Radon in Water: Testing, Treatment, and Health Risks Explained

Most discussions about radon focus on soil gas entering homes through foundations, but radon can also dissolve into groundwater and enter homes through the water supply. For households served by private wells, waterborne radon can contribute to indoor air radon levels and, in rare cases, pose an ingestion risk.

This guide explains how radon enters water, the EPA's proposed water quality standards, the difference between well water and municipal water, testing methods, treatment options, and when testing your water supply makes sense. All information is based on guidance from the U.S. Environmental Protection Agency and state water quality programs.

How radon enters water

Radon is a gas, but it is also soluble in water. When groundwater flows through bedrock, fractured rock, or soil that contains uranium and radium, radon gas produced by radioactive decay dissolves into the water. The amount of radon that dissolves depends on the uranium content of the surrounding geology, the water's contact time with radon-producing rock, water temperature, and pressure.

Bedrock aquifers, particularly those in granite, shale, and other igneous or metamorphic formations, tend to have higher radon concentrations in groundwater. Wells drilled into these formations can draw water with dissolved radon levels ranging from a few hundred to tens of thousands of picocuries per liter.

Once radon-laden water is pumped into a home, the gas is released from the water into indoor air whenever the water is agitated. Showering, running faucets, flushing toilets, dishwashing, and clothes washing all aerate the water and liberate dissolved radon. Because radon is a gas at room temperature and atmospheric pressure, it escapes readily, contributing to the total radon concentration in indoor air.

Surface water sources—lakes, rivers, and reservoirs—typically have very low radon levels because radon escapes into the atmosphere before the water is collected and distributed. This is why municipal water systems that draw from surface water rarely have radon concerns. Groundwater-fed municipal systems may have detectable radon, but most public water treatment processes involve aeration, filtration, and storage, all of which reduce radon before the water reaches consumers.

EPA water standards

In 1999, the EPA proposed a regulation that would establish a maximum contaminant level (MCL) for radon in public drinking water systems. The proposed rule included two possible standards:

• 300 pCi/L: The default MCL, intended to limit radon in drinking water to a level that corresponds to minimal added indoor air radon exposure.
• 4,000 pCi/L (Alternative MCL or AMCL): A higher threshold allowed if the state has developed and implemented an enhanced indoor air radon mitigation program. The logic is that if a state is aggressively addressing radon in indoor air (the larger contributor to exposure), a higher waterborne threshold is acceptable.

As of 2026, this rule has not been finalized or enforced at the federal level. However, the proposed MCL of 300 pCi/L is widely used as a guideline by state health departments, water testing laboratories, and well water treatment professionals. Several states have adopted their own drinking water radon standards or guidance levels based on the EPA proposal.

It is important to note that these standards apply to public water systems. Private wells are not regulated by the EPA or most state agencies for radon content. Well owners are responsible for testing and treating their own water if radon levels are a concern.

Well water vs. city water

The risk of radon in your drinking water depends almost entirely on whether you have a private well or receive water from a municipal system.

Private wells

Private wells draw water directly from bedrock or unconsolidated aquifers without treatment. If the aquifer contains radon, the well water will contain radon. There is no aeration, filtration, or storage step to reduce radon before it reaches your tap. Wells drilled into granite, metamorphic rock, or uranium-bearing formations are at highest risk.

If you have a private well, you are responsible for water quality testing. Radon is not typically included in standard well water tests for bacteria or nitrates, so you must request a radon-in-water test specifically.

Municipal (city) water

Municipal water systems that draw from surface water sources (rivers, lakes, reservoirs) rarely have radon problems because radon dissipates into the atmosphere before water is collected. Groundwater-fed municipal systems may have radon in the source water, but most treatment processes—aeration in particular—remove much of it before distribution.

Public water systems are required to test and report water quality under the Safe Drinking Water Act, but radon testing is not currently mandatory. If you are on city water and concerned about radon, you can contact your water utility and ask whether they have tested for radon or request a Consumer Confidence Report (CCR), which summarizes water quality test results.

In practice, radon in municipal water is rarely a significant issue. The primary concern is private wells in areas with high soil and bedrock radon potential.

