Gross alpha-particle activity and high 226Ra concentrations do not correspond with high 210Po in the Atlantic and Gulf Coastal Plain aquifers of the United States

²¹⁰Po, which is of human-health concern based on lifetime ingestion cancer risk, is indirectly regulated in drinking water through the U.S. Environmental Protection Agency’s gross alpha-particle activity (GAPA) maximum contaminant level of 15 pCi/L (picocuries per liter). This regulation requires independent measurement of ²²⁶Ra for samples exceeding the GAPA screening level of 5 pCi/L. There is no such requirement for ²¹⁰Po. Co-occurrence of ²²⁶Ra and ²¹⁰Po, alpha-emitting ²³⁸U-decay-series progeny, might be helpful in locating high-²¹⁰Po waters but is unverified. Relations among ²¹⁰Po, ²²⁶Ra, and GAPA evaluated for samples from 257 public-supply wells from Coastal Plain aquifers showed that concentrations of ²²⁶Ra correlated with GAPA but neither correlated with ²¹⁰Po concentrations. The highest concentrations of ²²⁶Ra and ²¹⁰Po were found under differing geochemical conditions. The highest ²²⁶Ra occurred in low-pH oxidizing waters and in neutral-pH reducing waters, where geochemical conditions render Fe–Mn-hydroxide sorbents inefficient. ²¹⁰Po was highest (10.1 pCi/L) in reducing waters with high pH (>7.5, which results from progressive cation exchange), where ²²⁶Ra was lowest─exchanged to clay minerals. Because ²²⁶Ra and ²¹⁰Po did not co-occur, the GAPA screening might not be protective for ²¹⁰Po. Independent ²¹⁰Po analysis is prudent, especially where groundwater is reducing with high pH and low ²²⁶Ra concentrations.