Stanislaus County, CA
These figures come from wells tested in Stanislaus County, CA, not from any one well.
- Sulfate is common in wells in Stanislaus County, CA (18 of 160 tested wells were over that level), so it's worth testing for.
- Iron and manganese are not common in wells in Stanislaus County, CA (0 of 55 and 3 of 55 tested wells were over). Only a test tells you about your own well.
- Wells in Stanislaus County, CA are typically very hard (209 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:
- sulfate (common in wells here)
- iron
- manganese
- hardness
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 | 0.0% |
| Wells over the level / wells judged | 0 of 55 |
| Median | < 0.01 mg/L |
| Years sampled | 1968–2025 |
| Wells | 55 |
| Test results | 185 |
| Wells left out | 156 |
| Sources | USGS (98) |
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 | 5.5% |
| Wells over the level / wells judged | 3 of 55 |
| Median | < 0.01 mg/L |
| Years sampled | 1959–2025 |
| Wells | 55 |
| Test results | 180 |
| Wells left out | 168 |
| Sources | USGS (91) |
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 | 71% |
| Median | 249 mg/L as CaCO₃ |
| Years sampled | 1934–2025 |
| Wells | 209 |
| Test results | 615 |
| Wells left out | 235 |
| Sources | USGS (209) |
Sulfate Recent
| Level it's compared with | 250 mg/L |
|---|---|
| Share of wells over the level | 11% |
| Wells over the level / wells judged | 18 of 160 |
| Median | 20.25 mg/L |
| Years sampled | 1934–2025 |
| Wells | 160 |
| Test results | 496 |
| Wells left out | 218 |
| Sources | USGS (160) |