Lake San Marcos and San Marcos Creek Risk Assessment
Lake San Marcos, located in north-central San Diego County, CA, was originally constructed in 1946 to support agricultural water supply. In the early 1960s, the dam was modified and the shoreline recontoured to its current form. Today, the lake is surrounded by a residential community and is a popular destination for recreational activities such as boating, paddleboarding, and fishing.
But Lake San Marcos was classified as eutrophic (high in nutrients promoting growth of algae) in the early 2000’s and since February 2024 is under a Cleanup and Abatement Order from the local Regional Water Quality Control Board. This eutrophication has impaired beneficial uses due to excessive planktonic algae driven by elevated nutrient levels in both the lake and its watershed. The resulting increase in algal biomass can lead to drops in dissolved oxygen both at night, when algae shift from photosynthesis to respiration, and during fall/winter, as decomposing algae consume oxygen.
In addition to nutrients, the lake waters contained other constituents of concern including trace metals, pyrethroid and organochlorine pesticides, polycyclic aromatic hydrocarbons (PAHs), and total suspended solids (TSS).
To address these issues, the San Diego Regional Water Quality Control Board (SDRWQCB) issued a Cleanup and Abatement Order (CAO) requiring a comprehensive evaluation of the lake and its watershed. This effort aims to fill critical data gaps, establish a baseline for future comparisons, and guide effective remediation strategies.
The Lake San Marcos Trustees engaged GEI to lead the Phase 1 Risk Assessment Work Plan and Quality Assurance Project Plan (QAPP), and to implement the required studies in collaboration with listed parties, consultants, and the SDRWQCB. GEI’s key activities included:
- Completing comprehensive water and sediment quality monitoring, including chemistry and bioassay toxicity testing, to determine possible causations for algae issues and other constituents of concerns in the lake.
- Conducting stream bioassessments for macroinvertebrates and periphyton to determine overall watershed health and biodiversity.
- Sampling in-lake phytoplankton and zooplankton concentrations.
- Providing routine cyanotoxin monitoring in the lake and the adjacent golf course irrigation system to determine mitigation and management strategies.
Overall, the sampling program was designed to capture watershed dynamics, define land use variability, and support pollutant source identification. Our team also came up with the unique idea to host a fishing derby to involve residents in collecting fish for an ecological fish tissue risk assessment. This both allowed community members to learn more about management strategies and how we are aiming to keep the lake and watershed healthy.
The project will culminate in a comprehensive ecological and human health risk assessment report for the monitoring performed from 2024 to 2026, providing recommendations for future action. Findings from this first phase will also inform the program’s next phase, focusing on identifying pollutant sources through a storm sewer systems investigation plan and report. Ultimately, the goal of the program is to identify sources of nutrients and other pollutants in the lake and its watershed in order to pursue control measures and restore the lake to natural levels of algal biomass and a healthy ecological community.