Testing radon in water

Testing water for radon is straightforward and relatively inexpensive. It requires a water sample sent to a certified laboratory for analysis.

How to collect a sample

Water radon test kits are available from state radon offices, university extension programs, and private laboratories. The kit includes a sample vial (often a scintillation vial), instructions, and a prepaid mailer. Sample collection must be done carefully to avoid releasing radon before analysis:

• Run cold water from a faucet connected to your well for several minutes to clear the lines.
• Fill the sample vial slowly from the bottom to avoid aeration and bubbles.
• Cap the vial tightly and immediately to prevent radon from escaping.
• Ship the sample to the lab as soon as possible. Radon has a half-life of 3.8 days, so delays reduce accuracy.

Cost and turnaround

Radon-in-water testing typically costs between $25 and $100, depending on the laboratory and analysis method. Liquid scintillation counting is the most common analytical technique. Turnaround time is usually 1 to 2 weeks after the lab receives the sample.

Interpreting results

Results are reported in picocuries per liter (pCi/L), the same unit used for indoor air radon but measured in water instead of air. Use these benchmarks:

• Below 300 pCi/L: Generally considered low. Minimal contribution to indoor air radon.
• 300 to 4,000 pCi/L: Moderate. May warrant treatment if you want to reduce exposure, especially if indoor air radon is also elevated.
• Above 4,000 pCi/L: High. Treatment is recommended, particularly if you have not addressed indoor air radon.

Keep in mind that waterborne radon contributes to indoor air radon at roughly a 10,000-to-1 ratio. For example, 5,000 pCi/L in water adds about 0.5 pCi/L to indoor air. This is a smaller contributor than soil gas, but still measurable.

Health risk: water vs. air

Radon in water poses health risk through two pathways: inhalation and ingestion.

Inhalation risk (primary)

The primary health concern from radon in water is the radon gas released into indoor air during water use. When you shower, wash dishes, or run the washing machine, dissolved radon escapes from the water and mixes with the air you breathe. This increases your indoor air radon exposure and, over time, lung cancer risk.

The EPA uses a transfer ratio of approximately 1-to-10,000: for every 10,000 pCi/L of radon in water, indoor air radon increases by about 1.0 pCi/L. This ratio assumes typical household water use patterns. Homes with high water use (large families, frequent laundry, long showers) may see a slightly higher contribution; homes with low water use may see less.

Ingestion risk (minor)

Drinking water that contains radon exposes the stomach and digestive tract to a small amount of radiation. However, the ingestion risk from radon in water is considered much smaller than the inhalation risk. Radon is a gas and does not remain in the body long. The EPA estimates that ingestion of radon in water contributes less than 1 to 2 percent of the total radon-related health risk, with inhalation accounting for the rest.

For this reason, treatment and mitigation efforts focus on reducing radon release into indoor air rather than solely preventing ingestion.

Combined exposure

If your home has both elevated soil gas radon (from foundation entry) and elevated waterborne radon, the two sources are additive. For example:

• Indoor air radon from soil gas: 3.5 pCi/L
• Water radon: 8,000 pCi/L (contributing ~0.8 pCi/L to indoor air)
• Total indoor air radon: approximately 4.3 pCi/L

In this scenario, addressing soil gas radon with a sub-slab depressurization system and treating water radon with an aeration or GAC system would bring total exposure down significantly.

Treatment options

If testing shows elevated radon in your well water, two treatment technologies are proven effective: granular activated carbon (GAC) filtration and aeration.

Granular activated carbon (GAC)

GAC systems use activated carbon media to adsorb radon from water as it passes through the filter. These systems are typically installed at the point of entry (POE), meaning all water entering the home is treated. GAC is effective for radon levels up to about 5,000 pCi/L.

Pros:

• Lower upfront cost (usually $1,000 to $2,000 installed)
• Compact and easy to install in most homes
• Treats other contaminants (chlorine, organic compounds) in addition to radon

Cons:

• Carbon media accumulates radon decay products (radioactive lead and polonium), which requires proper disposal
• Filter must be replaced periodically (every 1 to 3 years depending on water use and radon level)
• Less effective for very high radon levels (above 5,000 to 10,000 pCi/L)

Aeration (air stripping)

Aeration systems work by spraying or bubbling water through a sealed chamber filled with air. Radon escapes from the water into the air, and the radon-laden air is vented outdoors through a small fan and exhaust pipe, similar to a radon mitigation system. Treated water is then delivered to the home.

