Corona City Council
June 3, 2026

Temescal Basin GSP 2-4

2.3. WATER SUPPLY
Water supply for municipal and industrial uses include groundwater and imported water from the Western Municipal Water District (WMWD). In addition, recycled water is used for non-potable uses. The water providers within the Basin and additional detail on their various water sources are described in the following sections.

2.3.1. Water Providers
Corona serves water to the majority of the population within the Basin. Norco and the Home Gardens County Water District (HGCWD) serve water to smaller portions of the Basin.

Corona provides water and wastewater services to residential, institutional, commercial, and industrial customers within the city as well as to the unincorporated communities of El Cerrito, Coronita, and parts of Temescal Canyon. Corona’s water service area encompasses approximately 39 square miles. Corona’s water sources include groundwater pumped from the Basin and the Coldwater Subbasin and imported water purchased from WMWD.

Norco is the sole water purveyor for the residents and businesses within its city boundaries, which encompass approximately 15 square miles. Norco purchases imported water from WMWD, purchases desalinated groundwater from the Chino Desalter Authority (CDA), and pumps groundwater from the Basin.

HGCWD serves water to a portion of the census-designated place of HGCWD and purchases all water from Corona.

The 2020 water supplies for each water purveyor from each water source are shown on Figure 2-4. Purchased imported water and groundwater from the Basin make up 53 percent and 47 percent of Corona’s supply, respectively (Michael Baker 2021). Purchased imported water and groundwater from the Chino Subbasin make up 93 percent and 7 percent of Norco’s supply, respectively (Norco 2021). Purchased imported water makes up 100 percent of the supply for HGCWD. Note that all of HGCWD purchased supply and a portion of Norco’s purchased supply are from Corona and are thus included in Corona’s total supply. It should be noted that these water supply distributions are based on year 2020 only and typically vary from year to year.

2.3.2. Water Supply Sources

2.3.2.1. Groundwater
Corona is the primary producer of groundwater in the Basin. Corona has 18 wells that extract water from the Basin for the purpose of potable water supply (Michael Baker 2021).

Norco has four active wells but they are located in the unadjudicated portion of the Chino Subbasin not the Basin.

A number of private wells were historically installed in the Basin. Well densities for domestic wells, production wells, public wells, and all groundwater wells completed and reported to DWR are shown on Figures 2-5, 2-6, 2-7, and 2-8 respectively. Well density varies throughout the Basin from 0 to 15 wells per square mile section. These well density maps

Temescal Basin GSP 2-5
show all the well completion reports that have been submitted to DWR over time. There are no records of which of these wells are currently active. However, the GSA agencies searched for existing active wells within the Basin. This search included reviewing water use records and contacting owners of large private properties (domestic, commercial, and industrial), inquiring about private wells in discussions with knowledgeable local residents and community leaders, and polling interested parties during public meetings. This effort indicated that the only private pumpers in the Basin are All American Asphalt, Dart Corporation, and 3M. No active private domestic wells were identified in this search.

Corona owns and operates the 10 million gallons per day (mgd) Temescal Desalter, a reverse osmosis (RO) treatment facility where groundwater from the Basin high in TDS is forced oneway through membranes that reject salts as waste brine. Corona then blends this water with locally produced groundwater. The location of the Temescal Desalter is shown along with other Corona water and wastewater facilities in Figure 2-9.

In addition to pumping groundwater from the Basin, Norco purchases groundwater from the CDA, which is extracted from the Chino Subbasin. This water purchase is further described in Section 2.3.2.3.

2.3.2.2. Local Surface Water
No surface water is used as a water supply source within the Basin. Just to the south of the Basin, Corona utilizes surface flows from Coldwater Canyon in percolation basins and then extracts groundwater from the Coldwater portion of the Bedford-Coldwater Subbasin.

2.3.2.3. Purchased or Imported Water
The Basin’s primary sources of imported water are supplied through WMWD, a member agency of Metropolitan Water District of Southern California (Met). Imported water supply from WMWD consists of treated surface water, untreated surface water and desalinated brackish groundwater.

WMWD supplies treated surface water via the Mills Pipeline from Henry J. Mills filtration plant. The Mills Pipeline delivers treated water directly to Corona through metered turnout WR-24. This connection has an effective capacity of 6.5 mgd (Michael Baker 2021). Norco also receives water from WMWD via the Mills Pipeline, which is then wheeled through a metered connection from Corona to Norco (Norco 2021).

WMWD supplies untreated surface water via the Lower Feeder. The Lower Feeder supplies raw water to Corona’s Lester Water Treatment Plant through metered turnout WR-19 and to Corona’s Sierra del Oro Water Treatment Plant through metered turnout WR-33. The Lester Plant has a peak capacity of 30 mgd, and the Sierra del Oro Plant has a peak capacity of 9.0 mgd (Michael Baker 2021).

WMWD supplies desalinated brackish groundwater via the Arlington Desalter to both Corona and Norco. Norco entered into a purchase water agreement with WMWD to purchase a minimum of 4,400 acre-feet per year (AFY) of treated groundwater annually from the Arlington Desalter reverse-osmosis treatment facility (Norco 2021). Excess production from the desalter is made available to Corona (Michael Baker 2021).

Temescal Basin GSP 2-6
Norco is a member agency of the CDA, a Joint Powers of Authority. Norco has an annual obligation to purchase 1,000 AFY of reverse osmosis treated potable groundwater water from CDA (City of Norco 2021).

The City of Corona operates well(s) for HGCWD and supplies them with all their water supply.

The reliability of imported water is documented in WMWD’s 2020 UWMP (WSC 2021). The WMWD UWMP details the potential constraints facing Met, the wholesaler that provides most of the imported water supply for WMWD, Corona, and Norco. Various past and ongoing actions address the water supply threats including water conservation, increased storage programs, and augmenting water supplies. Because of their robust planning efforts, WMWD’s UWMP indicates there would be 99 percent of supply available in a single dry year and 100 percent of supply in multiple dry years. In addition, Corona maintains a two-way connection with the City of Riverside that can be used in the event of an emergency.

2.3.2.4. Recycled Water
As shown on Figure 2-9, three wastewater reclamation facilities are located in the Basin.

Existing reclaimed water supply is provided by three Water Reclamation Facilities (WRF1, WRF2 and WRF3) and two non-potable wells owned and operated by Corona. The average annual production from these sources is approximately 11.35 mgd or 12,700 AFY. Corona is a member of the Western Riverside County Regional Wastewater Authority (WRCRWA), which operates a new wastewater reclamation facility in Eastvale. When WRCRWA is fully implemented, Corona’s level of recycled water production will stay the same. However, the location of sources of supply will shift to the north and Corona will have access to additional recycled water supply from WRCRWA (Corona 2018).

Norco is also a member of WRCRWA but does not currently receive and distribute recycled water.

2.3.2.5. Conjunctive Use/Managed Recharge/In-Lieu Recharge In 2013, Corona prepared a Recharge Master Plan (RMP) for the Basin that defines the groundwater management objectives for the Basin. The RMP lays out goals and alternatives for artificial recharge in the Basin. Implementation of the RMP is ongoing. Corona currently discharges tertiary treated effluent from its Wastewater Treatment Plants No. 1 and No. 2 to the Lincoln/Cota Ponds, where the effluent is either lost to evapotranspiration or percolated to groundwater (WEI 2013).

2.3.3. Water Use Sectors
Water use sectors are defined in the GSP Regulations as categories of water demand based on the general land uses to which the water is applied, including urban, industrial, agricultural, managed wetlands, managed recharge, and native vegetation.

The distribution of current land use types in the Basin is presented on Figure 2-10. While the land use types are more detailed than the water sector categories, the land use mapping provides relevant background information for understanding the various water uses and

Temescal Basin GSP 2-7
locations of these uses in the Basin. A significant portion of the Basin is characterized as single-family residential land use. The next most common land use type within the Basin is industrial. Water use and land use by sector for Corona, Norco, and the Basin are presented in Table 2-1.

Table 2-1. Water Use and Land Use by Sector (2020)
1 Water Use Sector Corona Water Use Norco Water Use Basin Land Use Urban2 87 percent 82 percent 70 percent
Industrial3 13 percent 18 percent 11 percent
Agricultural 0 percent 0 percent 1 percent
Managed Wetlands 0 percent 0 percent 0 percent
Managed Recharge 0 percent 0 percent 0 percent
Native Vegetation 0 percent 0 percent 18 percent
Notes:
1) Water use data is provided by Corona and Norco’s Urban Water Management Plans (UWMPs) (Michael Baker 2021 and Norco 2021) and land use data is based on an analysis of the land use parcels included in the Basin as shown in Figure 2-10.
2) Urban water use for Corona does not include commercial uses, which is reported as combined with industrial. 3) Industrial water use includes commercial uses.

2.4. WATER RESOURCES MONITORING PROGRAMS
This section summarizes the following water resources monitoring activities in the Basin:
• Climate
• Surface Water Flow
• Surface Water Quality
• Groundwater Levels
• Groundwater Quality
• Groundwater Production
• Conjunctive Use/Managed Recharge
• Recycled Water
• Imported Water
• Land Use
• Land Subsidence
• Incorporation of Existing Monitoring into GSP
Several ongoing monitoring programs provide data and information relevant to the Basin.

Corona, Norco, other local agencies, state agencies and federal agencies are responsible for the various monitoring programs, which are summarized briefly below (Sections 2.4.1 through 2.4.12).

Temescal Basin GSP 2-8

2.4.1. Climate
The State Water Resources Control Board (SWRCB) Division of Drinking Water (DDW) compiles climate data in the California Irrigation Management Information System (CIMIS).

This database includes total solar radiation, soil temperature, air temperature/relative humidity, wind direction, wind speed, and precipitation. While the CIMIS database is a comprehensive source for climate data, there are no CIMIS stations in the Basin. The closest CIMIS stations are:
• Chino No. 255 - This station is located north of the Basin (Latitude: 33.985350, Longitude: -117.656528).
• U.C. Riverside No. 44 - This station is located east of the Basin (Latitude: 33.964942, Longitude: -117.33698).

2.4.2. Surface Water Flows
United States Geological Survey (USGS) owns and operates two streamflow gauges in the Basin. These include:
• TEMESCAL C AB MAIN ST A CORONA CA (11072100) - This station is located on the Temescal Creek near Main Street in Corona.
• SANTA ANA R BL PRADO DAM CA (11074000) - This station is located along the Santa Ana River below Prado Dam.

2.4.3. Surface Water Quality
Corona and Norco are both members of the Middle Santa Ana River Watershed Total Maximum Daily Load (TMDL) Task Force. The Task Force is a collaborative effort of publicand private-sector agencies and interests focused on the development of pathogen TMDLs for Santa Ana River Reach 3, its tributaries, and other water bodies in the Chino Basin area, located immediately north of the Basin. Formed in 2007, the Task Force has been working on several pathogen-related activities and studies for the Chino Basin. The objectives of this Task Force are to implement a number of tasks identified by the Regional Board in their 2005 Amendment to the water quality control plan (Basin Plan) (SWRCB 2020a). These include the implementation of a watershed-wide monitoring program to assess compliance with water contact recreation (REC-1) beneficial use water quality objectives for fecal coliform, evaluate numeric targets established for E. coli, and identify and implement measures to control sources of impairment. The Task Force works with the Regional Board in the formulation of pathogen TMDL allocation and implementation strategies (SAWPA 2018).

The Upper Temescal Valley Salt and Nutrient Management Plan (SNMP) developed by Elsinore Valley Municipal Water District and Eastern Municipal Water District includes several management actions, one of which is the implementation of a monitoring program.

This monitoring program includes seven surface water monitoring sites, one of which is in the GSP Area. This privately-owned continuous flow gage is located at the All American Aggregate pit in Corona, the discharge point of the Temescal Wash (WEI 2017).

Temescal Basin GSP 2-9
Data is also collected by OCWD and other monitoring entities associated with local habitat conservation programs (HCPs) throughout the Santa Ana River region, including in the Prado Management Area. These data have been and will continue to be incorporated into the GSA’s database.

Releases to the Temescal Wash are monitored by various dischargers through NPDES permit requirements (Todd and AKM 2008), and these data also have been and will continue to be incorporated into the GSA’s database.

2.4.4. Groundwater Levels
Corona has monitored water quality in production wells in the Basin to protect water quality and to comply with regulations over time. Since 1998, Corona has conducted a monitoring program including water level measurements in about 19 production wells, maintaining these data in a water level database. In 2006, Corona expanded the water level monitoring program to include wells that are not currently pumping (or pump on a limited basis). These wells include inactive irrigation wells, inactive or periodically used production wells, and dedicated monitoring wells installed by Corona (Todd and AKM 2008).

In addition, groundwater levels are measured in and around the Basin by Western Riverside County Regional WastewaterAuthority, OCWD, Chino Basin Watermaster, monitoring programs through the Upper Santa Ana River Habitat Conservation Plan (SARHCP). Data from these ongoing programs are used to supplement GSA collected data and inform the understanding of the Basin.

2.4.5. Groundwater Quality
Groundwater quality monitoring occurs at Corona’s active production wells on a continuous basis, ranging in frequency from semi-monthly to semiannual depending on the water quality constituent. However, no formal water quality monitoring program has been established at the monitoring wells, primarily because of an inability to pump some of the wells. Additional groundwater quality is available from neighboring basins including OCWD and Chino Basin Watermaster. The SWRCB groundwater ambient monitoring program (GAMA) Groundwater Information System (SWRCB 2020b) also compiles available water quality data from cooperating agencies. Data from these sources has been compiled and assessed as needed.

2.4.6. Groundwater Production
Corona’s groundwater pumping accounts for most groundwater production from the Basin, however, there are also a few known private pumpers. WMWD serves as the Santa Ana Watershed water master and records annual production for the watershed.

According to Watermaster records, other current and historical pumpers include:
• All American Asphalt
• Dart Corporation
• 3M Company (formerly Minnesota Mining and Manufacturing Company).

Temescal Basin GSP 2-10

2.4.7. Conjunctive Use/Managed Recharge
Corona currently discharges tertiary treated effluent from its Wastewater Treatment Plants No. 1 and No. 2 to the Lincoln/Cota Ponds, where the effluent is either lost to evapotranspiration or percolated to groundwater. Effluent discharge quantity is monitored and recorded by Corona (WEI 2013).

2.4.8. Recycled Water
Corona records recycled water flows and quality at the three reclamation facilities: WRF1, WRF2 and WRF3. Corona also records recycled water deliveries to the 282 metered connections in the recycled water service areas for landscape irrigation, toilet flushing via dual plumbed systems, firefighting, dust control and various construction applications.

2.4.9. Imported Water
Corona maintains records of imported water purchases and deliveries from WMWD and water delivered to Norco and HGCWD.

2.4.10. Land use
Land use data for the Basin are available through the Southern California Association of Governments (SCAG), as well as the planning departments of the cities of Corona and Norco.

The most recent land use mapping data from SCAG are from 2016, while the latest general plans from Corona and Norco were adopted in 2004 and 2014, respectively. The current land use shows much of the Basin is now single-family residential homes with very little agricultural area.

The Basin was historically an agricultural area and has significantly urbanized since the middle 1980s. In the 1950s and 1960s, the Basin consisted mainly of irrigated agricultural lands with a variety of crops, especially citrus. The 1984 land use map on Figure 2-11 suggests that much of the southern part of the Basin continued to be used for agriculture, but most of this land was likely fallow or non-irrigated pasture by 1984.

The contributing watersheds that surround the Basin consist mostly of native vegetation or grasslands used for grazing. With the exception of urbanization of the small watershed on the northeastern side of the Basin, land use in the contributing watersheds has not changed significantly over the last 20 years.

2.4.11. Natural Resources
Additional monitoring from OCWD and other local HCP programs focus on natural resources including biological surveys and other information.

2.4.12. Land subsidence
While the potential for subsidence was recognized in the 2008 Groundwater Management Plan, it has not been a known issue in the Basin and ground surface elevations have not been monitored until recently. The TRE Altamira Interferometric Synthetic Aperture Radar

Temescal Basin GSP 2-11
(InSAR) Dataset, provided by the California Department of Water Resources (DWR) through the Sustainable Groundwater Management Act (SGMA) Data Viewer (DWR 2020), shows vertical ground surface displacement from June 2015 to September 2019 and indicates that the Basin has been characterized by uplift over that period, likely reflecting tectonic factors.

No known available sources of data indicate subsidence in the Basin.

2.4.13. Incorporation of Existing Monitoring into GSP
Data from existing monitoring programs have been collected and incorporated into the GSP.

The existing monitoring data and locations are discussed further as part of the Monitoring Plan, Chapter 7 of this GSP.

2.5. WATER RESOURCES MANAGEMENT
This section summarizes previous plans related to different aspects of water resources management in the Basin. Generally, this previous work falls into two main categories:
groundwater basin management and water resources management. The categorization helps to provide some context for the summaries that follow:
• Groundwater Basin Management - Plans and studies focusing on groundwater management include the 2008 GWMP, the monitoring program in the 2008 GWMP, and the 2013 RMP. Management of groundwater quality is described in general in the Water Quality Control Plan for the Santa Ana Basin. • Water Resources Management - There are a number of water resources planning documents. WMWD’s Updated Integrated Regional Water Management Plan (IRWMP) (Kennedy/Jenks 2008) and SAWPA’s One Water One Watershed Plan (OWOW Plan) (SAWPA 2018) provide information on water resources on a regional scale. However, WMWD’s IRWMP plan is over 10 years old and SAWPA’s OWOW Plan is very high level as it covers the entire Santa Ana River Watershed. Additional plans developed by Corona and Norco are more recent and more focused on the Basin. The 2020 Corona Urban Water Management Plan and the 2020 Norco Urban Water Management Plan include information on existing and future water demands and supplies, including groundwater, imported water, surface water, and recycled water (Michael Baker 2021 and Norco 2021). The 2020 Urban Water Management Plans (UWMPs) also identified water supply strategies for meeting future demands.

The Reclaimed Water Master Plan (Corona 2018) provides recommendations for expansion of Corona’s reclaimed water program.

2.5.1. AB3030 Groundwater Management Plan
The GWMP was prepared in June 2008 and includes the Basin and the Bedford-Coldwater Subbasin (Todd and AKM 2008). The goals of the 2008 GWMP included operating the groundwater basin in a sustainable manner for beneficial uses and increasing the reliability of water supply for basin users.

The major components of the 2008 GWMP included:

Temescal Basin GSP 2-12
• Data compilation and management
• State of the groundwater basins
• Corona water demand and supply
• Basin management objectives
• Basin management strategies
• Implementation plan.

The 2008 GWMP included a thorough evaluation of the groundwater conditions and conceptual model. The study found that the Basin was potentially in a state of overdraft from 2001 through 2004, when groundwater pumping in the Basin increased from a previous average of 10,000 AFY to an average of 20,000 AFY. The 2008 GWMP recommended numerous strategies for managing groundwater while maintaining groundwater production including:
• Develop new wells that will allow flexibility in pumping distribution and maintenance of water levels
• Enhance recharge directly into the Basin
• Provide the infrastructure necessary for the conveyance of water to recharge facilities
• Provide replacement water sources for a portion of the groundwater demand, potentially decreasing Basin production
• Increase monitoring of groundwater levels and storage for the tracking of overdraft mitigation.

Since 2008, Corona has added new wells, which allow flexibility in pumping distribution.

2.5.2. Groundwater Monitoring Program and Protocols
The 2008 GWMP included a groundwater monitoring program for the Basin and the Bedford-Coldwater Subbasin (Todd and AKM 2008).

Objectives of the 2008 GWMP monitoring program included:
• Characterize water levels and water quality basin-wide • Monitor areas of concern to address specific problems • Evaluate the performance of groundwater management activities • Track changes in groundwater levels, quality and storage over time.

2.5.3. Recharge Master Plan for the Temescal Basin
The RMP for Corona’s use of the Temescal Basin was prepared in September 2013 by Wildermuth Environmental to address the groundwater overdraft identified in the 2008 GWMP. The major components of the RMP included:
• Define goals for artificial recharge and develop planning criteria • Characterize potential source waters for artificial recharge • Characterize the universe of potential sites for artificial recharge

Temescal Basin GSP 2-13
• Develop alternatives for artificial recharge
• Evaluate and rank alternatives for artificial recharge.

The RMP recommended implementation of Alternative 1 (Divert base flow in Temescal Creek for Recharge at the Lincoln/Cota Ponds) and Alternative 4b (Stormwater and recycled water recharge at the Main Street and Oak Street basins), which would result in about 7,200 to 9,300 AFY of new recharge to the Temescal Basin. This would exceed the goal for the RMP to increase recharge by 4,000 AFY and would allow Corona to decrease its reliance on purchased imported water and decrease the total cost of its water supply.

Since 2013, Corona conducted research on Alternative 4b and found that the water quality analysis of stormwater is not high enough to use for recharge to the Basin. Implementing this alternative would require the additional use of clarifying equipment to address debris and silt in the stormwater runoff. Although it may be pursued in the future, Alternative 4b is not being pursued at this time.

2.5.4. Water Quality Control Plan for the Santa Ana River Basin The Water Quality Control Plan for the Basin Plan provides the framework for how surface water and groundwater quality in the Santa Ana Region should be managed to provide the highest water quality reasonably possible. The Basin Plan (i) designates beneficial uses for surface and ground waters, (ii) sets narrative and numerical objectives that must be attained or maintained to protect the designated beneficial uses and conform to the state's antidegradation policy, and (iii) describes implementation programs to protect all waters in the Santa Ana Region (SWRCB 2020a).

The Basin Plan includes site-specific objectives for un-ionized ammonia, cadmium, copper, and lead for the Santa Ana River System, which includes Temescal Creek. These objectives aim to prevent chronic toxicity to aquatic life in the Santa Ana River. The Basin Plan also states water quality objectives for the Temescal Groundwater Management Zone for 770 milligrams per liter (mg/L) TDS and 10.0 mg/L nitrate as nitrogen.

The Basin Plan outlines the statewide monitoring activities aimed at assessing attainment of water quality goals and objectives specified in the Basin Plan. The groundwater monitoring program relies on data collected by municipal supply districts. The Santa Ana Regional Water Quality Control Board (SARWQCB) contributes to the data collection effort.

2.5.5. Integrated Regional Water Management Plan Update Corona and Norco purchase imported water from WMWD. Therefore, it is relevant to track WMWD planning efforts that affect the Corona and Norco service areas or the imported water delivered to Corona and Norco.

WMWD completed its most recent Integrated Regional Water Management Plan in 2008 (Kennedy/Jenks 2008). The purpose of the IRWMP was to address long range water quantity, quality, and environmental planning needs within WMWD’s service area.

Temescal Basin GSP 2-14
The 2008 WMWD IRWMP focused on:
• Identifying and evaluating water management strategies that could increase local water supply, thereby improving water supply reliability. • Evaluating local and regional water quality, environmental, and disadvantaged community issues.

The IRWMP also includes discussion of other regional planning efforts that impact water management within the WMWD service area as well as compilation of estimates of water demands by member agencies, water supplies (e.g., local groundwater, recycled water, surface water, and imported water) available to the agencies, and efforts to coordinate investments in water management, as appropriate, between agencies.

The IRWMP included several projects relevant to Corona:
• New water wells
• Replacement water wells
• Groundwater blending program
• Improvement of groundwater quality/quantity monitoring program • Recharge basins within Oak Avenue detention basin
• Recharge basins within Main Street detention basin
• Upgradient injection wells
• Recycled water injection wells
• Lincoln and Cota street percolation ponds maintenance program.

Several of these projects include groundwater recharge projects that were also recommended in the 2013 RMP.

2.5.6. Santa Ana River Watershed One Water One Watershed Plan Corona, Norco, and HGCWD are involved in SAWPA, which in 2018 updated its One Water One Watershed Plan (OWOW Plan). The OWOW Plan’s goals for the entire Santa Ana River Watershed are as follows:
• Achieve resilient water resources through innovation and optimization • Ensure high-quality water for all people and the environment • Preserve and enhance recreational areas, open space, habitat, and natural hydrologic function
• Engage with members of disadvantaged communities and associated supporting organizations to diminish environmental injustices and their impacts on the watershed
• Educate and build trust between people and organizations • Improve data integration, tracking, and reporting to strengthen decision making.

The Plan includes ongoing water management projects and programs undertaken by Corona, Norco, and HGCWD.

Temescal Basin GSP 2-15

2.5.7. Corona and Norco Urban Water Management Plans
The California Urban Water Management Planning Act requires preparation of Urban Water Management Plans (UWMPs) by urban water providers with 3,000 or more connections.

The UWMPs, generally required every five years, provide information on water supply and water demand—past, present, and future—and allow comparisons as a basis for ensuring reliable water supplies. UWMPs examine water supply and demand in normal years and during one-year and multi-year droughts. UWMPs also provide information on per-capita water use, encourage water conservation, and present contingency plans for addressing water shortages.

According to its 2020 UWMP, Corona is in compliance with the state requirements to reduce per capita water use by 20 percent by 2020 (Senate Bill X7-7). The 2020 per capita daily water use of 180 gallons per capita per day (gpcd) was below the target of 213 gpcd (Michael Baker 2021). Per the UWMP, Corona should be able to meet demands through 2040 in normal, dry, and multiple-dry years using their existing water sources.

For the City of Norco, the 2020 per capita daily water use of 151 gpcd was currently below the target of 263 gpcd (Norco 2021). Per its 2020 UWMP, Norco is in compliance with Senate Bill X7-7 and should be able to meet demands through 2040 in normal, dry, and multiple-dry years using their existing water sources.

2.5.8. Reclaimed Water Master Plan
The purpose of the 2018 Reclaimed Water Master Plan (RWMP) (Corona 2018) was to assist Corona with meeting its goals for reclaimed water use by recommending the implementation of appropriate projects, programs, and additional studies. The RWMP identified, evaluated, prioritized, and scheduled 33 projects. The recommendations from the RWMP fell into four categories:
• Improvements involving receiving future supply from WRCWRA • Improvements to add demand for reclaimed water
• Enhancements to data collection
• Additional studies related to future uses of reclaimed water The RWMP does not include projects relating to recharge of the Basin with reclaimed water.

2.5.9. Water Resources Management Implementation Status Most of the previous plans summarized above have included recommendations for water resources management activities in the Basin. Since the time of publication, many of these recommendations have been implemented.

