Planning ............................................................................................................................... 7

2.3 Individual or Regional Planning and Compliance .............................................................................. 8

2.4 Reporting Years and Units of Measure ............................................................................................... 8

2.5 Coordination and Outreach ................................................................................................................. 8 Chapter 3 System Description....................................................................................................................... 9

3.1 General Description ............................................................................................................................ 9

3.2 Service Area Boundary Map ............................................................................................................. 10

3.3 Service Area Climate ........................................................................................................................ 10

3.4 Service Area Population and Demographics ..................................................................................... 11 Chapter 4 System Water Use ...................................................................................................................... 12

4.1 Recycled versus Potable and Raw Water Demand ........................................................................... 12

4.2 Past, Current and Projected Water Use by Sector ............................................................................. 12

4.2.1.1 Single family and multi-family residential ..................................................................................... 13

4.2.1.2 Commercial ..................................................................................................................................... 14

4.2.1.3 Industrial ......................................................................................................................................... 14

4.2.1.4 Institutional ..................................................................................................................................... 14

4.2.1.5 Landscape ....................................................................................................................................... 14

4.2.1.6 Sales to Other Agencies .................................................................................................................. 15

4.2.1.7 Conjunctive Use, Groundwater Recharge, Saline Intrusion Barriers .............................................. 15

4.2.1.8 Agricultural ..................................................................................................................................... 15

4.2.1.9 Distribution System Losses ............................................................................................................. 15

4.2.2.1 Estimating Future Water Savings ................................................................................................... 16

4.2.2.2 Water Use for Lower Income Households ...................................................................................... 17

4.3.1 Distribution System Water Losses .............................................................................................. 17

4.3.2 Progress Toward Meeting the Water Loss Performance Standard ............................................. 18

4.4 Climate Change ................................................................................................................................. 19 Chapter 5 SB X7-7 Baselines and Targets .................................................................................................. 20

2 5.1 Overview & Terminology ................................................................................................................. 20

5.2 Updating Calculations from the 2010 UWMP .................................................................................. 20

5.3 Baseline Periods ................................................................................................................................ 20

5.4 Service Area Population.................................................................................................................... 20

5.5 Gross Water Use ............................................................................................................................... 20

5.6 Baseline Daily per Capita Water Use ............................................................................................... 21

5.7 2015 and 2020 Targets ...................................................................................................................... 21

5.8 2020 Compliance Daily per Capita Water Use ................................................................................. 21 Chapter 6 Normal-Year Supply Characterization ....................................................................................... 22

6.1 Purchased or Imported ...................................................................................................................... 22

6.2 Groundwater ..................................................................................................................................... 22

6.3 Surface Water .................................................................................................................................... 23

6.4 Stormwater ........................................................................................................................................ 26

6.5 Wastewater and Recycled Water ...................................................................................................... 26

6.6 Desalinated Water Opportunities ...................................................................................................... 29

6.7 Exchanges or Transfers ..................................................................................................................... 30

6.8 Supply from Storage ......................................................................................................................... 30

6.9 Future Water Projects........................................................................................................................ 30

6.10 Summary of Existing and Planned Sources of Water ..................................................................... 30

6.11 Climate Change Impacts to Supply ................................................................................................. 33

6.12 Energy Use and Intensity ................................................................................................................ 33 Chapter 7 Water Supply Reliability Assessment ........................................................................................ 36

7.1 Constraints on Water Sources ........................................................................................................... 36

7.1.1 Water Quality ............................................................................................................................. 36

7.2 Reliability by Type of Year .............................................................................................................. 36

7.2.1 Normal Water Year .................................................................................................................... 37

7.2.2 Single Dry Water Year ............................................................................................................... 38

7.2.3 Multiple Dry Water Years.......................................................................................................... 39

7.3 Supply and Demand Assessment ...................................................................................................... 39

7.4 Regional Supply Reliability .......................................................................................................... 40

7.5 Drought Risk Assessment ................................................................................................................. 41 Chapter 8 Water Shortage Contingency Planning ...................................................................................... 43

8.1 Water Supply Reliability Analysis .................................................................................................... 43

3 8.2 Annual Water Supply and Demand Assessment Procedures ............................................................ 44

8.3 Six Standard Water Shortage Levels ................................................................................................ 44

8.3.1 Stages of Action ............................................................................................................................. 44

8.4 Prohibitions on End Uses .................................................................................................................. 47

8.5 Penalties, Charges, Other Enforcement of Prohibitions .................................................................... 48

