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