2.6. GENERAL PLANS
This section presents elements of general plans and other land use planning in the Basin as relevant to groundwater sustainability. It focuses on planning goals and objectives that are aligned with potential groundwater management activities. In addition, this section

Temescal Basin GSP 2-16
highlights the potential for future changes in land use that may influence water demands and infiltration/recharge of the Basin.

The goals, objectives, policies, and implementation measures as described in the general plans for Riverside County, Corona, and Norco, which together encompass the Basin, are summarized below. The jurisdictional boundaries in the Basin are presented on Figure 2-2.

Applicable general plans include:
• The Riverside County General Plan - The entire Basin is within Riverside County (Riverside County 2015).
• Corona General Plan - Most of the Basin is within the Corona jurisdictional boundary. Corona’s General Plan includes plans and policies applicable to the entire city as well as its sphere of influence (Corona 2021).
• Norco - The northeastern portion of the Basin is within the Norco jurisdictional boundary (Norco 2009).

The goals and policies that are water resources related are summarized as follows.

2.6.1. Riverside County General Plan
The Riverside County General Plan was adopted in 2015. The General Plan covers the entire unincorporated portion of the County and also includes 19 detailed Area Plans covering most of the County.

The Multipurpose Open Space Element of the Riverside County General Plan addresses the conservation, development, and use of natural resources including water, soils, rivers, and mineral deposits. A number of policies are related to water supply and conveyance, water conservation, watershed management and groundwater recharge. Several of these policies are summarized in Table 2-2.

2.6.2. City of Corona General Plan
The City of Corona’s General Plan was updated in 2021 and covers the 37.6 square miles within City limits and provides guidance to Riverside County for the 35.2 square miles within the Corona Sphere of Influence. The General Plan chapters most relevant to water resource management are the chapters on Infrastructure and Public Services and Environmental Resources. Additional relevant policies are in the Land Use and Public Health and Safety chapters as well.

Relevant policies included in the General Plan are summarized in Table 2-3.

Table 2-2. Select Policies in the Riverside County General Plan Category Policy1
Water Supply and
Conveyance Balance consideration of water supply requirements between urban, agricultural, and environmental needs.

Provide active leadership in the regional coordination of water resource management and sustainability efforts affecting Riverside County.

Promote the use of recycled water for landscape irrigation.

Water Conservation Implement water-efficient landscape ordinance and policies.

Seek opportunities to coordinate water-efficiency policies and programs with water service providers.

Watershed
Management
Encourage wastewater treatment innovations, sanitary sewer systems, and groundwater management strategies that protect groundwater quality in rural areas.

Minimize pollutant discharge to storm drainage systems, natural drainages, and aquifers Where feasible, decrease stormwater runoff by reducing pavement in development areas, reducing dry weather urban runoff, and by incorporating “Low Impact Development,” green infrastructure and other Best Management Practice design measures.

Groundwater
Recharge Support efforts to create additional water storage where needed, in cooperation with federal, state, and local water authorities.

Participate in the development, implementation, and maintenance of a program to recharge the aquifers underlying the county.

Ensure that aquifer water recharge areas are preserved and protected.

Use natural approaches to managing streams, to the maximum extent possible, where groundwater recharge is likely to occur.

Discourage development within watercourses and areas within 100 feet of the outside boundary of the riparian vegetation, the top of the bank, or the 100 year floodplain, whichever is greater.

Notes:
1 : Some policy statements have been shortened for use in this table. The full text is included in the Riverside County General Plan.

T:\Projects\Corona GSP 46414\Deliverables\GSP\2 Plan Area\Tables 2-2 and 2-3 20210827.xlsx - Table 2-2 Riverside GeneralPlan Des by: EG

Table 2-3. Selected Policies in the City of Corona General Plan Category Policy1
Accommodate the types, densities, and mix of land uses that can be adequately supported by transportation and utility infrastructure (water, sewer, etc.) and public services (schools, parks, libraries, etc.)
Require new residential, commercial, office, and industrial development be designed to minimize consumption of and sustain scarce environmental resources through methods including drought-tolerant species and recycled water for irrigation in landscaping, capturing rainwater and using it onsite, and water efficient fixtures.

Establish guidelines and standards for water conservation and actively promote use of water conserving devices and practices in both new construction and major alterations and additions to existing buildings.

Encourage the use of recycled water by industrial, commercial, and institutional, users through incentives such as differential pricing.

Require the use of recycled water for landscaped irrigation, grading, and other non-contact uses in new developments, parks, golf courses, sports fields, and comparable uses, where feasible.

Encourage the use of rainwater capture and storage facilities in residential and nonresidential developments.

Prohibit the discharge of toxins, debris, refuse, and other pollution into watercourses, other drainages and groundwater basins.

Balance consideration of water supply requirements between urban, agricultural, and environmental needs so that sufficient supply is available to meet each of these different demands.

Provide active leadership in the regional coordination of water resource management and sustainability efforts affecting Riverside County and continue to monitor and participate in, as appropriate, regional activities to prevent overdraft caused by population growth.

Support efforts to create additional water storage where needed, in cooperation with federal, State, and local water authorities. Additionally, support and/or engage in water banking in conjunction with these agencies where appropriate, as needed.

In cooperation with Riverside County, participate in the development, implementation, and maintenance of a program to recharge the aquifers underlying Corona and SOI areas.

Retain storm water at or near the site of generation for percolation into the groundwater to conserve for future uses and mitigate flooding.

Use natural approaches to managing streams, to the maximum extent possible, where groundwater recharge is likely to occur.

Require new private or public developments to preserve and enhance riparian habitat and prevent obstruction of natural watercourses.

Consider wetlands for use as natural water treatment areas that will result in improvement of water quality Promote the collection of relevant data on groundwater levels and liquefaction susceptibility, as a basis for future refinement of liquefaction policies or procedures.

Use natural watercourses as Corona’s primary flood control channels, whenever feasible and practical.

Minimize the potential risk of contamination to surface water and groundwater resources and implement restoration efforts to resources adversely impacted by past urban and rural land use activities.

Notes:
1 : Some policy statements have been shortened for use in this table. The full text is included in the City of Corona General Plan.

Land Use
Infrastructure and
Utilities
Environmental
Resources
Public Health &
Safety
T:\Projects\Corona GSP 46414\Deliverables\GSP\2 Plan Area\Tables 2-2 and 2-3 20210827.xlsx - Table 2-3 Corona General Plan Des by: EG

Temescal Basin GSP 2-19

2.6.3. City of Norco General Plan
Norco’s General Plan Update includes several elements, of which Conservation is the most relevant for water resources planning (Norco 2014). Relevant policies included in the General Plan are listed in Table 2-4.

Table 2-4. Selected Policies in the City of Norco General Plan Category Policy(1)
Water
Supply
Continue to promote water conservation through the use of xeriscape designs in new development and public spaces where feasible.

Continue to provide information to the public on ways to conserve water and reduce consumption.

Monitor the demand for reclaimed water and file for Petitions of Change with the SARWQCB as-needed to reduce the amount of reclaimed water that is discharged from treatment facilities and make that water available for transmission into Norco’s reclaimed water infrastructure system.

Insure that there are adequate increases in water production and distribution capabilities to meet future growth demands.

Water
Quality
Develop and maintain inter-agency agreements and infrastructure improvements to have back-up water supply sources from adjoining water districts during times of emergencies and system maintenance requirements.

Continue public information campaigns to all residents with large animals to ensure awareness that manure spreading as a means of disposal is strictly prohibited to prevent contamination to groundwater supplies.

Notes:

1. Some policy statements have been shortened for use in this table. The full text is included in the City of Norco General Plan Update.

2.6.4. General Plan Influences on Groundwater Sustainability Agency Ability to Achieve Sustainability
The general plans for Riverside County, Corona, and Norco all include policies to increase water conservation and protect groundwater and surface water quality. They also include policies promoting the preservation of natural floodplains, which contribute to groundwater recharge. However, the planned growth in the Basin would convert open space uses that allow groundwater infiltration to more developed land use types with more impervious cover that will likely not allow the same amount of groundwater infiltration. Use of low impact development practices and stormwater best management practices (BMPs) that promote infiltration would help mitigate loss of infiltration due to land use changes.

Riverside County. The Riverside County General Plan addresses the importance of groundwater. The policies and implementation of the land use and public facilities/services elements indicate that the County role is to support and encourage local water agencies in ensuring that water supply is available. Similarly, with wastewater issues and protection of water quantity and quality, the County role is limited to encouragement of other agencies,

Temescal Basin GSP 2-20
developers, and landowners. The General Plan contains little policy to manage land use within the constraints of available water supply other than to encourage drought resistant plants and the use of recycled water.

Corona serves a population that is predicted to increase from 170,100 in 2020 to about 185,600 residents by 2045 (Michael Baker 2021). Some of this growth will be along the southern edge of Corona in the Eagle Creek area within and adjacent to the Basin. The UWMP anticipates future growth in the City will be offset by lower per capita water use.

However, the general plan indicates that Met may build an additional treatment plant in the area to meet increased water demand, if warranted. Corona land use policies generally are protective of agricultural land and hillsides, and conservation policies address water efficiency, water recycling, sustainability measures, and coordination with other agencies, including HGCWD and Norco.

The increased development included in the general plans was simulated by the numerical model described in Chapter 5. Based on these scenarios, the Basin remains sustainable even with future growth.

2.6.5. GSP Influences on General Plans
The Temescal Groundwater Sustainability Agency (Temescal GSA) agencies will work together to implement this GSP and rely on their portfolio of water supply to maintain sustainability. Future growth is expected to be limited based on the general plans.

City of Corona. Implementation of the GSP will support Corona in providing continued groundwater to its population. In addition, the GSP will ensure good quality water in sufficient quantities to serve its residents into the future, including drought periods.

Riverside County. The Riverside County General Plan generally assumes that local water agencies can ensure adequate high-quality water supplies into the future. The GSP provides additional specific information, documents potential challenges to water supply, and explores undesirable results that may occur with future increases in groundwater demand.

Undesirable results will be defined with sustainability criteria, and if identified, will be addressed with management actions. These management actions may have ramifications for County land use planning. For example, GSPs are authorized within the GSP Plan Areas to impose well spacing requirements and control groundwater pumping and control extractions by regulating, limiting, or suspending extractions from individual groundwater wells. Such regulation may present a constraint on potential land uses.

2.7. ADDITIONAL GSP ELEMENTS
The GSP requirements include a list of additional GSP elements from Water Code Section 10727.4 that may or may not be relevant to a GSP. As shown in Table 2-5, several of these elements are not applicable to the Basin. The elements that are applicable to the Basin, are presented in the sections below.

Temescal Basin GSP 2-21
Table 2-5. Additional GSP Elements included in Water Code Section 10727.4 Water Code Section 10727.4 Elements GSP Section or N/A
a) Control of saline water intrusion N/A
b) Wellhead protection areas and recharge areas 2.7.1
c) Migration of contaminated groundwater 2.7.2
d) A well abandonment and well destruction program 2.7.3 e) Replenishment of groundwater extractions N/A
f) Activities implementing, opportunities for, and removing impediments to, conjunctive use or underground storage
N/A
g) Well construction policies 2.7.3
h) Measures addressing groundwater contamination cleanup, groundwater recharge, in-lieu use, diversions to storage, conservation, water recycling, conveyance, and extraction projects
N/A
i) Efficient water management practices, as defined in Section 10902, for the delivery of water and water conservation methods to improve the efficiency of water use

2.7.4 j) Efforts to develop relationships with state and federal regulatory agencies

2.7.5 k) Processes to review land use plans and efforts to coordinate with land use planning agencies to assess activities that potentially create risks to groundwater quality or quantity

2.7.6 l) Impacts on groundwater dependent ecosystems 4.10 and 6.7

2.7.1. Wellhead Protection Areas and Recharge Areas
In 2002, Corona conducted an assessment of the vulnerability of their drinking water wells under the California Drinking Water Source Assessment Program. This program, developed by the California Department of Public Health, delineates the area around drinking water sources, such as wells, through which contaminants might reach the water supply. This assessment identified surface recharge areas in the vicinity of Corona’s wells. In addition, the analysis in the 2008 GWMP identified the main areas of basin recharge for the aquifers tapped by Corona’s wells. These areas include the entire footprint of the unconfined Channel Aquifer, recharge areas along washes and alluvial fans, and areas of subsurface inflow such as Temescal Canyon and Arlington Gap (Todd and AKM 2008).

2.7.2. Groundwater Contamination Migration and Clean-up There are several groundwater contaminated sites in the Basin in varied stages of remediation. The pollutants of concern for these sites include gasoline, diesel, and volatile organic compounds (VOCs). The status of each site is summarized in Table 2-6. The

Temescal Basin GSP 2-22
remediation activities for contaminated sites directly over the Basin are managed and tracked by the SARWQCB. GeoTracker is the SWRCB data management system for sites that impact groundwater or have the potential to impact groundwater. GeoTracker provides information on sites that require groundwater cleanup and the status of required clean-up activities. In the Basin, there are a number of closed sites (where clean-up activities have been completed) and five open sites, as shown on Figure 2-12.

Table 2-6. Status of Contamination Sites in the Basin
Site Contaminants of Concern Status
ARCO #1924 Gasoline OPEN – Eligible for closure
as of 2/18/2016
Thomas Ranch
(Schofield)
Benzene, other acid or
corrosive, other petroleum,
xylene
OPEN – Site assessment as
of 8/21/1986
Dry Clean Express Tetrachloroethylene (PCE),
Trichloroethylene (TCE)
OPEN - Inactive as of
2/13/2020
Private Residence Diesel OPEN - Inactive as of
11/17/2017
All American Asphalt
Landfill
Non-Specified OPEN – Operating as of
11/1/2014

2.7.3. Well Permitting, Construction, and Destruction Requirements The RCDEH is responsible for issuing well permits. Permits are required for the construction and/or abandonment of all water wells including, but not limited to driven wells, monitoring wells, cathodic wells, extraction wells, agricultural wells, and community water supply wells.

The process includes an application by the property owner and certified well driller, and a site inspection by the County. The wells are also inspected during different stages of construction to help verify standards are being met. All drinking water wells are evaluated once they complete installation to ensure they comply with State well standards and meet minimum drinking water standards. If found in compliance, the land or well owner is issued a clearance letter authorizing their use.

Corona and Norco have not developed their own well construction standards but do require compliance with DWR standards and RCDEH standards.

Through their Water Engineering Program, RCDEH requires that a permit be obtained for the abandonment of any well in the County (RC DEH 2020). Guidance for well abandonment procedures is consistent with the standards developed by DWR and included in the California Water Code (§ 13800 through 13806) for drilling and destroying wells in California. The 2008 GWMP recommended increased coordination with RCDEH Water Engineering Program regarding well abandonment procedures.

Temescal Basin GSP 2-23

2.7.4. Efficient Water Management Practices
Corona and Norco encourage and facilitate efficient water management practices, which are discussed at a high level in each city’s General Plan (Corona 2021 and Norco 2014). In addition, specific water conservation targets and demand management measures, including metering, conservation pricing, public education, water loss auditing, and other water conservation program activities, are documented in each city’s 2020 UWMP (Michael Baker 2021 and Norco 2021). As documented in Section 2.5.7 of this GSP, Corona and Norco have both met and exceeded their 2020 water efficiency goals.

Water conservation reduces reliance on potable water supplies, including groundwater.

Increasing water conservation through the implementation of water efficiency practices may reduce groundwater pumping and promote sustainable groundwater management.

2.7.5. Relationships with State and Federal Agencies
The Temescal GSA has developed an interested parties list, which includes stakeholders, neighboring water agencies, local groups, State and Federal agencies, and others who have expressed interest in the GSP process. Notices have been sent to these interested parties throughout GSP preparation. In addition, State and Federal agencies have had the opportunity to participate in the Technical Advisory Committee (TAC), attend public meetings, and review and comment on public drafts of the GSP.

2.7.6. Land Use Plan Coordination
Land use planning agencies have been invited as interested parties to the GSP planning process. The GSA recognizes the importance of the natural recharge areas, where stormwater is recharged into the Basin and has developed projects and management actions to further assess enhanced recharge in coordination with local land use planning efforts (see Chapter 8).

2.8. NOTICE AND COMMUNICATION
As described in this and later chapters, groundwater is a major source of supply in the Basin and supports a range of beneficial uses: municipal, industrial, commercial, agricultural, and environmental. To some degree in the Basin, all land and property owners, residents, businesses, employees, and visitors are potentially affected by groundwater use. This reflects the orientation of the communities in the Basin and the amenities for small-city living and recreation. While recognizing the critical importance of imported supply, reliable groundwater is essential.

The Temescal GSA has encouraged public participation in the ongoing planning and development activities supporting the GSP process. Corona organized a TAC to support the GSP process; regularly scheduled TAC meetings have been announced on the GSA website and have been open to the public. In addition, public workshops regarding development of the GSP have been conducted to encourage public participation and to provide educational outreach. Early in the GSP preparation process the GSA contacted potential interested partis, including private well owners, environmental stakeholder, local and regional community organizations, and the community at large. Parties that expressed interest were included on the list of interested maintained pursuant to Water Code Section 10723.2.

Temescal Basin GSP 2-24
Organizations and individuals that expressed interest throughout GSP preparation were added to this list. Meeting notices were provided to all those on the interested parties list in advance of all public meetings relating to the GSP and when draft portions of the GSP were made available on the GSA website. Additionally, GSP development information and meeting notices have been regularly posted to the GSA website.

The Communication Plan in Appendix D provides an overview of outreach to the public by means of public TAC meetings, public workshops, informational materials (e.g., Fact Sheets), focused outreach, and the GSA website. These inform the public about the GSP development and implementation process and encourage active involvement by interested parties.

The GSA developed and maintained an interested parties list and has communicated to the individuals and organizations on the list during GSP development. These parties represent a variety of interests and perspectives. Additionally, the interested parties group brings a variety of expertise, including public and private groundwater users, local business interests, public water systems, land use planning agencies, regulatory agencies, etc. These parties have been engaged throughout the development of this GSP to provide them with information about the purpose of the GSP, educate about Basin characteristics, and obtain input on sustainability goals and management actions. A list of public meetings held during development of the GSP, comments received on the draft GSP prior to adoption, and how those comments were addressed is included in Appendix E.

2.8.1. Technical Advisory Committee (TAC)
The Temescal GSA formed a TAC to provide input and guidance to the staff and consultant team of the GSA during preparation of the GSP based on their expertise, knowledge, resources, and understanding of their communities, environment, commerce, and applicable regulations. The intent of the TAC is to contribute community and stakeholder perspectives and interests in GSP planning and GSP and SGMA implementation in the Basin.

The TAC includes representatives from the following public and private organizations:
• 3M Industrial Mineral Products Division
• All American Asphalt
• Santa Ana Regional Water Quality Control Board (RWQCB) • City of Norco
• Home Gardens County Water District
• Riverside County Flood Control and Water Conservation District • Corona City Council
• City of Corona Department of Water and Power.

The TAC held quarterly meetings throughout the GSP preparation period that were open to the public. Notification for these meetings was posted on the GSA’s website prior to meeting dates and presentation materials and meeting summaries were posted following each meeting. Meeting summaries and presentation materials from the TAC meetings are included in Appendix F.

Temescal Basin GSP 2-25

2.8.2. Public Workshops
Three public workshops were held during preparation of the GSP to engage with interested parties and stakeholders. The GSA agencies publicized these public meetings through the GSA website, social media, and distribution of targeted bilingual Fact Sheets. The workshops were held virtually in 2020 and 2021 and all presentations and materials were presented simultaneously in English and Spanish. These workshops were also streamed on Corona’s website, Facebook, and YouTube channels and on Corona TV (locally Channel 29 on Time Warner Spectrum and Channel 99 on AT&T).

Meeting summaries, presentation materials, and associated Fact Sheets from the public workshops are included in Appendix G.

2.8.3. Directed Outreach and Coordination
The GSA focused significant outreach efforts to engage and inform important local and regional stakeholders. This included engaging community leaders in historically underserved communities in the Basin and coordination with neighboring basins and local agencies.

2.8.4. Disadvantaged Community Outreach
Areas of the Basin identified as disadvantaged and severely disadvantaged communities (DACs and SDACs) are shown on Figure 2-13. These DACs and SDACs are within the service areas of Corona and HGCWD and receive water supply from those agencies. There are no active private wells in these DAC and SDAC areas. This fact notwithstanding, the GSA worked to identify individuals and/or organizations in or representing these DACs and SDACs and engage them in the GSP process. Outreach to DAC and SDAC areas of the Basin included communication with and distribution of Fact Sheets to and through local churches and community centers in the DAC/SDAC areas and individual and group meetings with politically active individuals, community leaders, and community action organizations, and elected officials. This outreach focused on presentations regarding SGMA, the Basin, the GSP process and components, and encouraged participation in public meetings and GSP review. These meetings also generated feedback on additional outreach that the GSA could undertake, much of which was implemented. Notes from these meetings are included in Appendix H.

2.8.5. Neighboring Basin Coordination
The GSA held meetings to facilitate communication and coordination with groundwater basins neighboring the Temescal Basin. This included meetings with representatives of the Chino Basin, Riverside-Arlington Basin GSA, and Coastal Plain of Orange County Basin GSA.

The meetings focused on data sharing between basins, water budget coordination, and GSP preparation timelines. Summary notes from these meetings are included in Appendix H.

2.8.6. Comments and Responses on Draft GSP
On September 15, 2021, the GSA notified stakeholders, including local City and County agencies, of their intent to adopt this GSP after a 90-day review period. Two letters with

Temescal Basin GSP 2-26
comments on the Draft GSP were received in mid-December. These letters, along with responses from the GSA and indications of how the GSP has been modified are included in Appendix I.

Service Layer Credits: Esri, HERE, Garmin, (c)
OpenStreetMap contributors, and the GIS user
community

TEMESCAL
WASH
Coldwater Canyon
Wardlow Wash
Cucamonga
Cree
k
Main
Street
Was
h
Santa Ana River

BEDFORD-COLDWATER

SUBBASIN
Elsinore
Subbasin
CHINO

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OF ORANGE
COUNTY

RIVERSIDE-ARLINGTON

SUBBASIN

TEMESCAL

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Dam
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ORANGE COUNTY

RIVERSIDE COUNTY
UV91

CITY OF CORONA

CITY OF NORCO

CITY OF CORONA

CITY OF CORONA

SPHERE OF INFLUENCE

HOME GARDENS COUNTY

WATER DISTRICT
Lake
Mathews
Streams
Temescal Basin GSP Area
Coastal Plain of Orange County Basin
Bedford-Coldwater Subbasin
Chino Subbasin
Elsinore Valley Subbasin
Riverside-Arlington Subbasin
Cities
City of Corona Sphere of Influence
Figure 2-1
Temescal Basin
GSP Area
Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 2 - Plan Area\Figure 2-1 TemescalHydrogeology.mxd Disclaimer:

Features shown in this figure are for planning
purposes and represent approximate locations.

Engineering and/or survey accuracy is not implied.

Data Sources:

Cal-Atlas, DWR Bulletin 118, National Hydrography
Dataset, SAWPA
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Jurisdictional Areas
Disclaimer:

Features shown in this figure are for planning
purposes and represent approximate locations.

Engineering and/or survey accuracy is not implied.

Data Sources:

Cal-Atlas, DWR Bulletin 118, National Hydrography
Dataset, SAWPA
0 0.5 1
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(
N
Streams
Temescal Basin
City of Corona
City of Norco
Home Gardens County Water District
City of Corona Sphere of Influence

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user community
City of Norco
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National Forest
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Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 2 - Plan Area\Figure 2-3 TemescalFederalStateLands.mxd Figure 2-3
Federal and
State Lands
Disclaimer:

Features shown in this figure are for planning
purposes and represent approximate locations.

Engineering and/or survey accuracy is not implied.

Data Sources:

EVMUD, Cal-Atlas, DWR Bulletin 118, BLM
0 0.5 1
Miles
(
N
Department of Defense
State of California
Federal Lands - Non-Forest Service Land within USFS
Federal Lands - US Forest Service
Temescal Basin

Corona (including Home Gardens) Norco
0 20,000
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
Water Supply (AFY)
Imported Water
Temescal Basin Groundwater
Bedford-Coldwater Basin Groundwater
Chino Basin Groundwater
Water Supply Sources (2020)
Figure 2-4
Water Supply Sources
within Basin - 2020
Notes:

Corona has the ability to pump groundwater from the Coldwater portion of the Bedford-Coldwater Basin, but did not do so in 2020.

Norco produces groundwater from wells in the un-adjudicated portion of the Chino Basin; they currently have no wells in the Temescal Basin.

Service Layer Credits: Esri, HERE, Garmin,
(c) OpenStreetMap contributors, and the GIS
user community

ORANGE COUNTY

RIVERSIDE COUNTY
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Basin Domestic
Groundwater Wells
Disclaimer:

Features shown in this figure are for planning
purposes and represent approximate locations.

Engineering and/or survey accuracy is not implied.

Data Sources:

Cal-Atlas, DWR Bulletin 118, National Hydrography
Dataset
0 0.5 1
Miles
(
N
Domestic Wells by Section
1 - 5
6 - 10
11 - 15
16 - 20
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user community

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Basin Production
Groundwater Wells
Disclaimer:

Features shown in this figure are for planning
purposes and represent approximate locations.

Engineering and/or survey accuracy is not implied.

Data Sources:

Cal-Atlas, DWR Bulletin 118, National Hydrography
Dataset
0 0.5 1
Miles
(
N
Production Wells by Section
1 - 5
6 - 10
11 - 15
16 - 20
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user community

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Basin Public
Groundwater Wells
Disclaimer:

Features shown in this figure are for planning
purposes and represent approximate locations.

Engineering and/or survey accuracy is not implied.

Data Sources:

Cal-Atlas, DWR Bulletin 118, National Hydrography
Dataset
0 0.5 1
Miles
(
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Public Wells by Section
1 - 5
6 - 10
11 - 15
16 - 20
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user community

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All Basin
Groundwater Wells
Disclaimer:

Features shown in this figure are for planning
purposes and represent approximate locations.

Engineering and/or survey accuracy is not implied.

