Radionuclides
Radon in U.S. tap water
29 public water systems across 8 U.S. states report Radon in their annual Consumer Confidence Report. 1 sit at or above the federal limit.
What it is
A naturally occurring radioactive gas that can dissolve into groundwater.
Why it's regulated
No enforceable federal limit in drinking water yet; inhalation of released radon raises lung-cancer risk.
At or above the federal limit (1)
| Water system | Measured | vs. limit |
|---|---|---|
| Santa Rosa, CA 2024 annual report | 445–455 pCi/L | 4.5× the limit |
Within the federal limit (17)
| Water system | Measured |
|---|---|
| Ansonia, CT 2024 annual report | 0–1856 pCi/L |
| Bethany, CT 2024 annual report | 0–1856 pCi/L |
| Branford, CT 2024 annual report | 0–1856 pCi/L |
| Cheshire, CT 2024 annual report | 0–1856 pCi/L |
| Derby, CT 2024 annual report | 0–1856 pCi/L |
| East Haven, CT 2024 annual report | 0–1856 pCi/L |
| Hamden, CT 2024 annual report | 0–1856 pCi/L |
| Milford, CT 2024 annual report | 0–1856 pCi/L |
| New Haven, CT 2024 annual report | 0–1856 pCi/L |
| North Haven, CT 2024 annual report | 0–1856 pCi/L |
| Orange, CT 2024 annual report | 0–1856 pCi/L |
| Seymour, CT 2024 annual report | 0–1856 pCi/L |
| West Haven, CT 2024 annual report | 0–1856 pCi/L |
| Woodbridge, CT 2024 annual report | 0–1856 pCi/L |
| Lewisville, TX 2024 annual report | Not detected pCi/L |
| West Jordan, UT 2024 annual report | Not detected pCi/L |
| West Valley City, UT 2024 annual report | Not detected pCi/L |
Frequently asked
+What is Radon?
A naturally occurring radioactive gas that can dissolve into groundwater. No enforceable federal limit in drinking water yet; inhalation of released radon raises lung-cancer risk.
+What is the federal limit for Radon in drinking water?
The federal MCL for Radon is 5000 pCi/L. The EPA enforces this against the regulated reporting statistic (typically a running annual average or 90th percentile), not a single-sample spike.
+How many U.S. water systems have Radon over the federal limit?
1 of the 29 public water systems on The Water Map report Radon at or above its federal limit, spanning 8 U.S. states. The full list is on this page.
+How can I check if Radon is in my city's tap water?
Search your city on The Water Map (https://www.thewatermap.com/) or browse the list on this page. Every U.S. public water utility is required to publish an annual Consumer Confidence Report that lists every contaminant it measured, including Radon.