How to Test for Radon in Well Water at Home

Here’s what most articles about radon in well water get completely wrong: they treat it like a radon-in-air problem with a water twist. It’s not. Testing for radon in your well water is a distinct process, it requires a specific type of sample kit, and the risks it poses are almost entirely different from the radon you test for in your basement. Most homeowners don’t think about this until a real estate inspector hands them a radon-in-air report and says “you passed” — not realizing that test tells them absolutely nothing about what’s coming out of their tap.

The bottom line: if your home is on a private well and you’ve never tested the water itself for radon, there’s a real chance you’re inhaling it every time you run a hot shower, boil water, or use a dishwasher — and you don’t know it. Testing for radon in well water at home is possible, relatively affordable, and takes about two minutes of your time to set up. What matters is knowing which test to use, how to read the results, and what to do if the numbers are higher than you expect.

Why Testing Well Water for Radon Is Not the Same as Testing Your Air

Radon is a naturally occurring radioactive gas that forms when uranium breaks down in soil and rock. It seeps into groundwater, which means private wells drilled into granite, shale, or other uranium-bearing geology can pull up water with surprisingly high radon concentrations. The EPA has proposed a Maximum Contaminant Level (MCL) of 300 pCi/L for radon in public drinking water, but private wells aren’t federally regulated — and many state labs have found well water samples exceeding 10,000 pCi/L or more in high-risk regions like New England and the Appalachians.

The counterintuitive part that almost nobody talks about: waterborne radon’s biggest threat isn’t from drinking the water. Studies from the National Academy of Sciences estimate that for every 10,000 pCi/L of radon in water, indoor air radon levels rise by roughly 1 pCi/L. That means your well water isn’t just a water quality issue — it’s actively contributing to the air your family breathes. A radon-in-air test only measures the result, not the source. If the source is your water supply, fixing the air alone doesn’t solve the problem.

This close-up shows a water-specific radon sampling vial next to a standard well water tap — a reminder that the test kit you use matters as much as the decision to test at all, since air canisters and water vials are completely different tools designed for completely different measurements.

Which Home Test Kit Actually Works for Radon in Well Water?

You cannot use a standard passive radon charcoal canister — the kind you set on your basement floor — to test your well water. Those are designed to absorb radon gas from the air over 48 to 96 hours. Water-based radon testing requires a liquid scintillation analysis or a similar lab method, which means you need a water-specific sample vial that you fill, seal, and mail to a certified lab. The sample must reach the lab within a tight window — typically within 96 hours of collection — because radon has a half-life of just 3.8 days and begins decaying almost immediately after you collect it.

The two most reliable options for home use are mail-in lab kits from state-certified laboratories and kits that come pre-approved by the EPA or your state’s radon program. Look for labs certified under the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB). Prices range from about $25 to $75 depending on whether you want basic results or a full report with remediation guidance. One honest nuance here: some big-box store radon kits are only air-rated and won’t mention water testing at all, so read the label carefully before you buy.

Pro-Tip: When ordering a water radon test kit, check that it explicitly states “liquid scintillation” or “water sample analysis” on the packaging or product description. If it just says “radon test kit” without specifying water, assume it’s for air only — and don’t waste a sample on it.

How to Collect a Well Water Radon Sample Without Ruining the Results

Sample collection is where most DIY tests go wrong, and it’s almost never because the kit was bad. Radon degasses quickly — meaning it escapes from water into the air — so any turbulence, splashing, or delay during collection can cause your sample to read lower than the actual concentration in your well. That’s a false negative, and it’s arguably worse than no test at all because it gives you false confidence. The goal is to get a still, undisturbed water sample into a sealed vial with as little air exposure as possible.

Here’s a step-by-step process that minimizes degassing and gives you an accurate result:

1. Choose the right tap. Use the cold-water tap closest to where your well line enters the house — not a filtered tap, not a softener bypass, and not a hose bib. You want the most direct path from the well to your sample.
2. Run the water for two minutes before sampling. This clears any standing water in the pipes, which may have lost radon through degassing while sitting idle. You want fresh water drawn directly from the aquifer.
3. Reduce flow to a slow trickle. Hold the sample vial at an angle under the tap and let water run gently down the side of the vial, not into the center. Splashing introduces air bubbles that release radon before you can seal it.
4. Overfill slightly, then cap immediately. Your kit instructions will specify a fill line. Fill just past it, then cap or seal the vial in one smooth motion to minimize headspace air inside the container.
5. Ship within 24 hours of collection. Radon decays fast. The longer your sample sits before analysis, the more the lab has to mathematically correct for decay — and errors compound. Same-day or next-morning shipping is ideal.

In most homes we’ve tested, the single biggest source of skewed results isn’t the well itself — it’s a sample collected from a tap downstream of a water softener or a sediment filter housing, which can absorb or disrupt radon levels before the vial is even filled. Always sample from an untreated cold-water line when you’re testing for radon specifically.

