Groundwater in San Luis Obispo County contains multiple contaminants that well owners should watch for, including arsenic, boron, chloride, iron, manganese, nitrate, radon, uranium, and PFOA and PFOS chemicals. Many of these reach levels that exceed federal health standards, making them a real concern for your drinking water.
These contaminants come from a mix of sources in the county's geology and land use. Arsenic and uranium occur naturally in the underground rock and soil layers that make up the Central Valley aquifer system. Nitrate seeps down from agricultural fertilizers and animal waste. Chloride builds up from old seawater that mixed into the rock long ago, and salt contamination from irrigation. Radon comes straight from radioactive minerals in the surrounding rock. PFOA and PFOS are human-made chemicals from industrial and consumer products that have entered the groundwater.
Groundwater in this county is very hard, driven by elevated calcium and magnesium from the underground rock, and carries very high sulfate and sodium levels. The rock layers here naturally contain sulfur-bearing minerals that dissolve into water as it moves underground, while sodium from ancient seawater trapped in the rock mixes in over time. These characteristics are widespread across wells throughout San Luis Obispo County.
Wells in San Luis Obispo County commonly contain arsenic, uranium, nitrate, boron, radon, and PFOA/PFOS at levels that exceed EPA health standards. Arsenic and uranium are linked to cancer and organ damage with long-term exposure. Nitrate is especially dangerous for babies and can cause serious illness. Radon increases lung cancer risk. Boron and PFOA/PFOS can harm reproduction and child development.
The county's water is extremely hard with very high sulfate and sodium content. Hard water leaves white scale and stains on fixtures, dishes, and laundry. It shortens the lifespan of water heaters and dishwashers. High sulfate can give water a bitter or salty taste and a rotten egg smell. Soap doesn't lather well in this water.
Testing is the only way to know what is actually in your well, since every well is different and yours may have higher or lower levels than the county average. We recommend a comprehensive water panel to check for metals, minerals, bacteria, and harmful chemicals--this typically costs $200-400. A basic test for bacteria and nitrate runs $50-100. Treatment options like reverse osmosis, activated carbon, or ion exchange can address specific problems once you know what you're dealing with.
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 ⓘ |
|---|---|---|---|---|---|
| Radon | 21 | 90% | Moderate | High ⓘ | |
| Sulfate | 802 | 84% | High | High | |
| Manganese | 472 | 21% | High | High | |
| Boron | 23 | 13% | Moderate | Moderate | |
| Nitrate | 600 | 12% | High | Moderate | |
| Iron | 528 | 10% | High | Moderate | |
| Uranium | 86 | 9% | Moderate | Moderate | |
| PFOA ⓘ municipal | 128 | 9% | High | Moderate | |
| PFOS ⓘ municipal | 128 | 9% | High | Moderate | |
| Chloride | 945 | 5% | High | Moderate | |
| Arsenic | 87 | 2% | Moderate | Low | |
| PFHxS ⓘ municipal | 127 | 1% | High | Low | |
| Lead | 70 | 0% | Moderate | Low | |
| PFNA ⓘ municipal | 128 | 0% | High | Low | |
| Chromium (total) | 23 | 0% | Moderate | Low | |
| 1,2,3-Trichloropropane | 23 | 0% | Moderate | Safe | |
| Fluoride | 730 | 0% | High | Low | |
| HFPO-DA (GenX) ⓘ municipal | 128 | 0% | High | Safe | |
| Nitrite | 338 | 0% | High | Low | |
| Perchlorate | 23 | 0% | Moderate | Low | |
| Tds | 23 | — | Moderate | Low | |
| pH | 23 | — | Moderate | Low | |
| Sodium | 811 | — | High | Low | |
| Total Coliform | 2* | 0% | Low | Safe ⓘ | |
| PFBS ⓘ municipal | 127 | — | High | Low | |
| E. coli | 2* | 0% | Low | Safe ⓘ | |
| Hardness | 879 | — | High | 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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