210 Po in Nevada groundwater and its relation to gross alpha radioactivity

Abstract

Polonium-210 ( 210 Po) is a highly toxic alpha emitter that is rarely found in groundwater at activities exceeding 1 pCi/L. 210 Po 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 210 Po 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 210 Po is unsupported by 210 Pb, indicating that the 210 Po is mobilized from aquifer sediments. The only significant contributors to alpha particle activity in Lahontan Valley groundwater are 234/238 U, 222 Rn, and 210 Po. Radon-222 activities were below 1000 pCi/L and were uncorrelated with 210 Po activity. The only applicable drinking water standard for 210 Po in the United States is the adjusted gross alpha radioactivity (GAR) standard of 15 pCi/L. 210 Po was not volatile in a Nevada well, but volatile 210 Po has been reported in a Florida well. Additional information on the volatility of 210 Po is needed because GAR is an inappropriate method to screen for volatile radionuclides. About 25% of the samples had 210 Po activities that exceed the level associated with a lifetime total cancer risk of 1× 10 −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 210 Po may be justified to protect human health even though the maximum contaminant level for adjusted GAR is not exceeded.