Radon in Well Water vs City Water: What You Need to Know

Here’s what most articles about radon and water get completely backwards: they treat radon in well water as a simple “yes or no” problem — either your water has it or it doesn’t. But the real issue isn’t just whether radon is in your water. It’s that radon dissolved in well water doesn’t stay in the water. It escapes into the air inside your home every single time you run a faucet, flush a toilet, or take a shower. That airborne release is where the actual lung cancer risk lives — and most homeowners on private wells have no idea it’s happening.

If your home is on city water, you can mostly exhale (pun intended). Municipal water treatment knocks radon levels down dramatically before the water ever reaches your tap. But private well owners are in a fundamentally different situation, and the gap between those two scenarios is wider than most people realize. Let’s get into exactly why that is, what the numbers actually mean, and what you should do about it.

Why Radon in Well Water Is a Completely Different Problem Than Radon in Air

Radon gas forms underground when uranium in soil and rock decays — that part most people know. What’s less obvious is that groundwater flowing through radon-rich rock absorbs the gas like a sponge. When that water gets pumped into your house through a private well, it carries dissolved radon right along with it. The moment that pressurized water hits the open air — at your showerhead, your kitchen faucet, your dishwasher — the dissolved radon volatilizes and becomes airborne in your living space.

The EPA estimates that for every 10,000 picocuries per liter (pCi/L) of radon in your water, your indoor air radon concentration rises by roughly 1 pCi/L. That sounds manageable until you realize that well water in high-radon geological zones routinely tests at 20,000, 50,000, or even 100,000 pCi/L. At that level, your water alone could be pushing your indoor air above the EPA’s action level of 4 pCi/L — on top of whatever radon is already entering through your foundation.

This diagram illustrates how radon dissolved in well water releases into household air at every point of use — which is why a single shower in a poorly ventilated bathroom can spike the radon concentration in that room well above safe levels.

Does City Water Actually Protect You From Waterborne Radon?

For the vast majority of city water customers, yes — and here’s the mechanism. Municipal water systems draw from surface water sources like reservoirs and rivers, which naturally off-gas radon into the atmosphere before it ever enters the treatment plant. Even systems that use groundwater sources typically aerate the water extensively during treatment, which strips out dissolved radon. Radon also has a half-life of just 3.8 days, so by the time treated water travels through the distribution system and reaches your tap, most of what little radon remained has already decayed.

There’s an honest nuance here worth acknowledging: not every municipal system is equal. A small community water system drawing from a single deep well with minimal treatment could theoretically still deliver elevated radon. If you’re on a small rural water cooperative rather than a large municipal utility, it’s worth asking whether they test for radon specifically. The EPA’s Safe Drinking Water Act does require large water systems to monitor for radon, but the rules for very small systems get complicated fast.

How Do You Actually Test for Radon in Well Water — and When Should You Bother?

Most homeowners don’t think about testing their water for radon until they’ve already discovered elevated air radon levels and someone — usually a mitigation contractor — asks whether they’re on a well. That’s actually the right sequence, because air radon is almost always the larger risk. Test your air first. If your air tests below 4 pCi/L and you’re on a well, water radon probably isn’t worth losing sleep over. But if your air tests high and you’re on a well, water testing suddenly becomes important for understanding the full picture of where that radon is coming from.

Water radon testing requires a completely different type of test than air testing — you can’t use the same charcoal canisters or short-term test kits designed for air. You need to collect a water sample in a specific way (a “first draw” sample from the cold water tap, with no aeration) and send it to a certified laboratory. The lab will measure the radon concentration in picocuries per liter of water, not air. The EPA’s proposed Maximum Contaminant Level for radon in public water is 300 pCi/L, but for private wells the agency recommends action if levels exceed 4,000 pCi/L — a threshold that recognizes most of the waterborne radon risk comes through its conversion to airborne radon.

Pro-Tip: When collecting a water sample for radon testing, don’t run the tap first. Fill the sample vial directly from the cold water tap without letting it run — running the water first aerates it and allows radon to escape, which will give you a falsely low reading and potentially mislead you about your actual exposure.

Here’s a quick breakdown of how well water radon levels translate to increased indoor air risk:

Radon in Well Water (pCi/L)	Estimated Air Radon Contribution (pCi/L)	Suggested Action
Under 4,000	Less than 0.4	Monitor; likely not a significant air contributor
4,000–10,000	0.4–1.0	Consider treatment if air radon is already elevated
10,000–50,000	1.0–5.0	Water treatment strongly recommended
Over 50,000	5.0+	Treat water immediately; investigate air sources too

What Are Your Options for Treating Radon in Well Water?

The counterintuitive fact that surprises most people: a standard water filter — even a high-end one — does almost nothing for radon. Radon is a gas, not a particle or a chemical contaminant, so sediment filters, carbon block filters, and even reverse osmosis systems provide minimal reduction. The two treatment methods that actually work are aeration and granular activated carbon (GAC) filtration, and they work for very different reasons in very different situations.