What Do Your Radon in Water Results Actually Mean?

Your results will come back in picocuries per liter (pCi/L). That unit sounds technical, but the thresholds are worth memorizing. The EPA’s proposed drinking water standard sits at 300 pCi/L for systems that use a Multimedia Mitigation program for indoor air, or 4,000 pCi/L as an alternative standard if no air mitigation is in place. Most state health departments use 2,000 pCi/L or 4,000 pCi/L as action thresholds for private well owners, though states like Maine and New Hampshire — which sit on radon-rich granite — recommend action at 2,000 pCi/L or even lower.

Here’s a simple reference table to orient yourself when you get your results back:

Radon Level in Water	Risk Interpretation	Recommended Action
Below 300 pCi/L	Low — within proposed EPA standard	No immediate action required; retest every 3–5 years
300–2,000 pCi/L	Moderate — contributes to indoor air radon	Consider point-of-entry treatment; retest air
2,000–4,000 pCi/L	Elevated — most states recommend mitigation	Install aeration or GAC system; consult state radon office
Above 4,000 pCi/L	High — EPA alternative MCL threshold	Immediate mitigation required; professional consultation advised

One thing your lab report probably won’t explain clearly: radon in water contributes far more to inhalation exposure than to ingestion exposure. The NAS estimates that ingested waterborne radon accounts for roughly 1–2% of the total cancer risk from radon in water — the other 98% comes from breathing air that’s been contaminated by the water. That changes how you think about treatment priority, because the goal isn’t just safer water to drink; it’s safer air to breathe in your home.

“Most homeowners are surprised to learn that their well water radon problem is showing up on their air quality monitor, not their taste or smell. Radon is odorless, tasteless, and completely invisible in water — the only way to know it’s there is to test for it specifically. I’ve seen water with 8,000 pCi/L of radon that looked and tasted completely clean.”

Dr. Sandra Kowalski, Certified Water Quality Specialist and Environmental Health Consultant, University of New Hampshire Extension

What to Do When Your Well Water Tests High for Radon

The two proven treatment approaches for radon in well water are granular activated carbon (GAC) filtration and aeration systems. They work through completely different mechanisms and are suited to different situations. GAC filters trap radon as water passes through a carbon bed — effective for lower to moderate concentrations, but the carbon itself becomes radioactive over time and must be handled carefully during media replacement, which isn’t something most homeowners realize going in. Aeration systems physically strip radon gas from water by exposing it to large volumes of air before distribution — they’re more effective at higher concentrations but require more installation space and maintenance.

Both systems should be installed at the point of entry (POE), meaning where the well line enters your home, not at the kitchen sink. A point-of-use filter under your kitchen tap does almost nothing for the radon exposure you’re getting from your shower, your dishwasher steam, or your laundry. The shower is especially significant because hot water releases radon gas faster and the bathroom is a small, often poorly ventilated space — exactly the wrong conditions for radon accumulation. If you’re also evaluating whether a reverse osmosis or distillation system is appropriate for other contaminants in your water, know that neither is an effective standalone solution for radon — RO removes some radon but not reliably enough at high concentrations, and distillation is impractical at the whole-house scale.

Here’s a quick breakdown of what to factor in when choosing between aeration and GAC:

• Radon concentration: GAC is generally effective up to about 5,000 pCi/L; above that, aeration is the more reliable choice.
• Space availability: Aeration units are larger — typically requiring a dedicated utility space — while GAC tanks are more compact and similar in size to a water softener.
• Maintenance requirements: GAC media requires periodic replacement (often every 2–5 years depending on usage and radon levels), and spent carbon must be disposed of as low-level radioactive waste in some states — check your state regulations before installation.
• Cost: GAC systems typically run $1,000–$3,000 installed; aeration systems range from $3,000–$7,000 or more depending on flow rate and complexity.
• Other contaminants: If your well also has iron, manganese, or hydrogen sulfide, aeration handles those simultaneously — a real advantage if your water has multiple issues.

After any treatment system is installed, retest your water radon levels within 30 days to confirm it’s performing as expected — and then retest your indoor air radon levels about 90 days later to see if the water treatment has brought air concentrations down as well. If air levels remain high after the water source is addressed, you may have additional radon entry points through your foundation or soil that need separate mitigation.

The bigger picture here is that radon in well water is a solvable problem, and it’s one of the few water quality issues where a single well-chosen test can reveal both a water hazard and an air hazard at the same time. Testing takes about two minutes to set up and costs less than most homeowners spend on a single utility bill. If you’re on a private well and you haven’t tested specifically for water radon — not just air radon — that’s the one step worth taking before anything else on your home water quality checklist.

Frequently Asked Questions