Radon in Well Water: How It Gets In, How to Test, and How to Treat It
Radon in well water is a less-recognized but real source of indoor air radon contamination, particularly for households drawing private well water from uranium-bearing geology. The dissolved radon stays in the water until use at atmospheric pressure; showers, dishwashers, and washing machines then transfer it to indoor air. For most municipal water customers, this is not a concern. For private well households in radon-prone regions like Colorado’s Front Range and mountain communities, it can meaningfully add to indoor air radon levels. This guide walks through how it works, how to test, and what treatment costs.
InspectandTest is an independent editorial team. We are not licensed water treatment professionals or radon mitigation specialists. This guide synthesizes EPA, AARST, and U.S. Geological Survey documentation.
How radon gets into well water
Radon-222 is a radioactive gas produced by the decay of radium-226, which itself comes from uranium-238 in soil and rock. Both are present in trace amounts in many geological formations, with concentrations significantly elevated in:
- Granitic rock (Pikes Peak granite of the Colorado Front Range, similar formations in the Northeast and Mountain West)
- Uranium-bearing Cretaceous sedimentary formations (significant in the Western U.S.)
- Black shale and certain pegmatite formations
When groundwater passes through these formations, dissolved radium continues to decay and produces dissolved radon. The radon stays in solution under pressure at depth but is highly volatile – it readily transfers to air when water is exposed at atmospheric pressure. Inside a well, water table conditions trap the radon dissolved in groundwater. When water is pumped to the surface and used, dissolved radon releases into indoor air.
The water-to-air transfer ratio
Approximately 10,000 picocuries per liter of dissolved radon in water transfers approximately 1.0 pCi/L into indoor air through normal household water use. This rule of thumb varies with household water use patterns (high-use households transfer more), ventilation rates, and home size, but provides a reasonable basis for estimating the airborne contribution.
For waterborne radon to meaningfully contribute to indoor air radon, concentrations need to exceed roughly 5,000 pCi/L in the water. Below that level, the air contribution is typically less than 0.5 pCi/L, which is small relative to the EPA action level of 4.0 pCi/L for indoor air.
Who should test for waterborne radon
Should test:
- Households on private wells in known-uranium-bearing geology (Colorado Front Range, mountain communities, similar regions)
- Households with elevated indoor air radon (above 4 pCi/L) that has not responded to standard sub-slab depressurization mitigation
- Pre-purchase due diligence on rural and mountain properties with private wells
- Any household where indoor air radon mitigation has been performed but levels remain elevated
Generally do not need to test:
- Households on municipal water (city utility processing typically removes most dissolved radon)
- Households with low (below 4 pCi/L) indoor air radon – waterborne radon is a smaller incremental concern
- Households in regions with known low groundwater radon
Testing protocol
- Order a radon-in-water test kit from an NRPP-certified laboratory or your state environmental health department’s recommended provider. Cost: $30 to $60 per sample.
- Follow the lab’s specific sample collection instructions exactly. Samples must be collected without exposure to air during fill – the dissolved radon transfers to air on contact, so the bottle must be filled completely with water and sealed without headspace.
- Collect from a tap that draws water directly from the well system (typically a cold-water tap from a fixture closest to the well). Avoid taps with water softeners or filtration upstream that may have removed radon.
- Ship same-day to the laboratory; delays allow continued radon decay and bias results low.
- Receive results in 1-2 weeks via email or mail.
Treatment methods
Granular Activated Carbon (GAC) filtration. Whole-house carbon filter that absorbs dissolved radon from water. Effective for concentrations under approximately 5,000 pCi/L. Cost: $800 to $2,500 installed. Requires periodic carbon replacement (typically every 5-10 years). The installed carbon eventually accumulates radon decay products and becomes a low-level radioactive disposal item; specific disposal protocols apply.
Aeration systems. The water is exposed to air in a controlled chamber (spray, packed-tower, or bubble aerator), the radon strips out into vented air, and the treated water is returned to the household supply. Effective for very high concentrations (over 10,000 pCi/L) and produces treated water with residual radon below 100 pCi/L. Cost: $3,000 to $7,000 installed. Requires venting to outdoor air (typically a small fan and exhaust pipe). Lower long-term operating costs than GAC for high-concentration situations.
Cost-benefit considerations
For most private well households in radon-prone areas, the cost-benefit math is:
- Test the well water once ($30 to $60). Determines whether treatment is needed.
- If concentrations are low (under 5,000 pCi/L): standard indoor air radon mitigation handles the residual airborne contribution. No water treatment needed.
- If concentrations are moderate (5,000-15,000 pCi/L): GAC filtration ($800 to $2,500 installed) is cost-effective.
- If concentrations are high (over 15,000 pCi/L): aeration system ($3,000 to $7,000 installed) is the right approach.
The waterborne radon mitigation is often paired with indoor air radon mitigation; both pathways need to be addressed for households with multiple sources.
Regional context
Colorado’s Front Range and mountain communities have well-documented elevated waterborne radon in some private wells. The Colorado Department of Public Health and Environment maintains documentation on county-level patterns; mountain counties (Boulder canyon, Gilpin, Clear Creek, Park) have particularly elevated levels in some private wells. Northeastern U.S. (New Hampshire, Maine, Massachusetts) and parts of the Mountain West (Idaho, Montana) also have well-documented patterns.
For most homeowners, indoor air radon testing is the first priority; waterborne testing is appropriate when indoor air remains elevated despite standard mitigation, or when the geographic and well context suggests it.