San Joaquin County, California: drinking water report. San Joaquin County – Stockton and the northern San Joaquin Valley – serves about 790,000 residents.
San Joaquin County – Stockton and the northern San Joaquin Valley – serves about 790,000 residents. Water sources include the Calaveras River, Stanislaus River, groundwater from the eastern San Joaquin subbasin, and imported water through the Delta-Mendota Canal. The county's agricultural setting means groundwater quality is shaped by decades of farming activity, and its position at the Sacramento-San Joaquin Delta creates complex surface water quality dynamics.
Nitrate contamination from agricultural fertilizer use is the primary groundwater concern in San Joaquin County. According to the California State Water Resources Control Board, nine public water systems in the county exceeded the nitrate MCL during the most recent compliance cycle. Small community water systems serving rural and disadvantaged communities are the most affected.
UCMR5 data shows PFAS detections in several county systems, with the City of Stockton reporting low-level readings. The Sharpe Army Depot, a military logistics facility in Lathrop, is a potential PFAS source. Additionally, naturally occurring arsenic in some eastern county wells has required treatment or well closure in several systems.
Rural communities and small water systems face the most significant quality challenges in this county. City of Stockton water meets federal standards but carries the contaminant profile typical of Central Valley agriculture.
Check your water for data specific to your provider. For nitrate and arsenic, reverse osmosis is the most effective household treatment. Our water filter guide details certified systems. Pull your detailed report, and visit our California page for statewide data.