INVITED KEYNOTE: GEOCHEMISTRY AS A CRITICAL FACTOR IN DEFINING RADIONUCLIDE OCCURRENCE IN WATER FROM PRINCIPAL DRINKING-WATER AQUIFERS OF THE UNITED STATES

Naturally occurring radionuclides are relatively common contaminants in drinking-water aquifers in the United States. Identification of radionuclide sources and processes controlling their mobilization can guide monitoring programs to ensure health-based compliance standards (designed to decrease cancer risks) are met. To investigate geochemical factors controlling radionuclide occurrence, untreated water samples from 1270 wells from 15 aquifers throughout the United States were analyzed for concentrations of uranium, radon-222, radium isotopes, and ancillary constituents. Statistical relations between populations of radionuclide concentrations greater than median values and other water-quality parameters were quantified. High concentrations of uranium were associated with (1) oxygenated waters, (2) neutral to alkaline waters, pH 6.5 to 8.5, and abundant bicarbonate (with which uranium correlated positively in nearly every aquifer), and (3) waters with atypically high total dissolved solids if criteria (1) and (2) were met. High concentrations of radium were associated with (1) low dissolved-oxygen (anoxic) waters that contained high concentrations of iron or manganese, (2) acidic waters (pH 5.5 or less), and (3) high concentrations of divalent competing ions. Uranium mobilization is facilitated by formation of uranyl-carbonate complexes, which inhibit uranium adsorption onto iron- and manganese-hydroxides; cationic radium is released by competitive exchange or reductive-dissolution of the hydroxide coatings. High radon-222 related most to aquifer radioactivity. Water types, pH, and redox that controlled radionuclide distributions varied among the individual aquifers to the extent that no single simple geochemical classification system described radionuclide occurrence across the United States. Combinations of geochemical variables, geological information, and spatial position were commonly adequate for delineating areas of concern within individual aquifers. Construction of a decision-tree-type analysis on the basis of water-quality and aquifer-type information will help prioritize analytes for monitoring programs. Data from ongoing sample collection for polonium-210 and lead-210 will refine geochemical understanding of their occurrence.