8.6 Supply Augmentation ....................................................................................................................... 48

8.7 Determining Water Shortage Reductions.......................................................................................... 48

8.8 Revenue and Expenditure Impacts .................................................................................................... 48

8.9 Resolution or Ordinance ................................................................................................................... 49

8.10 Catastrophic Supply Interruption .................................................................................................... 49 Chapter 9. Demand Management Measures ............................................................................................... 50

9.1 Demand Management Measures for Retail Agencies ....................................................................... 50

9.1.1 Water Waste Prevention Ordinances ......................................................................................... 50

9.1.2 Metering ..................................................................................................................................... 51

9.1.3 Conservation Pricing .................................................................................................................. 51

9.1.4 Public Education and Outreach .................................................................................................. 51

9.1.5 Programs to Assess and Manage Distribution System Real Loss .............................................. 53

9.1.6 Water Conservation Program Coordination and Staffing Support ............................................. 53 Chapter 10. Plan Adoption, Submittal, and Implementation ...................................................................... 55

10.1 Inclusion of all 2025 Data ............................................................................................................... 55

10.2 Notice of Public Hearing ................................................................................................................. 55

10.3 Public Hearing and Adoption .......................................................................................................... 56

10.4 Plan Submittal ................................................................................................................................. 56

10.5 Public Availability .......................................................................................................................... 56

10.6 Amending an Adopted UWMP ....................................................................................................... 56

4 Table 2-1 Retail Only: Public Water Systems .............................................................................................. 7 Table 2-2: Plan Identification ....................................................................................................................... 8 Table 2-3: Supplier Identification ................................................................................................................. 8 Table 2-4: Retail: Water Supplier Information Exchange ............................................................................ 8 Table 3-1 Retail: Population - Current and Projected ................................................................................. 11 Table 4-1 Retail: Demand for Potable and Raw Water – Actual ................................................................ 13 Table 4-2 Retail: Demands for Potable and Raw Water - Projected ........................................................... 16 Table 4-3 Retail: Inclusion in Water Use Projections ................................................................................. 17 Table 4-4 Retail: Optional Table Not Included........................................................................................... 18 Table 4-5 Retail Only: Water Loss Audit Reporting .................................................................................. 18 Table 4-6 Retail: Progress Towards 2028 Water Loss Standard……………………………………… 19 Table 5-1: Baselines and Targets Summary ............................................................................................... 21 Table 6-1 Retail: Groundwater Volume Pumped........................................................................................ 26 Table 6-2 Retail: Wastewater Collected Within Service Area .................................................................... 28 Table 6-3 Retail: Wastewater Treatment and Outcomes Within Service Area 2025 .................................. 28 Table 6-4 Retail: Recycled Water Direct Beneficial Uses Within Service Area ....................................... 29 Table 6-5 Retail: 2020 UWMP Recycled Water Use Projection Compared to 2025 Actual ...................... 29 Table 6-6 Retail: Methods to Encourage Future Recycled Water Use ....................................................... 30 Table 6-7 Retail: Expected Future Water Supply Projects or Programs ..................................................... 31 Table 6-8 Retail: Water Supplies – Actual ................................................................................................. 32 Table 6-9 Retail: Water Supplies – Projected ............................................................................................. 32 Table O-1A: 2025 Drinking Water Distribution Energy Use and Intensity ……………... …………….34 Table O-2: 2025 Wastewater Collection, Treatment and Discharge Energy Use and Intensity…………. 35 Table 7-1 Retail: Basis of Water Year Data ............................................................................................... 37 Table 7-2 Retail: Normal Year Supply and Use Comparison ..................................................................... 38 Table 7-3 Retail: Single Dry Year Supply and Use Comparison ............................................................... 39 Table 7-4 Retail: Multiple Dry Years Supply and Use Comparison........................................................... 40 Table 7-5 Retail: Five-Year Drought Risk Assessment .............................................................................. 42 Table 8-1 Cross-reference for Standard vs Supplier Shortage Levels ........................................................ 44 Table 8-2 Retail: Supply Augmentation and Other Actions ....................................................................... 47 Table 8-3 Retail: Demand Reduction Actions ............................................................................................ 47 Table 10-1 Retail: Notification to Cities and Counties ............................................................................... 55 Figure 1: Evapotranspiration, Precipitation and Temperature Data for Eureka WSO City, California ..... 10 Figure 2: Map of R.W. Mathews Dam and Ruth Lake Reservoir ............................................................... 24 Figure 3: R.W. Matthews Dam and Ruth Reservoir ................................................................................... 25 Figure 4: Ranney well along Mad River ..................................................................................................... 25 Figure 5: Water Week Tabling and Outreach at the 2025 Arcata Farmer’s Market………………………53

