Radon in well water: how to test for it and what the levels mean

Radon is a radioactive gas that forms naturally as uranium in rock breaks down. Groundwater moving through that rock picks it up, so private wells in some regions carry far more radon than public supplies drawn from rivers and lakes. You cannot see, smell or taste it, and the only way to know your level is to test. This page explains what the numbers mean, how to sample correctly and how radon is removed.

Why radon in water matters

When radon-laden water comes out of a shower, washing machine or faucet, much of the gas escapes into the air you breathe. EPA's fact sheet on radon in drinking water estimated that of the cancer deaths it attributed to radon in water, 89% were lung cancers from breathing radon released to indoor air and 11% were stomach cancers from drinking it. It also noted that only about 1 to 2 percent of radon in indoor air comes from drinking water, and that EPA recommends testing the air in your home before testing the water. EPA (archive): Radon in Drinking Water Questions and Answers [EPA-RADON-QA source note]

In other words, radon in water is mainly an indoor-air problem with a water source. Penn State Extension's rule of thumb is that indoor air concentrations rise by about 1 pCi/L for every 10,000 pCi/L of radon in the water, which is why only very high water levels move indoor air much. Penn State Extension: Reducing Radon in Drinking Water [PSU-RADON source note]

Is there a legal limit for radon in well water?

No. EPA does not regulate private wells, and its radon rule for public water systems was never finalized. EPA's rulemaking record shows the proposed rule was published on November 2, 1999, and that a final rule has not been published. EPA: SBAR Panel, National Primary Drinking Water Regulations: Radon The proposal offered two levels: 4,000 pCi/L for systems in states running programs to reduce radon in indoor air, or 300 pCi/L otherwise. EPA (archive): Proposed Radon in Drinking Water Regulation The National Ground Water Association likewise notes there is currently no federally enforced drinking water standard for radon. NGWA: Radon, Private Wells and Public Water Systems [EPA-RADON-RULE source note] [EPA-RADON-PROPOSED source note] [NGWA-RADON source note]

Some states publish their own guidance for private wells:

Sources: Vermont Department of Health: Radon in Drinking Water, Maine CDC: Maximum Exposure Guideline for Radon in Drinking Water, Connecticut DPH: Radon in Your Water. Check your own state's health department, since many states publish none. [VT-RADON source note] [ME-RADON-MEG source note] [CT-RADON source note]

Where is radon in well water most common?

Radon follows the geology. Maine's health agency describes the state as part of a geological region relatively high in natural uranium. Maine CDC: Maximum Exposure Guideline for Radon in Drinking Water A USGS-listed study of private bedrock wells in Maine found that wells within 5 kilometers of granitic intrusions are at risk of high uranium and radon, with 29% of samples in the Augusta area above 4,000 pCi/L. USGS Publications Warehouse: Uranium and radon in private bedrock well water in Maine USGS data for Maine describe its aquifers as mostly metamorphic and igneous rock, and wells drilled into similar bedrock deserve a test; your state radon or private well program can tell you whether your area is known for it. [ME-RADON-MEG source note] [USGS-ME-RADON source note]

How to test well water for radon

Radon testing needs a special kit, because the gas escapes as soon as water meets air and decays within days. Order a kit from a certified lab or your state health lab; Vermont's health department sells a radon-in-water kit for $25. Vermont Department of Health: Radon in Drinking Water Typical lab instructions: [VT-RADON source note]

1. Run the cold water for several minutes so you sample water from the well, not the pipes or tank.
2. Fill the vial slowly, often underwater or until it overflows, so no air bubble is trapped. Samples with bubbles cannot be analyzed.
3. Cap it immediately and ship it the same day. Labs set short holding times, often only a few days.

A radiological package that adds uranium and gross alpha costs more; see well water testing cost. EPA suggests well owners test for radionuclides every three years. EPA RadTown: Natural Radionuclides in Private Wells [EPA-RADNUC source note]

How is radon removed from well water?

Treatment goes on the main line where water enters the house (point of entry), because the risk comes from every faucet and shower, not just drinking water. EPA's fact sheet describes two options: aeration, which bubbles the radon out of the water and vents it outdoors, and granular activated carbon (GAC), which is cheaper but collects radioactivity on the filter that may need special disposal. EPA (archive): Radon in Drinking Water Questions and Answers [EPA-RADON-QA source note]

Penn State Extension puts aeration at about $3,000 to $5,000 installed and carbon at about $2,000, and advises carbon only for water below about 30,000 pCi/L. Penn State Extension: Reducing Radon in Drinking Water Vermont's health department discourages GAC for radon because the radon collected on the filter could create a radiation hazard. Vermont Department of Health: Radon in Drinking Water Get quotes from a treatment contractor experienced with radon, and retest after installation. [PSU-RADON source note] [VT-RADON source note]

What to do with your result

1. Compare it with your state's guideline, if it has one, and with the 300 and 4,000 pCi/L figures from EPA's proposal.
2. Test your indoor air if you have not, since air is the bigger exposure. EPA recommends air testing first.
3. Retest if the result is near a decision point; radon in water varies.
4. Treat at the point of entry if you decide to reduce it, and test again afterward.

For the rest of your annual testing, see well water testing and how to read well water test results.