Well Water Contaminants
Nobody tests a private well for everything — the lab bill would be enormous and most of it would come back clean. These are the contaminants worth your money, why each one matters, and what USGS actually measured in wells across 2,119 U.S. counties.
No federal agency tests or treats a private well — that is the owner's job. Everything on this site is a modeled or measured figure for an area, covering 3,103 counties and 49 states. It is never a result for your own tap. Test your well to find out what is in yours.
The four to read first
- Arsenic in well water
Naturally occurring, odorless and tasteless, dissolved out of bedrock. A long-term cancer risk, and the contaminant this site maps county by county.
- Nitrate in well water
From fertilizer, manure and septic systems. The one contaminant that can harm an infant within days, so it is the first test for any household with a baby or a pregnancy.
- Lead in well water
Rarely in the groundwater itself — it comes from your own plumbing when the water is acidic. Low pH is the warning sign to look for.
- PFAS in well water
"Forever chemicals" from firefighting foam and industry. Not covered by any USGS well model, so proximity to a known source is the only practical screen.
What USGS measured, parameter by parameter
County medians from USGS well samples, counted only where at least 5 wells were sampled. The last column is the county with the highest share of sampled wells over the limit.
| Parameter | Limit | Counties sampled | Worst county sampled |
|---|---|---|---|
| Nitrate (as N) | > 10 mg/L (EPA MCL) | 1,027 | Terry County, TX — 60% of 20 wells |
| Arsenic | > 10 µg/L (EPA MCL) | 1,634 | Fulton County, KY — 89% of 18 wells |
| Manganese | > 300 µg/L (EPA health advisory) | 1,484 | Tensas County, LA — 100% of 10 wells |
| Ironaesthetic | > 300 µg/L (EPA secondary standard) | 1,137 | Washington County, MS — 100% of 22 wells |
| pHaesthetic | outside 6.5–8.5 (EPA secondary range) | 1,839 | Hickman County, KY — 100% of 46 wells |
Manganese, iron and pH have no page of their own — they are reported on each county page. Iron and pH are aesthetic standards, not health limits. Lead is not sampled in these datasets because it enters the water after the well, from plumbing; acidic pH is its proxy.
What actually removes each one
A carbon pitcher removes none of the contaminants on this page. Point-of-use reverse osmosis under the kitchen sink handles arsenic (above 10 µg/L), nitrate (above 10 mg/L), lead and most PFAS at the tap you drink from. Iron, manganese and corrosive pH are whole-house problems and need different equipment — an oxidizing filter, a softener, or an acid neutralizer. Test first, then match the treatment to the result, and re-test after installing.
Find out what's actually in your well
Private wells are not regulated by the EPA — nobody tests your well for you. Arsenic, nitrate and coliform bacteria are the three most common private-well problems, and none of them has a taste or smell. A certified lab test checks all of them at once; the CDC and EPA recommend testing every well at least once a year.
Tap Score lab kit · 50+ contaminants · results in days
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