Modeling and Sensitivity Analysis to Determine Dominant Drivers of Increasing Radium Concentrations in Public Supply Wells, Southeast Wisconsin

Elevated radium in drinking water is a chronic problem for municipalities with public supply wells completed in the Midwestern Cambrian-Ordovician aquifer system (MCOAS). In southeastern Wisconsin, radium concentrations in many municipal wells have increased rapidly in the past decade, yet the processes responsible are not well understood. In this study, a reduced-dimensional numerical model was developed to overcome experimental and computational challenges that hamper transport model parameterization and regional-scale modeling efforts. An ensemble-based modeling approach was used to provide new insights into radium transport behavior and sources within the MCOAS. Results suggest that while diffusive transport may be responsible for elevated background levels, advection of water from or through the Maquoketa shale is the likely mechanism responsible for the rapid rise in radium concentrations observed over the past decade. This advective transport could directly transport radium from the shale units or drive changes in groundwater geochemistry that have the potential to significantly impact radium sequestration and mobilization. Further modeling suggests that changes in well construction may mitigate radium concentrations in municipal wells in southeastern Wisconsin. This study highlights how intensive regional pumping drives large-scale disequilibrium, increasing water quality issues such as the transport and mobilization of geogenic radium.