Data Sources:

Cal-Atlas, DWR Bulletin 118, National Hydrography
Dataset
0 0.5 1
Miles
(
N
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1 - 10
11 - 20
21 - 40
41 - 80
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user community
WRF
WRF
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Q3
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Q3

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Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 2 - Plan Area\Figure 2-9 Corona_and_Norco_Facilities.mxd Figure 2-9
Corona and Norco
Facilities
Disclaimer:

Features shown in this figure are for planning
purposes and represent approximate locations.

Engineering and/or survey accuracy is not implied.

Data Sources:

Cal-Atlas, DWR Bulletin 118, National Hydrography
Dataset, SAWPA
0 0.5 1
Miles
(
N
Q3 Water Treatment Plant
WRF Water Reclamation Facility
Temescal Basin

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user community

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(
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Figure 2-10
Existing Land Use
Agriculture
Commercial
Industrial
Military Installations
Mixed Residential; Mixed Use
Mobile Homes and Trailer Parks
Multi-Family Residential
Open Space/Recreation
Public
Rural Residential
Single Family Residential
Specific Plan
Transportation/Utilities
Undevelopable/Protected
Under Construction
Unknown
Vacant
Water
Temescal Basin
Disclaimer:

Features shown in this figure are for
planning purposes and represent
approximate locations. Engineering
and/or survey accuracy is not implied.

Data Sources:

Cal-Atlas, DWR Bulletin 118, SCAG

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(c) OpenStreetMap contributors, and the GIS
user community

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1984 Land Use
0 0.5 1
Miles
(
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Data Sources:

Cal-Atlas, DWR Bulletin 118, National Hydrography
Dataset, CA GeoTracker

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Temescal Basin GSP 3-1

HYDROGEOLOGICAL CONCEPTUAL MODEL
This chapter describes the hydrogeologic conceptual model of the Temescal Subbasin (Basin) of the Upper Santa Ana Groundwater Basin, including the Basin boundaries, geologic formations and structures, and principal aquifer units. The chapter also discusses groundwater recharge and discharge areas. The hydrogeologic conceptual model presented here is a summary of relevant and important aspects of the Basin hydrogeology that influence groundwater sustainability. While the Chapter 1 Introduction and Chapter 2 Plan Area establish the institutional framework for sustainable management in the Groundwater Sustainability Plan (GSP), this chapter, along with Chapter 4 Groundwater Conditions and Chapter 5 Water Budget, sets the physical framework.

The hydrogeologic conceptual model and basin conditions description document the Basin’s hydrogeology as the technical foundation for management. Later sections addressing the water budget and sustainability criteria will refer to and rely on the technical material contained here.

3.1. PHYSICAL SETTING AND TOPOGRAPHY
The Temescal Basin as defined by the California Department of Water Resources (DWR) is bounded on the west by the Santa Ana Mountains and the east by low-lying El Sobrante de San Jacinto and La Sierra hills. Figure 3-1 illustrates the topography of the Basin.

The Basin is connected to three adjacent groundwater basins, the Chino and RiversideArlington Subbasins of the Upper Santa Ana Groundwater Basin and the Bedford-Coldwater Subbasin of the Elsinore Groundwater Basin. The boundary with the Chino Subbasin (DWR Basin No. 8-2.01) to the north is generally marked by the Santa Ana River and a series of low-lying hills in the Norco area. The Basin is connected to the Riverside-Arlington Subbasin (DWR Basin No. 8-2.03) in a narrow valley groundwater restriction between the El Sobrante de San Jacinto and La Sierra hills, referred to as the Arlington Gap. Groundwater flows into the Basin from the Riverside-Arlington Subbasin through the Arlington Gap. The southern boundary of the Basin is located at the Bedford Canyon where it connects with the BedfordColdwater Subbasin of the Elsinore Groundwater Basin (DWR Basin No. 8-4).

The floor of Basin slopes from about 1,500 feet above mean sea level (msl) along the base of the Santa Ana Mountains in the southwest to about 500 feet msl in the northwest. The ground surface elevation in the city center is about 650 feet msl. In the southeast where the Temescal Wash enters the Basin from the Bedford-Coldwater Subbasin, the ground surface elevation is approximately 850 feet msl (Figure 3-1).

The Basin receives runoff and recharge from over 8,000 acres of uplands in the adjacent Santa Ana Mountains. Watersheds contributing runoff from the east are almost as large but contribute less runoff because of lower elevations and corresponding precipitation.

Temescal Basin GSP 3-2

3.2. SURFACE WATER FEATURES
The Basin includes a portion of the Santa Ana River watershed and a main tributary to the Santa Ana River, Temescal Wash, which flows through the Basin from the southeast to northwest. Surface water in the Basin originates as runoff from undeveloped tributary watersheds on the eastern slopes of the Santa Ana Mountains, wastewater treatment plant discharges, urban runoff within the Basin, flow in Temescal Wash, and flow in the Santa Ana River where it arrives at the Prado (flood control) Basin. Temescal Wash originates at Lake Elsinore, 17 miles upstream of the Basin and passes from south to north through the Bedford-Coldwater Subbasin and then through the Basin before discharging into the Prado Basin wetlands. This waterway is ephemeral and dry much of the year, flowing mainly during the winter. Tributary streams in the Santa Ana Mountains adjacent to the west side of the Basin flow primarily in response to rainstorm events, with limited base flow that enters groundwater where streams enter the Basin.

Figure 3-2 shows surface water features including rivers, streams, lakes, and ponds. The subwatersheds that drain into and through the Basin are shown on Figure 3-3.

3.3. SOILS
Characteristics of soils are important factors in natural and managed groundwater infiltration (recharge) and are therefore an important component of a hydrogeologic system. Soil hydrologic group data from the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Soil Survey Geographic Database (SSURGO) (NRCS 2020) are shown on Figure 3-4. The soil hydrologic group is an assessment of soil infiltration rates determined by the water transmitting properties of the soil, which include hydraulic conductivity and percentage of clays in the soil, relative to sands and gravels. The groups are defined as:
• Group A – High Infiltration Rate: water is transmitted freely through the soil; soils typically less than 10 percent clay and more than 90 percent sand or gravel. • Group B – Moderate Infiltration Rate: water transmission through the soil is unimpeded; soils typically have between 10 and 20 percent clay and 50 to 90 percent sand. • Group C – Slow Infiltration Rate: water transmission through the soil is somewhat restricted; soils typically have between 20 and 40 percent clay and less than 50 percent sand.
• Group D – Very Slow Infiltration Rate: water movement through the soil is restricted or very restricted; soils typically have greater than 40 percent clay, less than 50 percent sand.

The hydrologic group of the soil generally correlates with the potential for infiltration of water to the subsurface. However, a correlation does not necessarily exist between the soils at the ground surface and underlying geology or hydrogeology.

Temescal Basin GSP 3-3

3.4. GEOLOGIC SETTING
The Basin is located within one of the structural blocks of the Peninsular Ranges of Southern California. The Basin occurs in a linear low-lying block, referred to as the Elsinore-Temecula trough, between the Santa Ana Mountains on the west and the Perris Plain on the east (Todd and AKM 2008). The trough extends from the City of Corona (Corona) to the southeast some 30 miles and was formed along an extensive northwest-southeast trending fault zone including the Elsinore, Chino, and related faults. The Elsinore and Chino fault zones bound the Basin on the west and trend along the mountain fronts. The surficial geology and the surrounding area are shown on Figure 3-5.

The oldest rocks in the Basin crop out in the Santa Ana Mountains. These uplands are composed principally of volcanic (including the Santiago Peak Volcanics) and metamorphic rocks (including the Bedford Canyon Formation) of Jurassic and Cretaceous age. A thin rim of younger sedimentary units of Tertiary age crops out along the mountain front generally lying between the Elsinore and Chino faults. This zone of sedimentary units broadens to the north and contains numerous mapped formations of Cretaceous and Tertiary age. The northeastern side of the valley is flanked primarily by granitic rocks of Cretaceous age.

Erosion of these units has filled in the trough over time resulting in quaternary-age alluvial fan, channel, and other deposits making up the permeable portions of the Basin (USGS 2004 and 2006).

The geologic map on Figure 3-5 shows the distribution of these units in the Basin (USGS 2004 and 2006). The main surficial deposits on the floor of the Basin include younger and older alluvial fans deposited from the erosion of volcanic rocks and Bedford Canyon Formation to the west. These units prograde across the Basin to the northeast and are truncated by channel deposits along Temescal Wash.

3.5. FAULTS
The Basin was formed along an extensive northwest-southeast trending fault zone including the Elsinore, Chino, and related faults. The Elsinore and Chino fault zones bound the Basin on the west and trend along the mountain fronts. Fault locations and orientations are shown on Figure 3-5.

3.6. AQUIFERS
The basin-fill alluvial deposits and, to some extent, the underlying sedimentary units make up the aquifers in the Basin. However, these deposits do not fall neatly into two categories of permeability, such as bedrock and basin fill. Aquifer packages composed of various geologic units have been defined based on depositional environment, degree of consolidation, groundwater production, and location throughout the Basin.

Three aquifer packages provide water supply to wells in Basin: the Channel Aquifer, the Alluvial Fan aquifers, and, to a lesser extent, consolidated sandstone aquifers (Todd and AKM 2008). Of these three aquifers, only the Channel Aquifer is a principal aquifer as it is the most productive aquifer and provides most of the groundwater supply in the Basin. The

Temescal Basin GSP 3-4
Alluvial Fan and consolidated sandstone are secondary aquifers with limited production capacity and historical use. These aquifers meet one another in multiple areas throughout the Basin along erosional and depositional contacts. These contacts are permeable, and the aquifers are hydraulically connected. The geometry of these aquifers within the Basin are shown in cross sections presented on Figures 3-6 through 3-9. The thicknesses of these units vary significantly across the Basin, as indicated in the cross sections.

3.6.1. Description of Principal Aquifer Units
The Channel Aquifer is the principal aquifer in the Basin. This aquifer is a package of relatively homogeneous and highly permeable sands up to 200 feet thick that have been encountered in many of the Corona wells in the northern half of Basin. This sand package is interpreted as channel deposits of an ancestral arm of the Santa Ana River and, as such, has been referred to as the Channel Aquifer (Todd and AKM 2008). The alignment of the aquifer suggests that an ancestral river channel had entered the Basin at Arlington Gap, eroding the sedimentary units and possibly older alluvial fan deposits in the area. Permeable channel sands were deposited in the eroded channel over time. From the Arlington Gap, the Channel Aquifer trends northwest toward Prado Dam.

The orientation of The Channel Aquifer is illustrated on cross sections A to A’, B to B’, and C to C’ on Figures 3-7, 3-8, and 3-9, respectively (Todd and AKM 2008). Cross Section A to A’ extends from the Santa Ana Mountains to the northeast across Temescal Wash to the bedrock high in the northeast. As shown on the section, the Channel Aquifer occurs in the northeastern portion of the Basin and has a saturated thickness that ranges from 125 to 150 feet along this section. As illustrated on the section, Channel Aquifer sediments lie directly above granitic bedrock beneath Temescal Wash and above the Sandstone Aquifer in other areas (Figure 3-7).

The Channel Aquifer at Arlington Gap is shown on Cross Section B to B’ (Figure 3-8). Here the saturated thickness is approximately 200 feet and well data indicate a thick and permeable sand package. The Channel Aquifer is underlain by the Sandstone Aquifer throughout most of this area.

Cross-section C to C’ is located north of A-A’ and extends from the Santa Ana Mountains through the Norco area (Figure 3-9). The Channel Aquifer is shown on the western side of the section southeast of the Prado Management Area. Similar to Cross Section A-A’, the saturated thickness of the Channel Aquifer is about 100 to 150 feet thick. The cross section also shows the absence of the Channel Aquifer in the Norco area and illustrates the shallow depth to bedrock there (generally less than 100 feet). The saturated thickness of alluvial sediments in Norco is generally less than 50 feet. Also indicated on the section is a groundwater divide in the Norco area (near Well 53-499) indicating possible groundwater outflow from the Norco area to the Santa Ana River (Figure 3-9).

Figure 3-10 shows estimated values of hydraulic conductivity (K) derived from test data on driller’s logs and/or Corona well aquifer testing data and the aerial extents of the Channel Aquifer. The K value is an indicator of the aquifer’s permeability and is expressed in gallons per day per square foot (gpd/ft2
) or feet per day (ft/day). As shown on Figure 3-10, the wells

Temescal Basin GSP 3-5
within the limits of the Channel Aquifer have the highest hydraulic conductivity values in the Basin. The lower K values shown within the extent of the Channel Aquifer area on Figure 3- 10 are generally from deeper wells tapping the underlying Sandstone Aquifer. The average K value of City of Corona production wells screened solely in the Channel Aquifer (Wells 7A, 8A, 9A, 17, 25, and 28) is 2,062 gpd/ft2 (276 ft/day) (Todd and AKM 2008).

The Channel Aquifer adjoins the secondary aquifers described below as shown in the cross sections on Figures 3-7 through 3-9 and the map on Figure 3-10. These adjoining aquifers do have hydraulic connection and there is groundwater flow between the aquifers where they meet.

3.6.1.1. Secondary Aquifers
The recent alluvial fan aquifers and sandstone aquifer are also present within the Basin and have historically been used to a lesser extent than the principal aquifer. These secondary aquifers are described below.

3.6.1.1.1. Alluvial Fan Aquifers
Both older and recent alluvial fans have been deposited through time along the mountain front on the western edge of the Basin. These fans have prograded across the Basin from west to east (Figure 3-5). Although these deposits are relatively thick, the entire unit is heterogeneous and cannot be considered one single aquifer. Rather, sand lenses within the deposits collectively form the Alluvial Fan Aquifers. Lithologic data from wells are insufficient to map out the extent of the aquifers or characterize the deposits. Limited data indicate relatively fine-grained textures throughout much of the area, especially with depth (Todd and AKM 2008).

The geometry of these units in the subsurface, including the contact with the Channel Aquifer, is illustrated on Cross Section A to A’ on Figure 3-7. The section illustrates the alluvial fan deposits that have infilled the Basin. The fans have prograded across the Basin and a thin veneer of these deposits likely overlies the Channel Aquifer at the surface (not shown on the section). Wells that penetrate the entire thickness of the Channel Aquifer in the east do not appear to encounter alluvial fan deposits on top of the Sandstone Aquifer.

The total thickness of the deposits is unknown but appears to exceed 1,400 feet in the central Basin.

Only limited data exist for estimating K values in the alluvial fan deposits of Basin. Sparse data from a few wells indicate a K value of generally less than 50 gpd/ft2 in the Alluvial Fan Aquifers and in the Norco area (Figure 3-10). Specific capacity data from a City of Corona production well (Well 27) drilled in the Alluvial Fan, indicated a lower K value of about 7 ft/day (PBS&J 2004).

3.6.1.1.2. Sandstone Aquifer
Some of the sedimentary units underlying the alluvial Basin provide sufficient well yields to categorize them as aquifers. Although generally grouped with other bedrock units, the subsurface sedimentary rocks of Tertiary age in the northeast Basin area contain sandstone layers that are screened in several Corona wells. The estimated K value is 22 gpd/ft2 (3 ft/day) for one Corona production well (Well 24) screened solely in the Sandstone Aquifer

Temescal Basin GSP 3-6
(below the Channel Aquifer) (Todd and AKM 2008). Due to the limited production, small areal extent, increasing depths, and relatively low permeability in most areas, the Sandstone Aquifer is not considered a primary source of water supply.

3.6.2. Description of Lateral Boundaries
The lateral boundaries of the Basin are formed by contacts with bedrock units and borders with neighboring basins. The entire western Basin boundary and much of the eastern boundary of the Basin are contacts between Basin sedimentary units and upland bedrock outcrops. Along the north, the Basin is bounded by the contact with the Chino Subbasin, which is generally marked by the Santa Ana River and a series of low-lying hills in the Norco area. The boundary between the Basin and the Riverside-Arlington Subbasin is in the Arlington Gap and there is some flow into the Basin through this boundary. The southern boundary of the Basin is located at the Bedford Canyon where it connects with the BedfordColdwater Subbasin of the Elsinore Groundwater Basin.

Within the Basin the Channel Aquifer is bounded by its physical extents which are controlled by erosion and deposition. Near the Temescal Wash, an unnamed fault truncates the Channel Aquifer with an indeterminate amount of offset. The lateral extents of the Channel Aquifer are shown on Figure 3-10.

3.7. STRUCTURES AFFECTING GROUNDWATER
The Basin is defined by the lateral extents of the alluvial material described above. This material is bounded by bedrock in the Santa Ana Mountain on the west and the Peninsular Ranges to the east. The southern and northern boundaries of the Basin are formed by areas of thin alluvial material over shallow bedrock in narrow valleys (Todd and AKM 2008 and WEI 2015). A topographic rise in the subsurface bedrock appears to make a groundwater divided in the Norco area. The units in the Basin are also truncated by an inferred unnamed fault as part of the Elsinore and Chino fault zone along the base of the Santa Ana Mountains.

The location and effect of the Elsinore and Chino fault zone on the units of the Basin are shown on cross sections on Figures 3-6 through 3-9.

3.8. DEFINABLE BASIN BOTTOM
The Basin bottom is defined by bedrock, which is shallow around the perimeter and deep in the center, as shown on Figure 3-11. Depth to bedrock ranges in depth from 10 feet to approximately over 1,000 feet (Todd and AKM 2008 and WEI 2015). The depth to the bottom of the alluvial materials in the Basin and the contact with the bedrock bottom of the Basin are shown in the contours presented in Figure 3-11.

The thickest portion of the alluvial Basin (the deepest depth to bedrock) occurs in the central-west portions of the Basin as seen on Figure 3-7. The formation of a trough along the Elsinore and Chino fault zone is indicated by the asymmetric basin geometry.

Unconsolidated sediments are estimated to be more than 1,000 feet thick in this area.

Temescal Basin GSP 3-7
Bedrock is much shallower in the eastern portion of the Basin, however there is a slight deepening near the Arlington Gap, as indicated on Figure 3-8. Here, unconsolidated sediments are approximately 250 feet thick. This area is interpreted to have been eroded by a branch of the ancestral Santa Ana River, accounting for the depth. Sediments throughout the northern portion of the Basin, including in the Norco area, are about 100 feet thick as shown on Figure 3-8. Outcropping bedrock in the northern and eastern portions of the Basin is further evidence of the thin alluvial sediments.

3.9. RECHARGE AND DISCHARGE AREAS
Recharge to the Basin occurs primarily from wastewater discharge and subsurface inflow from outside the Basin, and to a lesser extent from deep percolation of precipitation, urban return flows, and infiltration of agriculture irrigation runoff as shown in Figure 3-12.

Discharge from wastewater treatment and subsurface inflow are the largest inflows to the Basin. Recharge associated with wastewater occurs when treated wastewater is discharged to ponds. Subsurface inflow occurs along the Basin boundaries and is a significant source of recharge to the Basin (Todd and AKM 2008).

Deep percolation of precipitation is the process by which precipitation enters groundwater.

Recharge to groundwater from deep percolation occurs throughout the Basin (Todd and AKM 2008). To a more limited extent, Basin recharge comes from the infiltration of runoff from precipitation in the Santa Ana Mountains west of the Basin and the Peninsular Ranges east of the Basin. Large amounts of runoff from the mountains flows into channels and the shallow subsurface at the edges of the Basin and then into and through the Basin. The amount of water available for recharge varies annually with changes in rainfall and runoff.

Runoff into the Basin is subject to evapotranspiration, infiltration, and continued surface flow to and in the Temescal Wash. The watersheds contributing to the Basin include multiple drainages, all of which flow across the Basin in generally east-west orientations.

Wet years generate large amounts of water that exceed the recharge capacity of the Basin (Todd and AKM 2008).

Return flows are those portions of applied water (e.g., landscape irrigation) that are not consumed by evapotranspiration and hence return to the groundwater system through deep percolation or infiltration. Return flows associated with urban, industrial, and agricultural water uses all have the potential to contribute to recharge to the Basin (Todd and AKM 2008).

Discharge from the Basin is primarily from groundwater pumping. A significant discharge also occurs to the Santa Ana River near the Prado Management Area (Todd and AKM 2008).

3.10. PRIMARY GROUNDWATER USES
The primary groundwater uses from both the principal and secondary aquifers in the Basin include municipal, rural residential, small community water systems, and small commercial uses. Groundwater pumped from the Basin aquifers supplies water for urban, agricultural,

Temescal Basin GSP 3-8
and industrial uses. Municipal uses account for most of this groundwater production.

Groundwater pumping also represents most of the outflow from the Basin.

3.11. DATA GAPS IN THE HYDROGEOLOGIC CONCEPTUAL MODEL
The hydrogeologic conceptual model has not identified data gaps in available information.

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Corona areas
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Tcga, Conglomerate of Arlington Mountain
Tf, Fernando Formation
Tns, Sandstone of Norco Area
Tsa, Santiago Formation
Tp/Tpsc/Tpsq/Tpy, Puente Formation; Tplv
Tt, Topanga Group
Tvs/Tvss, Vaqueros and Sespe Formations,
undifferentiated
Tsi, Silverado Formation
Kc, Carbonate Silicate Rock
Kcg/Kcgq/Kcto, Cajalco Pluton
Kd, Diorite, undifferentiated
Kgu, Granite, undifferentiated
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Kl/Klbc/Klhsc/Klhs, Ladd Formation
Klst, La Sierra Tonalite
Kmhg, Mount Hole Granodiorite
Kmp/Kmpc, Micropegmatite
Kqd, Quartz diorite
Krg, Granite of Riverside area
Ks, Serpentinite
Kt, Tonalite, undifferentiated
Ktr, Trabuco Formation
Kvsp/Kvspi, Santiago Peak Volcanics
Kvem/Kvs, Estelle Mountain volcanics of
Herzig
Kwl, Williams and Ladd Formations
Kwps/Kwsr, Williams Formation
Jbc, Bedford Canyon Formation
Trmu, Rocks of Menifee Valley
Figure 3-5
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Figure 3-6
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Figure 3-9
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( 0 to 50 gpd/sq. ft
( 50 to 250 gpd/sq. ft
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Approximate Limit of Channel Aquifer
Temescal Basin
Figure 3-10
Channel Aquifer Extent
and
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Lake or Pond
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Temescal Basin
Figure 3-12
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and Discharge Areas
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Temescal Basin GSP 4-1

CURRENT AND HISTORICAL GROUNDWATER CONDITIONS
This chapter describes the current and historical groundwater conditions in the Basin. The Sustainable Groundwater Management Act (SGMA) requires definition of various study periods for current, historical, and projected future conditions. Current conditions, by SGMA definition, include those occurring after January 1, 2015 and accordingly, historical conditions occurred before that date. A historical period must include at least 10 years.

The study period 1990 through 2019 is based on the cumulative departure from mean precipitation at Riverside, Claremont-Pomona, and Lake Elsinore climate monitoring stations. This period is representative and includes droughts and wet periods, with an average annual rainfall of 9.34 inches, comparable to the long-term average of 9.48 inches (1961 to 2019). Accordingly, groundwater conditions over time are described through 2020.

Groundwater conditions are described in terms of the six sustainability indicators identified in SGMA; these include:
• Groundwater elevations
• Groundwater storage
• Potential subsidence
• Groundwater quality
• Seawater intrusion (which is not likely to occur in this inland basin) • Interconnected surface water and groundwater dependent ecosystems.

4.1. GROUNDWATER ELEVATIONS

4.1.1. Available Data
Groundwater elevation records were collected from multiple sources, including the City of Corona, United States Geological Survey (USGS) National Water Information System (NWIS), California Department of Water Resources (DWR) California Statewide Groundwater Elevations Monitoring (CASGEM), and others. All wells with water level data are shown on Figure 4-1. Data from these sources were collected, reviewed, and compiled into a single unified groundwater elevation dataset (USGS 2020a and DWR 2010). Wells with groundwater level data are not distributed evenly throughout the Basin, and most measurement points are within Corona. Many wells have historical water level observations but have not been measured in recent years. In addition, there are temporal gaps in some of the data records and these are discussed in the data gaps section below.

4.1.2. Groundwater Occurrence
As summarized in Chapter 3, groundwater is present in one principal aquifer and two secondary aquifers and these aquifers are hydraulically connected. Groundwater in the Basin occurs under unconfined conditions and there are insufficient data to define vertical zones and to provide zone-specific groundwater elevation hydrographs or maps.

Temescal Basin GSP 4-2

4.1.3. Groundwater Elevations and Trends
Hydrographs showing groundwater elevation trends over time were prepared for all 39 wells with regular water level measurements in the Basin (Figure 4-1); these hydrographs were reviewed to identify wells with long term data that could be used to present representative hydrographs. The selection of representative wells was based a quantitative approach that considered hydrographs with long records characteristic of an area and distribution of wells across the Basin. In brief, all available groundwater elevation data for these wells were plotted as hydrographs and well locations were plotted on a basin-scale map. All wells with water level data are shown in Figure 4-1. Representative wells with long term hydrographs were selected based the following criteria:
• Location – Wells were prioritized considering broad distribution across the Basin availability of other wells nearby.
• Ongoing and/or recent monitoring – Wells were selected that are part of the active monitoring network or have recent data.
• Trends – Each hydrograph was assessed for continuity of monitoring, representation of local or regional trends, and presence of outliers or unrealistic data.

Recent and historical water level data inconsistently identified groundwater level measurements that were recorded during or immediately after pumping. Most groundwater level records are not identified as either pumping or static measurements. Review of these data showed some records identified as pumping water levels to be closely related to water levels not correlated with pumping. As such, for this study all water levels excluding obvious reporting errors are shown to preserve the overall trends.

Hydrographs in Figures 4-2 through 4-9 show groundwater level trends over time. In general, water levels correlate to wet and dry hydrologic. In general, water levels have been less responsive to wet and dry periods since 2000. Wells in some portions of the Basin show relatively stable groundwater levels over the past 20 years, while others show non-pumping water level changes during this period by up to 25 feet. The hydrographs do not show dramatic changes in historical water levels in the Basin. The range of historical non-pumping water levels in most wells is under 50 feet.

Figure 4-2 is the long-term hydrograph for Corona Well 15, showing water level changes in the Basin from 1953 to 2020. Since 1953, water levels in Well 15 have fluctuated a total of about 45 feet, from an elevation of 560 feet msl to about 515 feet msl (assuming the spikes below that level are influenced by local drawdown in the pumping well).

