Radon in Well Water: The Numbers, the Real Risk, and When to Treat

Radon in well water matters mostly because it escapes into the air you breathe: as a rule of thumb, every 10,000 pCi/L in the water adds about 1 pCi/L to indoor air. There is no enforceable federal limit, so state guidance rules, from Maine's 4,000 pCi/L flag to treatment recommendations at 10,000 pCi/L. Here's how to read your number and what fixing it costs.

Why does radon in well water matter?

Radon is a radioactive gas that dissolves into groundwater from uranium-bearing rock. When you shower, do laundry, or run the dishwasher, much of it bubbles out into the house. Breathing that air raises lung cancer risk, which is why the water number matters mainly for what it adds to indoor air.

The standard conversion comes from the National Academy of Sciences' 1999 report on radon in drinking water: about 10,000:1. So 5,000 pCi/L in your well adds roughly 0.5 pCi/L to your air, against EPA's 4 pCi/L action level for indoor air. Massachusetts guidance stresses the ratio varies widely from house to house. In most homes, radon seeping up from the soil is the bigger source.

What level of radon in well water is safe?

No level is officially safe, and none is enforceable. EPA proposed a 300 pCi/L limit for public water systems in November 1999, with a 4,000 pCi/L alternative for states running indoor-air radon programs, but never finalized either. Private wells fall to state guidance, which mostly clusters between 4,000 and 10,000 pCi/L.

• Maine CDC — 4,000 pCi/L (interim guideline, set October 2006): a flag to review total radon, not an automatic trigger. Its treatment guide says maybe treat between 4,000 and 10,000, and treatment is recommended at 10,000 or higher.
• New Hampshire DES (fact sheet DWGB-3-12, 2016, reissued 2020): treat at 10,000 pCi/L or more, alongside air mitigation; between 2,000 and 10,000, treatment may be advisable if indoor air exceeds 4 pCi/L.
• Connecticut DPH — 5,000 pCi/L (fact sheet, 2012): reduce radon if the average of two tests reaches 5,000.
• Massachusetts — 10,000 pCi/L: a guideline from MassDEP's Office of Research and Standards, cited in a 2007 UMass Extension fact sheet.

EPA's 4,000 figure isn't arbitrary: it's the water level that adds about 0.4 pCi/L to indoor air, the national average for outdoor air. Rural rules differ by state in other ways too; see our septic rule guides for Maine and New Hampshire.

Is radon in drinking water a health risk?

Yes, but mostly through your lungs. EPA, using the National Academy of Sciences' 1999 risk assessment, estimated radon in drinking water causes about 168 U.S. cancer deaths a year: 89% lung cancer from radon released into indoor air, and 11% stomach cancer from swallowing it. Both risks rise with the concentration.

Smoking multiplies the lung risk. Maine CDC's 2006 analysis of the same report found that drinking the water accounts for roughly 7% of the total risk for smokers and 28% for never-smokers. Maine CDC notes the stomach estimate comes from modeling; population studies haven't been able to measure it directly.

Where is radon in well water common?

It's highest in drilled wells in granite and other crystalline bedrock: New England, the Piedmont from Pennsylvania south, the central and southern Appalachians, and parts of Colorado and other Rocky Mountain areas. Nationally, USGS found about 65% of domestic wells above 300 pCi/L but only about 4% above 4,000.

Regionally the picture is starker. In a USGS study of crystalline-rock aquifers in New England, New Jersey, and New York (1995–2007), 33% of 943 wells reached 4,000 pCi/L. In Pennsylvania, 14% of 1,041 groundwater samples topped 4,000, with the highest levels in the Piedmont's schist and gneiss. New Hampshire notes dug and point wells tend to have little radon, while drilled bedrock wells are where it shows up. Your well log, which our well depth guide shows how to find, tells you which kind you have.

How do you test well water for radon?

Test the air first; it's cheaper and the bigger risk. Then test the water if you have a drilled well in high-radon geology or your air is above 4 pCi/L. Order a radon-in-water kit from a state-certified lab, typically $25–$100, and take two samples, ideally about three months apart.

• Run the water first. Massachusetts guidance says to flush a little-used system for at least 20 minutes so the sample is fresh well water.
• Don't let it splash. Remove the faucet aerator, sample before any treatment, and fill the vial gently with a slow stream.
• No air bubble. Fill to the top so there's no headspace, then cap it tight.
• Ship it fast. Radon's half-life is about 3.8 days, so a slow sample reads low. Sample early in the week and send it the same day.

Add uranium and gross alpha to the order. New Hampshire recommends both for bedrock wells, since a high-radon well is more likely to have them. They're separate problems with enforceable public water limits: 30 µg/L for uranium (set by EPA in 2000) and 15 pCi/L for gross alpha. The same rocks can carry arsenic, so our well water testing guide covers a full panel. It all belongs on the list when buying a house with a well.

Radon in water levels and fixes at a glance

Use the average of two water tests and read it alongside your indoor air result. Below 4,000 pCi/L, most homes just retest. Between 4,000 and 10,000, your air result and your state's number decide. At 10,000 and up, Maine, New Hampshire, and Massachusetts guidance all point toward treating the water.

What removes radon from well water?

Whole-house treatment where the water enters the home. Aeration bubbles or sprays air through the water and vents the radon outdoors, removing 85% to 99% per Maine CDC. Granular activated carbon (GAC) traps radon in a large tank and suits lower levels. Connecticut limits carbon to water under 10,000 pCi/L; MassDEP advises against carbon above 5,000 pCi/L because of radiation buildup in the tank and disposal problems.

1. Aeration (about $5,000–$8,000 installed): the only effective method at or above 10,000 pCi/L, per Connecticut. A blower mixes air into the water in a tank, a vent pipe carries the radon above the roofline, and a pump repressurizes the water. Plan on annual service and electricity for the fan. Iron or manganese can oxidize inside the tank, so check your manganese and iron results first.
2. Granular activated carbon (about $1,500–$3,500 installed): simpler and quieter, with no vent pipe. The catch is radioactivity: Massachusetts warns radon decay builds up on the carbon, which can make the tank too radioactive to be near in a basement and expensive to dispose of. Install the tank away from bedrooms and play areas, and have a water treatment professional change and dispose of the media; spent carbon may need handling as radioactive waste.
3. Kitchen-tap filters (no help): reverse osmosis or pitcher filters treat one faucet, but the shower, washer, and dishwasher still release radon into the air.

Neither aeration nor carbon is a uranium fix; that takes treatment such as reverse osmosis or anion exchange. Our treatment cost guide prices each option. Retest the treated water after installation and periodically, since carbon wears out and fans fail.