Radioactivity in Groundwater
Radioactivity in groundwater occurs in a variety of different geologic settings within the United States (Zapecza and Szabo, 1988). Radium, one of the principal sources of radioactivity, is highest in quartz-rich sand aquifers in the Northern Atlantic Coastal Plain where low pH conditions release radium from the sediments (Szabo and dePaul, 1998). Radionuclides can enter the groundwater system by dissolution of minerals or desorption from sediment particle surfaces. In Maryland’s Coastal Plain aquifer system, high radium concentrations have been detected in the Magothy and Potomac Group (Patapsco and Patuxent Formations) aquifers in the upper Chesapeake Bay area (Bolton, 2000), and in the Surficial aquifer on the Eastern Shore (Denver and others, 2014). Radium isotopes that have been detected in Maryland Coastal Plain groundwater include radium-224 (Ra-224), radium-226 (Ra-226), and radium-228 (Ra-228) (Bolton and Hayes, 1999; Bolton, 2000). In Charles County, several cases of elevated radioactivity in water from the Upper and Lower Patapsco aquifer systems have been attributed to polonium-210 (Andreasen, 2015).
Radionuclides decay and emit gamma rays and alpha and beta particles at varying rates. Alpha particles cannot penetrate skin, but if radionuclides emitting alpha particles are ingested, the alpha particles can damage cell tissue, which could potentially lead to cancer. Ra-228 emits beta radiation, which can penetrate skin, but must be ingested to cause damage to internal cell tissue. Radium isotopes Ra-224 and Ra-228 are part of the thorium-232 decay series, whereas Ra-226 and Po-210 are part of the uranium-238 (U-239) decay series. The half-lives of Ra-224, Ra-226, and Ra-228 are 3.6 days, 1,600 years, and 5.8 years, respectively. Po-210 has a half-life of approximately 138 days. Exposure to low levels of Po-210 in drinking water, or in food products from animals raised with contaminated water, may have long-term biological effects on humans including possible damage to fetal and placental tissue (Seiler and Wiemels, 2012). The USEPA has established MCLs for radionuclides in drinking water as follows: GAPA, 15 pCi/L; gross beta-particle activity (GBPA), 4 millirems per year; combined radium (Ra-226+Ra-228), 5 pCi/L; and uranium, 30 micrograms per liter (µg/L) (U.S. Environmental Protection Agency, 2000). No individual drinking water standards have been established for Ra-224 and Po-210 by the state or USEPA, although they carry cancer risk upon ingestion nearly equivalent to that of Ra-226 and Ra-228 (U.S. Environmental Protection Agency, 1999).
All of the naturally-occurring radionuclides are Class A human carcinogens (U.S. Environmental Protection Agency, 1999), but some pose greater levels of cancer risk than others. USEPA assigns (cancer) risk coefficients to radionuclides applicable on the basis of water concentrations (U.S. Environmental Protection Agency, 1999). For Po-210, the USEPA has determined an activity level of concern of 1.1 pCi/L on the basis of a lifetime total cancer risk of 1 in 10,000 (U.S. Environmental Protection Agency, 1999). This level is near the lifetime total cancer risk for combined radium (Ra-226+Ra-228) of 5 pCi/L (the MCL). The GAPA MCL of 15 pCi/L is intended to limit risk from ingestion by presuming a “worst-case” scenario of a water sample with 5 pCi/L of Ra-226, 5 pCi/L of progeny of Ra-228, and 5 pCi/L of Po-210, the radionuclide that poses nearly equal dose risk to those of the radionuclides of radium (U.S. Environmental Protection Agency, 1999).
Andreasen, D.C., 2015, Preliminary investigation of elevated radioactivity in groundwater in Charles County, Maryland: Maryland Geological Survey Open-File Report 15-02-02, 36 p.
Bolton, D.W., 2000, Occurrence and distribution of radium, gross alpha-particle activity, and gross beta-particle activity in ground water in the Magothy Formation and Potomac Group aquifers, upper Chesapeake Bay area, Maryland: Maryland Geological Survey Report of Investigations No. 70, 97 p.
Bolton, D.W., and Hayes, M.A., 1999, Pilot study of carcinogens in well water in Anne Arundel County, Maryland: Maryland Geological Survey Open-File Report No. 99-02-10, 58 p.
Denver, J.M., Ator, S.W., Fischer, J.M., Harned, D.C., Schubert, Christopher, and Szabo, Zoltan, 2014, Water quality in the Northern Atlantic Coastal Plain Surficial Aquifer System, Delaware, Maryland, New Jersey, New York, North Carolina, and Virginia, 1988–2009: U.S. Geological Survey Circular 1353, 90 p.
Seiler, R.L., and Wiemels, J.L., 2012, Occurrence of 210Po and biological effects of low-level exposure: The need for research: Environmental Health Perspectives, vol. 120, no. 9, p. 1230-1237.
Szabo, Zoltan, and dePaul, V.T., 1998, Dissolved radium-226 and radium-228 in shallow ground water, southern New Jersey: U.S. Geological Survey Fact Sheet FS-062-98, 6 p.
Cancer risk coefficients for environmental exposure to radionuclides: Federal Guidance Report No. 13, EPA 402-R-99-001, Washington, D.C., 371 p.
Zapecza, O.S., and Szabo, Zoltan, 1988, Natural radioactivity in ground water—A review, in Moody, D.W., Chase, E.B., and Paulson, R.W., comp., National water summary 1986—Ground-water quality: Hydrologic conditions and events: U.S. Geological Survey Water-Supply Paper 2325, p. 50–57.