Contaminant Guide

Uranium in Well Water

Uranium occurs naturally in granite and other rock formations throughout the northern and western United States. It dissolves into groundwater through normal mineral weathering. Uranium poses both chemical toxicity (kidney damage) and radiological risk (bone cancer), and the EPA has set its MCLG at zero — meaning no safe level has been established.

What is uranium in well water?

Uranium is a naturally radioactive heavy metal. In well water it almost always comes from natural mineral dissolution, not industrial contamination. Granitic bedrock and some glacial deposits contain uranium-bearing minerals that slowly release uranium into groundwater. It is colorless and tasteless — undetectable without laboratory testing.

Where is it most common?

Uranium is most prevalent in wells drawing from bedrock aquifers in Michigan, Wisconsin, Maine, and New Hampshire, and from alluvial or volcanic aquifers in Wyoming and other western states. Granite-heavy terrain in New England is particularly associated with elevated uranium. Glacial outwash aquifers in the Upper Midwest also show elevated levels.

Health effects

  • Kidney damage (nephrotoxicity) — Uranium is a heavy metal that accumulates in the kidneys. Chronic exposure above the MCL is associated with kidney tubular damage; this is the primary driver of the 30 µg/L MCL.
  • Bone cancer — As a radioactive element, uranium and its decay products deposit in bone mineral and irradiate bone marrow over time. MCLG=0 reflects this radiological carcinogenicity.

The EPA limit: MCL = 30 µg/L, MCLG = 0

The MCL of 30 µg/L was set primarily based on kidney toxicity, not radioactivity. However, the MCLG is 0 because uranium is also a radionuclide — any level of ionizing radiation exposure carries some cancer risk, and EPA sets MCLGs for known or probable carcinogens at zero. Compliance with the MCL does not mean zero health risk.

Testing

Uranium is measured by ICP-MS (EPA Method 200.8) at a certified laboratory, reported in µg/L (parts per billion). Note that this measures chemical uranium concentration, not radioactivity directly. If you are in a granite-heavy area or your bedrock well has never been tested for uranium, test at least once. Uranium levels tend to be stable over time in the same well.

Find a certified lab and learn how to collect a sample

Treatment

  • Reverse osmosis (RO) — removes >90% of uranium; effective point-of-use solution.
  • Anion exchange (strong-base anion resin) — highly effective for uranyl carbonate complexes common in groundwater; used in whole-house point-of-entry systems.
  • Activated alumina adsorption — effective at pH 6–8; can be used at point of entry.
  • Lime softening — effective at municipal scale; not practical for residential use.

Compare uranium treatment systems for private wells

Regulatory framework

MCL: 30 µg/L. MCLG: 0. Uranium Rule finalized 2000 (65 Fed. Reg. 76708), effective 2003. The MCL of 30 µg/L was set based on kidney toxicity modeling (the chemical endpoint), not radiological risk. The MCLG=0 acknowledges that uranium is also a Class A human carcinogen via its radioactive decay chain.

Uranium regulation presents a dual-hazard challenge: chemical toxicity dominates at drinking water concentrations (30 µg/L ≈ 0.7 pCi/L alpha activity, below the radium MCL threshold), but radiological risk still informs the zero MCLG.

Detection

ICP-MS (EPA Method 200.8) measures total dissolved uranium in µg/L. Radiochemical methods (alpha spectrometry) measure radioactivity in pCi/L and can distinguish U-234, U-235, U-238 isotopes. For regulatory compliance under the Uranium Rule, the chemical concentration (µg/L) standard applies — not a radioactivity standard. Certified lab required.

Geochemistry

Uranium in groundwater primarily occurs as the soluble uranyl ion (UO₂²⁺), commonly as uranyl carbonate complexes under oxic, high-alkalinity conditions. Uranium mobility increases with higher pH, higher dissolved oxygen, and higher carbonate alkalinity. Under reducing conditions, U(VI) is reduced to insoluble U(IV), limiting mobility — the opposite of iron and manganese behavior.

Data access

Access our data API and methodology

References

  1. Kurttio, P., Auvinen, A., Salonen, L., et al. (2002). Renal effects of uranium in drinking water. Environmental Health Perspectives, 110(4), 337-342. https://doi.org/10.1289/ehp.02110337
  2. Nolan, J., & Weber, K.A. (2015). Natural uranium contamination in major U.S. aquifers linked to nitrate. Environmental Science & Technology Letters, 2(8), 215-220. https://doi.org/10.1021/acs.estlett.5b00174
  3. Ayotte, J.D., Gronberg, J.M., & Apodaca, L.E. (2011). Trace elements and radon in groundwater across the United States, 1992-2003. USGS Scientific Investigations Report 2011-5059. https://pubs.usgs.gov/sir/2011/5059/