Radon in Water: How to Test a Well, What the Levels Mean, and How Mitigation Works
By SubcontractorHub Editorial Team·Published September 2026
Short version: radon in water is mainly a private well issue in granite and bedrock regions. Most of the risk comes from breathing radon released by showers and laundry, not from drinking it. The rule of thumb is that 10,000 pCi/L in water adds about 1 pCi/L to indoor air. There is no final federal limit; many states use around 4,000 pCi/L as a treatment threshold. Test the air first, collect the water sample without bubbles, and if treatment is needed, aeration is the method that works at any level.
Radon is a radioactive gas produced by the natural decay of uranium in rock and soil. Most people know it as a basement air problem, and that is still where most household exposure comes from. But radon also dissolves into groundwater as it moves through fractured bedrock, and a drilled well pulls that water straight into the house.
This guide covers who actually needs to worry about radon in well water, how to take a radon water test that gives a real number, how to read the result, and what mitigation involves. It is written for homeowners and for the water treatment and water purification dealers and radon contractors who get asked about it.
1. Who Has Radon in Their Water
Radon in water is overwhelmingly a groundwater issue. Surface water from lakes and rivers loses its radon to the air long before it reaches a tap, and large utilities that treat and store water give radon time to decay and escape. Private wells drilled into bedrock are the highest-risk group, because the water comes out of the rock with its radon still dissolved and reaches the house within minutes.
The geology matters more than anything else. The highest levels in the United States are associated with granite and other uranium-bearing crystalline rock, which is why state health departments in New England, particularly Maine and New Hampshire, have long-running private well radon programs. Parts of the Appalachian region, Colorado and other areas with granitic bedrock also see elevated results. Neighbouring wells can differ widely, because radon concentration depends on the specific fractures a well intersects.
The only way to know is to test. Your state radon program or health department is the best place to start, since many publish maps of well results and some offer discounted test kits.
2. Why the Risk Is Mostly in the Air
Radon is a dissolved gas, and it comes out of solution whenever water is agitated or heated. Showers, dishwashers, washing machines and running taps all release it into the indoor air. The EPA and the National Academy of Sciences have both concluded that inhaling that released radon accounts for most of the health risk from radon in water, with lung cancer as the concern. Drinking radon-bearing water carries a smaller risk, which the EPA associates with stomach cancer.
Rule of thumb: 10,000 pCi/L of radon in water ≈ 1 pCi/L added to indoor air
EPA indoor air action level: 4 pCi/L (the EPA also suggests considering a fix between 2 and 4 pCi/L)
That ratio is an average, and the real contribution depends on how much water the household uses, the size of the house and how well ventilated it is. It still explains why a result of 2,000 pCi/L is usually a minor contributor while a result of 30,000 pCi/L can be a significant one. It is also why you cannot evaluate a water result in isolation from an air test.
3. What the Regulations Actually Say
This is where most online advice goes wrong. There is no final federal drinking water standard for radon. In 1999 the EPA proposed a maximum contaminant level of 300 pCi/L for community water systems, with an alternative maximum contaminant level of 4,000 pCi/L for states that adopted programs to reduce radon in indoor air (the multimedia mitigation approach). That proposal was never finalized.
Even if it had been, it would not have covered private wells, which are not regulated under the Safe Drinking Water Act at all. In practice, private well owners rely on state guidance. Several states publish their own recommended action levels, and 4,000 pCi/L is a common figure, but thresholds and recommendations vary and change over time. Check your own state health department or radon program before deciding a result is fine or needs treatment.
4. How to Take a Radon Water Test
A radon water test is cheap, but it is easy to ruin, and almost every error reads low. Radon escapes from a sample the moment it splashes, and it decays with a half-life of about 3.8 days, so both collection and shipping are time-critical.
- Test the indoor air first, or at the same time. The air result decides how much the water number matters.
- Get a kit from a certified lab, ideally through your state radon program. Home test strips are not suitable for radon in water.
- Sample raw water. Use a cold tap ahead of any treatment equipment, or put the equipment in bypass. A carbon filter will strip radon and give a falsely reassuring result. The bypass steps in the well shock guide apply here too.
- Remove the faucet aerator and run the cold water for several minutes so the sample comes from the well, not from water that has sat in the pipes or pressure tank.
- Fill the vial without aeration. Slow, smooth flow, no bubbles, no splashing, and cap it with no air space. Follow the lab’s instructions exactly; some ask you to fill the vial under water in a container.
- Ship it the same day for next-day delivery, and avoid collecting on a Friday. The lab needs to analyze it within a few days.
A high first result is worth confirming with a second sample before buying equipment. And radon is only one radionuclide: the same bedrock can put uranium and radium into the water, which require a separate test and different treatment. The EPA limit for uranium in public water is 30 micrograms per litre, and many state programs recommend testing private wells in high-radon areas for it.
5. What Your Result Means
The table below reflects typical guidance, not a rule. Your state’s recommendation takes precedence, and the indoor air result should always be read alongside it.
If the indoor air is above 4 pCi/L and the water is low, the radon is coming from the soil, and the fix is a sub-slab depressurization system, not water treatment. If both are high, the home may need both.
6. Radon in Water Mitigation: Aeration vs Carbon
Radon mitigation for water has to be point-of-entry, treating all the water entering the house, because the risk comes from every shower and appliance. There are two proven options.
Aeration
An aeration system moves well water into a tank where air is bubbled up through it or the water is sprayed, which strips the dissolved radon. The radon-laden air is fanned out through a vent pipe that terminates outdoors above the roofline, and a repressurization pump sends the treated water on to the house. Residential units commonly achieve 95 to 99 percent or better removal, which is why aeration is the standard answer at high levels.
