210Po in Nevada Groundwater and Its Relation to Gross Alpha Radioactivity

Polonium-210 (Po-210) is a highly toxic alpha emitter that is rarely found in groundwater at activities exceeding 1 pCi/L. Po-210 activities in 63 domestic and public-supply wells in Lahontan Valley in Churchill County in northern Nevada, United States, ranged from 0.01 +/- 0.005 to 178 +/- 16 pCi/L with a median activity of 2.88 pCi/L. Wells with high Po-210 activities had low dissolved oxygen concentrations (less than 0.1 mg/L) and commonly had pH greater than 9. Lead-210 activities are low and aqueous Po-210 is unsupported by Pb-210, indicating that the Po-210 is mobilized from aquifer sediments. The only significant contributors to alpha particle activity in Lahontan Valley groundwater are U-234/238, Rn-222, and Po-210. Radon-222 activities were below 1000 pCi/L and were uncorrelated with Po-210 activity. The only applicable drinking water standard for Po-210 in the United States is the adjusted gross alpha radioactivity (GAR) standard of 15 pCi/L. Po-210 was not volatile in a Nevada well, but volatile Po-210 has been reported in a Florida well. Additional information on the volatility of Po-210 is needed because GAR is an inappropriate method to screen for volatile radionuclides. About 25% of the samples had Po-210 activities that exceed the level associated with a lifetime total cancer risk of 1x10(-4) (1.1 pCi/L) without exceeding the GAR standard. In cases where the 72-h GAR exceeds the uranium activity by more than 5 to 10 pCi/L, an analysis to rule out the presence of Po-210 may be justified to protect human health even though the maximum contaminant level for adjusted GAR is not exceeded.