Isotopic Tracing of Sources and Cycling of Hazardous Metal Mixtures

Project 2 Summary Native American communities surveyed in the Strong Heart Study (SHS) show markedly higher arsenic (As), uranium (U) and selenium (Se) concentrations in urine relative to the average US population. Much of the exposure to hazardous metal mixtures comes from drinking groundwater contaminated with metals often above safety standards. Tribal lands in the US often coincide with mineral deposits, U mining, and metal(loid) contamination in aquifers. In the Northern Plains, elevated concentrations of U and As in drinking water reflect naturally occurring enrichment in aquifers, as well as the legacy of mining near tribal communities. Project 2 of the Columbia University Northern Plains Superfund Research Program (CUNP-SRP) will use isotopic compositions of U and Se, as these elements are sensitive to redox changes in the aquifer, to infer sources and processes controlling contaminant exposures. Redox chemistry of U and Se controls their aqueous concentrations in groundwater; the oxidized species are mobile and toxic whereas the reduced species are insoluble. Concentrations of dissolved U and Se are affected by multiple processes (e.g., dilution, adsorption, dispersion) and are poor indicators of redox-induced fate and cycling in groundwater. Thus, isotope ratios of dissolved U and Se in groundwater, in tandem with concentrations, are invaluable for studying redox-induced (im)mobilization, and tracing uptake and human exposure. We will use U and Se isotope ratios to characterize redox reactions in groundwater, acquiring unique mechanistic information about the reaction progress. These data will help us define local and more distal sources, and the transport and cycling processes of metals in groundwater within the SHS communities in North/South Dakota. Furthermore, we will develop isotopic tracers of U and Se cycling in humans impacted by environmental exposures. The specific aims are to: (1) Determine spatial distribution pattern of isotope ratios or “isoscapes” of U and Se in groundwater samples. This will use high-resolution data and models from Project 1 to target wells within and around U, As and Se hotspots for isotopic characterization. We will also identify local versus distal sources of contamination around these hotspots. (2) Determine the levels of naturally occurring reductive U and Se removal from groundwater and identify conditions for potential As release based on the temporal evolution of U and Se isotope ratios. We will characterize the redox transformation of U and Se and potential mixing of multiple groundwater sources (if any). (3) Determine human exposure from the environmental contamination using isotopic tracers. We will develop a novel non-invasive method to track metal exposure from U and Se in drinking water. For this, we will measure U and Se isotope ratios in urine to constrain primary exposure sources and affected populations by linking those to water isoscapes and the observational data from Project 3. Our results will help evaluate risk of exposure to hazardous metal mixtures, monitoring and management of contaminated aquifers, and linking exposures to environmental sources using a non-invasive diagnostic tool.