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. Simulations with a recently revised county-scale flow model indicate that the presence of multi-aquifer wells and drawdown in the confined aquifer generated by decades of pumping result in mixing of shallow, oxic groundwater into the confined, anoxic system. PHREEQC was used to model this interaction, with an emphasis on the mobilization of hexavalent chromium, manganese, and iron. Measured hexavalent chromium concentrations, although generally less than 3 ppb, tend to be higher in groundwater from multi-aquifer wells, and aqueous manganese and iron concentrations tend to be higher in confined aquifer wells, with manganese concentrations highest near the boundary between oxic and anoxic conditions and iron most prevalent under completely anoxic conditions. 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 oxides in the Mount Simon formation. Additionally, whether the chromium is adsorbed onto the surface of sand grains or, instead, bound within the mineral structure may impact mobilization and release rates in the confined Mount Simon aquifer. Extractions performed using well cuttings reveal that only a small portion of total chromium is available for mobilization, and that solid-phase chromium is associated with iron-rich intervals, potentially iron-bearing clays. No connection was observed between chromium and manganese concentrations, questioning the role of manganese oxides in the oxidation of chromium in the groundwater system and potentially indicating a greater dependence on dissolved oxygen. Our findings suggest that in this hydrogeologic setting, pumping practices and alterations to redox conditions are impacting the mobility of hexavalent chromium, manganese, and iron.