San Joaquin County, CA
These figures come from wells tested in San Joaquin County, CA, not from any one well.
- Manganese is common in wells in San Joaquin County, CA (10 of 62 tested wells were over that level), so it's worth testing for.
- Iron and sulfate are not common in wells in San Joaquin County, CA (2 of 62 and 28 of 345 tested wells were over). Only a test tells you about your own well.
- Wells in San Joaquin County, CA are typically hard (372 wells tested).
- Only a test of your own well tells you about your water.
What to test for
Start with the four this site covers:
- manganese (common in wells here)
- iron
- hardness
- sulfate
Most well owners also test for these. We point to the source and don't interpret the results.
- arsenic: occurs naturally in groundwater in many regions. USGS: arsenic and drinking water
- nitrate: common near farmland and septic systems. EPA: private drinking water wells
- total coliform bacteria: the standard yearly well check. EPA: private drinking water wells
- lead and copper: from household plumbing, especially when pH is below 6.5. EPA: lead in drinking water
- PFAS: several states run private-well sampling programs. EPA: PFAS
- a certified lab: states certify labs and some run their own private-well programs. California's certified-lab list (from your state drinking-water program)
The numbers
County figures describe the wells sampled there, not any one well. Exact counts, sample years and sources are below.
Iron Recent
| Level it's compared with | 0.3 mg/L |
|---|---|
| Share of wells over the level | 3.2% |
| Wells over the level / wells judged | 2 of 62 |
| Median | < 0.018 mg/L |
| Years sampled | 1959–2025 |
| Wells | 62 |
| Test results | 247 |
| Wells left out | 133 |
| Sources | USGS (187) |
More than half the wells had results below the lab's detection limit, so the median is reported as below that limit.
Manganese Recent
| Level it's compared with | 0.05 mg/L |
|---|---|
| Share of wells over the level | 16% |
| Wells over the level / wells judged | 10 of 62 |
| Median | < 0.01 mg/L |
| Years sampled | 1956–2025 |
| Wells | 62 |
| Test results | 236 |
| Wells left out | 127 |
| Sources | USGS (179) |
More than half the wells had results below the lab's detection limit, so the median is reported as below that limit.
Hardness Recent
| Level it's compared with | none; hardness is shown in bands |
|---|---|
| Share of wells very hard | 44% |
| Median | 150 mg/L as CaCO₃ |
| Years sampled | 1947–2025 |
| Wells | 372 |
| Test results | 661 |
| Wells left out | 198 |
| Sources | USGS (372) |
Sulfate Recent
| Level it's compared with | 250 mg/L |
|---|---|
| Share of wells over the level | 8.1% |
| Wells over the level / wells judged | 28 of 345 |
| Median | 14 mg/L |
| Years sampled | 1944–2025 |
| Wells | 345 |
| Test results | 475 |
| Wells left out | 184 |
| Sources | USGS (345) |