The highest water levels in wells with long-term data were measured in the early 1980s in response to a wet hydrologic cycle that began in 1978. These higher levels also correlate to a period of relatively low pumping in the Basin. During a later wet cycle from 1992 to 1998, water levels did not recover to 1980s levels, likely related to an increase in Basin pumping.

Groundwater elevation responses to changes in pumping and precipitation patterns are discussed further in Chapter 5 – Water Budget.

Temescal Basin GSP 4-3
The lowest groundwater levels generally correspond to dry periods and periods of increased pumping, though the responses throughout the Basin are not uniform. Hydrographs from most wells show lowering water levels during 2000 to 2004, a period that was not hydrologically dry but had increased pumping in Corona. In the long-term hydrograph from Corona Well 15 (Figure 4-2), within Corona, the lowest water levels occurred during the 2015 to 2017 period, after very low rainfall during 2011 through 2015. From 2010 to 2015, water levels declined 10 to 15 feet. Slight increases occurred in 2018 through 2019, likely the result of increased precipitation after 2015. Current levels are near record lows.

Overall, other wells in the Basin follow similar trends, although some wells have more variation in water levels in response to wet and dry periods. The westernmost hydrograph in the Basin is from Corona Well 11 and it shows very little groundwater level change from 2002 through 2020 (Figure 4-3). The wells further east in the Channel Aquifer show similar patterns, including Corona Well 22 (Figure 4-4), Corona 19 (Figure 4-5), Corona 17 A (Figure 4-7), and Corona 8a (Figure 4-8). Groundwater elevations in these wells were at their highest elevations in 2010 and declined at slow but steady rates through the most recent drought period of 2014 through 2016. Water level declines ranged from 10 to 20 feet from 2011 to 2015. Water levels in these four wells have remained stable or increased since 2018.

Corona Well 26 (Figure 4-6) is located on the northeastern part of the Channel Aquifer.

Groundwater elevations in this well were also high in 2010, but then decreased sharply in 2013, perhaps due to increased local pumping. The pumping water level in this well is significantly lower than the static water level, which could indicate lower specific capacity on the edges of the Channel Aquifer. Since 2013, water levels have been stable or increasing in Corona Well 26.

Corona Well 13 (Figure 4-9), located on the southeastern part of the Channel aquifer shows little change in groundwater levels from 2014 through 2020

4.1.4. Groundwater Flow
Figures 4-10 and 4-11 are groundwater elevation contour maps constructed to examine current groundwater flow conditions using data from fall 2015 and spring 2017. These time periods were chosen to represent dry and wet conditions, respectively. Contours were developed based on available groundwater elevation data for all wells. The median water levels during each season were used. These contours were prepared assuming no barriers to horizontal groundwater flow, including local faults. Due to limited water level data in the southern portion of the Basin, there is a higher level of uncertainty in groundwater flow direction and gradient in the south. Contours in zones with a higher level of uncertainty are shown with dashed lines.

Groundwater flow in the Basin is generally from the surrounding uplands toward Temescal Wash and then north and northwest toward the groundwater and surface water discharge location at Prado Dam. The fall 2015 groundwater elevation contours (Figure 4-10) indicate flow from south to north in the Basin. The groundwater elevations in this period represent relatively dry conditions at the end of a drought period. A small depression is depicted in the northern portion of the Basin, most likely due to pumping.

Temescal Basin GSP 4-4
Spring 2017 groundwater elevation contours (Figure 4-11) look very similar to groundwater conditions in fall 2015, indicating almost identical groundwater flow conditions. Spring 2017 was a wet period following the 2011 to 2015 dry period. This groundwater elevation surface also indicates flow generally south to north with a small depression in the Corona area. This period was selected because every well with water level data during fall 2015 also had data collected during spring 2017. In areas with similar data availability, spring 2018 groundwater elevation contours also look like the spring 2017 contours.

The similarities between these two groundwater elevation surfaces suggest that the groundwater levels and flow direction in the Basin are not entirely controlled by wet and dry periods and the groundwater flow conditions have been relatively constant in recent years.

Several hydrographs support this, with many showing little change in water levels over the past five years.

4.1.5. Vertical Groundwater Gradients
The current monitoring network for groundwater elevations provides little information about vertical head (groundwater elevation) gradients within the Basin. Available data are almost entirely from water supply wells, which typically have long screened zones and are not appropriate for evaluating vertical groundwater gradients. The potentiometric head at the depth of the well screens can be different from the true water table, which is the first zone of saturation reached when drilling down from the ground surface.

Vertical head gradients are an important factor affecting the viability of riparian vegetation.

As discussed in greater detail in Section 4.10.3, Riparian Vegetation, phreatophytic vegetation along streams generally survives droughts even when groundwater elevations are tens of feet below the ground surface for two or more years. This suggests that some shallow zones of saturation persist even when the water level in deep aquifers declines. This implies the presence of large vertical head gradients within the aquifer system.

4.2. CHANGES IN GROUNDWATER STORAGE
Change in storage estimates based on evaluation of groundwater elevation changes have not historically been completed for the Basin. Such storage change estimates are based on available groundwater elevation data that are limited geographically and temporally and thus include uncertainty. In addition, the storativity, or storage coefficient (the volume of water released from storage per unit decline in hydraulic head), is largely unknown across the Basin. The volume of groundwater storage change over time is sometimes calculated by multiplying the groundwater elevation changes during a period by the storage coefficient.

Storage coefficient values and storage change estimates representing the Basin were developed for the numerical model, as described in Appendix J. The numerical model is the best tool for estimating groundwater storage changes. The resulting change in storage estimates are presented in the Water Budget chapter.

Temescal Basin GSP 4-5

4.3. LAND SUBSIDENCE AND POTENTIAL FOR SUBSIDENCE
Land subsidence is the differential lowering of the ground surface, which can damage structures and facilities. This may be caused by regional tectonism or by declines in groundwater elevations due to pumping. The latter process is relevant to the GSP. In brief, as groundwater elevations decline in the subsurface, dewatering and compaction of predominantly fine-grained deposits (such as clay and silt) can cause the overlying ground surface to subside.

This process is illustrated by two conceptual diagrams shown on Figure 4-12. The upper diagram depicts an alluvial groundwater basin with a regional clay layer and numerous smaller discontinuous clay layers. Groundwater elevation declines associated with pumping cause a decrease in water pressure in the pore space (pore pressure) of the aquifer system.

Because the water pressure in the pores helps support the weight of the overlying aquifer, the pore pressure decrease causes more weight of the overlying aquifer to be transferred to the grains within the structure of the sediment layer. If the weight borne by the sediment grains exceeds the structural strength of the sediment layer, then the aquifer system begins to deform. This deformation consists of re-arrangement and compaction of fine-grained units1
, as illustrated on the lower diagram of Figure 4-12. The tabular nature of the finegrained sediments allows for preferred alignment and compaction. As the sediments compact, the ground surface can sink, as illustrated by the right-hand column on the lower diagram of Figure 4-12.

Land subsidence due to groundwater withdrawals can be temporary (elastic) or permanent (inelastic).

Elastic deformation occurs when sediments compress as pore pressures decrease but expand by an equal amount as pore pressures increase. A decrease in groundwater elevations from groundwater pumping causes a small elastic compaction in both coarse-and fine-grained sediments; however, this compaction recovers as the effective stress returns to its initial value. Because elastic deformation is relatively minor and fully recoverable, it is not considered an impact.

Inelastic deformation occurs when the magnitude of the greatest pressure that has acted on the clay layer since its deposition (preconsolidation stress) is exceeded. This occurs when groundwater elevations in the aquifer reach a historically low groundwater elevation. During inelastic deformation, or compaction, the sediment grains rearrange into a tighter configuration as pore pressures are reduced. This causes the volume of the sediment layer to reduce, which causes the land surface to subside. Inelastic deformation is permanent because it does not recover as pore pressures increase. Clay particles are often planar in form and more subject to permanent realignment (and inelastic subsidence). In general, coarse-grained deposits (e.g., sand and gravels) have sufficient intergranular strength and do not undergo inelastic deformation within the range of pore pressure changes 1 Although extraction of groundwater by pumping wells causes a more complex deformation of the aquifer system than discussed herein, the simplistic concept of vertical compaction is often used to illustrate the land subsidence process (LSCE et al. 2014).

Temescal Basin GSP 4-6
encountered from groundwater pumping. The volume of compaction is equal to the volume of groundwater that is expelled from the pore space, resulting in a loss of storage capacity.

This loss of storage capacity is permanent but may not be substantial because clay layers do not typically store significant amounts of usable groundwater. Inelastic compaction, however, may decrease the vertical permeability of the clay resulting in minor changes in vertical flow.

The following potential impacts can be associated with land subsidence due to groundwater withdrawals (modified from LSCE et al. 2014):
• Damage to infrastructure including foundations, roads, bridges, or pipelines; • Loss of conveyance in canals, streams, or channels;
• Diminished effectiveness of levees;
• Collapsed or damaged well casings; and
• Land fissures.

Inelastic subsidence has not been a known issue in the Basin.

4.3.1. Interferometric Synthetic Aperture Radar (InSAR) InSAR data are provided by DWR on its SGMA Data Viewer (DWR 2020) and document vertical displacement of the land surface across a broad area of California from June 13, 2015 to September 19, 2019. The TRE Altamira InSAR data, shown on Figure 4-13, shows land surface deformation between 2015 and 2019.

The TRE Altamira InSAR data indicates effectively no change in ground surface elevation within the Basin (Figure 4-13). Further review of the TRE Altamira InSAR data shows that ground surface elevations in the Basin rose by up to 0.08 feet (0.96 inches) between June 2015 and September 2019, with most of the Basin rising by about 0.02 feet (0.24 inches). A few small areas within the Basin subsided by up to 0.08 feet (0.96 inches). Given this data and the understanding of the hydrogeological conceptual model, there is no issue with subsidence at this time.

4.4. GROUNDWATER QUALITY
The natural quality (chemistry) of groundwater is generally controlled by interactions between rainwater and rocks/soil in the vadose zone and aquifer (Drever 1988). As rainfall infiltrates the soil column, anions and cations from sediments are dissolved into the water.

These changes are influenced by soil and rock properties, weathering, organic matter, and geochemical processes occurring in the subsurface. Once in the groundwater system, changing geochemical environments continue to alter groundwater quality. A long contact time between the water and sediments may allow for more dissolution and overall higher salinity level in groundwater (Drever 1988). The natural groundwater quality in a basin is the net result of these complex subsurface processes that have occurred over time. Under natural conditions, older, deeper groundwater often has higher salinity than shallow groundwater because of a longer residence time.

Temescal Basin GSP 4-7
Human processes can increase soil salinity and introduce higher levels of nitrate, inorganic chemicals, and organic compounds to soils in the vadose zone. When recharging water flows through saline soils, ions are dissolved into the infiltrating water and the salinity of shallow groundwater increases.

Most of the groundwater pumped in Temescal Subbasin (Basin) by the City of Corona is treated at the Temescal Desalter, a reverse osmosis membrane treatment facility. The facility treats nitrates, per-fluorinated compounds, 1,2,3-Trichloropropane (1,2,3-TCP), perchlorates, and suspended and dissolved solids. The remaining groundwater is treated at the City Park Ion Exchange Treatment Plant which utilizes two different types of resin, the first treats for perchlorates and the second for nitrates. Water delivered to municipal users is tested regularly to ensure all drinking water standards are met (Corona 2019). There are no other active domestic users of groundwater in the Basin, see Sections 2.3.2.1 and 6.2.

The City of Corona recognizes the human right to water and is committed to providing safe drinking water to City residents and has expanded service to the Home Gardens County Water District (HGCWD) service area.

The water quality of the groundwater discussed in this section is the ambient water quality of Basin and does not reflect the treated water delivered to customers by Corona.

Groundwater quality data for this study were sourced from the California Water Boards Groundwater Ambient Monitoring and Assessment (SWRCB 2020a) datasets (which includes data collected by Division of Drinking Water, USGS, DWR, and Regional Water Quality Control Board). In addition, water quality data collected by Corona were included in the analysis. Figure 4-14 shows the location and data source of the 113 wells with water quality data since 2010 that are in the Basin. The distribution of wells within the Basin is not uniform. Water quality data are primarily available for the Corona wells in the north-central portion of the Basin. In total, 22 wells with recent water quality data were used to assess water quality in the Basin.

Additional monitoring wells for facilities regulated by the Regional Water Quality Control Board do exist in several clusters within the Basin, but these wells were excluded from this groundwater quality assessment. The Regional Water Quality Control Board wells monitor facilities with point source contamination, and their measurements may not be representative of the ambient water quality in the Basin.

A 2008 analysis of the inorganic water quality in the Basin showed that water quality is primarily a sodium/calcium-bicarbonate water type (Todd and AKM 2008). However, the major ion concentration ratios can vary by region. By analyzing the ion ratio characteristics of different areas, the 2008 Corona Groundwater Management Plan identified regions with groundwater mixing and supported the groundwater flow paths identified in the conceptual model. The inorganic major ion analyses identified the following regional trends:
• Groundwater in the Bedford Canyon portion of Temescal Wash or Temescal Canyon has a higher ratio of calcium-to-sodium and sulfate-to-chloride than wells located in Arlington Gap. Groundwater in the Temescal Wash area upgradient of the Norco area has relative cation concentrations that are most like that of the Arlington Gap

Temescal Basin GSP 4-8
groundwater. However, the relative cation concentrations do suggest some mixing with waters from the Temescal Canyon area.
• Groundwater in wells located in the Norco area have a lower ratio of calcium-tosodium and sulfate/bicarbonate-to-chloride than most other areas. • Groundwater in wells located in the southwestern alluvial fan have the highest ratio of calcium-to-sodium and sulfate-to-chloride/bicarbonate compared to groundwater in other areas. The water type in the alluvial fan may result from geochemical interaction between rainfall runoff and the outcropping Santiago Peak volcanics in the western catchment area of Basin prior to aquifer recharge along the base of the mountains.
• Cation concentrations indicate that groundwater in wells located in Temescal Wash downgradient of the Norco area appear to be mixtures of groundwater from three sources: Temescal Wash upgradient of the Norco area, Arlington Gap, and the western alluvial fan.

This water quality assessment indicates the major sources of water by analyzing the blending of different water quality from different areas. Identifying major areas of inflow and outflow is critical to developing a strong conceptual model of the aquifer. These results are particularly useful given the sparse water level data available in the southern part of the Basin. Based on water quality type, the groundwater in the Channel Aquifer appears to be derived mainly from Arlington Gap and to lesser extent Temescal Wash. In addition to these sources, the western Channel Aquifer also receives inflow from the Alluvial Fan.

4.5. KEY CONSTITUENTS OF CONCERN
The review of available water quality data indicates that total dissolved solids (TDS) and nitrate are the primary constituents of concern (COCs) in the Basin. Other substances known to contribute to poor groundwater quality were reviewed and are discussed later in this chapter.

Elevated TDS concentrations in groundwater are common, resulting from dissolution of minerals from soil and rocks. TDS in groundwater can also be an indicator of anthropogenic impacts from sources such as urban runoff, agricultural return flows, and wastewater disposal. TDS data are available for both inflows and outflows from the Basin.

Nitrate is the primary form of nitrogen detected in groundwater. While natural nitrate levels in groundwater are generally very low, elevated concentrations of nitrate in groundwater are associated with agricultural activities, septic systems, landscape fertilization, and wastewater treatment facility discharges.

Recent water quality results indicate average TDS concentrations of 785 milligrams per liter (mg/L) and nitrate concentrations of 42.8 mg/L (All nitrate concentrations are reported in terms of nitrate as NO3). These values represent the average concentrations of these constituents from the most recent water quality data for all drinking water and ambient groundwater monitoring events between water year 2010 and water year 2019. Water

Temescal Basin GSP 4-9
quality samples from regulated facilities were not included in the analysis. These average conditions serve as a snapshot of water quality conditions within the Basin.

4.5.1. Total Dissolved Solids (TDS)
Groundwater in the Basin is somewhat mineralized, with high TDS concentrations in many monitored wells. The recent average TDS concentrations in the Basin referenced previously are above the 500 mg/L lower secondary maximum contaminant level (SMCL) for drinking water, but below the upper SMCL of 1,000 mg/L. The SMCLs are based on aesthetic considerations (such as taste) and are not health-based.

Most of the recent maximum TDS concentrations from monitored wells in the Basin were above the 500 mg/L SMCL, as indicated on Figure 4-15. In total, all but two of the 20 wells with data have TDS concentrations over 500 mg/L, and most of the recent TDS measurements were similar to the median and mean TDS concentrations reported for the respective wells in the 2000 to 2019 period. The highest TDS concentrations on Figure 4-15 are in wells near the City of Corona, where concentrations from several wells exceed 1,000 mg/L. A total of three wells in the Basin have median TDS concentrations over 1,000 mg/L.

TDS concentrations in some wells have fluctuated by several hundred mg/L during the 2010 to 2019 period. The two wells on Figure 4-15 with TDS concentrations less than 250 mg/L, Corona Wells 11A and 13, have only shown low TDS concentrations in recent years. Prior to 2016, TDS measurements in these wells were generally greater than 700 mg/L.

4.5.2. Nitrate as Nitrate (NO3)
Elevated nitrate concentrations have been documented in the Basin since at least the 1950s.

Recent data indicate that the average nitrate concentration in the Basin is 42.8 mg/L. The maximum contaminant level (MCL) for nitrate as NO3 in drinking water is 45 mg/L.

The most recently reported nitrate as NO3 concentrations for wells in the Basin are shown on Figure 4-16. Water quality data indicate nitrate concentrations ranging from less than 1 mg/L to 100 mg/L. Nine wells in the Basin have recent median nitrate concentrations greater than 45 mg/L. The highest nitrate concentrations are those associated with wells at the Arlington Gap. Eight water supply wells in the Basin have had nitrate concentrations exceeding the MCL and have required treatment and/or blending to meet regulatory requirements.

Nitrate contamination in groundwater is commonly related to activities at the ground surface (e.g., fertilizer application, septic systems), and as a result, shallow groundwater typically has higher concentrations than deep groundwater. The wide range of nitrate concentrations in wells in the Basin could be due to vertical variations in nitrate concentrations, but well construction information for monitored wells are limited so it is not possible to adequately assess nitrate concentration variation with depth.

Temescal Basin GSP 4-10

4.6. OTHER CONSTITUENTS
While recent water quality data are limited, available data do not indicate that other constituents of concern pose a significant threat to beneficial uses of groundwater in the Basin. The ambient water quality is discussed in Section 4.4 but this does not reflect on the quality of available drinking water. As noted, groundwater pumped from the Basin for domestic and municipal use is treated at the Temescal Desalter or the City Park Treatment facility prior to distribution by Corona. There are no other domestic groundwater users, either public or private, in the Basin. Nonetheless, these and other naturally occurring and emerging anthropogenic constituents will continue to be monitored and analyzed.

4.6.1. Naturally Occurring Contaminants
Arsenic, uranium, fluoride, and hexavalent chromium are chemicals that can naturally occur at elevated concentrations in groundwater. These contaminants originate in the eroded rocks that make up aquifer sediments and enter groundwater through reactions between groundwater and the sediments. In general, the occurrence of arsenic, uranium, fluoride, and hexavalent chromium depend on regional geology and local groundwater conditions. As documented in this section, no naturally occurring contaminants were identified as widespread constituents of concern in the Basin. However, continued monitoring of these chemicals is recommended.

4.6.1.1. Arsenic
Arsenic in a known carcinogen with a MCL of 10 micrograms per liter (µg/L). Elevated arsenic concentrations occur in groundwater throughout the United States, often in aquifers with low-oxygen (reducing) conditions or high pH levels (USGS 2020b). In the Basin, groundwater in all but one well recorded arsenic concentrations under 5 µg/L. The one well showing groundwater with high arsenic concentrations (32 µg/L) is located near the Arlington Gap (HGCWD Well 5) and it is near a well with arsenic concentrations less than 2 µg/L. This suggests that arsenic may be depth-dependent in the Arlington Gap, but the depths of both wells are unknown.

4.6.1.2. Uranium
Uranium in California groundwater is often derived from eroded granite, such as the Mesozoic granites east of the Basin (Jurgens et al. 2010). The MCL for uranium is 20 picocuries per liter (pCi/L), equivalent to about 30 µg/L. At this concentration, the effect of radiation is negligible, but the chemical properties of uranium can cause kidney damage.

Uranium often occurs in shallow, oxygen-rich groundwater (Jurgens et al. 2010). Uranium has been measured in 18 wells in the Basin since 2010. Groundwater in two wells, one in the Arlington Gap region (34 pCi/L) (HGCWD Well 5) and one in Corona (20.8 pCi/L) (Corona Well 19), indicate uranium concentrations greater than the 20 pCi/L MCL. The well with high uranium concentrations in the Arlington Gap is adjacent to a well with groundwater with a uranium concentration of 12.7 pCi/L.

Temescal Basin GSP 4-11

4.6.1.3. Fluoride
Fluoride is a necessary component of a healthy diet to prevent dental cavities, and a fluoride concentration of 0.7 mg/L in drinking water is recommended by the United States Department of Health and Human Services (USDHHS 2015). At extremely high concentrations, however, fluoride can cause mottling of teeth and damage bones.

Groundwater in aquifers with sediment originating from igneous rocks can often have fluoride concentrations above the 2 mg/L MCL for fluoride. One well in the Basin had a recent fluoride concentration above the MCL, with a recent concentration of 2.7 mg/L (Corona 26).

4.6.1.4. Hexavalent Chromium
Hexavalent chromium, the oxidized form of the metal chromium, occurs in oxygen-rich groundwater in western California, near chromium-bearing rocks. Hexavalent chromium in California drinking water is currently regulated along with total chromium; the MCL for total chromium is 50 µg/L. In 2014, California adopted a 10 µg/L MCL for hexavalent chromium, but this was overturned in 2017 due to a ruling that the California Department of Public Health had failed to consider the economic feasibility of complying with the MCL (SWRCB 2020c). All 18 wells recently monitored for hexavalent chromium showed groundwater concentrations under 4 µg/L, far below the 50 µg/L MCL.

4.6.2. 1,2,3- Trichloropropane
1,2,3- Trichloropropane (1,2,3- TCP) is a human-made chemical used in pesticide products and as a cleaning and degreasing solvent. It has a high chemical stability and can remain in groundwater for long periods of time (SWRCB 2020d). 1,2,3-TCP has been shown to cause cancer to laboratory animals and is believed to be carcinogenic to humans. California OEHHA established a 0.0007 ug/L public health goal (PHG) for 1,2,3-TCP in 2009. The notification level of 1,2,3-TCP is 0.005 ug/L. In total, 24 wells in the Basin have been tested for 1,2,3-TCP. Seven wells have detected 1,2,3-TCP above the notification level and public health goal. These wells are located near or in the City of Corona in the central part of the Basin.

Water pumped from wells with high concentrations of 1,2,3- TCP have been identified and all water produced by these wells is treated using RO technology at the Temescal Desalter before delivery to customers (Corona 2019). There are no active domestic wells in the Basin and no future domestic pumping is expected. Corona will continue to treat groundwater for 123-TCP and other constituents and provide the and HGCWD with safe drinking water.

4.6.3. Per- and Polyfluoroalkyl Substances (PFAS)
Per- and polyfluoroalkyl substances (PFAS) are a group of emerging contaminants that may pose a danger to reproductive, developmental, immunological, and renal health in humans.

Contaminants of emerging concern, or emerging contaminants, are chemicals that have only recently been identified as being present in soil and groundwater or were not previously monitored or detected but pose a risk to human health (USEPA 2019). The two most common PFAS are perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA).

Currently, California has a drinking water response level of 10 parts per trillion (ppt) for

Temescal Basin GSP 4-12
PFOA and 40 ppt for PFOS. PFAS have been used in products including firefighting foams, nonstick cookware, and stain- and water-repellant fabrics for many decades. PFAS contamination of groundwater often occurs near firefighting training facilities or landfills.

The California State Water Resources Control Board has undertaken PFAS monitoring throughout the state, measuring PFAS concentrations in groundwater and identifying point sources of PFAS contamination (SWRCB 2020e). A study of PFAS in the Santa Ana River Watershed has identified elevated PFAS concentrations in groundwater and contamination sources within the Basin (Behrooz 2020). Because of the emerging nature of PFAS, these studies are ongoing. Additionally, the state is still developing guidelines and regulatory limits for PFAS in water supplies.

4.6.4. Monitoring Networks
City of Corona
The Corona water system includes water supply wells in the Basin that are actively monitored for water quality. Since 2010, Corona has routinely collected water quality data from 19 active and inactive wells in the Basin.

Division of Drinking Water
Public drinking water systems in the Basin report water quality data to the State Water Resources Control Board (SWRCB) Division of Drinking Water (DDW). Each system monitors and reports water quality parameters to DDW and is required to participate in the Drinking Water Source Water Assessment Program (DWSAP) to assure wells are not subject to local contamination. While most of the public supply well water quality data was received directly from Corona, some additional data are available from DDW.

Orange County Water District (OCWD)
OCWD also monitors groundwater quality near Temescal Basin in the Prado area. OCWD is currently installing more than a dozen shallow monitoring wells in the Prado area to provide more information on shallow groundwater conditions. These wells will provide additional data on impacts interconnected surface water and groundwater dependent ecosystems in the Prado Area.

Other Agencies
The Santa Ana Regional Water Quality Control Board (RWQCB) monitors clean-up sites throughout the Basin. Water quality data from 91 wells in this system have been collected.

However, data from these wells are not used in this analysis because they often represent point source contamination and cannot accurately capture the ambient water quality in the Basin.

Wells with water quality data from all available sources are shown on Figure 4-14.

Temescal Basin GSP 4-13

4.7. THREATS TO WATER QUALITY

4.7.1. Regulated Facilities
The Santa Ana Regional Water Quality Control Board (RWQCB) regulates thirteen cleanup sites in the Basin. These sites include a military site, leaking underground storage tanks, and dry cleaning facilities. Since 2010, 91 wells at regulated facilities have been monitored for chemical constituents.

4.7.2. Septic Systems
Limited areas of the Basin are not served by municipal sewers and rely on on-site wastewater treatment (OWTS or septic systems). These represent sources of TDS and nitrate loading to groundwater, as well as potential sources of other contaminants.

