Modeling and\nSensitivity Analysis to Determine Dominant\nDrivers of Increasing Radium Concentrations in Public Supply Wells,\nSoutheast Wisconsin

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