Radon in Well Water in Wisconsin

You are on a private well in Wisconsin, you have heard that radon rises out of the soil, and now you are wondering whether the water coming out of that well is worth testing too. The question lands hardest in the granite country of northcentral and northwestern Wisconsin, where the same uranium-bearing bedrock that pushes radon up through basement floors can also carry it into groundwater. A well drilled into or near that rock can deliver water with dissolved radon in it, and that radon does not simply stay in the glass. This guide is the deeper companion to our overview on radon in water: it works through how waterborne radon behaves, who actually needs to test, how a water test differs from an air test, and how the two common treatment methods compare. Badger State Radon is a free matching service, not a contractor.

How radon gets from water into the air

Radon is a naturally occurring radioactive gas produced as uranium breaks down in rock and soil. Where groundwater flows through uranium-bearing rock, some radon dissolves into the water and stays in solution under the pressure inside your well and pipes. The moment that water is agitated or exposed to air, the radon comes back out of solution and mixes into the room. A showerhead spraying warm water, a dishwasher filling and draining, a washing machine cycling, even a running faucet all release some of it.

The rule the Wisconsin DNR uses to size the effect is that about 10,000 pCi/L of radon in water adds roughly 1 pCi/L to indoor air. The uses that aerate warm water release the most, so bathrooms and laundry areas see the biggest short-term jump while the water runs. The practical takeaway is that a private well can quietly add to the indoor air level you would read on a radon air test, layered on top of whatever radon is already rising through the foundation. Because the gas escapes into the air during ordinary use, the main way waterborne radon reaches you is by breathing it, not by drinking it. That is also why a well can matter even in a household that drinks bottled or filtered water: the shower and the laundry still release radon into the same air everyone breathes.

Who should test

This is a private-well issue. Public and municipal water systems in Wisconsin are treated and typically aerated before delivery, which strips out most dissolved radon well before the water reaches your home, so households on city water rarely need to think about radon in their water at all. A private well draws straight from the aquifer with no such step, and no utility monitors it for you. Testing is the well owner's responsibility.

Within the private-well population, dissolved radon is not spread evenly. It tracks the geology, and the highest levels in the state sit in the granite and granitic sand and gravel formations of northcentral and northwestern Wisconsin. That puts communities like Wausau in Marathon County and Stevens Point in Portage County among the areas where a home on a private well is worth a look at the water, not just the air. Even there, plenty of wells test low, so the goal is not to assume the worst but to measure. The Wisconsin DNR's guidance on testing a private well is the place to start if you are deciding whether and how to sample yours.

How to test your water

A water radon test is a separate test from an air test, and one does not substitute for the other. An air test measures the gas in the room air and reports in pCi/L of air. A water radon test measures the dissolved radon in a collected water sample, which a laboratory analyzes and reports in pCi/L of water. The scales are different: water results come back in the thousands, which is why the DNR treatment threshold sits far above the 4.0 pCi/L figure used for air. A low air reading does not prove the water is clear, and a high water number does not translate one to one into an air number.

Collecting a valid water sample takes some care. Radon leaves the water quickly once it is disturbed, so the sample has to be drawn to avoid aeration, sealed with no air bubble, and delivered to the lab within a short holding time. The Wisconsin DNR walkthrough for testing a private well covers how to collect the sample, keep it valid, and where to send it. Because no one tests a private well for you, the simplest path is often to line the water sample up with an air test you were already planning: if you are arranging radon testing for the air, ask whether the same provider can collect a water sample on the same visit.

Treatment options compared

When a water test comes back high, treatment happens at the water, not the air. The Wisconsin DNR suggests considering treatment when radon in well water runs above 4,000 pCi/L. Both common methods are point-of-entry systems, installed where the water line enters the house so every fixture is covered. Treating only the drinking-water tap would leave the shower and laundry releasing radon into the air, which defeats the purpose.

• Aeration: the water passes through a tank where air is bubbled through it, releasing the radon so it can be vented outdoors before the water is used inside. The DNR describes aeration as the most effective method for radon, and a system runs about $3,500. It adds moving parts and a vent, but it removes radon rather than storing it.
• Granular activated carbon (GAC): the water passes through a carbon filter that captures radon. GAC can work and is sometimes less expensive up front, but it usually needs pretreatment to handle iron, sediment, or other water conditions, and the captured radon means the filter can accumulate radioactivity over time. At higher radon levels that buildup can require careful handling or shielding, and the media eventually needs replacement and proper disposal.

Whichever route fits your well, a water radon treatment system is a plumbing installation. It needs plumbing approval from the Wisconsin Department of Safety and Professional Services (DSPS), and the DNR asks that you call the department before you install one so the setup meets code. This is work for a qualified professional, not a weekend project: sizing the system to your radon level and water quality, venting the released gas outdoors, and keeping the unit serviceable are all part of a proper install. That is one more reason to test the water first and let the numbers, rather than a sales pitch, decide whether treatment is warranted and which method fits.

Air first, then water

For most homes the larger exposure by far is soil gas rising through the foundation, not the water, so the order matters. Radon is the leading cause of lung cancer among people who do not smoke and is linked to about 21,000 US lung cancer deaths a year, which is why the air side comes first. Test the air, and if a test reads at or above 4.0 pCi/L, address it with a mitigation system. Then, if you are on a private well in one of the higher-radon areas, treat the water as the second step, tested and, if needed, treated separately from the air.

For the full picture of how radon moves through a Wisconsin home and how the numbers fit together, see the Wisconsin radon guide. When you are ready to act, Badger State Radon connects you with independent local radon professionals who can look at both the air and the water, so you get one coordinated plan instead of two disconnected quotes.