On the basis of lifetime cancer risks, lead-210 (²¹⁰Pb) and polonium-210 (²¹⁰Po) ≥ 1.0 and 0.7 pCi/L (picocuries per liter), respectively, in drinking-water supplies may pose human-health concerns. ²¹⁰Pb and ²¹⁰Po 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 (²²²Rn) concentrations > 4000 pCi/L had ²¹⁰Pb ≥ 1.0 pCi/L. ²¹⁰Pb is mobilized by alpha recoil associated with the decay of ²²²Rn and short-lived progeny. ²¹⁰Pb concentrations ≥ 1.0 pCi/L occurred most frequently where acidic groundwaters inhibited ²¹⁰Pb readsorption (felsic-crystalline rocks) and where reducing alkaline conditions favored dissolution of iron–manganese- (Fe–Mn-) oxyhydroxides (which adsorb ²¹⁰Pb) and formation of lead–carbonate complexes (enhancing lead (Pb) mobility). ²¹⁰Po 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.