What is radon in well water?
Radon (Rn-222) is a colorless, odorless, tasteless radioactive gas produced by the decay of uranium-226 in rock. It seeps into groundwater that flows through uranium-rich granite and bedrock. When you turn on a tap or shower, the water agitates and releases radon gas into indoor air.
The health risk from waterborne radon is primarily through inhalation, not ingestion. Drinking water with radon contributes a small amount of stomach radiation; breathing radon released from water use is the dominant exposure pathway.
Where is it most common?
Radon in well water is most prevalent in Maine, New Hampshire, Vermont, Michigan, and Pennsylvania — states with significant granite and crystalline bedrock geology. New England has some of the highest waterborne radon levels in the country due to its granitic terrain. If your area has elevated indoor radon (above 4 pCi/L from soil entry), your well water likely also contains radon.
Health effects
- Lung cancer from inhalation of waterborne radon — Radon decays into solid radioactive progeny (Po-218, Po-214) that attach to airborne particles and are deposited in lung tissue when inhaled, delivering alpha radiation to the bronchial epithelium. The EPA estimates that radon is responsible for approximately 21,000 lung cancer deaths per year in the U.S., making it the second-leading cause after smoking. Waterborne radon contribution to indoor air radon is typically 1–2% of total indoor radon (soil entry is the main source), but in homes on well water with very high radon levels it can be a significant fraction.
The general rule of thumb: every 10,000 pCi/L of radon in water contributes approximately 1 pCi/L to indoor air radon.
Is there an EPA limit?
No. The EPA proposed a radon MCL of 300 pCi/L (or 4,000 pCi/L with a state multimedia mitigation program) in 1999, but the rule was never finalized. The MCLG is 0 — no safe level of radon exists. Many states have set their own guidance levels, typically 300–4,000 pCi/L.
Testing
Test both your water and indoor air:
- Water radon: liquid scintillation counting or emanation method; measured in pCi/L; certified lab required. Collect sample in a glass vial without headspace (special collection required — follow lab instructions precisely).
- Indoor air radon: alpha track detector (90-day test, most accurate) or charcoal canister (short-term, 2–7 days). Place in lowest livable area. EPA action level: 4 pCi/L.
If your indoor air radon exceeds 4 pCi/L, test your water. If water exceeds 5,000 pCi/L, waterborne radon may be a significant contributor to indoor air.
Find a certified lab and learn radon sampling protocol
Treatment
- Point-of-entry aeration — the most effective treatment (>99% removal); aerates all household water before it enters the plumbing system. Requires venting radon-rich air safely to the outside. Best choice for high radon levels.
- Granular activated carbon (GAC) at point of entry — also effective for radon removal, but the carbon bed accumulates radioactive decay products over time and requires special disposal as low-level radioactive waste. Generally preferred only for moderate radon levels.
- Point-of-use treatment is NOT sufficient — treating only the kitchen tap does not reduce radon released into air during showering or clothes washing. Full point-of-entry treatment is required.