OCCURRENCE AND GEOCHEMISTRY OF LEAD-210 AND POLONIUM-210 IN PUBLIC-DRINKING-WATER SUPPLIES FROM PRINCIPAL AQUIFERS OF THE UNITED STATES

Concentrations of lead-210 (²¹⁰Pb) and polonium-210 (²¹⁰Po) in over 1,100 samples of untreated water from public-supply wells located across 18 principal aquifers (PAs) were determined since 2012 as part of the U.S. Geological Survey’s National Water Quality Assessment project. These 18 PAs account for a majority of the groundwater withdrawn for public-supply purposes in the United States. The U.S. Environmental Protection Agency has estimated maximum contaminant levels (MCLs) of 1.0 and 0.7 picocuries per liter (pCi/L) for ²¹⁰Pb and ²¹⁰Po, respectively, based on lifetime cancer risks. These estimated MCLs, however, have not been promulgated due in part to a lack of monitoring data.

²¹⁰Pb concentrations greater than 1.0 pCi/L occurred most frequently in samples from the Piedmont (23%), North Atlantic Coastal Plain (NACP) (10%), Colorado Plateaus (8%) and Coastal Lowlands (7%) PAs. Among these PAs, concentrations of ²¹⁰Pb were most strongly correlated with radon-222 (²²²Rn) concentrations in the Piedmont, indicating ²¹⁰Pb occurrence in this aquifer system is controlled primarily by the aqueous concentration of ²²²Rn. ²²²Rn has a half-life of 3.8 days and decays through three short-lived progenies to form polonium-214 (²¹⁴Po) and then ²¹⁰Pb in about 49.5 minutes. In the Piedmont, which had the greatest ²²²Rn concentrations of all PAs, elevated ²¹⁰Pb concentrations commonly occurred in young (post 1950), oxic, calcium-magnesium – bicarbonate water types with pH values near 7.0 (neutral) – geochemical conditions that otherwise favor sequestration of lead. However, alpha-recoil from the decay of ²¹⁴Po adsorbed on aquifer solid surfaces mobilizes ²¹⁰Pb into solution accounting for the high frequency of concentrations greater than 1.0 pCi/L.

²¹⁰Po concentrations greater than 0.7 pCi/L occurred most frequently in samples from select Coastal Plain aquifers in the NACP (27%), Southeast Coastal Plain (6%) and Coastal Lowlands (3%) PAs. Elevated concentrations of ²¹⁰Po in these aquifers were associated with old (pre 1950), reducing, sodium – bicarbonate water types with pH greater than 8. Under these geochemical conditions, sorption capacity is diminished and ²¹⁰Po forms neutral species that are inefficiently sorbed, which allows an increasing fraction of ²¹⁰Po to accumulate in groundwater.