If your Connecticut home is on a well, CT DPH recommends testing the water for radon, not just the air. The state's recommended action level is 5,000 pCi/L, based on the average of two samples taken at the same time from the same tap (CT DPH, revised February 2025).
● Key Takeaways
CT DPH recommends a radon in water test for every home on a well, even if indoor air tested low. Its recommended action level is 5,000 pCi/L (average of two simultaneous samples). Roughly 10,000 pCi/L in water adds 1 pCi/L to indoor air. Carbon treatment fits levels below 10,000 pCi/L; aeration is the only effective option at or above 10,000 pCi/L. Test at least once and ideally every 5 years.
Why Test Well Water for Radon?
Radon is a radioactive gas formed when uranium in rock decays. It has no color, smell or taste, and CT DPH calls it the leading cause of lung cancer in nonsmokers (CT DPH, revised February 2025). Groundwater picks it up from bedrock, so homes on wells can have radon in their water. Homes on lakes, rivers or reservoirs generally do not, because the gas escapes before the water arrives (CT DPH Radon in Water).
Uranium is common in Connecticut bedrock. A USGS study with CT DPH found uranium at 1 µg/L or more across about 44.9% of the state (USGS Open-File Report 2020-1111). That is one reason CT DPH puts radon on its list of tests for every private well: at least once, and ideally every five years, alongside arsenic and uranium (CT DPH testing recommendations).
How Does Radon Get From Water Into the Air?
Radon stays dissolved in water under pressure in your well and pipes. When the water is heated or agitated, in a shower, washing machine or dishwasher, the gas escapes into the room. CT DPH says radon in the air rises for a short time whenever those appliances run (CT DPH, revised February 2025).
CT DPH estimates that every 10,000 pCi/L of radon in water adds about 1 pCi/L to indoor air (CT DPH Radon in Water). For comparison, CT DPH's troubleshooting guide treats indoor air above 4 pCi/L as a reason to test well water for uranium, radium and radon (CT DPH testing recommendations).
The bigger source of indoor radon is usually the soil and rock under the foundation, so CT DPH recommends that every Connecticut home test its air. But a clean air result does not clear the water. CT DPH says it is possible to have elevated radon in water even when indoor air is low, and "the only way to know for sure is to test for radon in both air and water."
Is Radon in Water a Drinking Risk or a Breathing Risk?
Mostly breathing. CT DPH says radon you drink can slightly raise the risk of stomach and other cancers, but that risk is "almost insignificant" compared with the lung cancer risk from radon in air (CT DPH Radon in Water). An archived EPA estimate based on 1999 National Academy of Sciences work put deaths from radon in drinking water at about 168 a year nationally, 89% of them lung cancers from breathing radon released from water and 11% stomach cancers from drinking it (EPA, 1999).
That is why radon treatment goes on the whole house. A drinking-water filter at one tap does not stop the gas released by the shower.
What Does the 5,000 pCi/L Action Level Mean?
Radon results are reported in picocuries per liter (pCi/L), a measure of radioactivity. CT DPH's recommended action level is 5,000 pCi/L: take action to reduce radon if the average of two water samples drawn simultaneously from the same location is at or above that level (CT DPH, revised February 2025).
Two things to know about that number:
- It is a recommendation. There is no enforceable radon limit for Connecticut private wells. In 1999 the EPA proposed a 300 pCi/L limit with a 4,000 pCi/L alternative for states running indoor-air programs, but the proposal applied to community water systems, not private wells (EPA, 1999).
- Results move. CT DPH says radon in water varies from test to test with water use and seasonal changes in the water table, so you may choose to test more than once.
How Do I Test My Well Water for Radon?
CT DPH recommends using a qualified radon measurement professional to collect the sample and a laboratory approved by DPH for radon in water (CT DPH Radon in Water). The DPH Radon Program lists nationally certified measurement and mitigation professionals, and DPH's Environmental Laboratory Certification Program lists labs certified for radiological drinking water analysis.
Because radon comes from uranium, it is sensible to test uranium at the same time. CT DPH recommends testing for uranium at least once and suggests asking the lab for uranium by ICP-MS (CT DPH uranium fact sheet). Uranium matters when swallowed; radon matters when breathed. They need different treatment.
How Is Radon Removed From Well Water?
If the two-sample average is 5,000 pCi/L or higher, CT DPH recommends one of two whole-house systems, installed by a nationally certified radon mitigation professional (CT DPH, revised February 2025):
| System | When CT DPH says it works | Installed cost (CT DPH, 2025) |
|---|---|---|
| Granular activated carbon (GAC) | Effective below 10,000 pCi/L | $1,500 to $3,500 |
| Aeration | The only effective method at or above 10,000 pCi/L | Average $5,000 to $7,000 |
GAC traps radon in a tank of carbon. The carbon also catches other contaminants, which shortens its life, and it must be replaced on a schedule by a professional. Aeration agitates the water so radon escapes and vents it outside. CT DPH notes that iron and manganese in the water have to be considered before choosing an aeration system.
After treatment, retest the treated water to confirm the system works, and keep the result with your other well records.
Need a radon in water test?
A water specialist can arrange certified sampling and lab analysis for radon and uranium in your well water.
CheckYourTap is an independent water-quality service, not your town or utility. We connect you with a vetted local partner. Your online report is free. Testing and treatment are paid services, and there's no cost to talk.
Keep Reading
- Private well testing in Connecticut: the complete guide
- Arsenic and uranium in Connecticut wells, by region
- Radon in well water: why the real risk is the air
- Radon in your shower: Litchfield County
Sources: CT DPH, Radon in Your Well Water (revised February 2025); CT DPH, Radon in Water; CT DPH, Educational materials: testing recommendations for private wells (updated May 2026); CT DPH, Uranium in Private Well Water (April 2024); CT DPH Environmental Laboratory Certification Program; Gross and Brown, USGS Open-File Report 2020-1111; EPA, Proposed Radon in Drinking Water Rule fact sheet, EPA 815-F-99-006 (October 1999, archived).
About this guide: CheckYourTap is an independent water-quality service funded by Valiant Energy Solutions, a Connecticut water quality and energy services company established in 1930. Valiant offers paid well testing and radon treatment, which is what the request button on this page connects you with. You can also hire any nationally certified radon professional or CT DPH-approved laboratory directly. The facts on this page come from the public sources cited above.
