Contra Costa County, CA
These figures come from wells tested in Contra Costa County, CA, not from any one well.
- Iron is common in wells in Contra Costa County, CA (5 of 46 tested wells were over that level), so it's worth testing for.
- Manganese is common in wells in Contra Costa County, CA (25 of 41 tested wells were over that level), so it's worth testing for.
- Sulfate is common in wells in Contra Costa County, CA (6 of 53 tested wells were over that level), so it's worth testing for.
- Wells in Contra Costa County, CA are typically very hard (55 wells tested).
- Most of these county tests are older than 2000, so treat them as a rough guide.
- Only a test of your own well tells you about your water.
What to test for
Start with the four this site covers:
- iron (common in wells here)
- manganese (common in wells here)
- sulfate (common in wells here)
- 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 Older tests
| Level it's compared with | 0.3 mg/L |
|---|---|
| Share of wells over the level | 11% |
| Wells over the level / wells judged | 5 of 46 |
| Median | < 0.05 mg/L |
| Years sampled | 1958–2023 |
| Wells | 46 |
| Test results | 52 |
| Wells left out | 16 |
| Sources | USGS (46) |
More than half the wells had results below the lab's detection limit, so the median is reported as below that limit.
Figures for this county come mostly from older survey wells: fewer than 10 of its wells have a sample since 2000. In Pennsylvania and Wisconsin, where we could compare, such wells showed iron and manganese over the limit more often than homeowner samples did. Test before deciding anything.
Manganese Older tests
| Level it's compared with | 0.05 mg/L |
|---|---|
| Share of wells over the level | 61% |
| Wells over the level / wells judged | 25 of 41 |
| Median | 0.07 mg/L |
| Years sampled | 1974–2023 |
| Wells | 43 |
| Test results | 46 |
| Wells left out | 16 |
| Sources | USGS (43) |
Figures for this county come mostly from older survey wells: fewer than 10 of its wells have a sample since 2000. In Pennsylvania and Wisconsin, where we could compare, such wells showed iron and manganese over the limit more often than homeowner samples did. Test before deciding anything.
Hardness Older tests
| Level it's compared with | none; hardness is shown in bands |
|---|---|
| Share of wells very hard | 85% |
| Median | 370 mg/L as CaCO₃ |
| Years sampled | 1957–2023 |
| Wells | 55 |
| Test results | 84 |
| Wells left out | 18 |
| Sources | USGS (55) |
Sulfate Older tests
| Level it's compared with | 250 mg/L |
|---|---|
| Share of wells over the level | 11% |
| Wells over the level / wells judged | 6 of 53 |
| Median | 130 mg/L |
| Years sampled | 1957–2023 |
| Wells | 53 |
| Test results | 72 |
| Wells left out | 18 |
| Sources | USGS (53) |