Groundwater in San Mateo County contains arsenic, chloride, iron, manganese, nitrate, PFOA, PFOS, radon, and sulfate. Multiple contaminants in your county exceed EPA health standards, so testing your well is important.
These contaminants come from a mix of sources. Arsenic and some metals occur naturally in the rock and soil layers below. Chloride, nitrate, and PFOA/PFOS enter groundwater from road salt, fertilizers, septic systems, and industrial activities on the land above. Radon comes from uranium naturally present in the rock.
Groundwater in this county is very hard, driven by high calcium and magnesium from the rock below, along with elevated sodium and sulfate. The coastal basin rocks here contain minerals that dissolve into water as it moves through the ground. These mineral concentrations are widespread across wells throughout the county.
Wells in San Mateo County show concerning levels of several contaminants that exceed EPA health standards. Arsenic, radon, and PFOA and PFOS chemicals are known to increase cancer risk with long-term exposure. Nitrate and manganese can harm infants and children, affecting oxygen in the blood and causing neurological problems. Chloride at high levels can be dangerous for people with heart disease or high blood pressure.
The county's very hard water creates real problems in your home. You'll see white scale buildup on faucets, in pipes, and on shower doors. Hard water also makes soap less effective for cleaning and laundry. The high mineral content can shorten the life of water heaters, dishwashers, and other appliances. Some people notice a salty or mineral taste in their water.
Every well in this county is different, and your water could have higher or lower levels than what's common here. Testing is the only way to know what you're actually drinking. We recommend a comprehensive water panel that checks for metals, minerals, bacteria, and chemicals--testing typically costs $200-$400. A water softener or whole-house filter system can help treat the hardness and mineral concerns once you know your specific situation.
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 ⓘ |
|---|---|---|---|---|---|
| Manganese | 36 | 56% | Moderate | High | |
| Radon | 11 | 55% | Low | High ⓘ | |
| Iron | 36 | 14% | Moderate | Moderate | |
| Arsenic | 18 | 11% | Moderate | Moderate | |
| Chloride | 54 | 9% | Moderate | Moderate | |
| Nitrate | 60 | 7% | Moderate | Moderate | |
| Sulfate | 53 | 6% | Moderate | Moderate | |
| PFOS ⓘ municipal | 103 | 1% | High | Low | |
| PFOA ⓘ municipal | 103 | 1% | High | Low | |
| PFHxS ⓘ municipal | 103 | 0% | High | Low | |
| Lead | 15 | 0% | Moderate | Low | |
| Uranium | 18 | 0% | Moderate | Low | |
| Nitrite | 60 | 0% | Moderate | Low | |
| HFPO-DA (GenX) ⓘ municipal | 103 | 0% | High | Safe | |
| Fluoride | 53 | 0% | Moderate | Low | |
| Boron | 2 | 0% | Low | Low ⓘ | |
| Chromium (total) | 2 | 0% | Low | Low ⓘ | |
| 1,2,3-Trichloropropane | 2 | 0% | Low | Safe ⓘ | |
| PFNA ⓘ municipal | 103 | 0% | High | Safe | |
| Perchlorate | 2 | 0% | Low | Low ⓘ | |
| Tds | 2 | — | Low | Low ⓘ | |
| PFBS ⓘ municipal | 103 | — | High | Safe | |
| Sodium | 54 | — | Moderate | Low | |
| Hardness | 54 | — | Moderate | Low | |
| pH | 63 | — | Moderate | 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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