PROPOSITION 1 - ROUND 2 INTEGRATED
REGIONAL WATER MANAGEMENT
IMPLEMENTATION GRANT
Technical Report
Lake Elsinore, California
Submitted to:
City of Lake Elsinore
130 South Main Street
Lake Elsinore, CA 92530
Submitted by:
GEI Consultants, Inc.
5901 Priestly Drive, Suite 301
Carlsbad, CA 92008
November 19, 2025
Project No. 2502831
________________________________________
John Rudolph
Senior Aquatic Ecologist
________________________________________
Chris Stransky
Senior Aquatic Scientist
GEI Consultants, Inc. 1 Table of Contents
Acronyms and Abbreviations .................................................................................... 3
1. Introduction ......................................................................................................... 4
1.1. Background 4
2. Methodology ........................................................................................................ 6
2.1. Monitoring Methods – Data Sources 6
2.1.1. Moleaer In-Situ Sondes 6
2.1.2. EVMWD In-Situ Sondes 8
2.1.3. TMDL DO Measurements at EVMWD In-Situ Sonde Locations 9
2.1.4. LEAMS Effectiveness Water Quality Profiles 9
2.1.5. Synoptic Lake Wide DO Measurements 9
3. Results ................................................................................................................. 13
3.1. Moleaer In-Situ Sondes 13
3.2. EVMWD In-Situ Sondes 14
3.3. TMDL DO Measurements at EVMWD In-Situ Sonde Locations 19
3.4. LEAMS Effectiveness Water Quality Profiles 20
3.5. Synoptic Lake Wide Bottom DO Measurements 25
4. Conclusion ......................................................................................................... 28
5. References ......................................................................................................... 30
List of Tables
Table 2-1. Monitoring Methods ...................................................................................................... 6 Table 2-2. Moleaer In-situ Sonde Locations .................................................................................... 7 Table 2-3. EVMWD In-Lake Monitoring Buoys ................................................................................ 9 Table 2-4. TMDL Station Location ................................................................................................... 9 Table 2-5. Synoptic Lake Wide DO Monitoring Sites .................................................................... 11 Table 3-1. In-Situ TMDL Bottom DO Measurements Pre- and Post-Moleaer Unit Installation at EVMWD Sonde Locations ............................................................................................. 19
List of Figures
Figure 2-1. Moleaer Nanobubble Barge and Sonde Locations, and EVMWD Buoy Locations ....... 8 Figure 2-2. Synoptic Lake-wide DO Locations ............................................................................... 12 Figure 3-1. Pilot Barge Sonde Monthly Average Bottom DO – 2024/2025................................... 13
GEI Consultants, Inc. 2 Figure 3-2. Pilot Barge 250’ NW Sonde Monthly Average Bottom DO – 2024/2025 .................... 14 Figure 3-3. Monthly DO (5-6m depth) Averages Reported by the EVMWD In Situ Sondes - July 2023 to August 2025 .................................................................................................... 15 Figure 3-4. Monthly (5-6m depth) DO Ranges Reported by the EVMWD In Situ Sondes - July 2023 to August 2025 .................................................................................................... 16 Figure 3-5. Annual Combined Bottom (5-6m depth) DO Measurements Reported by the EVMWD In Situ Sondes - July 2023, 2024, and 2025 .................................................................. 17 Figure 3-6. Annual Combined DO Measurements (5-6m depth) Reported by the EVMWD In Situ Sondes - August 2023, 2024, and 2025 ........................................................................ 18 Figure 3-7. TMDL In-Situ DO Measurements at EVMWD Sonde Locations 0.5-m from the Bottom - July 2019 to October 2025 ......................................................................................... 20 Figure 3-8. LEAMS Effectiveness Vertical Water Quality Profiles - 2021 to 2025 ......................... 22 Figure 3-9. LEAMS Effectiveness Vertical Water Quality Profiles - 2021 to 2025 ......................... 24 Figure 3-10. LEAMS Effectiveness Vertical Water Quality Profiles – Mean Dissolved Oxygen Pre and Post Nanobubble Installation at Each Meter Depth Across Stratified and Destratified Periods ...................................................................................................... 25 Figure 3-11. Lake-wide Synoptic DO Measurements Near the Sediment Surface - June 2025 .... 26 Figure 3-12. Lake-wide Synoptic DO Measurements Near the Sediment Surface - September 2025 .............................................................................................................................. 27
GEI Consultants, Inc. 3 Acronyms and Abbreviations
DO Dissolved Oxygen
EVMWD Elsinore Valley Municipal Water District
HABs Harmful Algal Blooms
LESJWA Lake Elsinore & San Jacinto Watersheds Project Authority mg/L Milligram per liter
ORP Oxidation Reduction Potential
TMDL Total Maximum Daily Load
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1. Introduction
The purpose of this document is to evaluate the effectiveness of the Moleaer nanobubble units deployed at Lake Elsinore as part of Proposition 1, Round 2 (Prop 1, Round 2) Integrated Regional Water Management Implementation (IRWM) Grant funding. The data collection and subsequent analysis was performed according to Lake Elsinore Prop 1, Round 2 Project Monitoring Plan (PMP).
