MEASURED AND MODELED ARSENIC SPECIES VARIABILITY IN MIDWESTERN GROUNDWATER

Arsenic contamination in upper Midwestern groundwater is widespread and naturally occurring. A recent study in western Minnesota found that over 50% of the 900 sampled private drinking water wells had arsenic over 10 µg/L. Upper Midwestern groundwater arsenic contamination is spatially coincident with a specific regional geologic feature: Des Moines lobe glacial sediment. However, Des Moines lobe till is not the source of arsenic contamination. Rather, this feature's distinctive physical characteristics cause hydrogeochemical conditions favorable for arsenic mobilization.

Current research includes building a comprehensive database of past upper Midwestern groundwater analytical results, collection and analysis of groundwater and sediment samples, and data analysis/model-building. Preliminary results show that groundwater arsenic concentration is not directly correlated to sediment arsenic concentration, has significant temporal and spatial variability, increases over time in new wells, and is linked to certain physical well characteristics.

Additional sediment and water samples are being collected during 2003 to provide a better understanding of the controlling geochemical mechanisms. Groups of wells with specific common characteristics are being targeted to examine differences in groundwater geochemistry associated with high arsenic wells, low arsenic wells, new wells, and wells with known temporally variable arsenic concentrations. Water samples are being analyzed for inorganic arsenic species (As³⁺ and As⁵⁺) and other parameters of potential interest. Sediment samples are being sequentially extracted to quantify the form of arsenic in sediment.

Geochemical and transport modeling is being performed to investigate mechanisms and transformation processes that may explain observed arsenic species ratios, changes in species ratios over time, and patterns of spatial variability in arsenic concentration. The models are being developed using measured parameter concentrations and literature values related to arsenic and hydrogeology. Current findings, including potential controls and mechanisms, will be presented.