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 measure-ment 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.