1.1. Background
Lake Elsinore (Lake) is the largest natural freshwater lake in Southern California and supports a range of beneficial uses, including water contact and non-contact recreation, commercial and sportfishing, and aquatic and terrestrial habitat for a wide range of species. Despite its natural origin, the Lake has undergone significant hydrological and structural modifications over time including the construction of a levee at its southern end and the ongoing supplementation of up to 6.0 million gallons per day of treated recycled water by the Elsinore Valley Municipal Water District (EVMWD) to support recreational uses and maintain water levels. However, the Lake continues to experience significant water quality challenges due to its unique physical characteristics.
Lake Elsinore is a shallow, terminal, water body that lacks a recurrent natural outflow. These characteristics allow nutrients to accumulate over time leading to eutrophication, including an overgrowth of algae, low levels of dissolved oxygen (DO), and occasional fish kills. Historically, the Lake has suffered from persistently low DO levels, particularly in the deeper areas and during warmer months as the lake stratifies. These hypoxic and anoxic conditions have promoted the internal release of nutrients, such as phosphorus, from the lakebed sediment further fueling harmful algal blooms (HABs).
The 2004 Total Maximum Daily Load (TMDL) established a numeric target for DO in Lake Elsinore of no less than 5.0 mg/L within the bottom one meter of the water column. Despite targeted management actions, including reducing the lake’s surface area and implementing a lake-wide aeration system, achieving this DO target has remained challenging. The most recent 2024-2025 Lake Elsinore and Canyon Lake Watersheds Nutrient TMDL Annual Monitoring Report records a lake-wide average DO of
3.1 mg/L in the bottom 1-meter of the water column, with the previous five years reporting lake wide averages in the bottom 1-meter ranging from 2.9 - 4.8 mg/L. To address this deficiency, the City of Lake Elsinore, in coordination with the Lake Elsinore and San Jacinto Watersheds Authority (LESJWA), has implemented multiple in-lake water quality improvement projects. Most notably, the City of Lake Elsinore purchased and deployed three Moleaer nanobubble units in 2024, with one in February and two in October of that year.
A pilot study was conducted in 2022 with funding from the State of California Prop 1, Round 1 IRWM Implementation Grant, to explore new algal bloom mitigation strategies for Lake Elsinore. The study evaluated several in-situ technologies for their ability to reduce algal biomass and improve water quality within mesocosms under real-world conditions during a historic peak bloom. These included the Moleaer nanobubble unit, biological treatments, chemical algaecides, and algal biomass harvesting technologies. All of the treatments evaluated, though showing limited success during this pilot study, showed some level of effectiveness as being part of an ultimate solution for Lake Elsinore based on a
GEI Consultants, Inc. 5 history of use and documented success stories in other water bodies. A companion stand-alone report was recommended to further assess the feasibility and costs for each of the treatment alternatives for larger-scale application in Lake Elsinore. The Moleaer treatments in this study did provide some benefit in the form of stabilizing the DO concentration throughout the water column, preventing stratification of DO and also preventing substantial drops in DO. Based on promising results from small-scale nanobubble technology, Moleaer’s nanobubble units were scaled up for a Phase 2 lake-wide implementation to evaluate its overall effectiveness and feasibility in increasing DO and improving water quality.
The implementation of Moleaer nanobubble systems aims to increase DO levels throughout the lake and reduce internal release of phosphorus. The system operates by generating pure oxygen from air, dissolving into pumped lake water, and returning supersaturated DO water to the lake. Nanometer-sized bubbles are formed in the dissolution process that are neutrally buoyant and remain suspended throughout the water column due to random Brownian motion. The underlying principle is that these nanobubbles, with their high surface area and extended residence time, facilitate more efficient DO transfer compared to traditional aeration systems. By increasing DO levels lake-wide, particularly at the sediment-water interface, the system helps suppress internal nutrient loading from sediments, which has been identified as contributing up to 75% of the Lake’s annual nutrient input, while at the same time reducing the likelihood of fish kills and contributing to overall improvements in lake water quality and ecosystem health.
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2. Methodology
2.1. Monitoring Methods – Data Sources
Five data types were selected to quantify the effectiveness of the Moleaer nanobubble units as it relates to improvements in DO throughout the Lake. A summary of the data types is listed in Table 2-1.
1. Continuous in-situ DO sondes installed by Moleaer
2. Continuous DO data in-situ DO sondes installed by EVMWD
3. TMDL DO measurements recorded at the EVMWD in-situ sonde locations during routine monitoring events (8 per year)
4. Full-depth water quality profiles at Station LE02 (mid-lake) record during monthly monitoring events to evaluate the effectiveness of the existing Lake Elsinore Aeration and Mixing System (LEAMS).
