Radionuclide
Radium and radionuclides in drinking water
Radium-226 and radium-228 are naturally occurring radioactive elements released into groundwater from rocks and soils, especially in granite-bearing geology. EPA's MCL for combined radium-226/-228 is 5 pCi/L. Gross alpha activity is regulated at 15 pCi/L and uranium at 0.030 mg/L. All are co-regulated under the Radionuclides Rule. All MCLGs are zero.
Where radium in drinking water comes from
Radionuclides in U.S. drinking water are almost entirely natural. The uranium decay series, which slowly produces radium-226, radium-228, radon, and other radioactive products, is present in many rock formations:
- New England (granite-rich)
- Parts of the Piedmont: Virginia, the Carolinas
- Parts of the upper Midwest: Wisconsin, Iowa, Minnesota
- Parts of the Mississippi alluvial aquifer
- Localized hot spots elsewhere
Industrial sources (uranium and phosphate mining, oil and gas brines) contribute in specific locations but are not the dominant U.S. story. Most radium-containing wells are simply drawing from rock that has always had it.
Why it matters for health
Radium is a known human carcinogen. The longest-running human data come from early-20th-century radium-dial workers, who ingested or inhaled radium and developed bone cancer at high rates. At drinking-water exposure levels, the primary concern is bone cancer (osteosarcoma): the body deposits radium in bone analogously to calcium.
Uranium in drinking water is primarily a kidney toxicant at the levels typically seen; its radioactivity contributes less to risk than its chemical kidney effects at relevant exposures. The MCL is set on the kidney injury endpoint.
Gross alpha activity is a screening measurement that captures alpha-emitting radionuclides other than uranium and radon. High gross alpha that cannot be accounted for by known sources triggers further testing.
EPA regulation
| Contaminant | MCL | MCLG | Units |
|---|---|---|---|
| Combined Radium-226/-228 | 5 | 0 | pCi/L |
| Gross Alpha (excluding U and Rn) | 15 | 0 | pCi/L |
| Uranium | 0.030 | 0 | mg/L |
| Beta and photon emitters | 4 | 0 | mrem/year (annual dose) |
All MCLGs are zero because radionuclides are carcinogens with no demonstrated safe level.
Radon is regulated separately
Radon enters homes through indoor air (the dominant exposure pathway) and through groundwater that off-gasses radon when water is run or showered. EPA’s proposed radon-in-water MCL has been pending for years; state-level guidance varies. The dominant household radon risk is soil-gas intrusion, not water. EPA recommends indoor-air radon testing as the primary screen for any home, regardless of water source.
What to do
- If your home is on a private well in granite or uranium-bearing geology, test for radionuclides every three to five years.
- Treatment:
- Ion exchange (radium, uranium)
- Reverse osmosis (radium, uranium)
- Lime softening (radium, at the treatment-plant level)
- Aeration plus carbon (radon)
- Carbon filtration alone does not remove radium or uranium.
This page is general information, not medical advice. If you have a documented high-radium well, talk to your clinician about whether follow-up health screening is appropriate.
Sources
- US EPA, “Radionuclides Rule”: https://www.epa.gov/dwreginfo/radionuclides-rule
- US EPA, “Radon”: https://www.epa.gov/radon
- US Geological Survey, “Radium in Groundwater of the United States”: https://www.usgs.gov/publications/radium-and-radon-groundwater-united-states
- ATSDR Toxicological Profile for Radium (1990): https://www.atsdr.cdc.gov/toxprofiles/tp144.pdf
Editorial review: reviewed 2026-05-11 by RK 2026-05-11. Editorial standards.