LONG-TERM ALTERATIONS IN GROUNDWATER CHEMISTRY INDUCED BY MUNICIPAL WELL PUMPING

Dane County, Wisconsin, hosts a number of high-capacity, multi-aquifer wells that were drilled through the regional Eau Claire aquitard and draw from both the confined and unconfined aquifers. These wells are excellent study sites in which to test hypotheses related to pumping-induced changes in redox conditions and mobilization of trace elements. Simulations with a recently revised county-scale flow model indicate that long term pumping and the presence of multi-aquifer wells result in mixing of shallow groundwater into the confined system. Mixing of waters of varying age and chemistry alters the geochemical conditions. PHREEQC is being used to model this interaction, with a particular focus on the fate and transport of hexavalent chromium. Concentrations of hexavalent chromium measured in groundwater from multi-aquifer wells in the study area exceed those measured in confined aquifer wells. Model simulations indicate that while chromium mobilization may be occurring in the confined system, aqueous chromium concentrations are limited by the sorption potential of iron in the Mount Simon formation. Though no federal drinking water standard currently exists for hexavalent chromium, it is a suspected carcinogen. It is a List 1 contaminant on the US Environmental Protection Agency’s third Unregulated Contaminant Monitoring Rule (UCMR 3), and its prevalence in public water supplies is of interest. Our findings suggest that in this hydrogeologic setting, redox conditions are a strong control on the mobility of hexavalent chromium.