Is radon in well water dangerous?

Radon in well water is a real health concern, but most of the risk comes from breathing it, not drinking it. When you shower, run the dishwasher, or wash clothes, radon escapes from the water into indoor air. The EPA estimates about 89% of radon-related cancer deaths from waterborne radon result from lung cancer caused by inhalation. Long-term ingestion of radon at high levels is associated with increased stomach cancer risk, but that's the smaller portion of the risk.

How do I test my well water for radon?

Order a radon-in-water test from a certified lab. It's a separate test from the radon-in-air test kits used for basements, don't confuse the two. Most certified radon-in-water tests cost $20 to $40. Your state health department can direct you to certified labs, and many state programs offer low-cost testing for well owners.

How does radon get into well water?

Radon is a radioactive gas produced naturally by the decay of uranium in rock and soil. When groundwater flows through uranium-containing rock formations, especially granite, it picks up dissolved radon. Private wells that draw from deep aquifers in these rock types can have significant radon concentrations. Surface water isn't affected because radon escapes to the atmosphere before it can accumulate.

Does radon in water cause lung cancer?

Radon from well water can contribute to lung cancer risk through inhalation, when water use releases radon gas into indoor air. The EPA estimates that waterborne radon accounts for a small fraction of total radon-related lung cancer deaths, with soil radon (entering through foundation cracks) being the much larger contributor for most homes. If your home has both high radon in air and radon in water, address the air pathway first, then the water.

Radon in Well Water: The Risk You're More Likely to Breathe Than Drink

Q: What removes radon from well water?

Aeration is the most effective treatment. A point-of-entry aeration system strips dissolved radon gas from water before it enters the house, removing 95 to 99% of the radon. Granular activated carbon (GAC) filters also remove radon effectively, but the carbon accumulates radioactive decay products over time and requires special disposal. For high radon levels, aeration is generally the better choice.

Health disclaimer: Radon is a radioactive gas and a known carcinogen. This page provides general information only, not medical or radiation safety advice. Contact your state health department or a certified radon professional for guidance specific to your home.

Most of the health risk from radon in well water doesn’t come from drinking it. It comes from breathing it.

That’s the fact that shapes everything about this contaminant. When you take a shower, run the dishwasher, or do laundry, radon dissolved in your well water escapes into the air inside your home. The EPA estimates that about 89% of radon-related cancer deaths from waterborne radon result from lung cancer caused by inhalation, not stomach cancer from ingestion.

This doesn’t mean you ignore it. It means you understand where the risk actually lives.

What Radon Is

Radon is a naturally occurring radioactive gas. It forms from the decay of uranium in rock and soil. It’s colorless, odorless, and tasteless. There’s no way to detect it without a test.

It dissolves into groundwater when water moves through uranium-containing rock formations. Private wells that draw from deep aquifers in those formations can pick up meaningful concentrations. Surface water, lakes, rivers, reservoirs, isn’t a concern because radon escapes to the atmosphere too quickly to accumulate.

This is a private well issue. If you’re on a municipal water supply, your water comes from treated surface water or groundwater that’s been aerated as part of treatment. Radon isn’t your concern there.

Who’s at Risk

Geography matters more than almost any other factor with radon.

The highest-risk areas are regions with granite bedrock. Granite contains uranium, and uranium decays into radon. New England has some of the highest radon-in-water levels in the country, Maine, New Hampshire, Vermont, and Massachusetts see frequent detections above 1,000 pCi/L in private wells. Parts of the Mid-Atlantic and the Appalachians are also elevated. Some areas of the Midwest and Southwest have uranium-bearing rock formations as well.

Your state health department or the USGS can tell you whether your local geology puts you at higher risk. Many states publish maps of radon-prone areas.

If you have a private well and you’re in New England or any granite-bedrock region, radon testing is worth doing. It costs less than most restaurant meals and takes one sample.

The Regulatory Picture

The EPA proposed a Maximum Contaminant Level (MCL) of 300 pCi/L for radon in public water supplies in 1999. That rule was never finalized at the federal level. As a result, there’s no federal MCL for radon in drinking water today.

Some states have set their own limits. Maine, for example, has an action level of 4,000 pCi/L. Other states use the EPA’s proposed 300 pCi/L as an informal guide. Check your state health department for the current standard in your area.

At 300 pCi/L in water, the EPA estimated a risk level comparable to long-term smoking exposure. That’s not a soft comparison. At 10,000 pCi/L, which does occur in high-risk geology, the risk is substantially higher.

For reference: the EPA recommends mitigating radon in indoor air when levels exceed 4 pCi/L (that’s air, not water). The two measurements use the same unit but describe different things. A rough conversion suggests that 10,000 pCi/L in water contributes about 1 pCi/L to indoor air. Water radon is usually a smaller contributor to indoor air levels than soil radon from foundation cracks, but it adds to the total.

The Full Radon Picture in Your Home

Soil radon and waterborne radon are two separate pathways, and they add up.

Radon from soil enters homes through cracks in foundations, gaps around pipes, and sump pits. For most homes, even those in high-radon geology, this is the larger contributor to indoor radon levels. The standard radon-in-air test you’d do for a basement measures total indoor radon from all sources.

If your home already has elevated radon in air, adding a waterborne radon source raises the total indoor concentration further. The EPA’s guidance: address the air source first. Then test the water contribution separately.

If your radon-in-air test is low but you have a private well in high-risk geology, a radon-in-water test still makes sense. Indoor air levels can be low even when water radon is high, depending on how well-ventilated your home is.

Testing

A radon-in-water test from a certified lab typically costs $20 to $40. This is separate from the radon-in-air test kits you’d use for a basement, those don’t measure dissolved radon in water.

To find a certified lab, contact your state health department. Many states have subsidized testing programs for well owners. The EPA also maintains resources for finding certified labs at its private wells page.

Collect the sample according to the lab’s instructions. Most require a cold water sample from a tap that’s been running for 30 to 60 seconds. Overnight stagnation isn’t recommended for radon sampling, unlike lead testing, where first-draw samples are standard.

Treatment

Aeration is the first-line treatment for radon in well water. A point-of-entry aeration system exposes the water to air before it enters your home’s plumbing, which releases the dissolved radon gas. Removal rates are typically 95 to 99%. The radon exhausts to outdoor air, not into your home. These systems require installation by a licensed contractor and need periodic maintenance.

Granular activated carbon (GAC) filters also remove radon effectively and can be installed as whole-house point-of-entry systems. The catch: GAC accumulates radon decay products over time, making the filter material itself mildly radioactive. Disposal requires following your state’s guidelines for low-level radioactive waste. For this reason, aeration is generally preferred when radon levels are high.

For lower radon concentrations, near the proposed 300 pCi/L limit, a GAC system is a practical option. For higher levels, aeration is the more appropriate technology.

Point-of-use filters at the tap don’t solve the problem. Remember: the bigger risk is inhalation during water use throughout the house, not ingestion from the kitchen tap. Treatment needs to happen at the point of entry.

For water testing options, see best mail-in water tests.

If you’re working through a full well water testing plan, the well water testing guide covers which tests to run based on your location and well type.

Sources:

EPA Radon in Drinking Water: https://www.epa.gov/radtown/radon-water
EPA Natural Radionuclides in Private Wells: https://www.epa.gov/radtown/natural-radionuclides-private-wells
EPA Radionuclides Rule: https://www.epa.gov/dwreginfo/radionuclides-rule

Health disclaimer: WaterAnswer.com provides general information only. Radon is a known carcinogen. For guidance on testing and mitigation specific to your home, contact your state health department or a certified radon professional.