5 List of Acronyms
AFY Acre-Feet per Year
AMWS Arcata Marsh and Wildlife Sanctuary
AWWA American Water Works Association
AWWTP Arcata Wastewater Treatment Plant
CII Commercial, Industrial, and Institutional
CIMIS California Irrigation Management Information System CWC California Water Code
DDW Division of Drinking Water (SWRCB)
DOF California Department of Finance
DWR California Department of Water Resources
EDD Employment Development Department
GIS Geographic Information System
GPCD Gallons per Capita Day
GPSCPD Gallons per Service Connection Per Day
IPR Indirect Potable Reuse
JCWD Jacoby Creek Water District
MCSD McKinleyville Community Services District
MG Million Gallons
MGD Million Gallons per Day
MGY Million Gallons per Year
NOAA National Oceanic and Atmospheric Administration
PWS Public Water System
RCEA Redwood Coast Energy Authority
RUWMP Regional Urban Water Management Plan
SCADA Supervisory Control and Data Acquisition
SWRCB State Water Resources Control Board
WSCP Water Shortage Contingency Plan
List of Appendices
Appendix A –Checklist Arranged by Chapter
Appendix B – Coordination and Outreach Documents
Appendix C – General Plan Land Use 2025 Map
Appendix D – Service Area Boundary Map
Appendix E – Population Estimate Discussion
Appendix F – Population Projections
Appendix G – Water Demand Projections
Appendix H – 2021-2025 Water Audits
Appendix I – SB X7-7 Compliance Form
Appendix J- Annual Water Supply and Drought Assessment Reporting Tables: 2025-2026 Appendix K – Humboldt Bay Municipal Water District Water Shortage Contingency Plan Appendix L – Draft Water Shortage Emergency Resolution
Appendix M – Ordinance 1462
Appendix N – Plan Adoption, Submittal, and Implementation Documents

6 Chapter 1 Introduction and Overview
The City of Arcata 2025 Urban Water Management Plan (UWMP) has been prepared in accordance with the California Urban Water Management Planning Act of 1983 (AB 797) as amended, including amendments made per the Water Conservation Act of 2009 (SB X7-7). The objective of an UWMP is to document an urban water supplier’s water supply, demands, and conservation efforts. This UWMP contains information required by California Water Code (CWC), Division 6, Parts 2.55 and 2.6.

The data used for preparing this report comes primarily from City of Arcata (City) Finance Department billing records and water system statistics reported monthly and annually to the Department of Water Resources (DWR) and State Water Resources Control Board (SWRCB). Figures relating to climate were obtained from the National Oceanic and Atmospheric Administration (NOAA) and the California Irrigation Management Information System (CIMIS). Current and projected population figures were calculated based on the City of Arcata’s General Plan 2045, adopted on July 17, 2024. DWR’s 2025 Urban Water Management Plan Guidebook was used to develop this UWMP. 2025 UWMP Standardized Tables (Final), supplied by DWR, were used to report 2025 UWMP data via the Water Use Efficiency data online submittal tool. Modified versions of the standardized tables are used within this report. Modifications were limited to excluding blank and/or not applicable information, and formatting changes.

The checklist of specific UWMP requirements provided by DWR in the 2025 UWMP Guidebook for Urban Water Suppliers was utilized to assist DWR in review of the City of Arcata 2025 Urban Water Management Plan. UWMP Checklist Arranged by Subject is included as Appendix A.

Lay Description
The source of drinking water for the City is water purchased from Humboldt Bay Municipal Water District (HBMWD). HBMWD draws source water from wells below the bed of the Mad River northeast of Arcata.

The water-bearing ground below the river is an aquifer. The wells, called Ranney Wells, draw water from the sands and gravel of the aquifer at depths of 60 to 90 feet, thereby providing a natural filtration process.

The water that HBMWD provides to its customers, both domestic and industrial, ultimately comes from the Ruth Lake Reservoir and the Mad River watershed located below R.W. Matthews Dam at Ruth, CA.

The reservoir was designed for a safe yield of 75 MGD per year, using the 1923-24 drought of record (HBMWD Water Shortage Contingency Plan). The District delivers potable water to seven municipalities via its Domestic Water System which is capable of supplying approximately 20 million gallons per day (MGD) of treated drinking water. Current production of treated drinking water by HBMWD for municipal purposes averages approximately 10 MGD (HBMWD Water Shortage Contingency Plan).

