Radon in Water: The Invisible Risk in Your Shower

Most Americans know radon as a basement problem — the radioactive gas that seeps from soil and rock into homes through foundation cracks. The EPA estimates radon is responsible for about 21,000 lung cancer deaths annually in the US, making it the second leading cause of lung cancer after smoking.

What far fewer people know is that radon can also enter homes through the water supply — and when it does, it creates a different but equally real exposure pathway that most radon testing programs completely miss.

The mechanism is straightforward: radon dissolves into groundwater as it percolates through uranium-bearing rock (granite, shale, and similar formations). When that groundwater is pumped into your home and flows through pipes, faucets, and particularly the shower, the radon degasses out of the water and into your household air. It's the same radon that causes lung cancer when breathed — it just arrives via a different route.

The waterborne radon problem is almost exclusively a private well issue. Municipal water treatment — particularly large surface water systems — loses virtually all radon before water reaches consumers. But for the 43 million Americans on private groundwater wells, particularly those in the granitic geology of New England, the Appalachians, and the Rocky Mountain states, waterborne radon is a genuine and largely unrecognised source of indoor radon exposure.

The distinction between ingesting radon in water and inhaling radon that has off-gassed from water matters enormously for understanding the health risk.

Ingestion risk When you drink water containing radon, the radon enters your digestive system. The risk is primarily stomach cancer — radon decays and emits alpha radiation directly to gastrointestinal tissue. However, the ingestion risk from waterborne radon is estimated to be about 1/10,000th of the inhalation risk for the same radon concentration. Drinking radon-containing water is far less dangerous than breathing air containing radon.

Inhalation risk: the bigger concern The primary risk from waterborne radon is not from drinking it — it's from the radon that off-gasses from the water during household use and enters the air you breathe. Activities that maximise water surface area and turbulence maximise off-gassing: • Showering: the greatest single off-gassing event, combining hot water, turbulence, and enclosed space • Dishwashing: significant off-gassing in kitchen air • Flushing toilets and running taps: smaller but cumulative contributions

A 2021 EPA technical assessment found that waterborne radon can contribute 1–4 pCi/L to indoor air radon levels in homes with contaminated well water — a meaningful increment when the EPA's action level for indoor radon is 4 pCi/L total. In other words, waterborne radon alone could push a home over the threshold at which remediation is recommended.

Waterborne radon risk is concentrated in specific geologies and regions — understanding where you fall on that map determines how urgently testing matters.

Highest-risk geology Radon in groundwater correlates directly with uranium content in bedrock. The highest-risk formations are: • Granitic bedrock: common across New England (particularly New Hampshire, Maine, Vermont), the Piedmont region of the mid-Atlantic, and parts of the Rocky Mountain states • Black shales: some Midwest and Appalachian regions • Phosphatic rock: parts of Florida and Idaho

New England and the Appalachians New England has some of the highest waterborne radon concentrations in the country. Studies of private wells in New Hampshire have found radon levels in water exceeding 10,000 pCi/L — far above the EPA's proposed MCL of 300 pCi/L for waterborne radon (a proposed rule that was never finalised). Maine, Vermont, Massachusetts, and Connecticut have similar geology.

A largely unregulated problem The EPA proposed a maximum contaminant level for radon in drinking water in 1999 — 300 pCi/L for systems without a complementary radon mitigation program, 4,000 pCi/L for systems with one. The rule was never finalised. Private wells are entirely unregulated for radon. Millions of families in high-risk regions have never tested their well water for radon and have no idea what they're breathing in their showers each morning.

Testing for radon in well water requires a specific water test — it's not included in standard well water panels and is different from the air radon test most people are familiar with.

How to test your well water for radon • Request a waterborne radon test from a state-certified laboratory — not all labs offer this test, so confirm before sampling • Collection requires special procedures: samples must be collected without aeration (to prevent loss of dissolved radon before the sample reaches the lab) and shipped immediately • Cost: typically $25–50 for a water radon test • The EPA suggests testing water if your indoor air radon level is elevated and the source isn't clearly soil entry, or if you're in a high-radon geology region

Interpreting results The EPA's guidance suggests that waterborne radon above 4,000 pCi/L warrants treatment to prevent meaningful contribution to indoor air radon. At lower levels, the contribution to indoor air is smaller — but even at 1,000–4,000 pCi/L, contribution to indoor air radon is detectable and may be relevant if indoor air radon levels are already near the action level.

Treatment options • Aeration treatment: The most effective approach — water is sprayed or aerated at the point of entry, releasing radon to the outside air before water enters the home. Removes 95–99% of waterborne radon. • Granular activated carbon (GAC) filters: Effective but accumulate radioactive decay products in the filter media, creating a low-level radioactive waste disposal issue. Used less commonly than aeration for this reason.

If you use PollutionProfile's Historical Exposure Recorder and have lived in New England or other high-radon geology for extended periods, this is worth adding to your exposure history.