Groundwater in Mariposa County contains iron, manganese, and chloride. These contaminants are present at levels high enough to exceed EPA health standards, so well owners should be aware of them.
These metals and salt come from the underground rock and soil layers in the Central Valley aquifer system. Iron and manganese dissolve naturally into water as it moves through rock and sediment over time. Chloride can come from natural mineral deposits in the aquifer or from agricultural and road salt that seeps down from the surface.
Groundwater in this county is soft, with low iron and minimal dissolved minerals overall. The water moves through sand, gravel, and clay layers that do not add much hardness or other minerals to the water. Most wells in the area show similarly soft water with low mineral content.
Wells in Mariposa County show chloride, iron, and manganese at levels that exceed EPA health standards. Chloride at high levels can raise blood pressure in some people, especially those on salt-restricted diets. Iron and manganese can affect the nervous system with long-term exposure, and manganese is of particular concern for young children and developing brains.
The good news is that the county's water is generally soft, so you probably won't see heavy scale buildup on faucets or inside pipes. Appliances like water heaters should have a normal lifespan. However, wells with elevated iron and manganese can cause brown or black staining on sinks, toilets, and laundry, and some people notice a metallic taste in their water.
We recommend getting your well tested to find out exactly what is in your water, since every well is different and yours could have higher or lower levels than the county average. Testing is the only way to know if you need treatment. A comprehensive metals and minerals panel typically costs between $200 and $400 and will show you what you're dealing with. Water softeners or iron and manganese filters can address these concerns effectively.
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 ⓘ |
|---|---|---|---|---|---|
| Iron | 55 | 11% | Moderate | Moderate | |
| Manganese | 55 | 9% | Moderate | Moderate | |
| Chloride | 55 | 7% | Moderate | Moderate | |
| Uranium | 5 | 0% | Low | Low ⓘ | |
| PFHxS ⓘ municipal | 4 | 0% | Low | Safe ⓘ | |
| Radon | 16 | 0% | Moderate | Low ⓘ | |
| Lead | 38 | 0% | Moderate | Low | |
| Nitrate | 5 | 0% | Low | Low ⓘ | |
| PFNA ⓘ municipal | 4 | 0% | Low | Safe ⓘ | |
| PFOA ⓘ municipal | 4 | 0% | Low | Safe ⓘ | |
| Arsenic | 38 | 0% | Moderate | Low | |
| Fluoride | 55 | 0% | Moderate | Low | |
| HFPO-DA (GenX) ⓘ municipal | 4 | 0% | Low | Safe ⓘ | |
| Nitrite | 5 | 0% | Low | Low ⓘ | |
| PFOS ⓘ municipal | 4 | 0% | Low | Safe ⓘ | |
| Sulfate | 55 | 0% | Moderate | Low | |
| Hardness | 55 | — | Moderate | Low | |
| Sodium | 55 | — | Moderate | Low | |
| pH | 85 | — | Moderate | Low | |
| PFBS ⓘ municipal | 4 | — | Low | Safe ⓘ |
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.
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