How to Test Well Water for Radon

By Steve Crews · Last updated June 20, 2026

Does radon in well water actually matter?

For most US homes, the soil beneath the foundation is the dominant radon source. Radon gas seeps up through cracks, sump pits, and floor joints and accumulates in indoor air. Water is typically a secondary contributor.

But secondary is not the same as negligible, especially for private wells. Municipal water systems draw from surface water or treat groundwater at a central facility, and by the time treated water reaches a tap, most dissolved radon has off-gassed. Private wells pull untreated groundwater directly into homes, and that water can carry significant dissolved radon depending on the local geology.

The EPA has documented a useful conversion: roughly every 10,000 pCi/L of radon in water adds about 1 pCi/L to indoor air concentrations. If your well returns 20,000 pCi/L and your air already reads 2 pCi/L from soil entry, the water pathway alone could push your total close to the EPA’s 4 pCi/L action level. At very high water levels (50,000 pCi/L and above, which do occur in parts of New England and the Appalachians), the water pathway alone can drive air concentrations into concerning territory.

Who should test their well water

Not every private well carries elevated radon. The risk correlates closely with geology. Uranium is present in many rock types but concentrates in granite and granite-derived soils, black shale formations, phosphate-bearing rocks, and certain sandstone basins.

If you live in Maine, New Hampshire, Vermont, Pennsylvania, or parts of North Carolina, Georgia, or the Mountain West and you use a private well, water testing makes sense regardless of what your air test shows. Your state radon program or state environmental agency will usually publish a geological risk map showing higher-risk counties.

Even outside those regions, testing is a reasonable step if your air radon levels came back elevated and you are on a well. You may be dealing with a soil problem, a water problem, or both, and knowing which source is larger helps you target the right fix first.

How to collect and submit a water sample

Testing well water for radon is not the same as testing tap water for lead or bacteria. Radon degasses quickly when water is agitated, so the sample collection method matters a great deal.

Get the kit first. Most certified labs ship you a collection kit: a small sealed vial (often 40 mL) designed to minimize air contact during filling, along with shipping materials and written instructions. Order the kit before you collect anything.

Collection steps. Run the cold water tap at a slow, steady flow for two to three minutes to flush the pressure tank and lines. Then fill the vial carefully, usually by submerging it directly in the slow stream or by following the lab’s specific instructions. The goal is to avoid splashing or turbulence. Cap it immediately, pack it as directed, and ship it the same day or first thing the next morning. Do not leave a filled vial sitting over a weekend; radon decays and the reading will be inaccurate.

Where to send it. Your state’s certified lab or radon program is the best starting point. Several states with documented high radon in groundwater (Maine, New Hampshire, and others) offer free or heavily subsidized water testing through their environmental agencies. Check your state program before spending anything. If your state does not offer a free option, a private certified lab typically charges $20 to $50 for a water radon analysis. AARST-NRPP maintains a directory of certified measurement professionals who can point you to accredited labs.

If you prefer a mail-in kit you can order online, confirm the included analysis comes from a state-certified lab before purchasing. Check price on Amazon

Understanding your results

Your lab report will give you a concentration in pCi/L for water. This uses the same unit as air radon testing but describes a completely different thing: radioactivity dissolved in liquid rather than suspended in air.

There is no finalized federal maximum contaminant level for radon in private well water. The EPA proposed rules for community water systems years ago but never issued a final standard, and private wells fall under state regulation rather than federal law. State guidance levels vary. Some states recommend treatment above 2,000 pCi/L in water; others use different thresholds. Your lab report should note your state’s current guidance, or you can contact your state radon program directly.

To estimate what a given water level means for indoor air, apply the 10,000:1 rule of thumb as a rough guide. A result of 5,000 pCi/L in water suggests an air contribution of about 0.5 pCi/L. A result of 30,000 pCi/L suggests a contribution of about 3 pCi/L. These are estimates, not precise calculations, but they help you judge whether water is a meaningful part of your overall radon picture.

Pairing your water test with an air test gives you the full picture. Our guide on how to test your home for radon covers air testing options in detail, including what kind of test to run and where to place it.

Treatment options if levels are high

Two technologies handle radon in well water reliably, and they suit different situations.

Aeration. An aeration system sprays or agitates incoming water to release dissolved gases before it enters your home’s distribution lines. It is the EPA’s preferred approach for higher water radon concentrations. A well-designed aeration system can remove 95 percent or more of dissolved radon. The trade-off is cost: a professionally installed aeration system typically runs $3,000 to $6,000, and the equipment needs periodic maintenance. For high water levels, it is usually the better long-term choice.

Granular activated carbon (GAC) filtration. A point-of-entry GAC filter removes radon as water passes through a carbon bed. Installation costs less than aeration, often in the range of $1,000 to $2,500, and the system handles moderate radon concentrations well. The catch: the carbon concentrates radon decay products over time and eventually becomes low-level radioactive waste that must be disposed of according to state or local regulations. At high water radon levels, spent GAC carbon can accumulate enough activity to require careful handling. For this reason, most experts and the EPA favor aeration when water levels are elevated.

Both treatment types should be installed at the point where water enters the house, not just at one kitchen tap. Radon releases into air primarily during showering, dishwashing, and laundry, so whole-house treatment is the only approach that addresses the inhalation pathway.

After any treatment system is installed, retest both your water and your indoor air to confirm it is actually working.

Water radon vs. soil radon: what to tackle first

Soil is the dominant radon pathway for the vast majority of US homes. Even in a region with moderate water radon, the air concentration from soil entry is usually the bigger exposure. This matters because if your air radon test comes back elevated, the primary fix is sub-slab or crawl-space mitigation, not water treatment.

A practical sequence: test the air first. Air tests capture contributions from all sources combined. If air levels are below 2 pCi/L and you are on a private well, water testing is still worthwhile in high-risk geologic areas, but is unlikely to reveal anything alarming. If air levels are elevated, test water alongside air to understand how much each source contributes, then prioritize the larger one.

For help interpreting what your numbers actually mean, see our guides on how to read radon test results and short-term vs. long-term radon tests.

FAQ

Do I need to test my well water if my air test came back low? A low air result (under 2 pCi/L) is genuinely reassuring. Water radon is unlikely to be a meaningful contributor when air levels are already that low. That said, if you are in a geological high-risk area like northern New England or parts of Appalachia and want a complete picture, a water test costs little enough that doing it once is reasonable.

How often should I retest my well water for radon? There is no universal federal requirement for private wells, but many state programs suggest retesting every two to three years or after any significant change to the well system, such as a pump replacement, nearby drilling activity, or a period of very low water use. Aquifer conditions and radon concentrations can shift over time.

Can I just put a filter under my kitchen sink instead of treating the whole house? No. A point-of-use filter only treats the water at that single tap. The primary radon exposure from water comes through inhalation during showering, dishwashing, and laundry, not from drinking. Whole-house treatment at the point where water enters the home is necessary to reduce the air exposure pathway.

Is tap water from a city or town a radon concern? Generally not. Public water systems are regulated under the Safe Drinking Water Act and typically draw from surface water or treat groundwater centrally. Any dissolved radon off-gasses during treatment and storage before reaching homes. Radon in drinking water is largely a private well issue.