Protecting Water Quality at the Source

Source water assessment updates are required by the State Water Board and are referenced in the annual drinking water quality report. LADWP completed an initial source water assessment in 2002 and is required to provide an updated assessment every five years through a watershed sanitary survey. Watershed sanitary surveys examine possible contamination to sources of drinking water and recommend actions to better protect these water sources.

Below is an update of LADWP’s source water assessment.

Surface Supply

In 2021, LADWP completed an assessment of the Owens Valley and Mono Basin watersheds that supply the Los Angeles Aqueduct. These sources are most vulnerable to geothermal activities that release naturally occurring arsenic into creeks which feed the Owens River. Other activities that impact water quality in these watersheds are livestock grazing, wildlife, and unauthorized public use of storage reservoirs. The impact to water quality from these activities is deemed to be minimal.

LADWP has monitored for Cryptosporidium and Giardia. Results indicate that their presence is extremely infrequent and in the rare instances these microbes are found, they are at very low levels. The ultraviolet treatment provided at the Los Angeles Aqueduct Filtration Plant safeguards against Cryptosporidium and Giardia. Neither were found in treated water in 2025.

Groundwater Supply

Assessment of groundwater sources in the San Fernando Basin was updated in 2020. Assessment of groundwater sources in the Central and Sylmar Basins was completed in 2019. Located in highly urbanized areas, the wells within these aquifers are most vulnerable to the following activities: dry cleaning, manufacturing, metal finishing, septic systems, chemical processing, and storage of fertilizer, pesticides, and chemicals. These local water supplies are treated and blended with water from other sources to ensure compliance with drinking water standards.

Purchased Imported Supplies from Metropolitan Water District

The most recent surveys for Metropolitan Water District’s (MWD) source waters are the Colorado River Watershed Sanitary Survey – 2022 Update, and the State Water Project Watershed Sanitary Survey – 2021 Update. Each source water used by MWD — the Colorado River and State Water Project — has different water quality challenges. Both are exposed to stormwater runoff, recreational activities, wastewater discharges, wildlife, fires, and other watershed-related factors that could affect water quality. Treatment to remove specific contaminants can be more expensive than measures to protect water at the source. This is why MWD, and other water agencies invest resources to support improved watershed protection programs.

Three of the five MWD treatment plants: F.E. Weymouth, Robert B. Diemer, and Joseph Jensen supply water to the Los Angeles area. MWD tests its water for nearly 400 constituents and performs about 250,000 water quality tests per year on samples gathered from its vast distribution system. Analysis of these samples is undertaken at Metropolitan's state-of-the art water quality laboratory. Results from MWD are provided to LADWP and are included in the report on Tables I, II, and III.

Visit LADWP’s Water Quality webpage to learn more about water quality projects and issues. For more information on the latest watershed sanitary surveys contact (213) 367-3182.