Radon in Well Water: How to Test It and When to Take Action

Most people don’t think about radon in their well water until a home inspector mentions it, or worse, until someone in the family starts asking questions about lung health. Radon is one of those hazards that’s genuinely invisible — no smell, no taste, no color — and if your home runs on a private well, there’s a real chance it’s flowing right out of your kitchen tap. That’s not meant to scare you. It’s just the reality of drawing water from bedrock, and understanding it means you’re already ahead of the problem.

What Radon Is and Why Well Water Is the Problem

Radon is a naturally occurring radioactive gas. It’s produced when uranium and radium decay in rock and soil — a process that’s been happening since the Earth formed and isn’t stopping anytime soon. In most homes, the bigger concern is radon seeping up through the foundation from the ground. But when your water supply comes from a drilled well that taps into granite, shale, or other uranium-bearing rock formations, the water itself can carry dissolved radon directly into your home. Surface water sources — lakes, rivers, reservoirs — lose radon quickly to the atmosphere, which is why municipal tap water is rarely the concern here. Private wells are a different story entirely.

The mechanism matters. Radon dissolves readily in cold groundwater under pressure. Deep aquifers in contact with radon-producing rock act almost like storage tanks for the gas. When you run the hot shower, fill a pot for boiling pasta, or run the dishwasher, that dissolved radon gets released into the air inside your home. This is called the “water-to-air transfer” effect, and studies have estimated that for every 10,000 picocuries per liter (pCi/L) of radon in water, indoor airborne radon increases by roughly 1 pCi/L. That’s not a trivial number when well water radon concentrations in high-risk areas can exceed 100,000 pCi/L. It’s also worth noting that radium in drinking water often comes from the same uranium decay chain as radon, so if you’re investigating one, it’s smart to look at the other at the same time.

Who Is Most at Risk and Where Radon Levels Run High

Geography plays a huge role in whether radon in well water is even on your radar. Certain geological formations are far more likely to produce elevated radon than others. Granite-heavy regions — think New England, the Appalachian Mountains, parts of the upper Midwest, and portions of the Rocky Mountain states — tend to have significantly higher concentrations than areas sitting on sedimentary rock like limestone or sandstone. That said, this isn’t a hard rule. Local geology is variable enough that two neighboring counties can have dramatically different radon profiles, which is exactly why testing your specific well matters more than assuming based on regional averages.

Beyond geography, a few other factors push your risk higher. Here’s what tends to increase radon concentrations in private well water:

1. Well depth: Deeper drilled wells penetrate further into bedrock, spending more time in contact with radon-producing mineral formations. Shallow dug wells that pull from surface aquifers tend to have lower radon levels because the water has had more exposure to open air.
2. Rock type: Granite, gneiss, and other igneous or metamorphic rocks are the primary culprits. Uranium concentrations in these formations can be 10 to 20 times higher than in sedimentary rock, which directly feeds radon production.
3. Well casing condition: Older or cracked well casings allow more contact between water and surrounding soil, increasing the surface area exposed to radon-generating material.
4. Water temperature: Colder groundwater holds dissolved radon more effectively than warmer water. This means wells in northern climates or at higher elevations can carry more radon into your home before it off-gasses.
5. Household water use patterns: Homes with high hot water use — large families, frequent laundry, long showers — release more radon into indoor air simply because they’re agitating and heating the water more often.

How to Test Your Well Water for Radon

Testing for radon in water is not the same as the air radon test kits you might have seen at hardware stores. Those charcoal canisters measure airborne radon — useful for evaluating what’s seeping up through your basement floor, but they won’t tell you anything about what’s coming out of your tap. Water-specific radon testing requires a water sample collected in a special vial and sent to a certified laboratory. The EPA’s state radon contacts page lists approved labs by state, and many state health departments offer testing through their own programs, sometimes at low or no cost for private well owners.

When collecting a water sample for radon testing, the method matters a lot. Radon off-gasses rapidly once water is exposed to air, so the sample has to be collected carefully to avoid losing dissolved gas before it reaches the lab. Here’s what a proper collection process looks like:

• Use the certified lab’s specific sample vial — don’t substitute a generic container.
• Run your cold water tap for at least 2 minutes before sampling to clear the pressure tank and pull fresh groundwater.
• Fill the vial from a slow, steady stream with the tube submerged in the vial to minimize turbulence and air contact.
• Cap the vial immediately and without trapping air bubbles — any headspace allows radon to escape the sample.
• Ship the sample to the lab the same day it’s collected, or as close to it as possible — radon has a half-life of about 3.8 days, so delay directly reduces the accuracy of results.
• Test during a period of normal household water use, not after the well has sat unused for days, which can artificially inflate concentrations.