Riverside County Department of Environmental Health is the permitting agency for septic systems and wells in the County. The Riverside County Department of Environmental Health maintains an inventory of septic system installations. While it is unclear how many of these septic systems still exist, it is assumed minimal because most of the Groundwater Sustainability Agency (GSA) area is served by municipal wastewater collection systems.

4.7.3. Non-point Sources
Nonpoint source (NPS) pollution is defined by the SWRCB as contamination that does not originate from regulated point sources and comes from many diffuse sources. NPS could occur when rainfall carries contaminants to surface waterways or percolates contaminants to groundwater. One example is loading to groundwater of nitrate from agricultural or landscaping land applications. While groundwater may have natural salinity, increasing TDS concentrations from soil salinization is another common non-point source pollution.

4.8. VERTICAL VARIATIONS IN WATER QUALITY
Water quality monitoring programs in the Basin do not show a distinct difference of water quality in depth, in part because most of the ambient monitoring wells have long screened intervals or are collected from wells with unknown construction.

4.9. SEAWATER INTRUSION CONDITIONS
The Basin is located approximately 25 miles inland from the Pacific Ocean and the lowest elevation at the northwestern boundary of the Basin is about 450 feet above sea level. No risk of seawater intrusion exists in the Basin given its location.

4.10. INTERCONNECTION OF SURFACE WATER AND GROUNDWATER
Interconnection of groundwater and surface water occurs wherever the water table intersects the land surface and groundwater discharges into a stream channel or spring.

These stream reaches gain flow from groundwater and are classified as gaining reaches.

Conversely, connection can occur along stream reaches where water percolates from the

Temescal Basin GSP 4-14
stream into the groundwater system (losing reaches), provided that the regional water table is close enough to the stream bed elevation that the subsurface materials are fully saturated along the flow path.

Groundwater pumping near interconnected surface waterways or springs can decrease surface flow by increasing the rate of percolation from the stream or intercepting groundwater that would have discharged to the stream or spring. If a gaining stream is the natural discharge point for a groundwater basin, pumping anywhere in the Basin can potentially decrease the outflow, particularly over long time periods such as multi-year droughts.

Because of the long dry season that characterizes the Mediterranean climate in Riverside County, vegetation exploits any near-surface water sources, including the water table along perennial stream channels, the wet soil areas around springs, and areas where the water table is within the rooting depth of the plants. Plants that draw water directly from the water table are called phreatophytes. They are able to continue growing vigorously during the dry season and typically stand out in summer and fall aerial photographs as patches of vegetation that are denser, taller and brighter green than the adjacent vegetation.

4.10.1. Stream Flow Measurements
Stream flow in the Basin includes runoff from undeveloped tributary watersheds on the eastern slopes of the Santa Ana Mountains, wastewater treatment plant discharges, urban runoff within the Basin, flow in Temescal Wash, and flow in the Santa Ana River where it arrives at the Prado (flood control) Basin. The flow regimes in these waterways are quite different. The locations of surface water features mentioned in this discussion are shown in Figure 4-17. Tributary streams in the Santa Ana Mountains adjacent to the west side of the Basin flow primarily in response to rainstorm events, but accretions of groundwater from fractured bedrock create a small, more persistent base flow. These small flows rapidly percolate where the creek enters the Basin and generally do not reach Temescal Wash.

None of the local tributaries is gaged, but a gage was installed in 2018 on Coldwater Canyon Creek about five miles south of the Basin, and its watershed is similar to those of the Basin tributaries. Daily flows at that gage and two Temescal Wash gages during water years 2013 through 2020 are shown in Figure 4-18.

Temescal Wash originates at Lake Elsinore, 17 miles upstream of Basin. It passes from south to north through the Bedford-Coldwater Subbasin and then through Basin before discharging into the Prado Basin wetlands. There are two stream gages on Temescal Wash, one below Lee Lake at the upstream end of the Bedford-Coldwater Subbasin (Temescal Wash at Corona Lake; USGS 11071900) and one at Main Street downstream of the wastewater treatment plant in Corona (Temescal Creek above Main Street at Corona; USGS 11072100). The flow regime at the outlet of Lee Lake is probably similar to the flow regime at the upstream end of Basin. Surface flow occurs primarily during and immediately following rainstorm events. No flow was recorded for three consecutive years during the recent drought.

Temescal Basin GSP 4-15
Reduction in wastewater treatment plant discharges to Temescal Wash over the preceding 10 to 20 years is thought to have contributed to the exceptionally low flows during the drought (Russell 2020). A comparison of total flow in Temescal Wash with recycled water discharges entering the wash confirms that base flow did decrease after 2012 but by only a small amount relative to total flows entering the Prado Wetlands. Figure 4-19 shows monthly average flows at the gauge above Main Street in Corona and monthly recycled water discharges to Temescal Wash from three water reclamation facilities. Gaged flows above Main Street experienced many more peak flow events than seen at gages farther upstream. Most of these additional flow events probably derive from impervious runoff in the surrounding urban area. Base flow closely tracks the discharge from Corona Wastewater Reclamation Facility 1 (WRF-1), which is located less than 1 mile upstream of the gage on the concrete-line Temescal Wash channel. There might be additional contributions of socalled nuisance water (for example, sprinkler overspray onto paving, or pipe leaks). Average discharges from WRF-1 decreased by about 2 cubic feet per second (cfs) following a SWRCB decision approving the City of Corona’s petition to decrease minimum discharges from 4.57 cfs to 2.25 cfs (SWRCB 2012). In December 2012, the RWQCB issued Order R8-2012-0028, which allowed the Temescal Valley Water District (TVWD) to recycle or percolate all reclaimed water at the Lee Lake WRF and cease all discharges to Temescal Wash. That WRF is located 4 miles upstream of Temescal Basin. Those discharges had already been decreasing and were less than 1 cfs during 2010 to 2012, which means they would have been consumed entirely by percolation and evapotranspiration before reaching Temescal Basin. The decrease in Lee Lake WRF discharges would not have affected Temescal Wash inflow to the Prado Wetlands. By the same token, discharges from the City of Corona WRF-3 (located 2.2 miles upstream of the Temescal Basin) were also too small to affect inflow to Prado Wetlands. For comparison, median annual outflow from Prado Dam decreased by 129 cfs, or fifty times more than the decrease in Temescal Wash base flow at the Main Street gage in Corona.

A review of 27 high-resolution aerial photographs (Google Earth 2021) between 1994 and 2020 revealed localized flowing or ponded reaches of Temescal Wash along a 2-mile reach where the Wash traverses bedrock between the Bedford-Coldwater Subbasin and the Basin.

The location of this reach along with vegetation and estimated depth to groundwater in spring 2017 are shown in Figure 4-20. Open water was also visible in the Wash channel in some of the air photos from the Minnesota Road bridge down to Temescal Wash Lake, a 33- acre lake that is a former gravel mining pit. The greater resistance of bedrock to subsurface flow appears to force groundwater into the creek channel and/or riparian root zone as it crosses the bedrock. Upon entering the Basin a short distance downstream of the Minnesota Road bridge, surface flow percolates back into the ground. Groundwater levels are probably far below the creek bed in that area, however, sufficient surface flow reaches Temescal Wash Lake to make it a perennial water body. For 3.4 miles below Temescal Wash Lake, the creek flows in a cement-lined culvert, finally discharging into the outer fringes of Prado Basin at North Lincoln Avenue.

Aerial photographs from 1967 show almost no riparian vegetation along the bedrock reach of Temescal Wash between the Bedford-Coldwater Subbasin and Temescal Basin.

Precipitation had been consistently below-average since 1947 and pumping along Temescal

Temescal Basin GSP 4-16
Wash in the Bedford-Coldwater Subbasin during that period was 167 percent of recent pumping (WEI 2015). Thus, dense riparian vegetation has not been a constant feature of Temescal Wash but has waxed and waned over the past several decades in response to changes in surface flow and groundwater levels.

4.10.2. Depth to Groundwater
Depth to groundwater provides a general indication of locations where gaining streams and riparian vegetation are likely to be present. However, available data are of limited use for this purpose due to insufficient vertical and geographic coverage. Available data are almost entirely from water supply wells, which are typically screened far below the water table. The groundwater elevation (potentiometric head) at the depth of the well screen can be different from the true water table, which is the first zone of saturation reached when drilling down from the ground surface. Because recharge occurs at the land surface and pumping occurs at depth, deep alluvial basins such as this one typically have large downward head gradients within the aquifer system. Thus, water level information from wells can potentially underestimate the locations where the water table is shallow enough to support phreatophytic riparian vegetation. Conversely, in areas where groundwater discharges into streams or wetlands—such as in the Prado Basin—vertical water-level gradients are typically upward.

The geographic coverage of water-level data for the Basin is limited because the wells with data are clustered near the north-central part of the Basin. The closest well to Temescal Wash is about 0.5 mile away. The error associated with extrapolating water levels to Temescal Wash could easily be greater than 10 feet. Horizontal water table gradients can be high near losing streams. Creeks and rivers that lose water commonly form a mound in the water table near the creek. The height and width of the mound depends on the transmissivity of the shallowest aquifer. For example, groundwater elevations in a shallow well adjacent to the Arroyo Seco in the Salinas Valley rose 5 to 10 feet more than groundwater elevations in wells 1,000 feet away when the river started flowing (Feeney 1994). A groundwater ridge up to 12 feet high develops beneath Putah Creek in Yolo County during the flow season, but the width of this ridge was estimated to be only a few hundred feet (Thomasson et al. 1960). These examples suggest that shallow wells within 100 to 200 feet of a stream channel would be needed to confirm the presence of hydraulic connection between surface water and groundwater.

Groundwater does not discharge into streams unless the water table is equal to or higher than the elevation of the stream bed. In addition, the water table does not provide water to phreatophytic vegetation unless it is at least as high as the base of the root zone. The depth of the root zone is uncertain, partly because the relatively few studies of rooting depth have produced inconsistent results and partly because rooting depth for some riparian species is facultative. This means that the plants will grow deeper roots if the water table declines.

Many species (including cottonwood and willow) germinate on moist soils along the edge of a creek in spring. As the stream surface recedes during the first summer, the seedlings survive if the roots grow at the same rate as the water-level decline. Over a period of years, roots grow deeper as the land surface accretes from sediment deposition and/or the creek

Temescal Basin GSP 4-17
channel meanders away from the young tree or shrub. For screening purposes, a depth to water of less than 30 feet in water supply wells near streams was selected as a threshold for identifying possible phreatophyte areas. This depth allows for 10 to 15 feet of root depth, 5 feet of elevation difference between the water level in the well and the overlying true water table, and 15 feet of topographic elevation difference between well heads and the bottoms of nearby creek channels where the vegetation is located.

In spite of these accuracy limitations, contours of depth to water measured in wells—in combination with depth to water data for the downstream end of the Bedford-Coldwater Subbasin (also shown in Figure 4-20)—indicates that there are only two areas in or near the Basin where depth to water is likely shallow enough to be within the root zone of vegetation or possibly discharge into stream channels or wetlands (Figure 4-20). One of the areas is the 2-mile bedrock reach of Temescal Wash between the Bedford-Coldwater Subbasin and Basin, and the other is the Prado Wetlands, where contouring suggests groundwater discharges into the wetlands. Depth to water in spring of 2017 was less than 20 feet downstream of about North Lincoln Avenue.

Depth to water in the Corona area was incorrectly characterized in a shallow groundwater and evapotranspiration assessment competed for the Upper Santa Ana River Integrated Model summary report (Ballau 2018). The map of shallow groundwater areas in slide 8 of Appendix E of that report shows shallow groundwater conditions extending up Temescal Wash from Prado Basin to the center of Corona. Shallow groundwater conditions were inferred from the presence of perennial flow in the Wash, as shown on the National Hydrography Dataset map. There is perennial flow, but it consists almost entirely of wastewater discharges from the Corona WRF-1 treatment plant, which is located about 1 mile upstream of the Main Street gage. All wells in that area—including deep supply wells and shallow monitoring wells at cleanup sites—have water levels generally more than 70 feet below the ground surface. Furthermore, the reach of Temescal Wash that is perennial is lined with concrete and not suitable for riparian habitat.

4.10.3. Riparian Vegetation
Vegetation data provides evidence that the water table near some reaches of Temescal Wash is shallow enough to supply water to phreatophytes. Where tree and shrub roots are able to reach the water table, riparian vegetation is typically denser and greener than along reaches where vegetation is supplied only by stream flow or residual soil moisture from the preceding wet season. Patches of dense riparian vegetation are visible in multiple historical photographs and are indicated by a crosshatch pattern in Figure 4-21. The figure also shows the distribution of vegetation classified as Natural Communities Commonly Associated with Groundwater (NCCAG) by the Nature Conservancy. Based on multiple historical vegetation surveys, the Nature Conservancy prepared detailed statewide mapping of NCCAG vegetation that is accessible on-line (DWR et al. 2020). Note that the NCCAG map does not include the corridor of dense riparian trees and shrubs along the bedrock reach of Temescal Wash between the Bedford-Coldwater Subbasin and the Basin. It does include 44 acres of red willow and 12 acres of cottonwood between Minnesota Road and Temescal Wash Lake, which is a reach where surface flow and shallow groundwater are probably leaking

Temescal Basin GSP 4-18
downward to a deeper regional water table. Red willow is a facultative phreatophyte, which means it will exploit a water table if it is within a reachable depth but otherwise survive on soil moisture (typically with smaller stature and greater spacing between plants).

Another waterway mapped as supporting riparian vegetation in the NCCAG database is the lower reach of Wardlow Wash, which drains the northwest corner of the Basin to the Santa Ana River. Most of the mapped vegetation is facultative phreatophytes (mainly sycamore), but one polygon of Fremont cottonwood (an obligate phreatophyte) is mapped about 0.5 mile from the southern edge of the Prado Wetlands, in an area where shallow depth to water is plausible.

An additional test for groundwater dependence of riparian vegetation was to compare changes in groundwater elevation with changes in vegetation health during the recent drought. Vegetation health can be detected by changes in the way the plant canopy absorbs and reflects light. The spectral characteristics of satellite imagery can be processed to obtain two metrics commonly used to characterize vegetation health: the Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Moisture Index (NDMI). Both are calculated as ratios of selected visible and infrared light wavelengths. The Nature Conservancy developed a second on-line mapping tool called GDE Pulse that provides annual dry-season averages of NDVI and NDMI for each mapped NCCAG polygon for 1985- 2018 to assist with the identification of groundwater dependent ecosystems (GDEs) (TNC 2020). For the Fremont cottonwood polygon, NDVI and NDMI declined by 0.25 and 0.26, respectively, from 2012 to 2017, and for the red willow areas, the metrics declined by 0.15 and 0.16. These fairly substantial declines were clearly related to the drought. Field observations of riparian vegetation documented riparian tree mortality of approximately 80 percent between 2014 and 2016 along the downstream end of Bedford-Coldwater Subbasin, the bedrock reach, and the Basin reach down to Temescal Wash Lake (Russell 2020 and Google Earth 2021). The question is whether the cause of the moisture stress was reduced rainfall, reduced streamflow or lower groundwater levels. After the relatively wet winters of 2019 and 2020, the stands of riparian trees are now recovering.

In summary, riparian vegetation along the bedrock reach of Temescal Wash is very likely phreatophytic and therefore affected by groundwater levels in the thin ribbon of channel deposits along the Wash. Once the Wash enters the Basin, however, the regional water table is far below the channel. The depth to water in the three wells with historical water level data closest to the Temescal Wash vegetation (south of well Corona 13 in Figure 4-1) was historically 50 to 150 feet below the ground surface. Depth to water increases from the center of the Channel Aquifer area toward the margins of the Basin because the ground slope (0.04-0.13 ft/ft from Figure 3-1) is four or more times steeper than the water table slope (about 0.01 ft/ft from Figures 4-10 and 4-11). Thus, the depth to water at the Temescal Wash riparian vegetation is likely greater than the 50 to 150 foot range of the three wells farther north. Groundwater elevations at the vegetation location is not considered a data gap because available data indicate that the regional water table could not plausibly be less than 30 ft below the ground surface. The riparian vegetation along that reach is probably supported by perched groundwater along the channel sustained by percolation of surface flow in Temescal Wash as it exits the bedrock reach.

Temescal Basin GSP 4-19

4.10.4. Wetlands and Interconnected Surface Water
The north end of the Basin is beneath the Santa Ana River and surrounding Prado Wetlands, which is a managed wetland maintained by operation of Prado Dam. Prado Dam impounds the river to regulate flood flows in winter and sustain a perennial wetland. In some previous reports, the impoundment area was referred to as the Prado Flood Control Basin. “Basin” in this case refers to a surface water feature. The RWQCB Basin Plan designates the area behind Prado Dam up to an elevation of 566 feet as the “Prado Basin Management Zone”.

This GSP refers to the dense wetland and riparian vegetation within that area as the Prado Wetlands. Surface water behind the dam is maintained at a specified elevation, which currently is 505 feet. The extent of wetland vegetation has increased from 1.8 square miles (mi2
) in 1960 to about 6.8 mi2 today, with most of the increase occurring prior to 1985 (WEI 2020). Approximately 1.4 mi2 of the total is within the Basin.

Evapotranspiration is higher in wetland and riparian vegetation areas where plant roots can access the water table than in areas where the water table is too deep to be accessed by roots. Thus, maps of remotely-sensed evapotranspiration (ET) show where the water table is shallow and being utilized by plants. Color-coded ET maps based on spectral analysis of Landsat imagery are available annually, and the maps from 1986 through 2016 consistently show a very sharp and stable boundary between high- and low-ET regions defining the southern edge of the Prado Wetlands (Ballau 2018). Temescal Basin along the lower reach of Temescal Wash between the wetlands and downtown Corona did not exhibit high ET between 1986 and 2016. This further confirms that the assumption of shallow groundwater in that region based on perennial flow in the channel was erroneous, as discussed earlier.

A systematic comparison of factors potentially related to groundwater levels in the Prado Wetlands was completed for this GSP. The Prado Basin Habitat Sustainability Program includes monitoring of groundwater levels and quality in scores of wells in and north of the Prado Wetlands, including 18 monitoring wells constructed specifically to detect changes in the shallow water table elevation within the wetlands (WEI 2020). It was found that wetland vegetation and riparian vegetation along the lower reaches of two north-side tributary creeks were associated with depths to water of 15 feet or less. Figure 4-22 compares water levels in shallow wells in Prado Wetlands with water levels to the north and south of the wetlands. For the wells in and to the north of the wetlands, the wells show the maximum depth to water between 2010 and 2020. In the central part of the wetlands, the maximum depth to water was 13 feet or less at all wells, which is shallow enough to be accessible to roots of established riparian vegetation. The most common species mapped in that area is red willow. In contrast, the minimum depths to water in wells in Temescal Basin south of the wetlands were all greater than 40 feet (beyond the reach of vegetation roots) except for the well closest to the wetlands, where it was 23 feet (within the possible rooting depth range of some riparian tree species). Those water levels are from water supply wells, which are relatively deep. To check for the possible presence of a shallow aquifer with higher water levels, data for shallow monitoring wells at groundwater contamination sites were obtained from the SWRCB Geotracker database and reviewed (SWRCB 2021). GeoTracker has

Temescal Basin GSP 4-20
information for six sites in the central part of Corona, and in all cases the depths to water were 42 to 172 feet, which is roughly consistent with the water supply wells and beyond the reach of vegetation roots. Thus, somewhere near the southern edge of the Prado Wetlands there is an abrupt transition to deeper water levels.

Various factors that could potentially affect shallow groundwater levels in the Prado Wetlands were evaluated for this GSP by comparing their variations over time between 2000 and 2019. Figure 4-23 shows depth to water hydrographs for four Prado Wetland wells, annual pumping at the Chino Basin desalter wells and in all wells in the Temescal Basin, water levels in several wells in the Temescal Basin, annual discharge in the Santa Ana River below Prado Dam and annual precipitation in Riverside. The 2012 through 2016 period is highlighted in the figure for discussion purposes. The Prado wells all show water-level declines from 2012 to 2015 followed by a rise in 2016. If groundwater pumping caused the declines during 2012 to 2015, it would have been above average during that period.

However, pumping at the Chino desalter wells (locations shown in Figure 4-22) was relatively constant during that period, and Temescal Basin pumping actually declined.

Furthermore, the large step increase in desalter pumping from 2005 to 2007 was not associated with a corresponding decrease in Prado Wetland groundwater levels.

Groundwater elevation trends in the Temescal Basin also show no correlation with shallow groundwater levels in the Prado Wetlands. The most common trend was a steady decline of 10 to 20 feet during 2012 through 2016. This is counterintuitive given the decrease in Temescal Basin pumping during that period. It suggests that sources of recharge—primarily percolation from Temescal Wash and stormwater retention basins on other streams— decreased during the drought by a total amount greater than the decrease in pumping.

A variable that does correlate with Prado Wetland groundwater levels is annual discharge in the Santa Ana River at the gage below Prado Dam. The flow at that location is a direct measure of the amount of surface water flowing through the wetlands. Annual discharge declined during 2011 through 2015, increased slightly in 2016 and even more in 2017.

Median and average annual discharge are shown in the figure and exhibit similar patterns.

Median discharge emphasizes moderate, steady flows such as discharges from wastewater treatment plants. Average discharge also includes the effects of runoff during large storm events. In wetter years such as 2011 and 2017 the average flow is considerably larger than the median flow. Annual precipitation at Riverside also correlates with the Prado groundwater trends. Precipitation was high in 2010 through 2011 and 2017 and low during 2012 through 2016.

The correlation of precipitation and river flow with Prado groundwater levels and the lack of correlation with groundwater pumping north and south of the wetlands indicates that the wetlands are primarily sustained by surface inflows.

Another evaluation of factors potentially correlated with changes in NDVI in the Prado Wetlands was presented in the 2019 annual report of the Prado Basin Habitat Sustainability Committee (WEI 2020). Using a spatially and temporally detailed statistical analysis of trends in time series plots for 1984 through 2019, no correlation was found between NDVI and groundwater levels. However, in some years changes in NDVI correlated with annual

Temescal Basin GSP 4-21
precipitation, growing-season average maximum and minimum temperatures, wastewater discharges, vegetation management activities or wildfires. These results are consistent with those presented above and support a conclusion that the wetlands are now primarily supported by surface inflows including storm runoff and reclaimed water discharges.

Prado Dam operation also strongly influences water availability in the wetlands because the impounded pool of water is more perennial than it would be in a natural condition.

Modeling completed for other studies projected large water-level declines in the Temescal Basin, with an implication that groundwater is being over-exploited and potentially impacting Prado Wetlands. However, actual water-level data from Temescal Basin show that the simulated declines are incorrect. The simulated declines were first presented in the Prado Basin Adaptive Management Plan (WEI 2016; see Figure 1-4 of that report). The same results were presented again in the 2019 Annual Report of the Prado Basin Habitat Sustainability Committee (WEI 2020; see Figure 1-3). The simulations indicated 20 feet of cumulative water level decline from 2005 to 2030 near the southern edge of the Prado Wetlands, increasing to 60 feet near downtown Corona. In reality, water levels in the Temescal Basin have shown no net increase or decrease from 2005 to 2020 (see, for example, Figure 4-23). It is possible that the groundwater model used for those simulations included the southward propagation of drawdown from the Chino desalter wells but did not fully include recharge from Temescal Wash and small streams in the Temescal Basin.

The low importance of groundwater as a factor in managing Prado Wetlands is also implicit in the Upper Santa Ana River Habitat Conservation Plan (HCP) (ICF 2020). Groundwater modeling completed for the HCP projected declining groundwater levels for the region surrounding the Prado Wetlands. However, none of the management actions in the HCP target groundwater pumping or levels beyond simply monitoring them. The actions focus on establishing an HCP Preserve, enhancing channel morphology, substrate, and in a few places flow to improve habitat quality. The thirty-three “avoidance and mitigation measures” listed in the HCP deal exclusively with land and vegetation disturbance and related construction activities. Most of the proposed actions are along the mainstem of the Santa Ana River. The surface hydrology model, for example, extended only about 2 miles up Temescal Wash—a reach that is concrete-lined, has no habitat value and is not connected to groundwater.

Small wetlands might be present outside of the Prado Wetlands. The Nature Conservancy NCCAG mapping includes a wetland layer separate from the riparian vegetation layer. It indicates the locations of possible wetlands outside of the Prado Wetlands area. Along Temescal Wash, the largest mapped polygons are within the riparian vegetation polygons between Minnesota Road and Temescal Wash Lake. Additional small polygons are shown along the shore of the lake. As stated earlier, water levels in wells are thought to be far below the creekbed and lake at that location, which would indicate that the wetlands and lake are sustained by surface discharges, not groundwater. The mapping also shows two strips of wetland in the channelized reach of the Wash downstream of the lake. The channel has a cement bottom, so wetlands are not likely present (i.e., there is a mapping error). The mapping shows no off-channel wetlands in the Basin, which is not surprising given its largely urban land cover.

Temescal Basin GSP 4-22
The Western Riverside County Multiple Species Habitat Conservation Plan (MSHCP) was reviewed for additional information regarding plant species that might be affected by groundwater (RCRCA 2020). Two large regions mapped as narrow endemic plants and criteria area species partially overlap the Basin. However, those categories together contain 16 upland plant species that are unaffected by groundwater.

4.10.5. Animals and Interconnected Surface Water
Animals that depend on groundwater include fish and other aquatic organisms that rely on groundwater-supported stream flow, amphibious or terrestrial animals that lay their eggs in water and birds that inhabit riparian vegetation. Management of habitat for animals typically focuses on species that are listed as threatened or endangered under the state or federal Endangered Species Acts. That convention is followed here. Flow in Temescal Wash is too ephemeral to support migration of anadromous fish, although the population of rainbow trout in Coldwater Canyon Creek above the Bedford-Coldwater Subbasin is thought to be the remnant of a steelhead trout population present as recently as the 1930s (Russell 2020). No native fish species presently inhabit Temescal Wash. Resident fish are nonnatives such as bass, bullhead, sunfish, and carp. Arroyo chub (Gila orcuttii) is a native fish listed as a species of special concern that is present in Prado Wetlands and could potentially inhabit some reaches of Temescal Wash. The Riverside-Corona Resource Conservation District implemented the Temescal Wash Native Fish Restoration Project during 2007 through 2009.

