Radon in Water vs Radon in Air

Radon is a colorless, odorless, tasteless radioactive gas produced by the natural decay of uranium in rock and soil. Most people know it as a basement air problem: radon seeps up through the foundation from the soil underneath the house. That soil pathway is, by a wide margin, the biggest source of radon exposure in most homes. The EPA estimates that only about 1 to 2 percent of the radon in indoor air comes from drinking water.

But if your water comes from a private well drilled into uranium-bearing bedrock, the water itself can carry dissolved radon into the house. Every time that water gets agitated or heated (a shower, a dishwasher cycle, a load of laundry, even a running faucet), some of the dissolved radon escapes into the air you breathe. This is called off-gassing, and inhalation of that released radon is the primary health concern. There is also a smaller, secondary risk from actually drinking radon-laden water, which the National Academy of Sciences has linked to a modest increase in stomach cancer risk. Radon in air is tied to lung cancer, and it is the second-leading cause of lung cancer in the United States after smoking.

The 10,000-to-1 ratio is the number that puts the whole topic in perspective. It comes from the National Academy of Sciences and is the standard conversion used by the National Radon Program Services and state health departments. A well testing at 5,000 pCi/L is contributing roughly half a picocurie to your indoor air. A well testing at 40,000 pCi/L (not rare in parts of New England) pushes your air up by about 4 pCi/L all by itself, before counting anything coming through the foundation.

Who Should Test (and in What Order)

Radon in well water is a geology problem. The wells that carry high radon are drilled into granite and other crystalline bedrock where uranium naturally occurs. The classic hot zones are New England (Maine, New Hampshire, Rhode Island have some of the highest well-water radon readings in the country), the Appalachian spine, and the crystalline-rock belts running through Pennsylvania, Maryland, and Virginia. That said, radon shows up in all 50 states, so geography raises or lowers suspicion, it does not settle the question.

Here is the order of operations, and it is worth getting right because it saves most people money:

1. Test your indoor air first. A charcoal canister test costs $15-30 at any hardware store, and some state radon programs give them away free. Air is where most of your radon exposure comes from regardless of the source, and the air number tells you whether there is a problem worth chasing at all.
2. If air is at or above 4 pCi/L and you are on a private well, test the water. The water test tells you how much of the air problem is coming through the plumbing versus through the foundation. That distinction decides whether you need a soil mitigation system, a water treatment system, or both.
3. If air is low, water testing is optional. If the air in the lowest lived-in level of the house reads well under 4 pCi/L, the water is, by definition, not creating a meaningful air problem. Some homeowners in granite country still test the water once for the record, which is reasonable, but it is not urgent.

How to Test Well Water for Radon

Unlike iron, hardness, or pH, there is no meaningful DIY strip test for radon in water. Radon is a dissolved gas measured by liquid scintillation counting, which is lab equipment, full stop. Your realistic options:

Option 1: State or university lab programs

States with known radon geology run cheap, well-organized programs. Maine's state lab sells a radon water test directly to homeowners. New Hampshire, Vermont, and North Carolina publish lists of accredited labs, and Vermont has sold kits through its health department for around $25. University extension labs (the University of Georgia is a good example) offer radon water testing by mail. If your state has a program, this is usually the cheapest certified answer, typically $25-60.

Option 2: Commercial radon labs

Dedicated radon labs like AccuStar and general mail-in labs sell radon-in-water kits in the $30-130 range. You get two small glass vials, fill them following a strict no-splash procedure, and overnight them back. Results usually come back within days of the lab receiving the sample. This is the right option if your state does not run a program.

Option 3: Add radon to a certified full-panel test

Some certified lab networks can add a radon vial to a broader well water panel if you request it at order time. This matters because radon almost never travels alone: the same uranium-bearing bedrock that produces radon also tends to produce uranium, gross alpha radioactivity, arsenic, and low pH. New Hampshire's environmental agency specifically recommends that any bedrock well with high radon also be tested for uranium and gross alpha. If you are testing a bedrock well for the first time, a full panel plus a radon add-on answers the whole question in one shipment. Our what to test for in well water guide covers the full baseline list.

How to Collect a Radon Water Sample

Radon is a gas trying to leave your sample from the second you draw it. Radon-222 also has a half-life of about 3.8 days, meaning half of it decays away in under four days even in a perfectly sealed vial. Both facts drive the collection rules, and breaking them gives you a falsely low number:

1. Sample from a cold-water tap before any treatment, ideally the spigot nearest the pressure tank. Softeners, carbon filters, and especially any system that aerates the water will strip radon and hide the true well number.
2. Run the water 5-10 minutes first so you are sampling fresh well water, not water that has been sitting in the pipes losing radon.
3. Reduce to a slow, smooth stream with no bubbles. Splashing and turbulence off-gas radon before the vial is even capped. Some labs have you fill the vial under a slow stream while tilted; follow their instructions exactly.
4. Fill the vial completely, with zero headspace. Any air bubble trapped in the vial pulls radon out of the water during shipping. Cap it underwater or overflow-fill per the lab's procedure.
5. Ship it the same day, overnight. Because of the 3.8-day half-life, labs correct for decay based on your recorded collection time, but only within a validity window. A sample that spends a week in transit is worthless.
6. Record the exact date and time of collection on the chain-of-custody form. The lab cannot do the decay correction without it.