5. Synoptic lake wide bottom water DO measurements recorded during two monitoring events performed by GEI Consultants in June and September 2025 Table 2-1. Monitoring Methods
Data Type Depth Measurement Frequency Responsible Party Moleaer In-Situ Data Sondes Surface, Mid, and Near
Bottom Every 2 hours City of Lake Elsinore
EVMWD In-Situ Data Sondes 1-m increments surface
to 6-m Hourly EVMWD
TMDL Monitoring Water Quality
Profiles
1-m increments from
Surface to 0.5-m from
bottom
Eight monitoring events annually:
Monthly during the summer period (June-
September), bi-monthly during the
remainder of the year (October-May)
LESJWA
LEAMS Effectiveness Water Quality
Profiles
1-m increments from
Surface to 0.5-m from
bottom
Monthly monitoring events EVMWD
Lake-wide Synoptic DO Monitoring 0.5 meters from bottom Two monitoring events: June and September 2025 at 29 lake wide stations City of Lake Elsinore
2.1.1. Moleaer In-Situ Sondes
Three Moleaer nanobubble units have been installed in Lake Elsinore. The first unit, known as the Pilot Barge, was installed on the Lake’s eastern shore in January 2024 and began full operation as a pilot program in February 2024. The Pilot Barge delivers approximately 500 pounds of oxygen per day to the lake in the form of nanobubbles. In October 2024, two additional barges of identical size were installed:
Launch Point Barge and Elm Grove Beach Barge. Each of these newer units injects 2,500 pounds of oxygen per day into the lake. Thus, a total of 5500 pounds of oxygen is delivered by the three nanobubble barges since October 2024. This amount of oxygen delivery amounts to about one third of
GEI Consultants, Inc. 7 the estimated lake wide summertime oxygen demand (~15,000 lbs/day) from lake bottom sediment (EVMWD, 2025a).
Multi-parameter in-situ sondes were installed 1-m above the sediment at eight locations, both adjacent to and more distant from the barges, to continuously monitor water quality. Measurements are recorded every two hours, providing high-frequency data to assess spatial and temporal variations in lake water quality conditions. Of the eight sondes deployed, only two have continuously collected data without requiring maintenance or repairs since installation including, 1) Pilot Barge sonde and 2) Pilot Barge 250’ NW sonde (see Table 2-2 and Figure 2-1). The Pilot Barge sonde was mounted near the output of the nanobubble unit, while the Pilot Barge 250’ NW sonde was mounted 250 feet to the northwest of the Pilot Barge. DO data from these two sondes spanning January 2024-May 2025 were accessed from the Moleaer HydroVu dashboard. Raw datasets from these near-bottom sondes were selected and exported in CSV format with parameters including date, time, and DO (mg/L). The CSV files were then imported into a central master workbook and organized by Site ID, Date, Time, and DO. All timestamps were standardized to Pacific Time and checked for consistency. DO data from January 2024-May 2024 and January 2025-May 2025 were selected for analysis as these overlapping seasonal periods enable direct comparison of conditions before and after the installation of all Moleaer units. Values outside the expected range for bottom DO in Lake Elsinore (i.e., outlier data) based on typical historic records (GEI Consultants, Inc., 2025b) were flagged and removed. Cleaned and validated DO records for bottom water were compiled into a summary table for each sonde location, with monthly averages calculated to support trend analysis and comparison to other data sources.
Table 2-2. Moleaer In-situ Sonde Locations
Sonde ID Latitude Longitude
Pilot Barge 250’ NW* 33.671617 -117.343892
Pilot Barge 250’ SE 33.670794 -117.342567
Pilot Barge 1000’ NW 33.672811 -117.346256
Pilot Barge* 33.671622 -117.343433
Site 2 33.665226 -117.344854
Site 3 33.652374 -117.346370
Launch Point 33.673208 -117.364331
Elm Grove Beach 33.664428 -117.335111
*Indicates data sondes used for this evaluation
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Figure 2-1. Moleaer Nanobubble Barge and Sonde Locations, and EVMWD Buoy Locations
2.1.2. EVMWD In-Situ Sondes
EVMWD maintains two long-term, fixed-location in-situ sondes in Lake Elsinore: the Lakeshore Sonde and the Grand Avenue Sonde (Table 2-3, Figure 2-1), located approximately 3,462 feet and 4,698 feet, respectively, from the nearest nanobubble unit. These instruments collect hourly measurements of multiple water quality parameters, including dissolved oxygen (DO), at 1-meter depth intervals beginning 1 meter below the surface. During the evaluation period, the sondes recorded data to depths of 5 or 6 meters, depending on the lake’s water elevation. However, they did not capture bottom, near-sediment DO measurements during this period, with the deepest measurements remaining approximately 2-3 meters above the true lakebed at each sonde location. EVMWD staff provided pre-cleaned and annotated data for July 2023 to August 2025 in Excel spreadsheet format with notes regarding calibration events, inspection and maintenance activities, instrument downtime, and known data gaps or flags. Each dataset included date/time, depth, temperature, specific conductivity, total dissolved solids, pH, turbidity, DO (mg/L and sat %), and oxidation-reduction potential (ORP). For this evaluation, the following fields were extracted and retained: 1) date/time, 2) depth, and 3) dissolved oxygen (mg/L).
Additional screening steps included removing clearly erroneous values, if any, and extracting DO data for both sonde’s deepest depth measurements (5 to 6 meters). Monthly averages of the deepest depth measurements were calculated for comparative analysis pre- and post-installation of the Moleaer nanobubble units.
