Groundwater in San Joaquin County contains arsenic, boron, chloride, nitrate, uranium, and several other contaminants that exceed EPA health standards. These levels are serious enough that well owners in this county need to test their water and take action if results show contamination.
These contaminants come from a mix of sources. Arsenic and uranium occur naturally in the rock and sediment of the Central Valley aquifer system. Nitrate and nitrite seep down from agricultural fertilizers and animal waste spread across the county's farmland. Chloride builds up from old saltwater that got trapped in deeper layers of rock, and it moves upward as fresh groundwater gets pumped out. Industrial chemicals like PFOA and PFOS come from past manufacturing and military sites. Trichloropropane got into the water from pesticide use and improper disposal.
Groundwater in this county is hard, driven by calcium and magnesium that dissolve from the surrounding rock layers. The Central Valley aquifer system naturally produces hard water as water moves slowly through fine sediment and rock. Hard water is the most common characteristic across wells throughout this county.
Wells in San Joaquin County show a serious mix of contaminants above EPA health standards. Arsenic and uranium can cause cancer and kidney damage with long-term exposure. Nitrate and nitrite affect how your blood carries oxygen, especially in babies. Boron, chloride, and sulfate can harm your kidneys and digestive system. PFOA, PFOS, and PFHXS are industrial chemicals linked to liver damage, thyroid problems, and immune system issues. Trichloropropane is a pesticide that causes cancer. Manganese can damage your nervous system and brain development in children.
County wells tend to have hard water, which leaves scale buildup on fixtures and inside pipes and appliances. Hard water makes soap less effective and can shorten the lifespan of water heaters and dishwashers. You may notice spots on dishes, reduced water pressure over time, and stiffness in clothes after washing.
Testing is the only way to know what is actually in your well since every well is different--yours could have higher or lower levels than what is common in the county. We recommend a comprehensive metals and minerals panel, which typically costs between $200 and $400. Treatment options like reverse osmosis systems or activated carbon filters can address multiple contaminants at once, but the right choice depends on your specific test results.
Not sure if your well is affected? Get certified results in 5–7 days.
Test Your Well Water with Tap Score →| Contaminant | Samples ⓘ | % Above MCL ⓘ | Distribution ⓘ | Confidence ⓘ | Risk ⓘ |
|---|---|---|---|---|---|
| Manganese | 415 | 26% | High | High | |
| Arsenic | 409 | 24% | High | High | |
| Chloride | 1151 | 19% | High | High | |
| Nitrate | 752 | 10% | High | Moderate | |
| Sulfate | 772 | 9% | High | Moderate | |
| 1,2,3-Trichloropropane | 44 | 9% | Moderate | Moderate | |
| Iron | 432 | 6% | High | Moderate | |
| PFOS ⓘ municipal | 230 | 6% | High | Moderate | |
| PFOA ⓘ municipal | 230 | 5% | High |
Moderate
Elevated concentration, not % above limit
|
|
| Boron | 45 | 4% | Moderate |
Moderate
Elevated concentration, not % above limit
|
|
| Uranium | 160 | 4% | High |
Moderate
Elevated concentration, not % above limit
|
|
| PFHxS ⓘ municipal | 230 | 1% | High | Low | |
| Nitrite | 296 | 0% | High | Low | |
| PFNA ⓘ municipal | 230 | 0% | High | Safe | |
| Fluoride | 634 | 0% | High | Low | |
| HFPO-DA (GenX) ⓘ municipal | 193 | 0% | High | Safe | |
| Perchlorate | 40 | 0% | Moderate | Low | |
| Radon | 28 | 0% | Moderate | Low ⓘ | |
| Lead | 138 | 0% | High | Low | |
| Chromium (total) | 44 | 0% | Moderate | Low | |
| pH | 42 | — | Moderate | Low | |
| Tds | 44 | — | Moderate | Low | |
| Hardness | 1005 | — | High | Low | |
| E. coli | 37* | 0% | Moderate | Low | |
| Sodium | 1004 | — | High | Low | |
| PFBS ⓘ municipal | 230 | — | High | Low | |
| Total Coliform | 37* | 0% | Moderate | Low |
MCL = Maximum Contaminant Level (EPA limit for public water; used as benchmark for private wells). Distribution shows % of sampled wells in each concentration band. Methodology.
* Microbial contaminants (E. coli, coliform) reflect only samples from the last 10 years.
Data shows potential risk — a certified test confirms whether your water is affected.
Order a Tap Score Test →Population-level CDC data. Not individual risk prediction.
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