Stanislaus County, California: drinking water report. Stanislaus County sits in California's Central Valley, an agricultural powerhouse with a population
Stanislaus County sits in California's Central Valley, an agricultural powerhouse with a population of about 550,000 centered on the city of Modesto. Water here comes from a mix of surface diversions from the Tuolumne and Stanislaus Rivers and groundwater from the San Joaquin Valley aquifer system. Decades of intensive agriculture have reshaped the groundwater quality in ways that affect every resident, whether they live in the city or on a rural well.
Nitrate contamination is the defining water quality issue in Stanislaus County. According to the California State Water Resources Control Board, 15 community water systems in the county have recorded nitrate levels above 50% of the MCL, with several small systems exceeding it outright. The source is agricultural – fertilizers, dairy operations, and animal waste from the county's thousands of farms percolate through the sandy soil into the aquifer.
The EPA's UCMR5 program also detected PFAS at sampling points in the county, though at lower concentrations than military-adjacent areas. A 2024 UC Davis study of Central Valley groundwater found that 1,4-dioxane, a solvent used in industrial processes, was present in 22% of tested wells across the region, including sites in Stanislaus County.
If you are on a private well in Stanislaus County, testing for nitrate is not optional – it is the most likely contaminant you will encounter, and it is odorless and tasteless. Municipal systems treat for it, but small community systems sometimes struggle to keep up.
Check your water for the latest data on your area. For nitrate, reverse osmosis or ion exchange filters are effective at the household level. Our water filter guide covers which systems handle agricultural contaminants. Pull your detailed report for trend data, and visit our California page for statewide context.
Drinking water is one exposure route. These federal datasets also cover Stanislaus County, CA.
The EPA places Stanislaus 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.
49% of homes in Stanislaus 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 Stanislaus County, CA averages 11 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