Short definition
Radon in water is dissolved radon gas — a radioactive decay product of uranium found in groundwater drawn from uranium-bearing rock formations. The main risk isn’t drinking it; it’s the radon released into indoor air during showering, dishwashing, and laundry. Eastern Washington counties (Spokane, Pend Oreille, Stevens, Ferry) are EPA Radon Zone 1 areas where this matters more.
What it is
Radon (Rn-222) is a radioactive noble gas formed by the decay of radium-226 in soil and rock. Granitic and uranium-bearing geology — common in parts of eastern WA and the Cascade foothills — releases radon into groundwater. When that water comes into your home and gets agitated (showers, dishwashers, washing machines, faucets), dissolved radon transfers from water to indoor air.
The exposure math: roughly 1 pCi/L (picocurie per liter) of radon in water contributes about 0.1 pCi/L to indoor air. So 10,000 pCi/L of radon in water adds about 1 pCi/L to indoor air. EPA’s action level for indoor air radon is 4 pCi/L. That means waterborne radon becomes meaningful as an air-quality contributor at well-water levels around 4,000+ pCi/L.
There is no finalized federal MCL for radon in water. EPA proposed a 300 pCi/L MCL or, where states run an active radon-in-air program, an alternative MCL of 4,000 pCi/L. WA DOH treats radon-in-water testing as voluntary; air-side testing is the priority recommendation for any WA home.
Why it matters to a homeowner
Two reasons it matters in Washington:
Eastern WA is in Zone 1. Spokane, Pend Oreille, Stevens, and Ferry counties have the highest predicted indoor radon levels in the state. Adams, Lincoln, Whitman, and parts of Okanogan are also elevated. Western WA is mostly Zone 2 or 3.
Lung cancer is the actual risk. Radon is the second leading cause of lung cancer in the US after smoking. The risk is from inhaled radon (and its short-lived decay products), not ingested radon. Smokers in radon-elevated homes face dramatically elevated lung-cancer risk — combined risk is much higher than either alone.
If you’re in eastern WA: test the air in your home first. If the air radon comes back elevated and air-side mitigation (sub-slab depressurization) doesn’t reduce it to expected levels, then test the water. Water-side treatment is layered on top of air-side mitigation, not a substitute for it.
When you’ll encounter this term
- Spokane home with a private well — indoor air test is the priority
- Real estate transaction in a radon-zone county — many include an air radon test as a contingency
- New construction in eastern WA — passive radon-resistant construction (sub-slab vent stub) is recommended per WA energy code
- Family with smokers in the home — combined risk is dramatically higher; mitigation is high priority
Treatment options
| Method | Best for | Cost |
|---|---|---|
| Aeration (point-of-entry) | High waterborne radon | $4,000–$5,500 install |
| Granular activated carbon (POE) | Moderate levels | $1,000–$2,500 install |
| Sub-slab depressurization (air-side) | Indoor air radon | $1,500–$3,500 |
Aeration is the most effective for waterborne radon — it strips dissolved radon from the water and vents it to outdoor air, where it dilutes harmlessly. GAC is less expensive but accumulates radioactivity in the bed over time, so periodic media disposal is needed and the installation has location restrictions.
Common failure modes
- Test only air, not water, in an elevated-indoor-radon home with a private well — the water source may be contributing
- GAC filter accumulates radioactivity — bed must be replaced periodically; spent media disposal handled per local rules
- Showering with elevated radon water — this is the inhalation exposure pathway during shower aerosolization
- Conflating water radon with air radon — different tests, different mitigation
Common variants and disambiguation
- Radon-in-water vs. radon-in-air. Different tests, different mitigation, different rules. Air is the primary residential concern; water is secondary.
- Radon vs. radium. Radon is the gas; radium is the parent isotope (radium-226) that decays to radon. Some private wells in WA show elevated radium.
- EPA action level for indoor air: 4 pCi/L. WA DOH recommends mitigation at this level; “consider mitigation” is suggested at 2 pCi/L.
Cost data
- Radon-in-water test kit: $30–$80 mail-in lab.
- Air radon test kit (charcoal canister, 2-3 day passive): $15–$30.
- Aeration treatment system: $4,000–$5,500 installed.
- GAC water treatment: $1,000–$2,500 installed.
- Sub-slab depressurization (air-side mitigation): $1,500–$3,500.
Washington note
EPA Radon Zone mapping puts Spokane, Pend Oreille, Stevens, and Ferry counties in Zone 1 (highest predicted levels). Most of western WA sits in Zone 2 or 3. WA Department of Health runs a voluntary radon program with county-by-county risk data; verify the current page at https://doh.wa.gov/community-and-environment/contaminants/radon before relying on specific zone designations for a transaction.
EPA’s air radon action level is 4 pCi/L. WA DOH recommends mitigation at this level and suggests “considering” mitigation at 2 pCi/L. Mitigation is well-established work; choose an EPA / NRPP-certified contractor.
For private wells in radon zones: prioritize air radon testing. If air levels stay elevated despite air-side mitigation, then test the water. Water aeration is the most effective treatment for high waterborne radon, but it’s costly enough that you want the air-side answer first.