Merced County, California: drinking water report. Merced County sits in the heart of California's Central Valley with about 281,000 residents.
Merced County sits in the heart of California's Central Valley with about 281,000 residents. The city of Merced and surrounding communities rely almost entirely on groundwater, pumped from the deep alluvial aquifer beneath the valley floor. Agriculture dominates the economy and the landscape – almonds, dairy, sweet potatoes, and poultry generate billions in annual revenue but also generate the waste streams that define local water quality.
Nitrate is the primary drinking water contaminant in Merced County, driven by dairy operations and fertilizer use. According to the California State Water Board's 2024 groundwater quality report, 23 community water systems in the county have recorded nitrate above 75% of the MCL, with 11 systems exceeding it outright. The Central Valley Regional Water Quality Control Board's 2024 dairy program data shows that Merced County has more dairy cows per acre than almost any other county in California.
Castle Air Force Base, now closed, occupies a large area in the county. The base is a Superfund site with documented groundwater contamination from TCE and PFAS. According to EPA records, the TCE plume extends several miles downgradient from the base, and AFFF contamination has been confirmed in on-site monitoring wells.
Merced County residents on small community water systems face the highest nitrate risk. If your system has issued a nitrate violation, the water board may provide bottled water or point-of-use treatment – ask your provider about available assistance programs.
Check your water for current monitoring data. For nitrate and PFAS, reverse osmosis handles both contaminant types. Our water filter guide evaluates systems appropriate for Central Valley groundwater. Pull your detailed report for historical trends, and visit our California page for statewide context.
Drinking water is one exposure route. These federal datasets also cover Merced County, CA.
The EPA places Merced County, CA in Zone 3 - the EPA's lowest-risk category, with a predicted average indoor radon level below 2 pCi/L. Zone maps predict an area's radon potential from geology and existing indoor readings; they say nothing about a specific house. Testing is the only way to know your own level.
47% of homes in Merced County, CA were built before 1980, and 11% before 1950. Residential lead paint was banned in 1978, so housing age is the strongest available proxy for lead-paint exposure. This describes the county's housing stock, not any one home.
Modelled annual fine-particle pollution (PM2.5) in Merced County, CA averages 10 µg/m³, above the EPA's annual standard of 9 µg/m³. These are CDC modelled county estimates rather than readings from a specific monitor.
How radon zones work · Air quality by county · Lead exposure guide