Because radon in groundwater can swing between seasons and between pumping conditions, New Hampshire recommends two tests at least a month apart before spending money on treatment. That is good advice anywhere. One high test starts the conversation; two high tests justify equipment.

What Your Radon Number Means (in pCi/L)

Radon in water is reported in picocuries per liter (pCi/L), the same unit used for air, which trips people up: 4 pCi/L is the action level for air, while water numbers run in the thousands. There is no enforceable federal limit for radon in water. The EPA proposed a framework in 1999 (a 300 pCi/L limit for public water systems, or 4,000 pCi/L in states running EPA-approved indoor-air radon programs), and that rule was never finalized. It also never applied to private wells in the first place. What exists instead is a patchwork of state advisory levels: Maine advises action at 4,000 pCi/L, New Hampshire recommends treatment at 10,000 pCi/L and calls it advisable between 2,000 and 10,000 pCi/L when indoor air exceeds 4 pCi/L. The scale below reflects how those guidelines stack up in practice.

Treatment: Aeration vs Activated Carbon

Radon must be treated at the point of entry (whole-house), never at the kitchen tap. The risk is inhalation from every water fixture in the home, so an under-sink filter that cleans only your drinking glass misses the point entirely. Two technologies do the job:

The honest summary: aeration is the gold standard and the only sensible choice at high concentrations. GAC is cheaper upfront and works at modest levels, but it comes with a caveat most sellers skip: because the carbon holds the radon while it decays, the tank itself becomes mildly radioactive over its service life. New Hampshire's guidance is to change GAC media every 1 to 2 years specifically to prevent gamma radioactivity from accumulating in the filter bed, and to site the tank away from living spaces where possible. That is manageable, but you should know it going in. We stock whole-house carbon systems for chlorine, taste, and odor applications; if radon is the target, talk to Aidan first so the system and the maintenance schedule are actually matched to a radon job rather than a standard carbon application.

One more note: aeration and GAC only address radon. If your bedrock well also carries uranium or gross alpha (common companions), those are removed by different equipment, which is one more reason the full water picture matters before anything gets installed.

Why Radon Is Not on a Standard Well Panel (And What to Do About It)

Standard well panels, ours included, are built around contaminants that survive a normal sample bottle: metals, minerals, bacteria, nitrates, VOCs with proper vials. Radon fails that test twice. It escapes the sample during any splash or headspace exposure, and it decays with a 3.8-day half-life, so it needs its own overflow-filled glass vials and a same-day overnight shipment with a decay-corrected analysis on the other end. Bundling that into a general-purpose kit would make every kit more expensive and more fragile for the majority of buyers who do not need it.

So here is the straightforward playbook we recommend:

1. Get the baseline first. The Well Water Test Kit ($199) covers 53 contaminants through certified labs, including uranium, arsenic, pH, iron, and bacteria (our complete well water testing guide walks through the whole baseline process). On a bedrock well, those results already tell us a lot about whether your geology is the radon-producing kind: low pH, uranium hits, and arsenic are all fellow travelers of radon-bearing granite.
2. Test your air with a cheap charcoal kit if you have not already.
3. Add a dedicated radon water test through your state program or a commercial radon lab if your air is elevated. The kit's expert review includes Aidan looking at your full results, and if your chemistry and geography say radon testing is warranted, he will tell you exactly that and point you to the cheapest legitimate way to get it done. We would rather route you to your state lab for a $30 radon vial than pretend our standard panel answers a question it does not.

You can compare all of our testing options on the water testing collection page, and if you already have results in hand, our guide to reading well water test results walks through every line item.

4 Common Radon Water Testing Mistakes

1. Testing water before testing air

The air test is cheaper, faster, and measures the thing that actually harms you. A water-first approach can leave you staring at a 3,000 pCi/L water result with no idea whether it matters. Air first, water second. The only exception: a real estate transaction where the well is being tested comprehensively anyway.

2. Assuming radon was included in the well panel

It was not, unless you specifically ordered it and received separate glass vials with no-headspace fill instructions. Read the report: if there is no radon-222 line item, it was not tested. This surprises people constantly, including buyers who paid for "complete" testing during a home purchase. Our arsenic testing guide covers a similar trap: invisible contaminants only get found when someone deliberately looks.

3. Letting the sample splash, sit, or ship slow

Every hour of delay and every bubble in the vial pulls the number down. A falsely low radon result is worse than no result, because it closes the question incorrectly. Follow the lab's fill procedure exactly, note the collection time, and get it to the carrier the same day.

4. Making a treatment decision off one test

Radon in groundwater moves with seasons, rainfall, and how hard the well has been pumping. New Hampshire recommends at least two tests, a month or more apart, before installing anything. Aeration systems are four-figure purchases; a second $30-60 test is cheap insurance against treating a number that was never representative.

Frequently Asked Questions

Written by Aidan Walsh, owner of Mid Atlantic Water. 32+ years installing well water treatment systems across Pennsylvania, Maryland, Delaware, New Jersey, Virginia, and beyond. Article reviewed July 2026.