MOLECULAR ENVIRONMENTAL SCIENCE AT THE LANDSCAPE SCALE: X-RAY ABSORPTION SPECTROSCOPY STUDIES OF ARSENIC GEOCHEMISTRY IN GLACIAL TILLS

Arsenic concentrations exceeding the U.S. EPA maximum contaminant limit of 10 µg/L have been found in domestic water wells throughout west-central Minnesota. The purpose of our study is to measure the specific geochemical form of As in geologic strata, determine the source of the As, and identify the processes that drive the patterns of spatial heterogeneity observed in well-water As concentrations. The source of the As is thought to be naturally occurring geologic material, and the glacial tills of the Des Moines Lobe have been implicated. The area of interest lies in the footprint of the Des Moines Lobe ice-streams and has a complex depositional history. Previous work in west-central Minnesota has found a correlation between well-water As concentrations and proximity of the well-screens to glacial till. Wells with elevated As appear to tap local aquifers in contact with different Des Moines Lobe and older tills. However, the specific formation source of As and the mechanisms leading to its release to groundwater are not known. Arsenic concentrations in the impacted well water are high (10 to 100 µg/L) compared with As concentration of the sediments (< 10 mg/kg). This suggests that the As present in the sediments is in a chemical form that is easily mobilized by the groundwater. In the present research we use As X-ray absorption near edge structure (XANES) spectroscopy to identify the oxidation state of As at extremely low concentrations (less than 8 mg/kg) in natural sediments. All measured samples had a fluorescence peak consistent with arsenate (As (V)) and some samples had a second peak indicating the presence of another, more reduced As phase. The present work is unique among molecular environmental studies as it strives to assess As speciation at the low As concentrations of concern as well as integrate the results into a landscape-scale perspective on As fate and transport.