National Water-Quality Assessment (NAWQA) Program
Szabo, Zoltan, Fischer, J.M., and Hancock, T.C., 2012, Principal aquifers can contribute radium to sources of drinking water under certain geochemical conditions: U.S. Geological Survey Fact Sheet 2010-3113, 6 p.
Figure 1. Elevated radium concentrations occur most commonly in aquifers in the eastern and central United States for the wells sampled in 15 principal aquifers across the United States. About 3 percent of sampled wells had combined radium concentrations greater than the MCL. Ninety-eight percent of the wells that exceeded the combined radium (radium-226 plus radium-228) drinking-water standard of 5 picocuries per liter established by the U.S. Environmental Protection Agency were in aquifers east of the High Plains. The highest concentrations of combined radium were in the Mid-Continent and Ozark Plateau Cambro-Ordovician aquifer system and the Northern Atlantic Coastal Plain. More than 20 percent of sampled wells in these aquifers had combined radium concentrations that were greater than or equal to the MCL.