GEI Consultants, Inc. 9 Table 2-3. EVMWD In-Lake Monitoring Buoys
Buoy Latitude Longitude
Lakeshore Buoy 33.665714 -117.352880
Grand Avenue Buoy 33.660523 -117.352397
2.1.3. TMDL DO Measurements at EVMWD In-Situ Sonde Locations Depth profile readings for temperature, specific conductivity, DO, pH, turbidity, and ORP are recorded at 1-meter increments from the surface to 0.5-meter from the bottom at the Lakeshore and Grand Avenue Sonde locations during routine TMDL monitoring events. A handheld YSI ProDSS is used to take measurements to verify that the EVMWD sonde sensors are functioning correctly and providing accurate data. DO data recorded 1-meter from the bottom were compiled from field sampling logs included in previous TMDL annual monitoring report appendices spanning July 2019 to June 2025, as well as field sampling logs from Summer 2025 monitoring events. Although the EVMWD in-situ sondes collect DO measurements from the surface to a depth of 5-6 meters, these sondes do not provide a true bottom, near-sediment measurement. The depth profile readings taken during routine TMDL monitoring events provide near-sediment measurements (within 1-meter from the bottom) and were therefore used to compare bottom DO levels from Moleaer sondes and lake-wide surveys pre and post-Moleaer nanobubble unit installation.
2.1.4. LEAMS Effectiveness Water Quality Profiles
Depth profile readings for temperature, specific conductivity, DO, pH, turbidity, and ORP using a handheld YSI ProDSS are recorded at 1-meter increments from the surface to 0.5-meter from the bottom at the mid-lake TMDL monitoring location LE02 (Table 2-4). DO data recorded 1-meter increments top- to-bottom were compiled from field sampling logs spanning January 2021 to November 2025.
Table 2-4. TMDL Station Location
TMDL Station Latitude Longitude
LE02 33.663341 -117.354213
2.1.5. Synoptic Lake Wide DO Measurements
DO measurements were collected during two lake-wide synoptic sampling events in June and September 2025. Each lake-wide survey was conducted over two mornings within a 2-hour window to ensure complete spatial coverage while minimizing diel variability. During each event, DO was measured using a handheld YSI ProDSS within 0.5-meters of the bottom at 29 locations around the Lake (Table 2-5, Figure 2-3). Measurements were collected between 8:00 a.m. and 10:00 a.m. to capture early morning DO prior to daytime thermal stratification, algal photosynthetic activity, and potential wind-related mixing.
All measurements were recorded in field sampling logs and subsequently digitized into structured Excel
GEI Consultants, Inc. 10 spreadsheets for analysis. As part of the QA/QC process, all digitized entries were independently verified against original field logs to ensure transcription accuracy. Any questionable values were resolved using field notes and instrument calibration records. After QC validation, DO data were used to assess spatial variation in DO concentration within 1-meter from the bottom across the lake, as well as compare seasonal DO conditions during two key periods of the year when historical measures have shown DO concentration to be low.
GEI Consultants, Inc. 11 Table 2-5. Synoptic Lake Wide DO Monitoring Sites
Site ID Latitude Longitude
LE-DO-1 33.6678 -117.3774
LE-DO-2 33.6640 -117.3716
LE-DO-3 33.6601 -117.3657
LE-DO-4 33.6563 -117.3599
LE-DO-5 33.6525 -117.3541
LE-DO-6 33.6486 -117.3483
LE-DO-7 33.6448 -117.3425
LE-DO-8 33.6410 -117.3366
LE-DO-9 33.6372 -117.3308
LE-DO-10 33.6711 -117.3717
LE-DO-11 33.6684 -117.3675
LE-DO-12 33.6647 -117.3617
LE-DO-13 33.6609 -117.3558
LE-DO-14 33.6571 -117.3499
LE-DO-15 33.6534 -117.3440
LE-DO-16 33.6496 -117.3381
LE-DO-17 33.6458 -117.3323
LE-DO-18 33.6729 -117.3635
LE-DO-19 33.6692 -117.3576
LE-DO-20 33.6655 -117.3517
LE-DO-21 33.6617 -117.3458
LE-DO-22 33.6580 -117.3399
LE-DO-23 33.6543 -117.3339
LE-DO-24 33.6784 -117.3585
LE-DO-25 33.6747 -117.3527
LE-DO-26 33.6709 -117.3468
LE-DO-27 33.6671 -117.3409
LE-DO-28 33.6633 -117.3351
LE-DO-29 33.6596 -117.3292
GEI Consultants, Inc. 12 Figure 2-2. Synoptic Lake-wide DO Locations
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3. Results
3.1. Moleaer In-Situ Sondes
Following the installation of the second two Moleaer nanobubble units on Lake Elsinore in October 2024, average monthly bottom dissolved oxygen levels increased in 2025 compared to the same period in 2024. From January to May 2024, the bottom DO levels measured at the Pilot Barge Buoy sonde exhibited a declining trend from January (6.1 mg/L) through May (4.8 mg/L). Conversely, in 2025 bottom DO levels remained relatively stable and elevated relative to 2024, ranging between 6.0 and 7.5 mg/L across all five months (Figure 3-1). The Pilot Barge 250’ NW sonde exhibited a similar trend, with 2025 DO levels consistently exceeding those in 2024 from January to May (Figure 3-2). While these sondes sit near-bottom, they are located in the littoral zone in only 4-6 meters of water along the perimeter of the Lake, an area where wind mixing can bring higher oxygen water to the bottom. Despite the shallow depth of these sondes, interannual comparisons of dissolved oxygen pre and post-installation of Moleaer nanobubble units would provide valuable information on their effectiveness.