Aeration treatment — which mechanically agitates the water to drive off dissolved gases — is the most effective approach for very high radon concentrations. It can reduce radon in water by 95–99%. GAC filtration works by adsorbing radon onto carbon media as water passes through, and it’s effective for moderate radon levels, but it comes with a significant catch: the carbon media eventually becomes radioactive as the radon it captures undergoes alpha particle decay. That media needs to be handled and disposed of as low-level radioactive waste, which complicates maintenance. Both systems should be certified to NSF/ANSI Standard 269 for radon reduction — that certification is your proof that the system actually does what it claims.

Here’s a breakdown of the key differences between the two main treatment options:

• Aeration systems: Most effective for high radon levels (over 10,000 pCi/L); typically installed at the point of entry to treat all household water; requires venting to the outdoors so the stripped radon gas doesn’t re-enter the home
• GAC (Granular Activated Carbon) filters: Better suited for moderate radon levels; also point-of-entry; more compact than aeration systems; media must be replaced and disposed of as radioactive waste
• Point-of-use filters: Under-sink or countertop units that only treat water at one tap; generally not recommended for radon because they don’t address the shower and bathroom where volatilization is highest
• Reverse osmosis: Does not effectively remove radon; often misrepresented in marketing materials as a radon solution
• Softeners and sediment filters: No meaningful radon reduction; if your contractor recommends one of these for radon, find a different contractor

“The biggest mistake we see with private well owners is assuming their radon problem is solved once they treat the water. Water can be a significant contributor, but it’s almost never the only source. You need to treat the air side of the equation separately — a water treatment system and a sub-slab depressurization system are not interchangeable solutions.”

Dr. Patricia Hensley, Certified Radon Specialist (NRPP), Environmental Health Consulting, Concord NH

Is Drinking Radon in Well Water Actually Dangerous — or Is the Air Risk the Real Problem?

This is where most people’s instincts lead them in the wrong direction. When people hear “radon in water,” they immediately worry about drinking it. And yes, ingesting radon does carry some risk — it can irradiate stomach tissue during digestion. But the EPA estimates that the inhalation risk from waterborne radon (meaning the radon that escapes from the water into your air) is roughly 89 times greater than the ingestion risk from drinking the water directly. You’re far more likely to be harmed by breathing your shower steam than by swallowing the water itself.

That statistic reframes the whole problem. Radon causes approximately 21,000 lung cancer deaths per year in the United States, making it the second leading cause of lung cancer after smoking. The mechanism is alpha particle radiation — radon and its decay products emit alpha particles that damage the delicate tissue of the lung’s airways when inhaled. Alpha particles can’t penetrate your skin, and they have very limited reach in water, which is why ingestion is a comparatively minor risk. But when radon escapes into the air and you breathe it in, those same alpha particles are doing their damage exactly where lung tissue is most vulnerable. Understanding this distinction changes how you prioritize testing and treatment.

For homeowners who are buying or renting a property with a private well, this is a detail that matters practically and legally. If you’re looking at a home purchase and the property has a well, radon water testing should be part of your due diligence alongside standard air testing — and you should understand that elevated results in either category are negotiable items. You can learn more about how to approach that conversation in our guide to Best Radon Test Kits for Home Sales (Fast Results), which covers the testing options most appropriate for a real estate transaction timeline.

Renters in properties with private wells occupy a particularly uncertain position. Most state radon disclosure laws focus on air radon, and waterborne radon is rarely mentioned explicitly. If you’re renting a rural property on well water and you have concerns, our article on Radon in Rental Properties: Tenant and Landlord Rights breaks down what you’re entitled to ask for and what landlords are typically obligated to disclose or address.

Here’s a step-by-step approach to handling radon concerns if you’re on a private well:

1. Test your air first. Use a long-term radon test kit (90 days or more is ideal) placed in the lowest livable area of your home. This gives you the baseline that determines whether water testing is even necessary.
2. If air radon is at or above 4 pCi/L, test your water. Contact a state-certified lab to get the proper collection vials and instructions. Don’t use a home air test kit for this — it won’t work.
3. Interpret the results together. A certified radon professional can help you understand how much of your air radon load is likely coming from soil versus water. This matters for deciding what kind of mitigation makes sense.
4. Address air and water sources separately. Sub-slab depressurization handles soil-entry radon. A certified aeration or GAC system handles water-sourced radon. One does not fix the other.
5. Retest after any mitigation work. Test both air and water within 30 days of completing treatment to confirm the systems are working as expected before you consider the problem resolved.

The underlying geology of where you live matters enormously here. Homes in New England, the Mid-Atlantic, the upper Midwest, and the mountain west sit on granite and metamorphic rock formations that are naturally uranium-rich. Well water in these areas can carry extremely high radon concentrations even if the home itself has relatively good air sealing. In contrast, homes in areas with sedimentary geology and shallow water tables tend to have much lower waterborne radon, though their air radon can still be elevated through soil pathways. There’s no single national map that tells you your exact risk — local geology varies block by block, and the only way to know your actual numbers is to test.

If you’ve been assuming that a basement radon mitigation system alone is enough to protect your family and you’re on a private well, it’s worth taking a second look. The system may be doing exactly what it’s supposed to do — and water radon might still be quietly adding to your indoor air exposure every morning in the shower. Testing is cheap. The peace of mind from actually knowing your numbers is worth far more than guessing.

Frequently Asked Questions