Long-term Alterations in Groundwater Chemistry Induced by Municipal Well Pumping

The proposed research aims to determine the long-term effects of municipal well pumping on groundwater chemistry induced by changes in flow paths and mixing between aquifers. Of particular interest in Dane County, WI, are cross-connected wells that were drilled through the regional Eau Claire aquitard to increase productivity by drawing water from both the confined and unconfined aquifers. Mixing water of different ages and chemistry in this manner creates new geochemical environments that may allow for mobilization of undesirable trace elements. This process likely contributes to elevated concentrations of chromium, iron, and manganese observed in some Dane County wells. Understanding of the geochemical conditions being created by municipal well pumping can assist in identifying pumping strategies and well construction practices that improve water quality, thus protecting Wisconsin citizens and preserving these water resources for future use. The project will take advantage of a large body of groundwater chemistry data that has been previously collected as well as a recently revised, countyscale, groundwater flow model. The flow model will be used in conjunction with particle tracking to identify areas where flow paths have been altered from pre-development conditions in a way that is likely to have induced mixing of water from shallow sources with water in the deeper aquifer system. Solute transport modeling and geochemical modeling, constrained by available water chemistry data, will allow for additional quantification of mixing and changes in redox or other conditions. Preliminary model results will be used to identify wells from which additional water chemistry samples will be collected to improve calibration and test model predictions. An iterative modeling approach will be used with the ultimate goal of identifying hydrogeologic units that are the primary contributors to elevated concentrations of chromium, iron and manganese and to explore options to limit mobilization of these trace elements.