The focus of that effort was on eradication of nonnative plants and animals, particularly arroyo chub predators. Modifying flow conditions was not part of the project. No habitat areas for arroyo toad or red-legged frog are mapped within the Basin.

The Upper Santa Ana River HCP documents historical sightings and current potentially suitable habitat for a number of listed species, including six at various locations along Temescal Wash between Lake Elsinore and the Prado Wetlands: Arroyo chub, California glossy snake (Arizona elegans occidentalis), southwestern pond turtle (Emys pallida), yellowbreasted chat (Icteria virens), least Bell’s vireo (Vireo bellii pusillus), and southwestern willow flycatcher (Empidonax traillii extimus). In all cases there were either a number of historical sightings but little suitable habitat or vice versa. Apparently, habitat restoration opportunities are richer along the Santa Ana River than along Temescal Wash, and this led to the HCP’s focus on the former.

Two bird species that inhabit the Prado Wetlands are federally listed as endangered: least Bell’s vireo and southwestern willow flycatcher. Critical habitat areas have been delineated by the U.S. Fish and Wildlife Service for many listed species. Critical habitat maps for three species are shown in Figure 4-21. Critical habitat for the least Bell’s vireo and southwestern willow flycatcher in the Temescal Basin region more or less coincide with the extent of the Prado Wetlands. Critical habitat for the coastal California gnatcatcher (polioptila californica californica) includes areas on the eastern slopes of the Santa Ana Mountains that very slightly overlap the western edge of the Temescal Basin. The only vegetation in those areas that might utilize groundwater would be along tributary streams, where a small amount of base flow is sustained by groundwater discharging at a low rate from fractured bedrock.

That discharge would not be affected by pumping and water levels in the Basin, so Basin

Temescal Basin GSP 4-23
management would not impact the extent or health of vegetation along streams in the mapped habitat areas.

In summary, groundwater management is unlikely to impact habitat in the Prado Wetlands and along Temescal Wash with possible minor exceptions where Temescal Wash first enters the Basin from the bedrock reach and along the southern edge of the Prado Wetlands.

Additional data are needed regarding the presence of a shallow water table in those locations to reach a more definitive conclusion.

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Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 4 - GW Conditions\Figure 4-10 Groundwater Elevation Contours Fall 2015.mxd Well Monitored Fall 2015
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Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 4 - GW Conditions\Figure 4-11 Groundwater Elevation Contours Spring 2017.mxd Well Monitored Spring 2017
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Figure 4-12
Concepts of
Land Subsidence
Source: Galloway et al., 1999.

After LSCE et al., 2014.

Area of
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https://gis.water.ca.gov/arcgisimg/rest/services/SAR/Vertical_Displacement_TRE_ALTAMIRA_ v2019_Total_Since_20150613_20190919/ImageServer

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Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 4 - GW Conditions\Figure 4-17 Regional surface water.mxd WRF Water Reclamation Facility
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11042700 MURRIETA C NR MURRIETA CA 30
11042800 WARM SPRINGS C NR MURRIETA CA 55.5
11070500 SAN JACINTO R NR ELSINORE CA 723
11072100 TEMESCAL C AB MAIN ST A CORONA CA 224
11071760 COLDWATER CANYON C NR CORONA CA 4.2
11071900 TEMESCAL C A CORONA LK NR CORONA CA 57.9
11074000 SANTA ANA R BL PRADO DAM CA 1,490

Path: C:\Projects\Corona GSP 46414\GRAPHICS\Figure 4-18.gpj Figure 4-18
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Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 4 - GW Conditions\Figure 4-20 Depth to Water.mxd WRF Water Reclamation Facility
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Path: \\todd-file\Data\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 4 - GW Conditions\Figure 4-21 Critical Habitat Areas.mxd Dense Riparian Vegetation
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Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 4 - GW Conditions\Figure 4-22 Depth to Water Near Prado.mxd "/ Chino Desalter Wells
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Temescal Basin GSP 5-1

WATER BUDGET
A water balance (or water budget) is a quantitative tabulation of all inflows, outflows, and storage change of a hydrologic system. The Sustainable Groundwater Management Act (SGMA) requires that water balances be prepared for the groundwater system and surface water system of a basin. If a basin contains multiple management areas, separate balances must be developed for each of them. Management areas have not been defined for the Temescal Subbasin (Basin). Furthermore, water budgets must be developed for time periods representing historical, current, future no project (baseline), and future growth plus climate change (growth plus climate change) conditions.

This chapter presents the basis for selecting the water budget analysis periods for the Basin, describes modeling tools used to estimate some water budget items, and presents the surface water and groundwater budgets.

5.1. WATER BUDGET METHODOLOGY
Annual balances were developed for water years 1990 through 2018, the period simulated by the numerical groundwater model. The model is described in Appendix J and provides estimates for several items in the water balance for which direct measurements are not available: flows between groundwater and surface water bodies, flows to and from adjacent basins, evapotranspiration of riparian vegetation, and storage change. The numerical model allows a dynamic and comprehensive quantification of the water balance wherein all estimated water balance elements fit together and are calibrated to groundwater level changes over time. Accordingly, the numerical model is the best tool to quantify those water balance items. It will be updated regularly through the Groundwater Sustainability Plan (GSP) process, providing a better understanding of the surface water-groundwater system and a tool to evaluate future conditions and management actions.

5.2. DRY AND WET PERIODS
Dry and wet periods in historical hydrology can be identified on the basis of individual years or sequences of dry and wet years. GSP Regulations require that each year during the water budget analysis period be assigned a water year type, which is a classification based on the amount of annual precipitation. Figure 5-1 shows annual precipitation at Elsinore (National Oceanic and Atmospheric Administration (NOAA) Station GHCND:USC00042805) for water years 1899 through 2020. Water year types are also indicated and are assigned to five categories corresponding to quintiles of annual precipitation. The categories used here (dry, below normal, normal, above normal, and wet) accurately describe the quintiles but differ from the categories commonly used in the Central Valley (critical, dry, below normal, above normal, and wet). Those categories do not accurately describe quintiles and are based on the Sacramento River Index, which has little relevance to conditions in the Basin. The quintile divisions for precipitation during 1899 to 2020 at the Lake Elsinore station are shown in Table 5-1.

Temescal Basin GSP 5-2
Table 5-1. Water Year Type Classification
Water Year Type Range as Percent of Mean
Precipitation Range
(inches)
Wet W >139 > 16.5
Above Normal AN 101 to 139 12.0 to 16.5
Normal N 75 to 101 8.9 to 12.0
Below Normal BN 56 to 75 6.6 to 8.9
Dry D <56 < 6.6
Average precipitation for 1899 to 2020 was 11.89inches per year Individual wet and dry years are not particularly useful for groundwater management in basins where groundwater storage greatly exceeds annual pumping and recharge, which is the case in the Basin. In those basins, multi-year droughts and sequences of wet years are more relevant, because they relate to the amount of operable groundwater storage needed to support sustainable groundwater management. Multi-year wet and dry periods can be identified from a plot of cumulative departure of annual precipitation, which is also shown on Figure 5-1. Wet periods appear as upward-trending segments of the cumulative departure curve, and droughts appear as declining segments. By far the largest climatic deviations in this record were the sustained wet conditions from 1937 to 1944 and dry conditions from 1946 to 1965. These events pre-dated the most recent 30 years, which is the period the California Department of Water Resources (DWR) states should be used for determining year types (DWR 2016c). They also pre-date the period simulated by the groundwater model. However, large wet and dry events like those could recur in the future, and it is prudent to consider climate uncertainty in planning for groundwater sustainability.

5.3. WATER BALANCE ANALYSIS PERIODS
GSP regulations require evaluation of the water balances over historical, current, and future periods. The historical period must include at least 10 years, and the future period must include exactly 50 years. The duration of the current period is not specified, but to be consistent with SGMA concepts it needs to include several years around 2015, which was the implementation date of SGMA. Historical and current analysis periods for the Basin were selected from within the 1990 through 2018 modeling period. Ideally, each period is characterized by average precipitation and relatively constant land and water use. In the Basin, urbanization increase has been gradual throughout the 1990 to 2018 period. The historical period is represented by water years 1993 through 2007, and the current period by water years 2010 to 2013. Those periods had 101 percent and 102 percent of the 1899 to 2020 average annual rainfall, respectively.

Temescal Basin GSP 5-3
The future period is intended to represent conditions expected to occur over the next 50 years. The model simulation period is only 29 years (1990 to 2018). To obtain a 50-year period, simulations of future conditions used the 1993 through 2017 sequence of rainfall and natural stream flow repeated twice. Average annual precipitation during 1993 to 2017 was 94 percent of the long-term average. For the baseline scenario, no adjustments were made to the hydrologic sequence. Adjustments made to simulate future climate change are described in Section 5.5.3.

5.4. MANAGEMENT AREAS
As defined in the GSP regulations, a Management Area (MA) is an area within a basin for which the GSP may identify different minimum thresholds, measurable objectives, monitoring, or projects and management actions based on differences in water use sector, water source type, geology, aquifer characteristics, or other factors. The Channel Aquifer area is more permeable than the alluvial fan aquifer areas, and it is where almost all groundwater pumping now occurs. However, there is no reason that monitoring, sustainability criteria, and management actions need to be different for the Channel Aquifer and alluvial fan aquifer areas. Accordingly, the Channel Aquifer area is not designated as a management area, and the Temescal Basin is managed as a whole.

5.5. METHODS OF ANALYSIS
Complete, itemized surface water, and groundwater balances were estimated by combining raw data (rainfall, stream flow, municipal pumping, and wastewater percolation from septic tanks and wastewater treatment plant discharge) with values simulated using models2 .

Collectively, the models simulate the entire hydrologic system, but each model or model module focuses on part of the system, as described below. In general, the models were used to estimate flows in the surface water and groundwater balances that are difficult to measure directly or that relate to time-dependent groundwater levels. These include surface and subsurface inflows from tributary areas, percolation from stream reaches within the Basin, groundwater discharge to streams, potential subsurface flow to and from neighboring basins, the locations and discharges of pumping wells, consumptive use of groundwater by riparian vegetation, and changes in groundwater storage. Descriptions of the inflows and outflows to the surface water and groundwater models are included below in Sections 5.6 and 5.7.

5.5.1. Rainfall-Runoff-Recharge Model
This Fortran-based model developed over a number of years by Todd Groundwater staff simulates hydrologic processes that occur over the entire land surface, including 2 Water balance values are shown to nearest acre-foot to retain small items, but entries are probably accurate to only two significant digits.

Temescal Basin GSP 5-4
precipitation, interception3, infiltration, runoff, evapotranspiration, irrigation, effects of impervious surfaces, pipe leaks in urban areas, deep percolation below the root zone, and shallow groundwater flow to streams and deep recharge. The model simulates these processes on a daily time step for 286 recharge zones delineated to reflect differences in physical characteristics as well as basin and jurisdictional boundaries. Simulation of watershed areas outside the Basin are included to provide estimates of stream flow and subsurface flow entering the Basin. Daily simulation results were subtotaled to monthly values for input to the groundwater model. Additional details regarding the rainfall-runoffrecharge model can be found in Appendix J and the model code is available on request.

5.5.2. Groundwater Model
A numerical groundwater flow model of the Basin was completed in 2008 for the Groundwater Management Plan (Todd and AKM 2008). For this GSP, the model was revised, expanded to include the entire Basin, updated with new geological information, and updated through water year 2018.

The revised and updated model uses the MODFLOW 2005 code developed by the U.S.

Geological Survey that is a public domain open-source software as required by GSP regulation §352.4(f)(3). The model produces linked simulation of surface water and groundwater, as described below. Additional documentation of the model update and calibration is provided in Appendix J.

5.5.2.1. Surface Water Module
Stream flow in MODFLOW is simulated using the Streamflow Routing Package (SFR) where a network of stream segments represents the small streams entering the Basin from Temescal Wash and tributary watersheds.

Surface water inflows to Temescal Wash were obtained from a similar groundwater flow model of the Bedford-Coldwater Subbasin. Small stream inflows were estimated using the rainfall-runoff-recharge model. Each stream segment is divided into reaches, one per model grid cell traversed by the segment. Flow is routed down each segment from reach to reach.

Along each reach mass balance is conserved in the stream, including inflow from the upstream reach and tributaries, inflow from local runoff, head-dependent flow across the stream bed to or from groundwater, evapotranspiration losses, and outflow to the next downstream reach. Flow across the stream bed is a function of the wetted channel length and width, the bed permeability and the difference in elevation between the stream surface and groundwater at the reach cell. Wetted width and depth of the stream are functions of stream flow.

5.5.2.2. Groundwater Module
The MODFLOW groundwater model is constructed to cover the entire Basin. The model grid size is oriented north-south and has a uniform 100 feet (ft) horizontal grid spacing to 3 Interception refers to precipitation that does not reach the soil, but instead falls on (and is intercepted by) plant leaves, branches, and plant litter, and is subject to evaporation loss.

Temescal Basin GSP 5-5
provide sufficient resolution to resolve hydraulic gradients, well drawdown cones, and groundwater-surface water interactions in the Basin.

The model covers the entire Basin as delineated by DWR and also the southern part of the Chino Basin. Including part of the Chino Basin improves the ability of the model to simulate groundwater conditions beneath Prado Basin.

The numerical model has been constructed to reflect the hydrogeological conceptual model developed for the GSP, Chapter 3. The vertical extent of the Basin is based on the mapped depth to consolidated rock. The elevation of surface features and streambed elevations have been derived from geographic information system (GIS) files developed from the local topography and stream information.

Citrus orchards irrigated with groundwater were common in the Basin in the early 1990s, but except for one small grove those have all been replaced by urban development.

Agricultural irrigation pumping of the orchards was estimated by the rainfall-runoffrecharge model, with pumping assigned to a hypothetical irrigation well at the center of each irrigated recharge zone. This pumping was phased out over time as urban development occurred. Urban irrigation is supplied by the municipal water system, which uses imported water and local wells. Municipal well extractions are known and are entered directly into the model. All major pumpers in the Basin report their annual production to Western Municipal Water District (WMWD), which was the source of data for several nonmunicipal pumping wells. Pumping at private domestic wells is not reported and there is currently no private domestic groundwater use in the Basin so none is included in the model.

5.5.3. Simulation of Future Conditions
GSP regulations §354.18(c)(3) require simulation of three future scenarios to determine their effects on water balances, yield and sustainability indicators. The growth and climate change scenarios were combined, resulting in the following two scenarios:

Baseline. This represents a continuation of existing land and water use patterns, imported water availability, and climate.

Growth Plus Climate Change. This scenario implements anticipated changes in land use and associated water use, such as urban expansion, and anticipated effects of future climate change on local hydrology (rainfall recharge and stream percolation) and on the availability of imported water supplies.

Both of the future simulations assume that the level of development and related water demand are constant throughout the simulation. That is, development in the growth plus climate change simulation is not phased in over time but rather corresponds to 2068 development throughout the simulation. This is the best way to demonstrate whether 2068 land use is sustainable because it allows for assessment of the effects of variations in climatic conditions (wet and dry cycles) on groundwater conditions, avoids subjective decisions about the concurrent timing of droughts and development, and provides time for the full effect of future conditions on groundwater to become apparent.

Temescal Basin GSP 5-6

5.5.3.1. Baseline Scenario
The baseline simulation is a 50-year period, as required by SGMA regulations, with water budget components developed using the criteria and assumptions described below. Initial water levels are simulated water levels for September 2018 from the historical calibration simulation. That year represents relatively recent, non-drought conditions. These simulated water levels are internally consistent throughout the model flow domain and reasonably matched measured water levels at wells with available data (see Appendix J for discussion of model calibration).

Surface water and other inflows came from multiple sources. Monthly inflows in Temescal Wash were obtained from the baseline and growth plus climate change simulations produced by the Bedford-Coldwater Subbasin groundwater model (Todd, H&H, and Stantec 2021), which was used to develop the GSP for that subbasin. Small stream and bedrock inflows simulated for 1993 to 2017 of the calibration model period were repeated twice to obtain 50 years of data.

In the baseline scenario, land use remains the same as the current conditions. In the model, land use is represented by 2014 land use mapped by remote sensing methods and obtained from DWR (2017), adjusted for subsequent urbanization identified in Google Earth imagery (Google Earth 2021).

Municipal, commercial, and industrial (M&I) pumping was set equal to the estimated sustainable yield. M&I pumping was relatively high during 2002 to 2014 and exceeded the sustainable yield, as evidenced by the steady declines in groundwater storage during that period. Using the groundwater model, City of Corona (Corona) pumping (which represents 97 percent of the M&I total) was decreased until the future baseline scenario no longer produced long-term storage declines. The adjusted M&I pumping equaled 98 percent of the 2010 to 2018 average, or 15,615 acre-feet per year (AFY). Total municipal use was assumed to equal the 2010 to 2018 average. This reflects an assumption that the amounts of imported water are adjusted to make up the difference between total water demand and sustainable groundwater yield. In the groundwater model, total municipal water use was used only to estimate pipe leaks.

The Baseline scenario also assumes that wastewater percolation and recycling continue as they have in recent years. Discharges from Water Reclamation Facility 1 (WRF-1) and WRF-2 to percolation ponds, streams and recycled uses were estimated as the average amounts during 2010 to 2018.

5.5.3.2. Growth Plus Climate Change Scenario
The growth plus climate change scenario incorporated anticipated effects of climate change, urban development, and associated changes in water and wastewater management. In this scenario, rainfall and reference evapotranspiration (ET0) were adjusted to 2070 conditions using monthly multipliers developed by DWR based on climate modeling studies. The multipliers were applied to historical monthly data for the 1993 to 2017 hydrologic period used in the model. DWR prepared a unique set of multipliers for each four square kilometer (km2
) cell of a grid covering the entire state. Nine climate grid cells overlie the Basin and its tributary watershed areas. For each recharge analysis polygon in the rainfall-runoff-recharge

Temescal Basin GSP 5-7
model, multipliers from the nearest climate grid cell were used. The climate in 2070 is expected to be drier and warmer than it presently is.

Figure 5-2 compares average monthly precipitation and ET0 before and after applying the climate change multipliers. Simulations of irrigated turf in the rainfall-runoff-recharge model indicated that the combined effect of the warmer and drier climate will increase annual irrigation demand by about 10 percent.

In the growth plus climate change scenario, bedrock inflow and surface inflow from tributary streams along the perimeter of the Basin were re-simulated using the rainfallrunoff-recharge model to reflect the effects of urban development in some of the tributary watersheds and of climate change. Urbanization also increased surface runoff within the Basin, which was routed to small streams and Temescal Wash.

For inflows from Temescal Wash, Cucamonga Creek and Chino Creek (which were not simulated using the rainfall-runoff-recharge model) and future baseline flows were adjusted to 2070 conditions using DWR streamflow multipliers. The DWR data set ends in 2011.

Multipliers for 1987 to 1992 were used for 2012 to 2017 based on similarity of cumulative departure of precipitation for the two periods. Then 1992 to 2017 adjusted stream flows were used twice in succession to simulate 2019 to 2068. Surface discharges from the WRCRWA reclaimed water facility were from future projections developed during planning studies for that facility.

Land use in 2018 is shown in Figure 5-3. Land use maps for 1990, 2018 and 2068 were developed on the basis of Riverside County digital crop maps (1993 and 2000), Google Earth historical imagery (Google Earth 2021), a 2014 statewide crop map developed by DWR (DWR 2017), Corona General Plan 2020 to 2040 (Corona 2021), and Corona’s 2020 Urban Water Management Plan (Michael Baker 2021). Corona was one of the fastest growing cities in the United States during the past several decades. From 1990 to 2018, the dominant land use change was conversion of citrus groves and natural grassland to residential use. Table 5- 2 lists the acreages of several categories of land use in the Basin and tributary watersheds in 1990, 2018 and 2068.

The rate of growth is expected to slow considerably during the next few decades. Within the current Corona city limits, population is expected to increase by 11 percent between 2020 and 2040, and commercial/industrial building space by 18 percent. The Urban Water Management Plan (UWMP) directs more growth to its sphere of influence areas outside the Corona city limits, with a projected 55 percent increase in population and 490 percent increase in commercial/industrial building space. Redevelopment within Corona will have minor effects on groundwater recharge, but development in the sphere of influence areas will have a major effect.

Land use is held constant at the 2068 level of development throughout the 50-year simulation period. This approach avoids errors that can arise from the assumed timing of future droughts and provides a long hydrologic analysis period for assessing the sustainability of 2068 land and water use conditions.

Temescal Basin GSP 5-8
The 2020 draft UWMP (Michael Baker 2021) anticipates a steady decline in per-capita water use from 180 gallons per-capita per day (gpcd) in 2020 to 155 gpcd in 2045. This is plausible but possibly optimistic given that per-capita use has not been declining in recent years and rebounded slightly from drought-related decreases achieved during 2015 to 2016. It was conservatively assumed here that per-capita water use would not continue to decline during 2045 to 2068.

Combining the estimates of population and per-capita water use for 2068, total municipal water use in 2068 would be 34,490 AFY, or essentially the same as the 2010 to 2018 average (about 1 percent higher).

Pipe leaks were assumed to remain at the existing percentage of total water use in Corona and Norco. Municipal groundwater pumping was assumed to remain at the sustainable yield level, which was tentatively estimated to equal average production during 2010 to 2018.

Percolation of reclaimed water at WRF-2 was assumed to remain at the average for 2010 to 2018. This assumes that future decreases in per-capita water use will be achieved primarily through reductions in landscape irrigation. It also implies that future increases in wastewater generation due to population growth will be partially offset by increased indoor water conservation, and any remaining increase will become recycled water for irrigation.

Flow across the northern model boundary that cuts through the Chino Basin was set to zero, consistent with the mandated objective of hydraulic control. Hydraulic control is the elimination of groundwater discharge from the Chino Basin to the Prado Wetlands and Santa Ana River, achieved by pumping from a line of desalter wells located roughly parallel to and 2 to 4 miles north of the Santa Ana River. The objective of hydraulic control was included in the 2004 update of the Santa Ana River Basin Plan (SWRCB 2020a). Hydraulic control is considered necessary to maximize the safe yield and to prevent degraded groundwater from discharging from the Chino Basin to the Santa Ana River and impacting downstream beneficial uses (WEI 2005 and 2019).

Subsurface inflow from the Arlington Basin through Arlington Gap was assumed to be zero, consistent with long-term declining trends and modeling of future conditions in the Arlington Basin (Shaw 2020).

Table 5-2. Temescal Basini Land Use in 1990, 2018, and 2068 (acres) Channel Aquifer Alluvial Fan Aquifder Prado/Chino Area Tributary Watersheds 1990 2018 2068 1990 2018 2068 1990 2018 2068 1990 2018 2068 Citrus 0 0 0 2,997 0 0 0 0 0 29 0 0
Truck crops 0 0 0 0 0 0 93 93 93 0 0 0
Pasture 0 0 0 0 0 0 379 379 379 0 0 0
Non-irrigated grain 0 0 0 0 0 0 2,499 1,176 1,176 0 0 0 Grassland 47 86 86 193 190 190 406 406 406 72 72 72
Shrubs/Trees 782 782 782 0 0 0 3,719 3,719 3,719 0 0 0
Dense riparian 0 0 0 0 0 0 0 0 0 0 0 0
Sparse riparian 0 0 0 499 1,036 1,734 0 0 0 234 1,097 2,414 Open water 799 799 799 6,425 10,956 10,867 1,389 2,833 2,833 704 2,704 2,704 Low-density residential 37 103 103 100 231 231 25 25 25 121 247 247 Residential 1,138 2,431 1,978 434 2,987 2,718 0 174 174 204 538 538 Turf 98 98 573 5 52 717 0 0 0 0 0 1,105
Commercial 1 1 1 219 121 0 0 0 0 142 646 368
Industrial 11 11 11 884 332 166 0 0 0 0 91 0
Quarry 0 0 0 0 0 0 0 0 0 0 0 0
Vacant 0 0 0 0 0 0 0 0 0 0 0 0
Land Use
T:\Projects\Corona GSP 46414\Model\RCH\RechargeIn_Temescal.xlsx - RechargeZones Todd Groundwater Des by: GY

Temescal Basin GSP 5-10

5.6. SURFACE WATER BALANCE
This section describes and quantifies the water balance of creeks and rivers that cross the Basin. All significant inflows to and outflows from these surface water bodies are included in the water balance. The surface water balance shares two flows in common with the groundwater balance: 1) percolation from surface water to groundwater and 2) seepage of groundwater into surface water. Each of these is an outflow from one system and an inflow to the other. Key features of the surface water balances for each management area and analysis period are described below, followed by additional information about the methods used to quantify items in the water balances.

Historical annual surface water balances for the Temescal Basin during 1990 to 2018 are shown in Figure 5-4 (upper graph). Average annual surface water budgets for the model, historical, current, and future budget analysis periods are listed in Table 5-3 and detailed surface water budget tables are included in Appendix K. The largest inflows to the Temescal Basin are from Temescal Wash and tributary watersheds along the western and eastern edges of the Basin, and those occur predominantly in wet years. The only other surface flow of significance is the small but relatively steady discharge of reclaimed water from WRF-1 to Butterfield Drain, which enters Temescal Wash just upstream of the Prado Wetlands.

Outflow is almost entirely surface outflow from Temescal Wash to the Prado Wetlands, with some losses to percolation along unlined reaches of stream channels.

Surface flows in the Prado Wetlands and southern Chino Basin part of the groundwater model flow domain are generally steadier than surface flows in the Temescal Basin part of the model (Figure 5-4, middle graph). This is partly because the data shown in the graph are monthly flows used in the groundwater model, which may exclude some ephemeral high flow events. But in addition, flow in the Santa Ana River consists to a significant degree of discharges from wastewater treatment plants, which are relatively steady. Outflows from Prado Dam are also relatively steady because streamflow fluctuations upstream are absorbed to some extent by storage fluctuations in the wetlands.

A substantial amount of water has been imported into the Basin since before 1990. It is delivered directly to users and does not flow into streams or lakes. Use of imported water by Corona is shown in Figure 5-4 (bottom graph). Imported water consists of State Water Project (SWP) water purchased from the Metropolitan Water District of Southern California (Met) and delivered to Corona.

