Radon in well water

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What you need to know about radon

These counties see the most radon in their wells

Ranked by the share of tested wells with a result over the limit. We only list counties with at least 25 tested wells, so a few bad results can't skew the ranking.

* Likely overstated, because of who tested these wells, when or where.

See the full rankings and download the data

Radon comes from uranium breaking down in the rock

Radon is a gas with no color, smell or taste. It forms when uranium in rock and soil breaks down. Underground, it dissolves into groundwater, and the USGS says it's in most groundwater in the U.S.

Wells in granite and similar hard, crystalline rock have the most. In a national USGS study, radon topped 4,000 pCi/L in 25% to 50% of samples from the crystalline-rock aquifers of New England, the Piedmont and Blue Ridge, and Colorado's Front Range. Across all aquifers, it was 3.6%. About 62% of all samples were over 300 pCi/L.

Lakes and rivers lose their radon to the air, so the EPA says it's only a concern for water from underground.

The bigger risk is the radon you breathe after it leaves the water

When you shower, wash dishes or cook, radon escapes from the water into the air. Breathing it is the main risk. Radon breaks down into radioactive particles that lodge in the lungs and damage lung tissue over time.

Radon in indoor air, from all sources, is the second leading cause of lung cancer in the U.S. The EPA estimates about 21,000 lung cancer deaths a year. People who smoke have an even greater risk.

Water is a small share of that. The EPA says only about 1% to 2% of radon in indoor air comes from tap water. Based on a National Academy of Sciences report, it estimates radon in drinking water causes about 168 cancer deaths a year: 89% are lung cancer from breathing radon released from water, and 11% are stomach cancer from drinking it.

The EPA says water at 4,000 pCi/L adds about 0.4 pCi/L to your home's air. That works out to about 1 pCi/L in air for every 10,000 pCi/L in water. For comparison, the EPA recommends fixing your home when air tests at 4 pCi/L or higher.

In 1999 the EPA proposed a limit of 300 pCi/L for public water, or 4,000 pCi/L in states that run programs to lower radon in indoor air. The rule was never finished. There is no federal limit for radon in drinking water, and the proposal never covered private wells.

Test the air first, then the water

Because indoor air is the larger risk, the EPA recommends testing your home's air before your water. If you have a private well, the EPA recommends testing the water for radon too.

Radon escapes from water easily and loses half its strength in under 4 days, so the sample has to be collected carefully and reach the lab fast. Get a radon-in-water kit from a certified lab and follow its fill and shipping instructions exactly. Your state radon office can point you to labs.

Some states offer free or low-cost testing for private wells. Check what yours has.

Whole-house treatment removes it before it reaches the shower

The EPA says the most effective treatment removes radon right where water enters the house, since the risk comes from every tap and shower. There are 2 kinds.

Costs are equipment ranges from Penn State Extension. The EPA and Kansas State University's radon program both note that GAC costs less and that its radioactive buildup needs careful handling.

Questions people ask about radon