Figure 3-1. Pilot Barge Sonde Monthly Average Bottom DO – 2024/2025
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Figure 3-2. Pilot Barge 250’ NW Sonde Monthly Average Bottom DO – 2024/2025
3.2. EVMWD In-Situ Sondes
Both the Lakeshore and Grand sondes recorded increases in dissolved oxygen concentrations following the installation of the three Moleaer nanobubble units in October 2024 (Figure 3-3, Figure 3-4). At the Lakeshore sonde, mean DO levels at 6-meters depth increased from 2.6 mg/L in October 2024 to 8.5 mg/L in November 2024. Similarly, mean DO levels recorded at 6 meters at Grand sonde increased from
1.2 mg/L to 7.8 mg/L during the same time period. Across the period of interest, DO levels for both sondes remained above hypoxic levels throughout the winter and early spring months, with seasonal declines in the late spring as the lake began to stratify. Despite seasonal declines, monthly DO averages and ranges remained consistently higher in 2025 relative to those across the same period in 2024, with any seasonal declines somewhat muted after installation of the nanobubble units. Beginning in February 2025, the Grand Avenue sonde recorded consistently lower DO values compared to the Lakeshore Sonde. This trend could be influenced by Grand Avenue sonde location being further from the nearest nanobubble unit, and as the lake began to stratify in mid-Spring lower DO at this location became apparent. The Grand Avenue sonde is located between Synoptic Study locations 13 and 14 where the lowest lake wide DO levels were observed in the synoptic survey (see Section 3.4). Although these sondes do not collect true bottom DO measurements, they have been recording DO measurements
GEI Consultants, Inc. 15 consistently at 5 to 6 meters below the surface pre- and post-nanobubble unit installation, which allows for yearly comparisons at similar depth.
Mean DO levels at the Lakeshore and Grand Avenue sonde locations were consistently low in both July 2023 and July 2024 (<1.5 mg/L). Statistically significant increases in DO levels were observed in July 2025 relative to July 2024 at both Lakeshore (n=1468, p < 0.0001) and Grand (n=848, p < 0.0001) sondes after all three units were in operation (Figure 3-5). Dissolved oxygen concentrations at both the Lakeshore and Grand Avenue sondes remained consistently low during August 2023 and August 2024, with mean values below 2.5 mg/L. A statistically significant increase in DO was observed in August 2025 at the Lakeshore sonde (n = 1,328; p < 0.001), where the mean concentration rose to 6.5 mg/L, well above hypoxic thresholds (Figure 3-6). For most of Summer 2025, Grand Avenue sonde (further from barges) experienced malfunction and required various parts to be replaced. Staff were able to order and install replacement parts, and calibrate the sonde, which allowed for some data to be collected in July 2025, but the sonde continued to experience communication issues which kept the sonde inoperable through August 2025.
*No Grand Sonde data for May, June, and August 2025 due to sonde maintenance/repairs Figure 3-3. Monthly DO (5-6m depth) Averages Reported by the EVMWD In Situ Sondes - July 2023 to August 2025
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Figure 3-4. Monthly (5-6m depth) DO Ranges Reported by the EVMWD In Situ Sondes - July 2023 to August 2025 (boxes represent 25th and 75th percentiles, bar represents the mean, whiskers represent max/min values) * - No Lakeshore or Grand Sonde data for these months due to sonde maintenance and/or repairs
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Figure 3-5. Annual Combined Bottom (5-6m depth) DO Measurements Reported by the EVMWD In Situ Sondes - July 2023, 2024, and 2025
(boxes represent 25th and 75th percentiles, bar represents the mean, whiskers represent max/min values)
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Figure 3-6. Annual Combined DO Measurements (5-6m depth) Reported by the EVMWD In Situ Sondes - August 2023, 2024, and 2025
(boxes represent 25th and 75th percentiles, bar represents the mean, whiskers represent max/min values)
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3.3. TMDL DO Measurements at EVMWD In-Situ Sonde Locations In early 2024, bottom DO concentrations measured next to the Lakeshore and Grand sondes as part of the routine TMDL monitoring were well below historical averages. DO concentrations recorded in February 2024 in support of the TMDL monitoring were 0.27 mg/L at Lakeshore and 0.33 mg/L at Grand sonde locations, 2 weeks after the first nanobubble unit Pilot Barge became operable (Table 3-1).
