Division of Environmental and Community Health

Maine Center for Disease Control & Prevention

A Division of the Maine Department of Health and Human Services

DHHSMeCDCEnvironmental and Community HealthDrinking WaterCompliance & EnforcementRegulated Contaminants → Radionuclides


Page Index

Maximum Contaminant Level (MCL)

Health Effects


Additional Resources

A radionuclide is an atom with an unstable nucleus which, to become more stable, emits energy in the form of rays or high speed particles. This is called ionizing radiation because it can create "ions" by displacing electrons in the body (e.g., in the DNA), disrupting its function. The three major types of ionizing radiation are: alpha particles, beta particles and gamma rays.

Approximately 80% of our exposure to radioactivity is natural, with the remaining 20% coming from man-made sources, although more frequent use of diagnostic imaging involving radiation (x-rays, CT scans) is increasing exposure from this source. We are exposed to naturally occurring radiation for example from radon gas emanating from rocks and soil, and cosmic radiation from space. We also carry small amounts of potassium-40 in our bodies from the foods containing potassium. Depending on the type of rocks where you live, 55% to 70% of natural exposure comes from radon gas, while cosmic radiation (which is greater at higher altitude) represents about 11%, and potassium-40 about 5%. Radiation may exist in drinking water from nuclides dissolved in the water from natural sources in the earth or occasionally from releases from laboratories or nuclear power plants.

Radionuclides regulated for drinking water include (Adjusted) Gross Alpha Emitters, Beta Particle and Photon (gamma) Radioactivity, Radium 226 and Radium 228 (Combined), and Uranium.


Maximum Contaminant Level (MCL)

The Environmental Protection Agency (EPA) has established the following standards for Radionuclides in drinking water:

  • Combined Radium-226 and Radium-228:  5 picoCuries per liter (pCi/L)
  • Gross Alpha (excluding radon and uranium):  15 picoCuries per liter (pCi/L)
  • Beta Particles and Photon Emitters:  4 millirems per year (mrem/year)
  • Uranium:  30 ppb (micrograms per liter, µg/L)

Health Effects of Radionuclides

  • Combined Radium-226 and Radium-228:  Increased risk of cancer
  • Gross Alpha:  Increased risk of cancer
  • Beta Particles and Photon Emitters:  Increased risk of cancer
  • Uranium:  Kidney problems; increased risk of cancer


The following treatment method(s) have proven to be effective in removing radionuclides at levels below their MCLs:

  • Radium 226 and Radium 228 (Combined):  Ion exchange, reverse osmosis, lime softening
  • Gross Alpha Emitters:  Reverse osmosis
  • Beta particle and Photon Radiation:  Ion exchange, reverse osmosis
  • Uranium:  Ion exchange, reverse osmosis, lime softening, coagulation/filtration

The Drinking Water Program recommends that water systems seek advice from a water treatment professional to determine the most effective treatment based on the characteristics of their specific water system. Contact your Public Water System Inspector before installing or making any changes to treatment in your public water system.


Additional Resources

Updated 2/10/2023