Radon in Drinking Water (2026): Risks and Removal

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The short answer

Radon in water is a radioactive gas that dissolves into groundwater, so it’s a concern for water from wells and other underground sources, not lakes, rivers, or reservoirs. The biggest risk isn’t drinking it: Radon escapes from water during showers and other household use, and breathing it raises lung cancer risk, while swallowing it carries a smaller stomach cancer risk. There is no final federal limit; the EPA proposed one in 1999 and says a final rule has not been published. Some states, including Maine and Vermont, use a 4,000 pCi/L guideline. Because the main risk is in the air, treatment has to happen where water enters the house, using aeration or granular activated carbon, not at a single tap.

Where does radon in water come from?

Radon forms when radium decays, and radium comes from the decay of uranium in rock and soil (USGS). Most radon gets into indoor air from the soil under buildings, and only about 1 to 2 percent comes from drinking water, according to a 1999 federal estimate.

In well water, it’s common. A USGS study of domestic wells in 30 aquifers, sampled from 1991 to 2004, found radon above 300 pCi/L, the lower of the two proposed federal limits, in about 65 percent of wells, and above 4,000 pCi/L, the higher proposed limit, in about 4 percent.

Is radon in water harmful?

Yes, mainly through the air. California says breathing radon in indoor air can cause lung cancer, and drinking water with radon also carries a risk of internal organ cancers, mainly stomach cancer, though that risk is smaller. In 1999, the EPA cited a National Academy of Sciences estimate that radon in drinking water causes about 168 cancer deaths a year in the U.S., and that about 89 percent of those deaths were lung cancers from breathing radon released into indoor air and about 11 percent were stomach cancers from drinking the water.

The same holds over a lifetime: Breathing radon raises lifetime lung cancer risk, and consuming radon in water poses a stomach cancer risk over that span.

What are the limits for radon in drinking water?

There’s no enforceable federal limit. On November 2, 1999, the EPA proposed a radon regulation with a maximum contaminant level of 300 pCi/L and an alternative maximum contaminant level of 4,000 pCi/L for water systems covered by an approved indoor air radon program, called a multimedia mitigation program. The agency’s rulemaking page for the radon regulation, last updated June 9, 2026, says a final rule has not been published. The federal gross alpha limit for public water also excludes radon.

Some states set their own guidance for wells.

Radon in water and radon in air use the same unit but are different measurements. The EPA recommends radon mitigation when indoor air radon is at or above 4 picocuries per liter of air. California has no federal or state drinking water standard for radon.

How do you test for radon in water?

Radon in water needs its own kit. Water testing devices and procedures differ from those used for radon in air, so private well owners concerned about radon may want to consider testing both. Maine’s lab uses a special small vial that must be returned promptly for analysis. A Penn State Extension fact sheet (revised 2001) calls direct water sampling the most accurate method and says to remove the faucet aerator and run the water for three minutes before sampling.

Levels vary, so test more than once. New Hampshire recommends at least two radon tests, at least one month apart when possible, before deciding on treatment, and testing at different times of the year is also worthwhile. Use a lab certified for drinking water testing. More in our well water guide.

How do you remove radon from water?

Radon can escape from water anywhere in the home, so it should be removed where water enters the house, not at one tap. Two point-of-entry methods can do that:

• Aeration bubbles air through the water and vents the radon outdoors with a fan. The University of Georgia rates it up to 99 percent effective, at about $4,000 installed.
• GAC (granular activated carbon) traps radon in a carbon tank. It’s rated more than 95 percent effective when radon is below 10,000 pCi/L, and much less effective above that.
• Point-of-use filters, such as under-sink or pitcher filters, don’t solve the problem. The University of Georgia says point-of-use devices do not adequately address radon risks and are not effective in reducing the risk from breathing radon released into the air from water used throughout the home, such as in showers.

Carbon tanks can become radioactive. Radon decay products can build up in the carbon to levels that need special disposal, so it’s recommended to place GAC systems outside the home or in an isolated area (Vermont Department of Health). Vermont discourages GAC for radon for this reason. A shield may be needed when radon is above 30,000 pCi/L, and spent filters need safe disposal.

If you do use carbon, look for certification to NSF/ANSI Standard 53; NSF’s listings include a “Radon Reduction” claim under NSF/ANSI 53. Water treatment won’t fix radon coming up through the foundation; New Hampshire notes that radon mitigation is achieved mainly by reducing the passage of radon through foundations. See our whole house water filter picks.

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