Moderate increases were observed in April 2024 (3.15 and 3.71 mg/L) and June 2024 (1.74 and 2.72 mg/L). Following installation of the additional two nanobubble units in October 2024, DO concentrations exhibited an increase. Dissolved oxygen levels near the sediment surface in October 2024 increased to
3.32 mg/L at Lakeshore and 2.63 mg/L at Grand, exceeding the 2019 to 2023 historical average for that month. By December 2024, bottom DO concentrations reached the highest observed levels that year at
8.91 mg/L at Lakeshore and 10.22 mg/L at Grand.
While somewhat variable, improvements in bottom DO levels were observed in 2025. In February 2025, concentrations reached 3.65 mg/L at the Lakeshore site and 3.03 mg/L at Grand Avenue. While these values are below the historical mean, both were higher than levels recorded in February 2024. Bottom DO in April 2025 was notably lower than historical values likely due to seasonal variability, falling below both the historical mean and 2024 values in these months, however notable increases were observed in mid to late summer when the lake typically stratifies and bottom DO is very low. July, September, and October 2025 were the only summer and early fall months since 2015 (aside from a single occurrence in October 2018) during which DO concentrations at a mid-lake monitoring location met the TMDL target of greater than 5 mg/L at 1-meter above the lake bottom.
Table 3-1. In-Situ TMDL Bottom DO Measurements Pre- and Post-Moleaer Unit Installation at EVMWD Sonde Locations
Month
Historical Average DO 1m-
from bottom
(2019-2023)
2024 DO 1m-from bottom 2025 DO 1m-from bottom
Lakeshore Grand Lakeshore Grand Lakeshore Grand
February 8.22 8.58 0.27 0.33 3.65 3.03
April 6.25 6.45 3.15 3.71 0.35 0.36
June 0.48 0.79 1.74 2.72 0.19 0.32
July 0.61 0.15 0.18 0.22 5.20 3.10
August 0.66 1.27 1.28 3.25 1.21 0.79
September 2.31 2.83 1.91 2.32 6.71 7.45
October 0.34 0.25 3.32 2.63 5.08 5.83
December 4.75 3.85 8.91 10.22 --- ---
- Greyed cells indicate monitoring dates when all three nanobubble units were online.
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Figure 3-7. TMDL In-Situ DO Measurements at EVMWD Sonde Locations 0.5-m from the Bottom - July 2019 to October 2025
3.4. LEAMS Effectiveness Water Quality Profiles
Vertical dissolved oxygen (DO) profiles (Figures 3-8 and 3-9) were derived from monthly LEAMS effectiveness water column measurements collected at TMDL Station LE02, located near the lake’s central basin. While interannual variability in DO distributions is evident, likely influenced by weather- related factors such as precipitation and ambient air temperature, consistent seasonal and depth-related patterns in DO concentration are apparent. There appears to be a sustained increase in summer hypolimnetic DO beginning in mid-2024, then another step up in late 2024 that persists into 2025.
Analysis of summer hypolimnetic DO concentrations (≥4 m depth) from June through September reveals that during the 2021–2023 period, mean DO levels remained near 1.0 mg/L. Hypoxic conditions (DO ≤2 mg/L) were prevalent, affecting 77% of deep-water observations during these months. Bottom, near- sediment (with 1-meter of bottom) DO during peak stratification (July–August) was frequently anoxic, averaging approximately 0.4 mg/L.
Following the installation of the first nanobubble oxygenation system in February 2024, hypolimnetic DO during the summer of 2024 increased to a mean of 3.4 mg/L, with hypoxia frequency declining to 42%.
After two additional systems were installed in October 2024, an increase in DO was observed in November 2024, with a +6.45 mg/L rise relative to November 2023. However, a similar increase was
GEI Consultants, Inc. 21 observed between November 2021-2022 prior to any system installation, suggesting that seasonal destratification and cooling may also contribute to reoxygenation of the water column. Despite this confounding factor, the sustained improvement into summer 2025, where mean hypolimnetic DO reached 5.1 mg/L and hypoxia frequency dropped further to 26%, indicates a cumulative effect of oxygenation. This transition marks a shift from persistent summer hypoxia to improved oxygen conditions at depth, and while this shift was evident during cooler months when the lake was destratified, it was more pronounced during summer months (June-September) when the lake was stratified (Figure 3-10).
In addition to improvements in the hypolimnion, increases in DO were also observed in the mid-column and surface layers. Surface and near-surface waters (0–2 m), which were already the most oxygenated portion of the water column, showed further increases beginning in early 2024 and continuing into 2025.
While increases in these upper layers were largely restricted to stratification periods during summer months, mean summer DO in this layer ranged from 3.7 to 5.9 mg/L during 2021–2023, rising to 6.6 mg/L in 2024 and 7.4 mg/L in 2025. These gains were most pronounced during peak stratification months (July–August), when oxygen stress typically intensifies.
The upper mid-column (3–4 m depth), representing the shallow metalimnion, also exhibited improvement, again most pronounced during summer months. During July and August of 2021 and 2022, DO concentrations ranged from 1.8 to 3.9 mg/L. In 2023, this layer became nearly anoxic, with DO values of 0.18 and 0.13 mg/L, respectively for these two months. Following installation of the Moleaer nanobubble units, the DO in July and August 2024 increased to 6.7 and 5.2 mg/L, and further to 7.3 and
8.7 mg/L in 2025, respectively.