Table 5-3 Average Annual Surface Water Budgets
Inflow or Outflow
Historical
1993 to 2007
Current
2010 to 2013
Baseline1
2019 to 2068
Growth Plus
Climate Change1
2019 to 2068
Inflows
Temescal Wash 18,560 10,761 14,920 12,857
Tributary inflow 25,617 23,016 21,399 4,643
Wastewater discharges 3,644 2,761 2,895 2,895
Groundwater flow into streams 5,980 4,917 990 1,380
Total Inflows 53,801 41,455 40,206 21,776
Outflows
Stream percolation -10,046 -10,544 -1,661 -1,714
Surface outflows -44,001 -38,894 -38,544 -20,062
Total Outflows -54,048 -49,437 -40,206 -21,776
1 The 50-year future baseline simulation uses historical hydrology for 1993 to 2017 two times in succession. \\todd-file\data\Projects\Corona GSP 46414\Data\Water Budget Data\File ]Tables for GSP text Todd Groundwater Des by: GY

Temescal Basin GSP 5-12

5.6.1. Inflows to Surface Water

5.6.1.1. Precipitation and Evaporation
Precipitation and evapotranspiration on the land surface are accounted for in the rainfallrunoff-recharge model. Those processes are not included in the surface water balances, which address only water in stream channels, lakes, and imported water. Precipitation and evaporation on the surface of creeks and rivers are invariably miniscule percentages of total stream flow and are not included in the water budget.

5.6.1.2. Tributary Inflows
Tributary inflows to the Basin are from Temescal Wash and tributary watersheds along the east and west sides of the Basin. Temescal Wash inflows were obtained from the BedfordColdwater Subbasin groundwater model. Surface inflows from seven Santa Ana Mountain watersheds and four watersheds along the east side of the Basin were calculated on a daily basis by the rainfall-runoff-recharge model. Daily flows could not simply be averaged over each month to produce inflows for the groundwater model because the model would then overestimate the amount of stream recharge. This error stems from the ephemeral occurrence of stream flow and the nonlinear relationship between stream flow and percolation. The error can best be illustrated by a hypothetical example in which daily stream flows during a month consist of one day of flow at 60 cubic feet per second (cfs) and zero flow the rest of the days. If the percolation capacity of the stream reach over the groundwater basin is 10 cfs, total percolation for the month would be 10 cfs for one day, or

19.83 acre-feet (AF). The 50 cfs that exceeded the percolation capacity would flow out to the Santa Ana River. If the daily flows were simply averaged over a 30-day month, the monthly flow would be 2 cfs. The groundwater model would calculate that all of that water would percolate because 2 cfs is less than the percolation capacity of the channel. This would result in 2 cfs of percolation over the course of 30 days, or 119 AF, during the month.

To minimize this error, daily flows entering the Basin from each tributary were clipped at the estimated percolation capacity of the unlined reach of channel overlying the Basin. That is, daily flows in excess of the estimated percolation capacity were assumed to flow out to the Santa Ana River. Averaging the clipped daily flows produced monthly flows realistically capable of percolating. Figure 5-5 compares average annual stream flow with and without clipping. The largest decreases were where large watersheds discharged into channels that are cement-lined along most of their length overlying the Basin, leaving only a short reach where percolation can occur.

5.6.1.3. Valley Floor Runoff
The rainfall-runoff-recharge model simulates runoff from valley floor areas, which include impervious surfaces in urban areas. Runoff from valley floor areas was added to flows in tributary streams or Temescal Wash at several locations.

5.6.1.4. Wastewater Discharges
The only discharge of reclaimed water to surface waterways in the Temescal Basin is the discharge from Corona WRF-1 plant to Butterfield Drain, which enters Temescal Wash at the

Temescal Basin GSP 5-13
southern edge of the Prado Wetlands. In 2012, the State Water Resources Control Board allowed Corona to decrease the discharge from 4.57 cfs to 2.25 cfs (1,625 AFY).

5.6.1.5. Groundwater Discharge to Streams
Groundwater can discharge into streams when the water table next to the stream is higher than the stream bed or the water level in the stream. The depth to groundwater is tens of feet in the southern part of the Temescal Basin, but the depth decreases to the north (see Figure 4-22). At the Butterfield well near the southern edge of the Prado Wetlands, depth to water was 18 to 35 feet during 2012 to 2018 (the period of record for that well). The only natural outflow path from the Basin is discharge to the Santa Ana River near Prado Dam.

Somewhere between the Butterfield Well and Prado Dam the depth to water presumably decreases to zero. No shallow wells are available in that region to confirm and monitor depth to water.

5.6.2. Outflows of Surface Water

5.6.2.1. Net Evaporation
Evaporation from streams is almost always a negligible fraction of total flow and is not explicitly itemized in the water budgets or simulated in the model.

5.6.2.2. Surface Water Percolation to Groundwater
The lower reaches of almost all streams entering the Temescal Basin are concrete-lined. The only opportunity for percolation is along the unlined reaches near the Basin margin (see Figure 4-17). The percolation capacities of the unlined reaches of tributary streams were estimated to be approximately 5 cfs per mile (cfs/mi). This would be the percolation rate along a creek channel with a wetted width of 16 feet and a bed permeability of 5 feet per day. Temescal Wash was estimated to have a percolation capacity of 20 cfs/mi along the 2- mile unlined reach where it enters the Basin, based on greater channel width and permeability. These capacities were applied to simulated daily flows to obtain a time series of flows capable of percolating. Those flows were averaged to monthly values and used in the groundwater model, which included adjustments for shallow depth to groundwater (relevant only near Prado). Based on these assumptions and calculations, percolation from streams contributes on the order of 10,000 AFY of recharge to the Basin, which is about 18 to 20 percent of total recharge on an average annual basis.

5.6.2.3. Surface Outflow from the Basin
Surface outflow from the Temescal Basin equals stream inflows minus percolation losses plus groundwater discharge to streams. Over periods of months or years, storage change of surface water is negligible in the absence of lakes or reservoirs. Surface outflow is by far the largest outflow, especially in wet years. The values in Table 5-3 understate the dominance of this outflow because the table excludes peak flows that were not passed to the groundwater model.

Temescal Basin GSP 5-14

5.7. GROUNDWATER BALANCE
Annual groundwater inflows and outflows for the Basin for the 1990 to 2018 model simulation period are shown as stacked bars in Figure 5-6. Inflows are stacked in the positive (upward) direction and outflows are stacked in the negative (downward) direction. A similar stacked-bar chart for the baseline simulation is shown in Figure 5-7 and for the growth plus climate change simulation in Figure 5-8. Average annual groundwater budgets for the Channel Aquifer area and the alluvial fan aquifer area during each of the water budget analysis periods are listed in Table 5-4. Detailed groundwater budget tables are included in Appendix K. Highlights of the water budgets are described below, followed by additional information on methods used to quantify each budget item.

Percolation from streams and percolation of reclaimed water have been the largest sources of recharge to the Basin, followed by rainfall recharge in non-irrigated areas. Percolation from streams varies substantially from year to year but averaged about 34 to 39 percent of total inflows in the historical and current scenarios. In the baseline and growth plus climate change scenario stream percolation represented a slightly smaller portion of inflows.

Percolation of reclaimed water was of a similar magnitude in the historical and future scenarios but became a larger percentage of total inflow because of decreases in other inflows. Inflows from irrigation deep percolation, bedrock inflow and pipe leaks were of similar magnitudes in the historical and current periods (7 to 9 percent of total inflows).

Because of urbanization in recent years, irrigation deep percolation and pipe leaks became larger percentages of total inflow in the future scenarios. Inflow from the Chino Basin is the smallest inflow, amounting to only 2 to 6 percent of total inflows in all scenarios.

Pumping by municipal wells increased during the historical simulation, increasing from 43 percent of total outflows in the historical period to 59 percent in the current period.

Although municipal pumping was smaller in the future simulations, it represented a larger percentage (71 percent) of total outflows because of decreases in other outflows. The next largest outflows were of roughly similar magnitudes: groundwater discharge to streams, riparian evapotranspiration (ET) and subsurface outflow to the Chino Basin. These each accounted for 10 to 19 percent of total outflows during the historical and current periods and slightly smaller percentages in the future simulations. Pumping at agricultural wells decreased rapidly in the 1990s, dwindling to negligible amounts in the current period.

The Basin water budgets were negative for the historical and current analysis periods, due to a variety of reasons and reflecting the different time periods. During 2000 to 2011, relatively high amounts of municipal groundwater pumping contributed to a gradual decrease in storage. Since 2011, the predominantly dry climatic conditions have resulted in reduced inflows and thus a decrease in storage. As documented in Section 4.1, water levels in wells located in the Channel Aquifer decreased slightly over the historical period. The observed water level decline was not significant and did not impact beneficial users of the Basin. However, relatively high estimated storativity in the Channel Aquifer and the slight water level decline resulted in a net negative change in storage over the time period.

Storage declines during the early years of the simulation may have resulted from incorrectly estimated initial water levels. Initial conditions in the model are user defined and act as

Temescal Basin GSP 5-15
boundary conditions, if initial groundwater levels are set too high at the start of the simulation, the model will reduce water levels to reach equilibrium. The result is a calculated change in storage that may be an artifact of the prescribed starting water levels.

Geographically distributed water level data is not available pre-1990 and the initial conditions are largely set by interpolating sparsely observed water levels. Additional improvements and sensitivity analysis could provide more information on the effect of the initial conditions and should be considered for the next GSP update.

The future baseline scenario is intended to represent a continuation of existing conditions.

For most of the budget items that are inputs to the model (for example, irrigation deep percolation, pipe leaks, reclaimed water percolation and pumping), average values during 2010 to 2018 were used. An exception was municipal pumping by Corona, which was relatively high during 2002 to 2014. Using the 2010 to 2018 average produced storage depletion in the future baseline simulation. While the numerical model simulates declines in storage over the historical period, groundwater level declines have been relatively small and undesirable results relative to groundwater levels or storage have not occurred in the Basin.

Corona’s objective is to pump within the sustainable yield of the Basin and to supply the remaining municipal water demand with imported and recycled water. This policy is reflected in the generally decreasing amounts of municipal pumping from 2008 to 2017.

Through iteration, using the groundwater model, municipal pumping of 15,600 AFY was found to produce essentially no long-term storage change in the future baseline simulation.

This equals 98 percent of average Corona pumping during 2010 to 2018 but is more than the amounts of Corona pumping in 2016 and 2017.

However, adaptive management and continued assessment of pumping volumes will be critical to maintaining sustainability as inflow to the Basin can vary widely based on hydrology and the model simulation is only a forecast of future conditions.

Growth and climate change had relatively small effects that tended to offset each other. The warmer, drier climatic conditions tended to decrease stream percolation and rainfall recharge. Urban growth—much of which is projected to be in tributary watershed areas— tended to increase recharge because of irrigation deep percolation, pipe leaks and percolation of runoff from disconnected impervious areas. Notably, total water use and percolation of reclaimed water were assumed not to change appreciably, consistent with assumptions in the Corona’s UWMP that population growth will be offset by decreases in per-capita water use. Consequently, individual inflows and outflows in the growth plus climate change scenario were identical to or very close to the values in the future baseline scenario.

Table 5-4. Average Annual Groundwater Budgets
Temescal Basin
SGMA Historical
1993 to 2007
SGMA Current
2010 to 2013
25-Year Historical
1993 to 2017
Baseline1
2019 to 2068
Growth Plus Climate
Change2
2019 to 2068
Groundwater Inflow
Percolation from streams 8,112 9,942 7,976 7,918 8,817
Bedrock inflow 1,024 952 980 1,084 1,314
Dispersed recharge: non-irrigated land 4,921 4,380 4,331 2,742 2,668 Dispersed recharge: irrigated land 2,042 1,680 1,892 3,172 3,253 Pipe leaks 2,585 2,520 2,560 2,151 2,174
Reclaimed water percolation 8,915 6,200 7,885 6,122 6,122 Inflow from Adjoining Basins 2,003 1,400 1,895 1,026 126 Total Inflow 29,601 27,075 27,520 24,213 24,473
Groundwater Outflow
Wells - M&I and domestic -13,631 -17,239 -14,668 -15,615 -15,615 Wells - agricultural -3,622 -1,386 -2,722 -22 -23
Groundwater discharge to streams -4,545 -1,295 -3,179 -1,739 -1,504 Riparian evapotranspiration -4,980 -3,922 -4,482 -4,538 -4,997 Outflow to Adjoining Basins -2,966 -2,085 -2,664 -2,364 -2,301 Total Outflow -29,744 -25,927 -27,714 -24,278 -24,439
Net Change in Storage
Inflows minus outflows -143 1,148 -194 -65 34
Notes:
1 : The 50-year future baseline simulation uses historical rainfall and evapotranspiration for 1993 to 2017 two times in succession. 2 : Future baseline rainfall and evapotranspiration are adjusted for climate change in this scenario.

Water Balance Items
T:\Projects\Corona GSP 46414\Data\Water Budget Data\GW_budgets_Temescal_calibrationH.xlsx Tables - final one zone Todd Groundwater Des by: GY

Temescal Basin GSP 5-17

5.7.1. Inflows to Groundwater
Inflows to the Temescal Basin groundwater flow system include dispersed recharge from rainfall and irrigation, percolation from streams, percolation of reclaimed water and subsurface inflow. The methods and data used to calculate each of these flows is described below.

5.7.1.1. Dispersed Recharge from Rainfall and Irrigation Dispersed recharge from rainfall and applied irrigation water is estimated by the rainfallrunoff-recharge model. The model simulates soil moisture storage in the root zone, with inflows from rainfall infiltration and irrigation, and outflows to evapotranspiration and deep percolation. Simulation is on a daily basis. In recharge zones with irrigated crops—which includes urban landscaping and agricultural irrigation (citrus)—irrigation is assumed to be applied when soil moisture falls below a certain threshold. When soil moisture exceeds the root zone storage capacity, the excess becomes deep percolation. Rainfall and irrigation water comingle in the root zone and in deep percolation. For the purposes of displaying an itemized water balance, the amount of deep percolation derived from irrigation is estimated as a percentage of the simulated irrigation quantity, and the remainder of the dispersed recharge is attributed to rainfall. Deep percolation of applied irrigation water (irrigation return flow) is generally similar from year to year, whereas rainfall percolation varies significantly on an annual basis. Because urban landscape irrigation increased while agricultural irrigation decreased during the simulation period, total recharge on irrigated lands decreased only slightly. Water pipe leaks were estimated as the percentage of unaccounted for water listed in the 2015 Corona UWMP, which was seven percent of delivered water (KWC Engineers 2016), distributed uniformly over areas of urban land use.

Sewer pipes convey only water used indoors, and their leak rate was assumed to be half of the leak rate for water pipes. For input to the groundwater model, the one-dimensional dispersed recharge rates are mapped onto model grid cells overlying each recharge polygon on an area-weighted average basis.

Figure 5-9 shows a map of average annual dispersed recharge during 1993 to 2007, which is a relatively long averaging period that includes a wide range of year types. Most dispersed recharge occurs during relatively wet years. Average annual recharge rates ranged from less than 0.3 to slightly over 12 inches per year (in/yr). Much of the southern half of the Temescal Basin converted from citrus orchards to residential development during that period. Recharge from agricultural irrigation was replaced by irrigation from landscape irrigation, pipe leaks and percolation of runoff from disconnected impervious surfaces. As a result, average annual dispersed recharge in that part of the Basin was similar to recharge in the northern part, which was urbanized throughout the simulation period. Dispersed recharge in tributary watersheds appears to be low because most of the deep percolation beneath the root zone was assumed to become stream base flow rather than deep recharge.

5.7.1.2. Percolation from Streams
Inflows to the stream network in the surface water module of the groundwater model include a combination of simulated runoff from tributary watersheds and valley floor areas

Temescal Basin GSP 5-18
obtained from the rainfall-runoff-recharge model and simulated inflows from adjacent groundwater models of the Bedford-Coldwater and Chino Basins.

The surface water module of the groundwater model simulates percolation reach by reach along each stream that crosses the Basin. The percolation rate is a function of stream bed permeability, wetted area, and the difference in elevation between the stream surface and the underlying water table. Along reaches with natural channel materials, a permeability of 5 feet per day (ft/day) was assumed. Most of the streams that cross the Temescal Basin are lined with concrete along much of their length. Those reaches were assigned a permeability of zero. The natural channel reaches of the tributary streams are probably tens of feet above the water table, which means they are not hydraulically connected to groundwater.

Percolation is a function of wetted area and permeability only.

Converting from daily to monthly analysis can introduce large errors in estimated percolation. If daily flows are averaged over a month, estimated stream flow appears to be much more moderate and perennial than the actual stream flow. The groundwater model would overestimate percolation in that case. To avoid this error, monthly flows were obtained from daily flows by first clipping the daily flows to values less than or equal to the estimated percolation capacity of the unlined reach of channel. Those flows were then be averaged to obtain monthly flow for input to the groundwater model.

The Santa Ana River, Cucamonga Creek and Chino Creek are not lined. Monthly inflows to those waterways were obtained from an existing groundwater model of the Chino Basin (WEI 2019).

5.7.1.3. Reclaimed Water Percolation
Reclaimed wastewater is currently percolated at the Lincoln Cota Percolation Ponds, which are adjacent to the left bank of Temescal Wash between Lincoln Avenue and Cota Street.

They are just outside the Channel Aquifer area. Prior to 1998, reclaimed water was also percolated in ponds at WRF-1 next to the Prado Wetlands. Since then, water from that facility has been sent to the Lincoln Cota ponds. Measured percolation volumes are added directly to the top layer of the groundwater model.

5.7.1.4. Subsurface Groundwater Inflow
Subsurface inflows from tributary watersheds and neighboring basins are estimated by various methods. Subsurface flow from tributary watersheds is calculated by the rainfallrunoff-recharge model by partitioning rainfall deep percolation into stream base flow and subsurface inflow. The subsurface flow is added as specified monthly volumes of water to model cells adjacent to the tributary watershed. Although the Bedford-Coldwater Basin and Temescal Basin share a boundary, models of both basins indicated little flow across it because groundwater tends to flow parallel to the boundary toward Temescal Wash. Based on previous studies, a small amount of subsurface inflow from the Arlington Basin through the Arlington Gap was included in historical simulations. That flow is expected to decrease to zero in the future. Flow between the Temescal and Chino Basins is simulated by the groundwater model as a function of water-level gradients and permeability along the boundary between the basins.

Temescal Basin GSP 5-19

5.7.2. Outflows from Groundwater
Major outflows from the water budget analysis areas are groundwater pumping (municipal, industrial, and agricultural), subsurface outflow, groundwater discharge into streams, and evapotranspiration by riparian vegetation.

5.7.2.1. Pumping by Wells
Pumping from M&I wells has been measured and recorded for many years by Corona and WMWD. Those data are used in the groundwater model. Agricultural pumping to irrigate citrus orchards in the 1990s was estimated using the rainfall-runoff-recharge model, which produces estimates of irrigation demand based on reference evapotranspiration, crop type and growth state, and availability of soil moisture from rainfall. Ten percent of applied irrigation water was assumed to percolate past the root zone and return to the groundwater supply. As described in Section 5.7, Corona pumping for the future baseline and growth plus climate change scenarios was set at 98 percent of the 2010 to 2018 average historical pumping.

5.7.2.2. Subsurface Outflow
Subsurface outflows to the Chino Basin were calculated with the groundwater model by the same methods used to simulate subsurface inflows. There are no outflows from the Temescal Basin to the Arlington or Bedford-Coldwater Basins because the water level gradients are always toward the Temescal Basin.

5.7.2.3. Groundwater Discharge to Streams
Where streams are hydraulically connected to the water table, discharges of groundwater into the streams are simulated by the groundwater model based on streambed wetted area, permeability, and on the amount by which the simulated groundwater elevation in a model stream cell is higher than the simulated surface water elevation. This condition is present primarily along the lower reaches of the Santa Ana River and other channels within the Prado Wetlands.

5.7.2.4. Riparian Evapotranspiration
Evapotranspiration of groundwater by phreatophytic riparian vegetation is influenced by available soil moisture and by depth to the water table. Like other types of vegetation, phreatophytes use soil moisture supplied by rainfall when it is available. Any remaining evapotranspiration demand is met by drawing water from the water table. Phreatophyte use of groundwater is assumed to decrease from the maximum rate when the water table is at the land surface to zero when the water table is 20 feet or more below the ground surface. These calculations are applied at model cells within the Prado Wetlands. A patch of dense riparian vegetation is also present where Temescal Wash enters the Basin from an upstream bedrock reach. However, that vegetation appears to be supported by percolation from the Wash, not groundwater, because the water table appears to be more than 30 feet below the ground surface in that area. The water demand of riparian vegetation was assumed to equal reference evapotranspiration.

Temescal Basin GSP 5-20

5.8. CHANGE IN GROUNDWATER STORAGE
Figure 5-10 shows the cumulative change in Basin storage from the model during 1990 through 2068. The baseline and growth plus climate change scenario results for 2019 to 2068 are displayed as continuations of the historical storage changes from 1990 to 2018.

As shown, groundwater storage decreased fairly steadily during 1990 to 2018. Average annual storage changes for the current and two historical periods ranged from decreases of 194 AFY to an increase of 1,148 AFY, as shown in Table 5-4. Total outflows were between

0.7 percent greater and 4.2 percent lower than total inflows. Factors that could have contributed to this simulated decline include incorrectly estimated initial water levels, relatively high amounts of municipal groundwater pumping during the early 2000s, and predominantly dry climatic conditions since 2011.

Average annual storage changes during both future scenarios were very slightly positive, with total inflows about two percent greater than total outflows. This was the intentional result of adjusting Corona pumping to achieve close to zero net storage change during 2019 to 2068. The abruptness of the transition from historical to future conditions is primarily the result of decreased pumping, but also to the effects of drought conditions at the end of the historical period. The similarity of the future baseline and growth plus climate change scenarios is due to the small differences in all water budget items between those two scenarios. Also, the effects of urban growth tended to offset the effects of the warmer, drier climate.

5.9. ESTIMATE OF SUSTAINABLE YIELD
The sustainable yield is defined as the volume of pumping that the Basin can sustain without causing undesirable effects. It is not a fixed or inherent natural characteristic of a groundwater basin. Rather, it is influenced by land use activities, importation of water, wastewater and stormwater management methods, potential recharge with recycled water, and the locations of wells with respect to interconnected streams. The estimates of sustainable yield presented in this section reflect the current status of those variables under the historical and future scenarios.

A long analysis period is needed to evaluate yield because of the episodic nature of natural recharge. Whereas pumping, irrigation return flow, and pipe leaks are fairly constant from year to year, recharge from precipitation and streams varies widely. Because of evolving land use during 1990 to 2018, no subset of years is ideal for estimating sustainable yield. For the purposes of this GSP historical sustainable yield was calculated based on 1993 to 2017, which is representative of long-term average conditions in terms of precipitation and stream flow. Sustainable yield was estimated for the historical simulation (using 1993 to 2017) and the two future simulations (both using all 50 years of the simulation), as shown in Table 5-5.

Temescal Basin GSP 5-21
Table 5-5. Estimated Sustainable Yield
25-Year Historical
1993 to 20171 (AFY)
Baseline
2019 to 20682 (AFY)
Growth Plus Climate Change
2019 to 20682 (AFY)
17,195 15,572 15,672
1 For the historical sustainable yield estimate, average annual water budgets during 1993 to 2017 were used.
2 The 50-year future simulation uses historical hydrology for 1993 to 2017 two times in succession.

These sustainable yield estimates equal total pumping plus storage change. This simple method of estimating yield ignores the interaction of pumping with other head-dependent boundaries, including interconnected surface water, riparian vegetation ET and subsurface inflows and outflows. All four of those types of boundaries are present in the Temescal Basin, which means that increasing the amount of pumping can theoretically increase the yield of the Basin, by increasing head-dependent inflows and decreasing head-dependent outflows. Conversely, a decrease in pumping will not result in an equal decrease in the rate of storage depletion because the other head dependent boundaries absorb some of the change in pumping. The amount of pumping in the future baseline simulation accounts for these complex interactions. It was obtained by trial and error as the amount of Corona pumping that resulted in close to zero long-term storage change. That means it also results in zero long-term change in other head-dependent boundary flows such as groundwater discharge to the Prado Wetlands. Large changes in those flows could cause undesirable results for groundwater dependent ecosystems.

The yield estimates in the table are for total groundwater pumping in the Basin, not just Corona pumping. Although Corona pumping is currently about 93 percent of total pumping, several industrial users also pump groundwater and others might do so in the future.

The yield estimates based on the future scenarios are probably a better basis for planning than the estimates based on the historical and current periods because the latter periods were influenced by factors that do not apply to the future period and because the future scenarios have a long hydrologic averaging period. Sustainable yields calculated from the future scenarios are based on projections far into the future. Slight imbalances in estimated water budgets can result in large cumulative changes in storage, and hence in the calculated yields. By the same token, the long planning horizon provides ample time to adjust water management (recharge and pumping) to maintain basin operation within the sustainable yield if long-term rising or falling trends in cumulative storage in fact occur. In the context of this GSP, sustainable yield estimated from the water budget is contingent on the absence of undesirable results related to water levels, storage, subsidence, water quality, or depletion of interconnected surface water. Quantitative sustainability criteria are presented in Chapter 6 that define thresholds at which groundwater conditions become undesirable for each of those sustainability indicators. For example, if pumping at the above estimates of sustainable yield caused subsidence or significant impacts on riparian or aquatic habitats, the yield may need to be reduced to avoid those impacts. It should be noted that the future

Temescal Basin GSP 5-22
sustainable yield is calculated in the model using projected hydrological conditions.

Conditions vary widely in the Basin between wet years and dry years and the actual precipitation (along with ET and other inflows) would influence the available yield of the Basin.

Accordingly, this sustainable yield value is a broad indicator. It indicates no overdraft based on the water budget, but it must be interpreted through evaluation of undesirable results.