Understanding Your Test Results and the Action Levels That Matter

The EPA has proposed — but never finalized — a Maximum Contaminant Level (MCL) of 300 pCi/L for radon in public water supplies, with an alternative standard of 4,000 pCi/L for utilities that implement multimedia mitigation programs addressing airborne radon. For private well owners, these proposed numbers are still the most widely cited benchmarks, even though they technically don’t apply to you legally. Many state health departments use 300 pCi/L as their advisory threshold for action, while others set the line at 1,000 pCi/L or even 4,000 pCi/L depending on local risk assessments. This is one of those situations where the right response genuinely depends on your specific circumstances — your water concentration, your home’s existing air radon level, and how much water your household uses daily.

Here’s a breakdown of how radon water concentrations roughly translate to risk and recommended response:

Radon in Water (pCi/L)	Approximate Air Contribution	Recommended Action
Below 300	Less than 0.03 pCi/L added to air	No immediate action required; retest every 3–5 years
300 – 1,000	0.03 – 0.1 pCi/L added to air	Consider mitigation; also test indoor air radon levels
1,000 – 4,000	0.1 – 0.4 pCi/L added to air	Mitigation recommended; point-of-entry treatment preferred
4,000 – 10,000	0.4 – 1.0 pCi/L added to air	Mitigation strongly advised; address air radon simultaneously
Above 10,000	Above 1.0 pCi/L added to air	Immediate mitigation; consult a licensed radon mitigation specialist

It’s also worth keeping in mind that waterborne radon isn’t the only pathway to elevated indoor air radon. If your air radon test already shows levels above 4 pCi/L — the EPA’s action level for indoor air — your priority should be addressing that through sub-slab depressurization or other air-side mitigation first. The water contribution, while real, is usually smaller than what’s coming up through the foundation in high-radon homes. Both issues deserve attention, but sequencing matters for cost-effectiveness.

Treatment Options That Actually Work for Radon in Well Water

When your test results come back above the threshold you’re working with, two treatment technologies have a solid track record for removing radon from well water: aeration and granular activated carbon (GAC) filtration. These aren’t interchangeable — they work differently, have different maintenance requirements, and suit different situations. Understanding the tradeoffs helps you pick the right tool rather than just the cheapest one.

Aeration systems work by exposing water to large amounts of air, which allows dissolved radon to off-gas before the water enters your home’s distribution system. Packed tower aerators and diffused bubble aerators are the two most common designs, and they can remove 95% to 99% of dissolved radon when properly sized. The downside is that they vent radon-laden air outside, which requires careful placement so the exhaust doesn’t re-enter living spaces. GAC filters, on the other hand, adsorb radon onto activated carbon as water passes through. They’re simpler to install and don’t require venting, but the carbon media becomes radioactively contaminated over time and must be handled carefully during replacement — sometimes classified as low-level radioactive waste depending on concentration levels and state regulations. For homes with radon concentrations above 30,000 pCi/L, GAC becomes harder to justify from a maintenance standpoint, and aeration is generally the better call. Both technologies should be installed as point-of-entry systems — treating all water entering the house — rather than point-of-use filters at individual taps. Radon enters your body through inhalation far more than ingestion, so treating only the drinking tap misses the shower, the dishwasher, and every other water use that releases gas into your air. You might already be familiar with how certain water problems can seem minor on the surface but point to bigger underlying issues — much like when pink stains in your bathroom turn out to signal a water chemistry issue worth investigating properly.

Pro-Tip: If you’re ordering a radon water test, add uranium to the same sample submission. Uranium and radon come from the same decay chain, uranium doesn’t off-gas like radon does, and many certified labs will run both tests on a single sample for a modest additional fee. If you find elevated radon, there’s a reasonable chance uranium is worth knowing about too — and it has its own drinking water standard of 30 micrograms per liter (µg/L) under EPA regulations.

“The waterborne radon pathway is chronically underestimated by homeowners, partly because the health risk literature focuses so heavily on soil gas entry. But in a home with a high-radon well, daily showering and cooking can contribute meaningfully to cumulative inhalation exposure — and unlike sub-slab radon, it’s a source that’s almost entirely fixable with the right point-of-entry treatment. I always tell people: test the air and the water together, because you can’t fully understand one without the other.”

Dr. Margaret Holloway, Environmental Health Scientist and Certified Radon Measurement Specialist, University Extension Program in Environmental Quality

Radon in well water isn’t a reason to panic, but it is a reason to test — especially if you’re in a granite-heavy region, drawing from a deep drilled well, or if your home’s indoor air radon has already come back elevated. The fix exists, it works reliably, and the testing process is genuinely manageable for any homeowner willing to spend an afternoon on it. What you want to avoid is the assumption that because your water looks and tastes fine, there’s nothing worth checking. With radon, that’s exactly the kind of thinking that lets years of unnecessary exposure pile up quietly in the background.

Frequently Asked Questions