These results show that while a decreasing DO gradient with depth is still evident, collectively the data demonstrate a water column improvement in oxygen conditions following installation of nanobubble units, with the most pronounced and ecologically relevant changes occurring in the hypolimnion and upper metalimnion during the summer stratification period.
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Figure 3-8. LEAMS Effectiveness Vertical Water Quality Profiles - 2021 to 2025
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Figure 3-9. LEAMS Effectiveness Vertical Water Quality Profiles - 2021 to 2025
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Figure 3-10. LEAMS Effectiveness Vertical Water Quality Profiles – Mean Dissolved Oxygen Pre and Post Nanobubble Installation at Each Meter Depth Across Stratified and Destratified Periods
(Pre-Installation = Jan 2021 to Sept 2024, Post-Installation = Oct 2024 to Nov 2025)
3.5. Synoptic Lake Wide Bottom DO Measurements
A synoptic study was conducted in June and September 2025 in which bottom DO was measured at 29 locations across Lake Elsinore. In June 2025, lake-wide bottom DO concentrations for this effort ranged from 0.24 to 7.4 mg/L, with a lake wide bottom mean of 3.0 mg/L (Figure 3-11). Elevated DO concentrations were measured near the southwestern shoreline and in areas adjacent to the nanobubble units, while lower DO concentrations were found in the central and southern portions of the Lake, which are located farther from the nanobubble systems. During the June 2025 synoptic survey, a low value of 0.24 mg/L mid-lake corresponds with similar low near-sediment DO values in June measured as part of the TMDL monitoring program at the mid-lake LE02 location (0.22 mg/L) and the two EVMWD sondes (0.19 and 0.32 mg/L). An exception to the generally lower DO values in the extreme southern region of the lake were two measurements showing elevated concentrations (6.4 and
7.4 mg/L), likely attributable to the shallow depth (2.7 and 3.2 meters, respectively) in that area.
Reduced stratification and enhanced surface interaction in these shallow zones promote greater oxygen transfer, explaining the localized increase in DO. By September 2025, lake-wide bottom DO increased substantially, ranging from 3.5 to 8.8 mg/L, with a lake wide mean of 6.8 mg/L and most of the lake meeting the TMDL target of >5 mg/L measured 1-meter above the lakebed (Figure 3-12).
GEI Consultants, Inc. 26 As a separate evaluation, Dr. Michael Anderson (UC Riverside, retired) conducted an AEM3D model simulation for the dates of the September 2025 synoptic survey, which excluded oxygen inputs from the nanobubble systems. The model simulation produced a DO concentration pattern consistent with those observed during the September 2025 synoptic study. Notably, the model predicted elevated bottom DO concentrations mid-lake, closely matching field measurements from the synoptic survey. Air temperatures during the week of the synoptic study were substantially lower than the preceding and following weeks, potentially enhancing vertical mixing and facilitating oxygen transport to deeper layers.
These results suggest it is possible that natural convective mixing and short-term cooling could have contributed to the observed increases in bottom DO in September 2025.
Figure 3-11. Lake-wide Synoptic DO Measurements Near the Sediment Surface - June 2025
GEI Consultants, Inc. 27 Figure 3-12. Lake-wide Synoptic DO Measurements Near the Sediment Surface - September 2025
Across both synoptic surveys, the dissolved oxygen at the lake bottom was generally elevated relative to historical lake-wide mean values from 2019 to 2023. The spatial distribution of the increased DO influence appeared to exhibit a counterclockwise patt ern, with higher concentrations occurring “downstream” of the Moleaer nanobubble units along the eastern and northwestern shorelines and progressively lower concentrations toward the southwest and southeast section. This gradient suggests that currents in Lake Elsinore are transporting the oxygenated bottom water counterclockwise around the lake perimeter.
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4. Conclusion
Dissolved oxygen data collected across multiple monitoring programs in Lake Elsinore exhibited quantifiable improvements in near-bottom and mid-column oxygen conditions following the installation and operation of multiple Moleaer nanobubble oxygenation units. The five datasets used to evaluate the effectiveness of the nanobubble units showed similar patterns of increased DO after installation.
1. Localized monitoring near the Pilot barge showed DO concentrations near the barge and at the 250’ NW site increasing from 2-6 mg/L in 2024, to consistently exceeding 6 mg/L across all months in early 2025.
2. Monthly average DO values at 5-6 meters depth from the EVMWD in-situ Lakeshore and Grand sondes (the deepest data available from this source) increased, reaching 8-10 mg/L by late 2024 and maintaining DO concentrations above the hypoxic threshold through winter and early spring 2025. Additionally, statistically significant increases in DO in July and August 2025 compared to those same months in 2023 and 2024 show sustained oxygen enhancement under stratified summer conditions which historically exhibit severe oxygen depletion.