The City has a Right to 1,095 million gallons per year (MGY) and in 2025 used approximately 50% of this volume. Demand projections for the City for the next 25 years indicate that in 2050 the city will use approximately 69% of the water right volume. HBMWD’s Water Shortage Contingency Plan indicates that even during a long-term drought (5 consecutive years), inflow volumes into Ruth Reservoir during the winter months will be adequate to meet the water needs of the City and other municipal customers, even without enacting water conservation measures.

7 A secondary source of drinking water for the City is Heindon Well. Heindon Well has not been in use since it was taken offline in 2016 for conservation purposes and facility rehabilitation. After a groundwater well rehabilitation project currently in progress, the City anticipates that it may utilize 100 percent of the safe yield for Heindon Groundwater Well, however, Heindon Groundwater Well will be used an emergency water source only.

Chapter 2 Plan Preparation

2.1 Basis for Preparing an UWMP
CWC 10617 defines an “urban water supplier” as a public or privately owned supplier, providing water for municipal purposes either directly or indirectly to more than 3,000 customers or supplying more than 3,000 acre-feet annually. Urban water suppliers are required to complete a UWMP within one year of becoming a supplier and update the plan at least once every five years, on or before July 1, in years ending in six and one, incorporating updated and new information from the five years preceding each update. (CWC 10621).

The City provides water for, and manages, two Public Water Systems (PWS); the City of Arcata (PWS#1210001) and the Jacoby Creek Water District (JCWD) (PWS#1210021); collectively referred to as the Service Area. In 2025, the City water system had an average of 6,347 service connections, had 566 million gallons (MG) of water supplied and thus is required to prepare an UWMP (Table 2-1). In 2025, the JCWD had an average of 335 service connections and had 19 MG of water delivered. The JCWD does not meet the definition of an urban water supplier and is not required to prepare an UWMP; however, due to the contiguous nature of the two water systems and the need for thorough and reliable local water planning, data from both systems are combined for the purposes of this report (Table 2-1). In 2025, the Service Area had an average of 6,682 service connections.

Table 2-1 Retail Only: Public Water Systems

2.2 Regional Planning
The UWMP was developed in cooperation with HBMWD, the regional wholesaler, and with other Public Water Systems (PWS) who are also municipal customers of HBMWD. Each municipal customer shared resources and information to ensure compatible individual plans that provide a planning document for individual communities and if needed, could be merged to review regional needs. Coordinating agencies included the City of Eureka, McKinleyville Community Services District, Humboldt Community Services District and HBMWD.

CA1210001 City of Arcata 6,347 566
CA1210021 Jacoby Creek Water
District
335 19
Total 6,682 585
Public Water System
Number
Public Water System
Name
Number of Municipal
Connections 2025
Submittal Table 2-1 Retail: Public Water Systems
Volume of Water
Supplied 2025 (MG)

8 2.3 Individual or Regional Planning and Compliance
The City and other local agencies formed a workgroup to ensure individually prepared UWMPs are comparable and compatible. Each agency dedicated to preparing their individual UWMP in a similar format to allow for ease of comparison on a regional scale. It was determined that each individual agency was in compliance with SBX7-7 and a regional approach to compliance was not necessary (Table 2-2).

2.4 Reporting Years and Units of Measure
The 2025 UWMP is reported on a calendar year basis, consistent with previously prepared plans. Water volume is reported throughout the 2025 UWMP in million gallons (MG). These reporting units enable the City to compare water demand, water goals, and conservation figures to operational data and with other regional suppliers. (Table 2-3).

Table 2-2: Plan Identification
Table 2-3: Supplier Identification

2.5 Coordination and Outreach
The City provided notice to the JCWD, HBMWD, the City of Eureka, McKinleyville Community Services District, Humboldt Community Services District and Humboldt County on its intent to review and update its UWMP as required in CWC Section 10642 on April 3, 2026 (Table 10-1). Retail agencies, such as the City, are required to provide their wholesaler with projected water demand from the wholesaler for the next 20 years, in five year increments. Water demand projections for the next 20 years have been provided to HBMWD (Table 2-4). Copies of the aforementioned communications are provided in Appendix B.

Table 2-4: Retail: Water Supplier Information Exchange
Submittal Table 2-4 Retail: Water Supplier Information Exchange Water Code Section 10631(h)
The retail Supplier has informed the following wholesale supplier(s) of projected water use.

Wholesale Water Supplier Name
Humboldt Bay Municipal Water District