Pros:

• Highly effective, removing 95 to 99+ percent of radon regardless of initial concentration
• No radioactive waste accumulation (radon is vented outdoors)
• Long service life with minimal maintenance (fan and diffuser cleaning)

Cons:

• Higher upfront cost (typically $3,000 to $5,000 installed)
• Requires electricity to run the aeration fan continuously
• More complex installation, often requiring a separate equipment room or outdoor enclosure

Which system to choose

For water radon levels below 5,000 pCi/L, GAC is often the more cost-effective choice. For levels above 5,000 pCi/L or for homeowners who prefer a system with no radioactive waste handling, aeration is the better option. Consult with a water treatment professional who has experience with radon removal to evaluate your specific situation and water chemistry.

When to test your water

Not every home needs a radon-in-water test, but there are specific circumstances where testing is strongly recommended.

You have a private well

If your home is served by a private well, particularly one drilled into bedrock, testing is a good idea. Well water radon is most common in areas with granite, shale, and other uranium-bearing rock formations. If you live in a known high-radon region (consult EPA radon zone maps or state radon offices), testing your water provides a complete picture of radon exposure sources.

Indoor air radon is elevated but soil mitigation didn't fully resolve it

If you installed a radon mitigation system for soil gas but indoor air radon levels remain above 2.0 pCi/L after mitigation, waterborne radon may be the residual contributor. Testing your water can confirm whether this is the case.

You are buying or building a home with well water

Radon in water testing is not yet standard in most home inspections, but it is a reasonable addition if the property has a private well and is located in a high-radon area. Identifying water radon before closing gives you the opportunity to negotiate treatment costs or install a system before occupancy.

You want a comprehensive radon assessment

Even if indoor air radon is low, some homeowners prefer to test all potential radon sources as part of a thorough environmental health assessment. Water testing adds minimal cost and provides peace of mind.

For more on testing your home for all radon sources, see our complete guide to what radon is and how it enters homes.

Frequently asked questions

1. How does radon get into well water?

Radon dissolves into groundwater as it moves through rock and soil containing uranium and radium. When groundwater flows through fractures in bedrock or permeable soil layers where radon gas is present, the gas dissolves into the water. Private wells that draw from bedrock aquifers, particularly granite formations, are most susceptible to elevated radon concentrations.

2. What is a safe radon level in water?

The EPA proposed a maximum contaminant level (MCL) of 300 pCi/L for radon in water, or alternatively 4,000 pCi/L if the state has an enhanced indoor air radon program (Alternative Maximum Contaminant Level). Many states and water testing labs use 300 pCi/L as the guideline for recommended action. There is no federally enforceable standard for private wells.

3. Should I test my well water for radon?

Yes, if you have a private well and live in an area with known radon in soil or bedrock, testing your water is recommended. The EPA suggests testing well water for radon if indoor air radon levels are elevated or if your water comes from a bedrock aquifer. Testing costs $25 to $100 and provides important information about total radon exposure in your home.

4. How much does radon in water contribute to indoor air radon?

The EPA estimates that 10,000 pCi/L of radon in water contributes approximately 1.0 pCi/L to indoor air radon. This 10,000-to-1 ratio means waterborne radon is typically a smaller contributor than soil gas, but it can still add measurable exposure, especially in homes with high water radon levels and well water use for showering and other aerated water activities.

5. How do you remove radon from well water?

The two primary treatment methods are granular activated carbon (GAC) filtration and aeration. GAC systems adsorb radon onto carbon media and are typically installed at point-of-entry. Aeration systems strip radon from water by exposing it to air in a sealed chamber, then vent the gas outdoors. Aeration is generally more effective for high radon levels (above 5,000 pCi/L) and does not accumulate radioactive material like GAC.

Sources & disclaimer: Information in this guide is drawn from the EPA Radon in Water guidance, state water quality programs, and the National Ground Water Association. This content is educational only and does not constitute medical, legal, or professional advice. Consult qualified water treatment professionals for testing and treatment decisions.