Groundwater Geochemistry and Radionuclide Activity in the Cambrian-OrdovicianAquifer of Dodge and Fond du Lac Counties, Wisconsin

摘要

Analyses of groundwater from wells in the Cambrian-Ordovician aquifer of easternWisconsin indicate that regions of the aquifer contain elevated concentrations of dissolved solids, chloride and sulfate. Groundwater from several wells in the area also approach or exceed the current drinking water standard for combined radium activity. Significant changes in groundwater chemistry occur where the aquifer becomes confined by the Maquoketa shale. Concentrations of Cl(-), SO4(2-) and Na(+) increase in the confined region, and the highest combined radium activities are typically observed in the area. Geochemical modeling implies that the observed changes in major ion groundwater chemistry occur in response to the presence of the confining unit which may act as a source of SO4(2-), through gypsum dissolution, and Na(+), through cation exchange. A finite difference groundwater flow model was linked to a particle tracking routine to determine groundwater flow paths and residence times in the aquifer near the boundary between unconfined and confined conditions. Results suggest that the presence of the confining unit produces a vertically stratified flow regime in the confined region.