3. Bottom DO concentrations collected during TMDL monitoring events at Lakeshore and Grand sonde sites increased in summer 2025 compared to 2024 and historical averages. Notable increases in DO were observed in mid to late summer when these concentrations are typically very low. July and September 2025 marked the only instances since 2015 during summer TMDL monitoring (June to September) where bottom DO concentrations met the TMDL target of >5 mg/L measured 1-meter above the lakebed.
4. Full-depth water column profile DO measurements from the TMDL monitoring location LE02 showed increased DO throughout the water column, particularly during months when the lake would be stratified (June – September). The frequency of summer observations with hypoxic conditions (DO <2.0 mg/L) in the hypolimnion declined following the nanobubble system installations, from as high as 77% prior to implementation, to 42% after deployment of the initial pilot unit in February 2024, and declined further to 26% following installation of the two additional units in October 2024. Although the post-installation record includes only two summer seasons, both have demonstrated substantial improvement in deep-water oxygen availability.
5. Findings from the June and September 2025 synoptic study show lake-wide improvements in lake bottom DO. By September 2025 (typically a highly stratified period with anoxic bottom conditions), 24 of the 29 monitoring locations exceeded the TMDL target of >5 mg/L measured 1-meter above the sediment, with a mean lake-wide DO of 6.8 mg/L, and all monitoring locations exceeded the 2 mg/L threshold used in assessing the benefits of reduced internal nutrient releases.
Overall, monitoring results collectively demonstrate that installation of the three nanobubble units in Lake Elsinore (delivering an estimate of approximately 1/3 of estimated lake-wide sediment oxygen demand) corresponded with sustained increases in near-bottom DO. Prior to the installation and operation of the three nanobubble units, bottom DO concentrations routinely fell below the 2020 TMDL target of greater than 5 mg/L within 1-m of the bottom, with widespread hypoxia or anoxia observed during summer and early fall stratification periods. Improvements in bottom DO concentration were
GEI Consultants, Inc. 29 most pronounced in winter and early spring and extended into the typically anoxic summer period.
While some seasonal stratification and spatial variation remain, the data suggests that the units appear to be effectively mitigating some of the worst anoxia periods, improving oxygen levels near the sediment-water interface and within the water column, progressing the lake toward achieving the 2020 TMDL DO target.
While interannual weather variability may influence DO levels in the lake, statistical analysis indicates that observed increases in DO at the Lakeshore (r = –0.16, p = 0.59) and Grand Avenue (r = –0.13, p =
0.66) sondes during TMDL monitoring events in 2024 and 2025 were not significantly correlated with average monthly air temperature. Notably, summer 2024 experienced warmer conditions than both 2023 and 2025, yet this did not correspond with reductions in DO relative to historical levels. Increased precipitation is also unlikely to explain the improvements observed. Although the winter and spring of the 2023–2024 wet season, and in particular the early spring of 2024, were wetter than previous years and coincided with elevated DO in spring and summer 2024, the 2024–2025 wet season recorded the lowest rainfall of the three-year period yet DO concentrations in 2025 exceeded those of 2024. These findings suggest that external weather factors alone do not account for the observed DO improvements, and that a combination of natural variability and targeted in-lake interventions likely contributed to enhanced oxygen conditions.
Additionally, while modelling by Dr. Michael Anderson showed that the DO values observed in the September synoptic event could have been explained by natural convective mixing and short-term cooling, this evaluation was based on a single date in September 2025. Given the extended period evaluated herein with multiple lines of evidence, the collective results do suggest that deployment of the nanobubble oxygenation units appear to be playing a role in stabilizing DO throughout the water column, complementing natural processes. The current configuration of the nanobubble barges involves pumping of lake water from the middle of the water column for oxygen dissolution and delivery to the middle of water column. Because the temperature of the delivered oxygenated water is higher than that of the lake bottom, it may reduce the likelihood of dissolved oxygen reaching the sediment–water interface without sufficient mixing energy. Previous studies have found that vertical and lateral currents in Lake Elsinore are significantly impacted by wind, which could help to mix oxygenated water into the lake bottom (see Appendix B in GEI Consultants Inc., 2025a). To increase confidence in the preliminary findings presented in this summary report, GEI recommends the following:
1. Continued lake-wide mapping for dissolved oxygen concentrations near the lake bottom
2. The installation of an in-situ sonde at the bottom of the deepest, coldest zone of the lake where internal nutrient releases are the most prevalent
These efforts would help validate the observed improvements and help refine the ongoing assessment of the nanobubble unit’s effectiveness. Additionally, these recommendations would provide critical insight into the spatial and temporal variability of bottom water oxygenation, particularly in the areas most susceptible to hypoxia.
30 5. References
GEI Consultants, Inc. (2025a). “Study to Evaluate Future Alternatives for the Lake Elsinore Aeration and Mixing System,” reported prepared for Elsinore Valley Municipal Water District, July 2025.
GEI Consultants, Inc. (2025b). “Lake Elsinore and Canyon Lake Watersheds Nutrient TMDL Monitoring 2024-2025 Annual Report,” prepared for Lake Elsinore & San Jacinto Watersheds Project Authority., October 9.