Occurrence and Geochemistry of Lead-210 and Polonium-210 Radionuclides in Public-Drinking-Water Supplies from Principal Aquifers of the United States

On the basis of lifetime cancer risks, lead-210 (Pb-210) and polonium-210 (Po-210) >= 1.0 and 0.7 pCi/L (picocuries per liter), respectively, in drinking-water supplies may pose human-health concerns. Pb-210 and Po-210 were detected at concentrations greater than these thresholds at 3.7 and 1.5%, respectively, of filtered untreated groundwater samples from 1263 public-supply wells in 19 principal aquifers across the United States. Nationally, 72% of samples with radon-222 (Rn-222) concentrations > 4000 pCi/L had Pb-210 >= 1.0 pCi/L.Pb- 210 is mobilized by alpha recoil associated with the decay of Rn-222 and short-lived progeny. Pb-210 concentrations >= 1.0 pCi/L occurred most frequently where acidic groundwaters inhibited Pb-210 readsorption (felsic-crystalline rocks) and where reducing alkaline conditions favored dissolution of iron-manganese- (Fe-Mn-) oxyhydroxides (which adsorb Pb-210) and formation of lead-carbonate complexes (enhancing lead (Pb) mobility). Po-210 concentrations >= 0.7 pCi/L occurred almost exclusively in confined Coastal Plain aquifers where old (low percent-modern carbon-14) groundwaters were reducing, with high pH (>7.5) and high sodium/chloride (Na/Cl) ratios resulting from cation exchange. In high-pH environments, aqueous polonium (Po) is poorly sorbed, occurring as dihydrogen polonate (H2PoO3(aq)) or, under strongly reducing conditions, as a hydrogen-polonide anion (HPo-). Fe-Mn- and sulfate-reduction and cation-exchange processes may mobilize polonium from mineral surfaces. Po2+ occurrence in low-to-neutral-pH waters is attenuated by adsorption.