The trade-offs are space, electricity, a second pump and maintenance. Iron and manganese oxidize in the tank and foul it, so wells with those problems need pretreatment ahead of the aerator. The iron filter guide covers that stage, and the well water treatment system selector shows where each piece of equipment sits in the treatment train.
Granular activated carbon
A point-of-entry carbon tank adsorbs radon onto the media, with no vent or second pump. It is cheaper and simpler, but the radon decays inside the tank, so the bed builds up radioactive decay products, including lead-210, and emits gamma radiation. At higher water radon levels that exposure near the tank becomes a real concern, and spent media can raise disposal questions.
For that reason carbon is generally limited to lower levels, up to a few thousand pCi/L, and many state programs point people with higher results toward aeration. If carbon is used, the tank should be located away from living space and the installer should explain the replacement and disposal plan up front.
What does not work
Under-sink reverse osmosis units and pitcher filters are not radon mitigation. They treat a single drinking tap at best, and they leave the shower, laundry and dishwasher releasing radon into the air, which is where most of the risk sits. A whole-house sediment filter or softener does nothing for radon either. After any system is installed, retest the water from a tap after the treatment to confirm the reduction.
For Water Treatment Dealers and Radon Contractors
Radon in water sits where two trades overlap. Radon contractors see the high air test and the private well; water treatment dealers already have the well owner as a customer for softeners, iron filters and UV. The homeowner usually just wants one person to explain the air and water results together and quote a fix that is honest about which one is driving the problem.
The professional version of the job is test-led: confirm the water number, read it against the air result and the state guidance, and quote aeration or carbon on that basis with the pretreatment the well actually needs and a post-install retest included. Aeration is a several-thousand-dollar purchase, so showing a monthly financing option beside the cash price is often what lets the homeowner say yes on the visit. Much of that install work is plumbing: bypass loops, a second pump, and a vent run to the roof.
SubcontractorHub is software for water treatment dealers: quotes that carry the test results, good-better-best options, financing, e-signature and follow-up retest scheduling in one place.
Frequently Asked Questions
Is radon in well water dangerous?
It can be, but mostly not in the way people assume. Most of the health risk from radon in water comes from breathing it: radon is a dissolved gas, and it escapes into the indoor air when water is sprayed, heated or agitated in showers, dishwashers and washing machines. A smaller share of the risk comes from drinking it, which the EPA associates with stomach cancer. The common rule of thumb is that about 10,000 pCi/L of radon in water adds roughly 1 pCi/L to the air in the home, so water radon usually matters most when levels are in the thousands or tens of thousands of pCi/L.
What is a safe level of radon in water?
There is no final federal limit. In 1999 the EPA proposed a maximum contaminant level of 300 pCi/L for community water systems, with an alternative level of 4,000 pCi/L for states that ran programs to reduce radon in indoor air, but that rule was never finalized, and it would not have applied to private wells anyway. Several states publish their own guidance for private wells, and 4,000 pCi/L is a common threshold for recommending treatment. Follow your state health department or radon program, and test your indoor air as well, because the air result usually drives the decision.
How do you test well water for radon?
Order a radon water test kit from a certified laboratory, usually through your state radon program or health department. Take the sample from a cold tap that is ahead of any treatment equipment, remove the faucet aerator, and run the water for several minutes so the sample comes from the well rather than the pipes. Fill the vial slowly without bubbles or splashing, cap it with no air space, and ship it the same day. Radon has a half-life of about 3.8 days, so the lab needs to analyze the sample within a few days of collection or the result reads low.
How do you remove radon from well water?
The two proven whole-house methods are aeration and granular activated carbon. Aeration systems bubble or spray air through the water in a tank and vent the released radon outdoors above the roofline, and they typically remove 95 to 99 percent or more even at high levels. Granular activated carbon adsorbs radon in a point-of-entry tank and is cheaper, but it is generally limited to lower levels because the carbon bed accumulates radioactive decay products and gives off gamma radiation. Under-sink filters and pitcher filters do not solve the problem because they do nothing about radon released from showers and laundry.
How much does radon water mitigation cost?
A whole-house aeration system typically costs several thousand dollars installed, depending on the radon level, household water demand, how much pretreatment is needed for iron or manganese, and how difficult it is to run the vent. A point-of-entry carbon system usually costs less up front, but it is only suitable at lower radon levels and carries media replacement and disposal considerations. Get quotes from contractors who are certified or listed by your state radon program, and ask for a post-installation water test to be part of the job.
Should I test for radon in air or water first?
Test the air first. The EPA action level for indoor air is 4 pCi/L, and in most homes the radon in the air comes from the soil under the house rather than from the water. If the air result is high and you are on a private well, test the water to find out how much of that air radon the well is contributing. A home with elevated air radon from soil needs a soil depressurization system, and treating the water alone will not fix it. A home with high water radon and a high air reading may need both.
Does a radon water test also check for uranium?
No. Radon is a gas and uranium is a dissolved metal, and they are measured with different tests. The same granite and bedrock geology that produces radon can also put uranium, radium and other radionuclides into groundwater, so many state programs recommend a separate radionuclide panel, often uranium plus gross alpha, for private wells in high-radon areas. Uranium is removed by different equipment than radon, typically anion exchange or reverse osmosis, so the results change what gets installed.
Related Guides & Tools
- Well Water Treatment System Selector
- How to Shock a Well
- Iron Filter for Well Water
- Whole House Water Filter Cost Calculator
- Well Drilling Cost Calculator
- → SubcontractorHub for Plumbing
- → Water Treatment and Water Purification Contractor Software
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