A gas from the rock around your well
Radon is a radioactive gas that occurs naturally in rock and soil, and groundwater can pick it up on its way to a well. In its 1999 proposal for radon in drinking water, EPA said levels tend to be highest where groundwater is in contact with granite, and are generally highest in New England and in the Appalachian uplands of the Middle Atlantic and Southeastern states.
Radon is a groundwater issue, so it concerns private wells and public systems that use wells. If your home is on a well in one of those regions, it is worth testing for.
Most of the risk is breathed in
When well water with radon comes out of a shower, faucet, washing machine or dishwasher, some of the gas moves into the air. The Connecticut Department of Public Health estimates that every 10,000 picocuries per liter (pCi/L) of radon in water adds about 1 pCi/L to the indoor air.
EPA's 1999 proposal said only about 1 to 2 percent of radon in indoor air comes from drinking water; most radon enters indoor air from the soil under homes. It also cited a National Academy of Sciences estimate that, of the cancer deaths linked to radon in drinking water, about 89 percent were lung cancers from breathing it and about 11 percent were stomach cancers from drinking it.
So a radon-in-water result is best read together with a test of the air in your home.
- Water to airAbout 1 pCi/L in air per 10,000 pCi/L in water (Connecticut DPH)
- Share of indoor radon from waterAbout 1 to 2 percent (EPA, 1999)
- Proposed federal standards (1999)300 pCi/L, or 4,000 pCi/L with an indoor-air program; never adopted
- Vermont advisory level4,000 pCi/L in water
Proposed in 1999, not in force
In November 1999, EPA proposed a radon standard for community water systems: 300 pCi/L, or an alternative 4,000 pCi/L for systems in states with programs to reduce radon in indoor air. Those standards were never finalized. EPA's current table of drinking water standards includes no radon limit, and its radionuclides rule covers other radioactive substances.
The 1999 proposal only applied to community water systems anyway. Private wells have no federal radon limit, so the guidance that matters comes from your state. Vermont, for example, uses an advisory level of 4,000 pCi/L for water and suggests retesting in five years if you are below it.
Radon guidance for well water differs by state. Ask your state health department what level it uses before deciding on treatment.
A special sample, not a standard test
Radon is a gas, so it is not part of most basic well tests. You need a radon-in-water kit from a lab. Follow the lab's sampling steps closely.
Prices vary. The Vermont Department of Health Laboratory sells a radon-in-water test kit for $25, and the Wisconsin State Laboratory of Hygiene lists a radon-in-water test for $111.
- Test the air first or at the same time
EPA says most radon enters indoor air from the soil under homes, so an air test tells you more about your overall exposure.
- Order a radon-in-water kit from a lab
Your state health department or certified lab list will name labs that test for it.
- Compare the result with your state's guidance
There is no federal number for private wells, so use your state's level.
Aeration and carbon filters
EPA's 1999 proposal named high-performance aeration as the best available treatment for radon, and also allowed point-of-entry aeration or granular activated carbon (GAC) filters for small systems. Both treat the water where it enters the house, since radon escapes at every tap and appliance.
GAC has a drawback. The Vermont Department of Health discourages using GAC systems to remove radon, because the radon collected on the filter can create a radiation hazard.
Test the water and the air
If you live in New England, the Appalachian uplands, or another area with granite bedrock, add a radon-in-water test to your well testing, and test the air in your home too. For what other well tests cost, see our guide to well water testing prices.
- Private well water by stateTesting guidance and county estimates for your state.Pick your state →
- How much does well water testing cost?Real 2026 prices from state and university labs.See lab prices →
- Arsenic in well waterAnother natural contaminant from rock, with a federal limit.Read the guide →
- Read your test resultsCompare your lab numbers against EPA limits and standards.Read my results →
This article summarizes public information from EPA and state health departments on radon in private wells. It is not medical advice. If you are worried about radon exposure, talk to a healthcare provider, and contact your state radon or drinking water program.