Radon and Water

Radon is a noble gas and is unusually soluble in water compared to other gases like oxygen or nitrogen. The dimensionless Ostwald solubility coefficient for radon in water at 20°C is ~0.25, meaning radon is about 4 times more soluble than oxygen.

When groundwater flows through radon-bearing rock or soil, radon continuously diffuses from mineral grains and soil gas into the water until equilibrium is approached. The equilibrium radon concentration in water depends on the radium-226 content of the aquifer matrix, the water residence time, and the partition between water and rock.

Radon in water decays with the same 3.82-day half-life as in air, so only recently recharged groundwater or water in close contact with radium-bearing minerals maintains high radon. Deep, old groundwater or water that has been stored in tanks for days loses most of its radon.

Radon in groundwater varies over six orders of magnitude depending on geology, aquifer type, and well depth. Surface water (lakes, rivers) and treated municipal water from surface sources typically have very low radon (<10 Bq/L) due to atmospheric degassing and treatment.

Groundwater from wells, especially drilled bedrock wells in granite or uranium-rich formations, can have radon levels of 1,000–100,000 Bq/L or higher. Private wells in high-radon areas (New England granite, Appalachian shales, phosphate regions) are the highest concern.

When radon-bearing water is used indoors, radon transfers from the water phase into indoor air. The transfer occurs mainly during water use events: showers, washing machines, dishwashers, faucets — any activity that agitates or aerates water.

The water-to-air transfer factor is empirically determined to be approximately 10⁻⁴ (or 1 Bq/m³ air per 10,000 Bq/L water). This rule-of-thumb is based on typical residential water use (~300 L/day), air exchange rates (~0.5 ACH), and measured radon releases.

Example: A home using groundwater with 50,000 Bq/L radon will experience an indoor air increase of about 5 Bq/m³ from water. A home with 500,000 Bq/L water may add 50 Bq/m³ to indoor air — significant but still less than typical soil-gas contributions in radon-prone areas.

Shower contribution: Hot showers produce the highest instantaneous transfer because heat reduces solubility and turbulent spray maximizes air-water interface. Bathroom air radon can spike 5–10× during and immediately after a hot shower in homes with very high water radon.

Most countries regulate radon in water primarily to limit its contribution to indoor air radon, not for ingestion risk (which is negligible compared to inhalation).

US EPA proposed (but never finalized) a Maximum Contaminant Level (MCL) of 11,100 Bq/L (~300 pCi/L) for community water systems, with an alternative MCL of 148,000 Bq/L (4000 pCi/L) if a multimedia mitigation program is in place.

WHO Guidelines for Drinking-water Quality (2017) do not set a radon limit but note that radon in water "is generally not a significant contributor" to overall radon exposure unless water radon exceeds 100,000 Bq/L.

European Union Drinking Water Directive 2020/2184 sets a parametric value of 100 Bq/L as a screening level; exceedances trigger investigation and possible remediation.

On average, water contributes about 1–2% of total indoor radon. The US National Academy of Sciences (NAS) estimated that for the US population, water accounts for roughly 1–5% of radon-related lung cancer risk, with soil gas accounting for the vast majority.

In high-radon water areas (New England, parts of the Appalachians, Rocky Mountain granites), water can contribute 10–30% of indoor radon. However, even in these areas, soil gas is usually the dominant source if the building has a basement or slab-on-grade contact with radon-bearing soil.

Homes supplied by surface water or municipal systems that aerate water have negligible water contribution. Homes on private wells in granite or uranium-rich bedrock are the primary concern.

Radon in water poses two potential exposure pathways: inhalation of radon that has degassed into indoor air, and ingestion of radon dissolved in drinking water.

Inhalation risk dominates: The alpha dose to lungs from breathing radon and its progeny is orders of magnitude larger than the dose to the stomach from ingesting radon-bearing water. Most ingested radon is exhaled before it decays.

Ingestion dose estimates: UNSCEAR and ICRP estimate stomach dose from ingested radon at ~10⁻⁸ Sv/Bq ingested — roughly 1000 times less significant than inhalation on a per-Bq basis. Drinking water with 10,000 Bq/L daily yields a committed effective dose around 0.1 mSv/year from ingestion, compared to ~1 mSv/year inhalation dose if the water contributes 10 Bq/m³ to indoor air.

Because inhalation is the dominant pathway, radon-in-water mitigation focuses on preventing degassing into air, not on reducing ingestion.

Radon in water is measured by liquid scintillation counting (LSC) or by transferring radon from water into air and counting alpha emissions with a scintillation cell or continuous radon monitor.

Standard methods: EPA Method 913.0 (liquid scintillation), ASTM D5072 (scintillation cell), and ISO 13164 series. Sampling requires care to avoid radon loss: samples are collected in glass vials with no headspace, sealed tightly, and analyzed within a few days (before significant decay).

Home test kits are available: water is collected in a vial, sealed, and mailed to a lab. Results are typically reported in Bq/L or pCi/L.

Interpretation: If water radon exceeds 10,000 Bq/L and indoor air radon is measured above reference levels, water treatment (aeration, GAC filtration) may be cost-effective. If water radon is below 5,000 Bq/L, contribution to air is likely negligible.

Two primary methods remove radon from water: aeration (highly effective, 95–99% removal) and granular activated carbon (GAC) adsorption (70–95% removal, accumulates radioactivity).

Aeration (air stripping): Water is sprayed or cascaded through a chamber while air is blown through, transferring radon from water to air. The radon-bearing air is vented outdoors. Point-of-entry aeration systems treat all household water before distribution.

GAC filtration: Water flows through a tank of activated carbon, which adsorbs radon. Over time, the carbon accumulates radon progeny (especially lead-210, half-life 22 years) and becomes a low-level radioactive waste requiring special disposal. GAC is simpler and cheaper than aeration but less effective and poses a disposal issue.

Cost and practicality: Aeration systems cost $3,000–$7,000 installed; GAC systems $1,000–$3,000. Because water typically contributes a small fraction of indoor radon, soil-gas mitigation (sub-slab depressurization) is almost always more cost-effective as a first step.