Path: T:\Projects\Corona GSP 46414\GRAPHICS\Figure 5-1 Cumulative Departure of Precipitation.gpj Figure 5-1
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1900190519101915192019251930193519401945195019551960196519701975198019851990199520002005201020152020 Water Year
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Annual Precipitation 1899-2020 Mean Cumulative Departure Precipitation at Elsinore (NOAA Station GHCND:USC00042805)

11.89"
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Below Normal
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2010-2013

Path: T:\Projects\Bedford Coldwater GSP 80802\GRAPHICS\Figure 5-3 Effect of Climate Change on Precipitation and Evapotranspiration.gpj Figure 5-2
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Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 5 - Water Budget\Figure 5-3 2018 land use_r1.mxd 2018 Land Use
Citrus
Grain
Dense riparian vegetation
Sparse riparian vegetatin
Natural - grassland
Natural - shrubs
Pasture
Truck crops
Commercial
Industrial
Residential
Low-density residential
Turf
Mines
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Temescal Basin Figure 5-3
Land Use in 2018
0 0.5 1
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(
N

Path: T:\Projects\Corona GSP 46414\GRAPHICS\Figure 5-4 Annual Surface Water Budgets 1990-2018.gpj Figure 5-4
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19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012201320142015201620172018 0 30,000
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Southern Chino Basin Surface Water Budget
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Surface Outflow to Prado Basin
Temescal Basin Surface Water Budget

1. Joseph Cyn

2. Eagle Cyn

3. Main St Cyn

4. West Side 4

5. Tin Mine/Hagador Cyns

6. Mabey Cyn

7. Wardlow Cyn

8. Temescal Wash Tribs

9. Home Gardens

10. N Side Arlington Gap

11. Norco East

12. Norco West

13. Santa Ana River & Prado

14. Cucamonga Cr

15. Chino Cr
Watershed Number and Name
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From Raw Daily Flows
From Daily Flows Clipped To Percolation Capacity
Average Annual Tributary Watershed Discharge (1980-2018) Figure 5 5
Effect of Clipping
on Simulated Tributary
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Figure 5-6
Temescal Basin
Annual Groundwater
Budgets, 1990 to 2018
199019921994199619982000200220042006200820102012201420162018 Year
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Pipe Leaks
Reclaimed Water Percolation
Groundwater Discharge To Streams
Riparian Evapotranspiration
Wells: Agricultural
Wells: Municipal, Industrial, and Domestic
Outflow To Chino Basin

Figure 5-7
Annual Groundwater
Budgets
Future Baseline
2020202220242026202820302032203420362038204020422044204620482050205220542056205820602062206420662068 Year
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Pipe Leaks
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Groundwater Discharge To Streams
Riparian Evapotranspiration
Wells: Agricultural
Wells: Municipal, Industrial, and Domestic
Outflow To Chino Basin

Figure 5-8
Annual Groundwater
Budgets, Growth Plus
Climate Change
2020202220242026202820302032203420362038204020422044204620482050205220542056205820602062206420662068 Year
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Bedrock Inflow
Pipe Leaks
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Groundwater Discharge To Streams
Riparian Evapotranspiration
Wells: Agricultural
Wells: Municipal, Industrial, and Domestic
Outflow To Chino Basin

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Path: T:\Projects\Corona GSP 46414\GIS\Maps\Figures\GSP Figures\Chapter 5 - Water Budget\Figure 5-9 Dispersed Recharge_r1.mxd Average Annual Dispersed
Recharge 1993-2007 (in/yr)
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0.3 - 1
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Figure 5-9
Dispersed Recharge
0 0.5 1
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Temescal Basin

Figure 5-10
Cumulative
Storage Changes
1990 to 2068
19891991199319951997199920012003200520072009201120132015201720192021202320252027202920312033203520372039204120432045204720492051205320552057205920612063206520672069 -35,000
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Temescal Basin GSP 6-1

SUSTAINABLE MANAGEMENT CRITERIA
The Sustainable Groundwater Management Act (SGMA) defines sustainable management as the use and management of groundwater in a manner that can be maintained without causing undesirable results, which are defined as significant and unreasonable effects caused by groundwater conditions occurring throughout a groundwater basin:
• Chronic lowering of groundwater levels indicating a significant and unreasonable depletion of supply
• Significant and unreasonable reduction of groundwater storage • Significant and unreasonable seawater intrusion
• Significant and unreasonable land subsidence that substantially interferes with surface land uses
• Significant and unreasonable degraded water quality, including the migration of contaminant plumes that impair water supplies
• Depletions of interconnected surface water that have significant and unreasonable adverse impacts on beneficial uses of the surface water For these sustainability indicators4
, a Groundwater Sustainability Plan (GSP) must develop
quantitative sustainability criteria that allows the Groundwater Sustainability Agency (GSA) to define, measure, and track sustainable management. These criteria include the following:
• Undesirable Result – significant and unreasonable conditions for any of the six sustainability indicators
• Minimum Threshold (MT5
) – numeric value used to define undesirable results for each sustainability indicator
• Measurable Objective (MO) – specific, quantifiable goal to track the performance of sustainable management
• Interim Milestone – target value representing measurable groundwater conditions, in increments of five years, set by the GSA as part of the GSP.

Together, these sustainability criteria provide a framework to define sustainable management, delineate between favorable and unfavorable groundwater conditions, and support quantitative tracking that identifies problems promptly, allows assessment of management actions, and demonstrates progress in achieving the goal of sustainability. 4 If one or more undesirable results can be demonstrated as not present and not likely to occur, a GSA is not required to establish the respective sustainability criteria per GSP Regulations §354.26(d); in the inland Temescal Basin seawater intrusion is not present and not likely to occur. 5 The abbreviations for Minimum Threshold (MT) and Measurable Objective (MO) are provided because these terms are used often; however, the full unabbreviated term is used when helpful for clarity or when included in a quotation.

Temescal Basin GSP 6-2

6.1. SUSTAINABILITY GOAL
The sustainability goal can be described as the mission statement of the GSA for managing the Basin; it embodies the purpose of sustainably managing groundwater resources and reflects the local community’s values—economic, social, and environmental. The sustainability goal for the Temescal Subbasin (Basin), stated below, was developed through discussion at several public meetings with the GSA and the Technical Advisory Committee (TAC).

6.1.1. Description of Sustainability Goal
To sustain groundwater resources for the current and future beneficial uses of the Basin in a manner that is adaptive and responsive to the following objectives:
• Provide a long-term, reliable and efficient groundwater supply for municipal, industrial, and other uses
• Provide reliable storage for water supply resilience during droughts and shortages • Protect groundwater quality
• Support beneficial uses of interconnected surface waters, and • Support integrated and cooperative water resource management.

This goal is consistent with SGMA and is based on information from the Plan Area, Hydrogeologic Conceptual Model, Groundwater Conditions, and Water Budget sections of this GSP that:
• Identify beneficial uses of Temescal Basin groundwater and document the roles of local water and land use agencies
• Describe the local hydrogeologic setting, groundwater quality conditions, groundwater levels and storage, and inflows and outflows of the Basin • Document the ongoing water resource monitoring and conjunctive management of groundwater, local surface water, recycled water and especially imported water sources that help protect groundwater quality and maintain water supply.

6.1.2. Approach to Sustainability Indicators
The approach to assessing the sustainability indicators and setting the sustainability criteria has been based on 1) review of available information from the Plan Area, Hydrogeologic Conceptual Model, Groundwater Conditions, and Water Budget sections of this GSP and 2) discussions with Temescal Basin stakeholders and local agency representatives, for example at TAC meetings and workshops.

This approach has developed since mid-2020 and generally began with definition of what an undesirable result is; this initially has been exploratory and qualitative and based on plainlanguage understanding of what undesirable means. Potential minimum thresholds have been explored in terms of when, where, how long, why, under what circumstances, and what beneficial use is adversely affected. This step identified seawater intrusion as not present and not likely to occur.

Temescal Basin GSP 6-3
Beyond a qualitative identification of undesirable, the approach to defining sustainability indicators varies among the undesirable results. Several of the undesirable results are directly or indirectly related to groundwater levels, including conditions related to groundwater storage, subsidence, and interconnected surface water. The definition began in terms of groundwater levels in individual wells but has recognized that storage depletion, subsidence, and impacts on connected surface water occur as water levels decline. As a result, the sustainability criteria for those indicators are interrelated across space and time, coordinated and as consistent as is reasonable and as available data allow.

The consideration of the causes and circumstances of undesirable results is important in the Basin particularly for groundwater quality because general quality is poor throughout much of the Basin and has been poor for decades. Sustainable management relating to groundwater quality is all about use and management of groundwater without causing undesirable results but does not necessarily include reversing natural undesirable conditions. Moreover (per SGMA §10727.2(b)(4)), a GSP may but is not required to address undesirable results that occurred before and have not been corrected by the SGMA benchmark date of January 1, 2015.

While native groundwater quality is poor, salt and nitrate loading are recognized as potential sources of groundwater quality deterioration throughout much the Basin. Such loading has been occurring for more than 100 years, however changes in groundwater quality at depth (where groundwater typically is pumped) will lag behind the salt and nutrient loading at the ground surface by decades. This means that groundwater quality monitoring data can be misleading, sustainability criteria potentially could be reactive to decades-old land use conditions and insensitive to the future, and the effects of management activities will not be seen for decades. Given all that, implementation of management actions is recognized as needed and such actions will be helpful in the long term.

Another important aspect to defining sustainability criteria has been considering what we know and more importantly what we don’t know about undesirable results that may be detected or may potentially occur in the Basin. From a big picture perspective, the Basin is well managed—historical groundwater levels have been largely stable, subsidence has not been perceived, groundwater storage has been managed such that recent drought impacts have been minimized, local groundwater quality degradation is being addressed through treatment and blending, and inter-connected surface water and groundwater dependent ecosystems (GDEs) are being maintained. While water resource monitoring has been useful and adaptive, significant data gaps and uncertainties exist. Because groundwater conditions are regarded generally as good and because considerable uncertainties exist, the process of setting sustainability criteria has been directed toward open discussion of uncertainties, indepth identification of data gaps and the means to fill them, and a strong intention for flexibility and adaptive management.

The intent is to quantify and qualify sustainability criteria such that they guide good management without setting off false alarms or triggering costly, ineffective, or harmful management actions.

Temescal Basin GSP 6-4

6.1.3. Summary of Sustainable Management Criteria
This section documents the six sustainability criteria as relevant to the Basin and as guided by the Sustainability Goal. The GSAs have managed the Basin without experiencing undesirable results, but continuation and improvement is needed of existing management actions—most notably continuing to use imported water and its conjunctive use with groundwater. It also will include improvement and expansion of management actions and monitoring; these are addressed for each sustainability criterion’s Measurable Objective in a subsection, Discussion of Monitoring and Management Measures to be Implemented.

While significant and unreasonable undesirable results have not been experienced in the Basin, the following sustainability criteria are defined in this section because potential exists for undesirable results.
• The Minimum Threshold relative to chronic lowering of groundwater levels is defined at designated Key Wells by historical groundwater low levels. Undesirable results are indicated when two consecutive exceedances occur in each of two consecutive years, in 60 percent or more of the Key Wells. The Measurable Objective is to maintain groundwater levels above the MTs and to maintain groundwater levels within the historical operating range. • The Minimum Threshold for reduction of storage is fulfilled by the minimum threshold for groundwater levels as proxy. The Measurable Objective for storage is fulfilled by the MT for groundwater levels, which maintains groundwater levels within the historical operating range.
• The Minimum Threshold for land subsidence is defined as a rate of decline equal to or greater than 0.2 feet (ft) in any five-year period. This has been considered in terms of a potential cumulative decline equal to or greater than one foot of decline since 2015; 2015 represents current conditions and the SGMA start date. The extent of cumulative subsidence across the Basin will be monitored and evaluated using Interferometric Synthetic Aperture Radar (InSAR) data. Subsidence is closely linked to groundwater levels and it is unlikely that significant inelastic subsidence would occur if groundwater levels remain above their minimum thresholds. • The Minimum Thresholds for degradation of water quality address nitrate and total dissolved solids (TDS). The MT for nitrate is defined initially as no statistically significant increase in the percentage of wells with 5-year average concentrations exceeding the nitrate maximum contaminant limit (MCL) of 45 milligrams per liter (mg/L) based on current conditions (2015 through 2019). The MT for TDS is defined initially as no statistically significant increase in the percentage of wells with 5-year average concentrations exceeding the TDS Secondary MCL of 1,000 mg/L based on current conditions. The Measurable Objectives for both are defined as maintaining or reducing the percentage of wells with average concentrations exceeding the MTs. • The Minimum Threshold for depletion of interconnected surface water is defined as a depth to water of 15 feet in shallow monitoring wells in the southern Prado

Temescal Basin GSP 6-5
area, where declines to lower water levels are correlated with Temescal Basin pumping and/or water levels.

6.2. CHRONIC LOWERING OF GROUNDWATER LEVELS
Chronic lowering of groundwater levels can indicate significant and unreasonable depletion of supply, causing undesirable results to domestic, industrial, or municipal groundwater users if continued over the planning and implementation horizon. As a clarification, droughtrelated groundwater level declines are not considered chronic if groundwater recharge and discharge are managed such that groundwater levels recover fully during non-drought periods.

Declining groundwater levels directly relate to other potential undesirable effects (for example regarding groundwater storage, land subsidence and interconnected surface water); these are described in subsequent sections. Effects on well users are described here.

Groundwater elevation trends in Basin are documented in Groundwater Conditions Section

4.1; hydrographs of representative wells are presented for the Basin. The Basin is not characterized by overdraft with widespread chronic groundwater level declines.

Groundwater levels in broad areas of the Basin have been maintained at relatively high levels because of the availability of imported water supplies. In addition, while groundwater level declines still occur with dry and critically-dry years, recent drought-related declines have not been as rapid or deep as in previous droughts. Many areas of the Basin experienced record lows during the most recent drought. However, the Basin was not marked by reports of significant water level decline impacts to production wells.

6.2.1. Description of Undesirable Results
As groundwater levels decline in a well, a sequence of increasingly severe undesirable results will occur. These include an increase in pumping costs and a decrease in pump output (in gallons per minute). With further declines, the pump may break suction, which means that the water level in the well has dropped to the level of the pump intake. This can be remedied by lowering the pump inside the well, which can cost thousands of dollars.

Chronically declining water levels will eventually drop below the top of the well screen. This exposes the screen to air, which can produce two adverse effects. In the first, water entering the well at the top of the screen will cascade down the inside of the well, entraining air; this air entrainment can result in cavitation damage to pumping equipment.

The other potential adverse effect is accelerated corrosion of the well screen. Corrosion eventually creates a risk of well screen collapse, which would likely render the well unusable. If water levels decline by more than about half of the total thickness of the aquifer (or total length of well screen), water might not be able to flow into the well at the desired rate regardless of the capacity or depth setting of the pump. This might occur where the thickness of basin fill materials is relatively thin. While describing a progression of potential adverse effects, at some point the well no longer fulfills its water supply purpose and is deemed to have “gone dry.” For the purposes of this discussion, a well going dry means that the entire screen length (to the bottom of the deepest screen) is unsaturated.

Temescal Basin GSP 6-6
For purposes of setting a Minimum Threshold, undesirable results are defined as a well going dry. This appears to be a low standard and not protective of private wells; but this is an initial definition to start the analysis. The rationale is summarized as follows with more explanation in the following sections:
• There are very few active private wells in the Basin (see Section 2.3.2.1). The owners and operators of those wells are known and they have not reported any adverse effects to those wells in the past.
• None of the existing private well owners report that their wells went dry or were otherwise affected during the recent drought. Because of this, some flexibility exists for purposes of analysis.
• Responsibility for potential undesirable results to shallow wells is shared between a GSA and a well owner; there is a reasonable expectation that a well owner would construct, maintain, and operate the well to provide its expected yield over the well’s life span, including droughts.
• As discussed below, MTs are set at historical groundwater level lows. • No private wells have been reported to have water shortages for the Basin in the DWR led Household Water Supply Shortage Reporting System (DWR 2021), Including during recent dry periods corresponding to historical groundwater level lows in some monitored wells.

6.2.2. Potential Causes of Undesirable Results
For the Basin, the primary potential cause of groundwater level undesirable results would be reduction of surface water supplies and associated increase in groundwater use and reduction in groundwater recharge from return flows. Reduction of imported water could have direct adverse impacts on municipal and industrial water users throughout the Basin.

Given that the Basin is not characterized by basin-wide chronic groundwater level declines, then the undesirable results of a well losing yield, having damage, or “going dry” represent a more complex interplay of causes and shared responsibility.

Some of the potential causes are within GSA responsibility; most notably, a GSA is responsible for groundwater basin management without causing undesirable results such as chronic groundwater level declines. SGMA also requires that a GSA address significant and unreasonable effects caused by groundwater conditions throughout the basin. This indicates that a GSA is not solely responsible for local or well-specific problems and furthermore that responsibility is shared with a well owner. A reasonable expectation exists that a well owner would construct, maintain, and operate the well to provide its expected yield over the well’s life span, including droughts, and with some anticipation that neighbors also might construct wells (consistent with land use and well permitting policies). As indicated above, there are very few active private wells in the Basin and those wells have not shown impacts in the past.

Temescal Basin GSP 6-7

6.2.3. Definition of Undesirable Results
As context, the Basin Sustainability Goal has the objective to provide a long-term, reliable and efficient groundwater supply for municipal, industrial, and other uses.

In that light, the definition of undesirable results would be the chronic lowering of groundwater levels indicating a significant and unreasonable depletion of supply if continued over the planning and implementation horizon. This is defined by groundwater conditions occurring throughout the Basin. This definition also recognizes that chronic lowering of groundwater levels could affect groundwater flow to or from the hydraulically connected Bedford-Coldwater, Chino, and Coastal Plain of Orange County basins, and thereby potentially affect the maintenance of sustainability in those basins.

As documented in Groundwater Conditions Section 4.1, analysis of hydrographs reveals that the Basin is not characterized by basin-wide chronic groundwater level declines. While affected at times by drought, groundwater levels in broad areas of the Basin have been maintained at relatively high levels because of the availability of imported water. Moreover, the Basin has not been marked by reports of significant water level decline impacts to shallow supply wells. In the absence of reported well problems, it can be concluded that undesirable results for the chronic lowering of water levels are not occurring in Basin and that the Basin is managed sustainably relative to groundwater levels.

While water levels have declined slightly in recent years due to dry climatic conditions, modeling of future expected conditions show these declines are not expected to continue in the future (see Chapter 5). This finding is consistent with the water budget analyses that indicate (within the range of uncertainty) balanced inflows and outflows in the future.

6.2.4. Potential Effects on Beneficial Uses and Users
Groundwater is a significant source of supply in the Basin and supplies wells municipal, industrial, and other beneficial uses. Groundwater has been and is being used for the range of beneficial uses, even during drought, and with reasonable operation and maintenance by well owners. Historically, changes in water levels in production wells have not correlated with changes in vegetation health or density in the Prado Wetlands (see Sections 4.10.4).

The mutual consistency of the water-level MT and interconnected surface water MT is discussed in Section 6.7.4.

6.2.5. Sustainable Management Criteria for Groundwater Levels The general approach to defining sustainability criteria (minimum thresholds and measurable objectives) for groundwater levels has involved selection of representative monitoring wells (Key Wells), review of groundwater level data, and review of supply well location/construction information to gage potential undesirable effects on wells.

Specifically, this has included evaluating historical low levels in Key Wells. This approach is founded on the idea that undesirable results were not reported when groundwater elevations were at their minimum values and therefore returning to those minima should not cause undesirable results in the future.

Temescal Basin GSP 6-8

6.2.5.1. Selection of Key Wells
The approach includes selection of existing monitored wells within the Basin that are or represent active supply wells. Sustainability criteria would be defined for each of these Key Wells and each would be monitored for groundwater levels with respect to MTs and MOs.

The Key Wells (Figure 6-1) have been identified by reviewing groundwater level hydrographs from all currently monitored wells and selecting wells that have a long, reliable, and recent record of groundwater level monitoring, that represent local or regional trends, and that together provide a broad geographic distribution for the Principal Aquifer, the Secondary Aquifer, and the Basin as a whole. The distribution of these wells also has been reviewed with respect to maps showing density of wells across the Basin (e.g., Figures 2-5 through 2- 8). These wells are mostly production wells, which is not optimal for monitoring; on the other hand, they are generally representative of production wells.

Groundwater level data and hydrographs of each Key Well have been reviewed to identify the all-time lowest groundwater elevation at each Key Well. As discussed in Groundwater Conditions Section 4.1.3, historical minima in many wells were recorded with the most recent drought, which implies that most currently active wells in the Basin would have experienced those historical minima.

The identified historical low at each Key Wells (i.e., historical maximum depth to water) represents the first approximation of a minimum threshold, with the realization that the final selection of the MT for a Key Well could be adjusted upward to be more protective of nearby supply wells.

6.2.5.2. Evaluation of Existing Wells
Existing wells in the Basin were assessed in the development of water level sustainability criteria. The California Department of Water Resources (DWR) has developed a database of information relating to well locations, use, construction, yield, and other information. By way of background, information on local supply wells has been recorded on Water Well Drillers Reports and is available mostly as paper or scanned copies. DWR has identified 383 individual paper records for wells in the Basin. However, detailed information from most of these records has not been digitized.

Accurate data on the location and elevation of most wells is not available. Most of the wells identified by DWR within the Basin have only been located to the center of a Public Land Survey System (PLSS) section. As a result, precise locations relating to Basin aquifer units and Key Wells are unknown.

In addition, construction information on most wells has not been entered into databases where it can be analyzed readily, and the status of wells is not known. Currently, DWR only has digitized construction information for 53 domestic, agricultural, or other production wells within the Basin. The current status of these wells is unknown, and most are fairly old.

Of the 53 domestic, agricultural, or other production wells in the Basin with construction records in the DWR database, only seven were constructed after January 1, 2000 and 41 of the 53 were constructed before 1990. As described in Water Budget Section 5, land use and groundwater production has changed significantly since the late 1980s. Additionally, the GSA agencies are the municipal water purveyors in the Basin and in this capacity they have

Temescal Basin GSP 6-9
assessed the presence of private domestic wells in the Basin; no existing active private domestic wells were identified through this assessment.

Given the age of the existing wells, they should have been present during the recent historical groundwater level minima in the Basin. In fact, DWR records indicate that only three wells have been constructed in the Basin since the end of the recent drought. One of these wells (Corona Well 32) is a municipal supply well and the other two are listed as landscape irrigation wells. Given the age of most of the wells in the Basin, the historical minima in Key Wells are deemed to be protective with regard to groundwater level declines.

As discussed in Sections 6.2.1 and 6.2.2, groundwater level declines involve a continuum of potential impacts that range from those effects not noticed by the well owner to those that are noticed and reasonably handled by the well owner.

6.2.6. Minimum Thresholds
According to GSP Regulations Section 354.28(c)(1) the minimum threshold for chronic lowering of groundwater levels must be the groundwater elevation indicating a depletion of supply at a given location that may lead to undesirable results. MTs for chronic lowering of groundwater levels are to be supported by information on the rate of groundwater elevation decline based on historical trends, water year type, and projected water use in a basin. However, as documented in the Groundwater Conditions Section 4.1.3, groundwater levels are not chronically declining in the Basin. While groundwater levels decline in dry and critically-dry years, they have recovered in normal, above normal, and wet years.

Groundwater levels in many Key Wells were at historical lows during the recent drought (thereby defining the respective MT) but all have since recovered.

Under current conditions, groundwater levels in Key Wells are above the MTs and no undesirable results are known to occur. Nonetheless, MTs have been developed because the potential exists for chronic lowering of groundwater levels.

Using recent and reliable information on the construction of existing supply wells, the MT levels shown in Table 6-1 are protective of most supply wells, based on available information. The MTs are based on historical low groundwater levels or levels that are higher. Because of this, the MTs are not only protective of local wells but also would help minimize potential impacts on groundwater flow to or from other area, such as the neighboring basins.

Based on historical lows, the MTs account for historical groundwater level variations, and consideration has been given to supporting basin management flexibility, for example to avoid setting off false alarms or triggering costly, ineffective, or harmful management actions. However, MTs have not been adjusted downward at this time, although periods of record for some groundwater level hydrographs are short and may not include actual historical lows that could recur.

Table 6-1. Minimum Thresholds for Groundwater Levels
Local Well Name Earliest Monitoring Date
Average Depth to
Groundwater
(ft bgs)
Pump Intake Depth
(ft bgs)
Date of Static Maximum
Depth to Groundwater
Maximum Static Depth to
Groundwater
(ft bgs)
Corona 7A 6/1/2002 156.84 230 1/1/2003 178
Corona 8 12/13/2012 112.6 No Pump 5/4/2014 129.5
Corona 8A 1/1/1998 119.69 192 10/1/2001 131
Corona 9A 7/1/2002 80.72 220 7/1/2002 159
Corona 11 7/18/1959 134.14 180 9/13/2017 158
Corona 11A 12/6/2017 143.48 221.2 5/31/2014 155.2
Corona 12A 3/1/1993 158.59 280.3 11/2/2005 164
Corona 13 2/1/1977 141.19 182 6/1/1989 174
Corona 14 2/1/1924 184.92 250 5/1/2009 239
Corona 15 8/13/1952 116.63 171.6 12/1/2004 134
Corona 16 12/13/2012 140.3 No Pump 7/2/2018 159.5
Corona 17A 6/1/2002 110.63 182 5/13/2006 125
Corona 19 4/1/1992 102.73 200 9/1/2003 124.5
Corona 22 4/1/2001 150.19 387 5/1/2004 153.3
Corona 25 4/1/2001 61.71 180 7/1/2003 161.5
Corona 26 5/1/2001 136.86 333 10/1/2004 122
Corona 27 3/1/2003 154.19 436.7 3/3/2020 211
Corona 28 3/1/2003 90.59 174 9/6/2016 95.2
Corona 29 3/18/2009 88.63 230 8/1/2018 88.2
Corona 30 8/28/2009 56.9 No Pump 4/24/2014 70.6
Corona 31 3/18/2009 95.13 217 8/7/2009 132.2
Corona 33 3/13/2019 58.8 255 2/4/2020 68.1
Corona 10th/Lincoln 11/17/2011 197.5 No Pump 9/21/2013 204 T:\Projects\Corona GSP 46414\Deliverables\GSP\6 Sustainability Criteria\Sustainability Criteria Assessment and Wells.xlsx - Table 6-1 Todd Groundwater Des by: CT