San Luis Obispo County, CA
These figures come from wells tested in San Luis Obispo County, CA, not from any one well.
- Iron is common in wells in San Luis Obispo County, CA (4 of 33 tested wells were over that level), so it's worth testing for.
- Sulfate is common in wells in San Luis Obispo County, CA (22 of 50 tested wells were over that level), so it's worth testing for.
- Manganese is not common in wells in San Luis Obispo County, CA (2 of 33 tested wells were over). Only a test tells you about your own well.
- Wells in San Luis Obispo County, CA are typically very hard (51 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:
- iron (common in wells here)
- sulfate (common in wells here)
- 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 | 12% |
| Wells over the level / wells judged | 4 of 33 |
| Median | 0.0101 mg/L |
| Years sampled | 1942–2026 |
| Wells | 33 |
| Test results | 73 |
| Wells left out | 72 |
| Sources | USGS (43) |
Manganese Recent
| Level it's compared with | 0.05 mg/L |
|---|---|
| Share of wells over the level | 6.1% |
| Wells over the level / wells judged | 2 of 33 |
| Median | 0.00384 mg/L |
| Years sampled | 1980–2026 |
| Wells | 33 |
| Test results | 65 |
| Wells left out | 55 |
| Sources | USGS (38) |
Hardness Recent
| Level it's compared with | none; hardness is shown in bands |
|---|---|
| Share of wells very hard | 82% |
| Median | 480 mg/L as CaCO₃ |
| Years sampled | 1927–2026 |
| Wells | 51 |
| Test results | 158 |
| Wells left out | 107 |
| Sources | USGS (51) |
Sulfate Recent
| Level it's compared with | 250 mg/L |
|---|---|
| Share of wells over the level | 44% |
| Wells over the level / wells judged | 22 of 50 |
| Median | 205.75 mg/L |
| Years sampled | 1927–2026 |
| Wells | 50 |
| Test results | 140 |
| Wells left out | 96 |
| Sources | USGS (50) |