Stockton, California sits in the heart of the San Joaquin Valley — one of the most productive agricultural regions on Earth and one of the most contaminated groundwater basins in the United States. The city’s water story is a case study in the tension between farming, industry, and clean drinking water.
The City of Stockton serves about 320,000 residents through a complex water supply that draws from both surface water (via the Delta Water Supply Project, which pulls from the Sacramento-San Joaquin Delta) and groundwater wells. That dual-source approach exists partly because neither source alone can consistently meet quality and quantity needs.
Nitrate: Agriculture’s Price Tag
The San Joaquin Valley’s groundwater has a nitrate problem that’s been building for decades. Millions of acres of irrigated agriculture, concentrated dairy operations, and fertilizer application have driven nitrate levels above safe drinking water standards across wide areas of the valley.
Stockton’s municipal wells have been affected. Several city wells have been taken offline or require blending due to elevated nitrate levels. The EPA maximum contaminant level for nitrate is 10 mg/L (as nitrogen), and some monitoring wells in the Stockton area have exceeded that.
For the city’s municipal customers, blending high-nitrate groundwater with treated surface water keeps tap water nitrate levels within compliance. But the underlying contamination isn’t going away — it’s decades of accumulated agricultural chemicals slowly moving through the aquifer.
Small community water systems and private wells in San Joaquin County face even higher risks. California’s State Water Resources Control Board has documented hundreds of water systems in the Central Valley that struggle with nitrate compliance. Many serve disadvantaged communities with limited resources for treatment.
1,2,3-TCP: A California-Specific Nightmare
1,2,3-trichloropropane (TCP) is a synthetic chemical that was a contaminant in soil fumigants widely used in California agriculture from the 1940s through the 1980s. It’s a probable carcinogen, extremely persistent in groundwater, and California set its MCL at 5 parts per trillion in 2017 — one of the lowest regulatory limits for any contaminant.
TCP contamination is widespread in the San Joaquin Valley’s groundwater. Multiple Stockton-area wells have detected TCP, and some have been shut down because treatment is expensive and the chemical is difficult to remove. Granular activated carbon (GAC) treatment works but requires frequent media replacement due to TCP’s poor adsorption characteristics.
This is a contamination problem largely invisible in states that don’t test for TCP. California’s proactive regulation exposed the scale of the issue.
Delta Water Quality
Stockton’s surface water comes from the Sacramento-San Joaquin Delta — a vast estuary that’s the hub of California’s water conveyance system. Delta water quality fluctuates significantly by season. During dry periods, saltwater intrusion from San Francisco Bay pushes into the delta, increasing salinity and bromide levels. Higher bromide leads to higher formation of disinfection byproducts during chlorination.
The delta also receives agricultural drainage from the entire Central Valley, carrying pesticides, herbicides, and nutrients. The City of Stockton’s Delta Water Supply Project treatment plant uses ozone and biological activated carbon — more advanced treatment than many cities need — specifically because of the challenging source water quality.
Chromium-6
Hexavalent chromium (chromium-6), which gained public attention through the Erin Brockovich case, occurs naturally in Central Valley soils and groundwater. California established a public health goal of 0.02 ppb for chromium-6 but has struggled to set an enforceable MCL due to treatment cost concerns.
Some Stockton-area groundwater wells have shown chromium-6 levels above the state’s public health goal. At these trace levels, the health risk is debated, but it adds to the overall cocktail of contaminants that Stockton’s water system must manage.
Drought and Subsidence
The Central Valley’s groundwater overdraft problem is legendary. During droughts, pumping intensifies and the water table drops — sometimes dramatically. In parts of the San Joaquin Valley, the land surface has sunk more than 28 feet due to aquifer compaction from excessive pumping.
While Stockton’s urban area hasn’t seen the worst subsidence, the regional drawdown affects water quality by pulling deeper, sometimes more contaminated water into the production zone. California’s Sustainable Groundwater Management Act (SGMA) is supposed to bring basins into balance by 2040, but progress has been slow.
What Residents Can Do
- Know your source. Whether your water comes from wells, delta water, or a blend affects which contaminants are most relevant.
- Filter your water. A quality carbon filter handles most of Stockton’s common contaminants — DBPs, taste, and some organics. For nitrate, you need reverse osmosis.
- Private well owners in San Joaquin County: Test for nitrates, TCP, bacteria, and arsenic at minimum. Many county wells exceed safe limits for one or more of these.
- Consider reverse osmosis if you have infants or are pregnant. Nitrate is particularly dangerous for babies under 6 months.
- Stay informed. The City of Stockton publishes water quality data, and the State Water Board’s GAMA program provides groundwater quality data by area.
If you’re concerned about your water quality, a certified water treatment professional can test your water and recommend the right treatment system.
See also our coverage of Bakersfield water quality and Fresno water quality.
Sources: City of Stockton Water Utility, California State Water Resources Control Board, EPA SDWIS, USGS GAMA, San Joaquin County Environmental Health