CHARACTERIZING NUTRIENT RELEASE AND RELATIVE STORAGE TIMES IN AN UNCONFINED KARST AQUIFER IN NORTHEASTERN WISCONSIN

Groundwater flow in the karst aquifers formed in Silurian dolomite of northeastern Wisconsin is ultimately guided by the solution-enlarged cracks and fractures that form the conduits of its circulation. While it is known that flow rates in conduits are more rapid and can lead to increased vulnerability to pollution, less is known about the interconnectedness of the conduits and storage time in these karst aquifers. Being able to recognize relative flow path length and storage time presents considerable value as surface contaminants, such as phosphorus, have greater opportunity to be redistributed from areas of high concentration to more vulnerable regions. Nutrient pollution and redistribution are of particular interest in northeastern Wisconsin where farmland is a major land use. We address these issues at a series of springs discharging from the Silurian dolomite in Brown and Door Counties, Wisconsin. Specific conductivity, here a proxy for relative storage time, ranged from 533 to 1703 µS/cm during our initial sampling in June 2022. Stable isotopic values vary inversely with specific conductivity with more depleted values of δ¹⁸O and δD found in high concentration groundwaters indicating more “winter-like” precipitation values and therefore longer storage times (ranges: -10.18 to -7.93 ‰ (δ¹⁸O) and -70.09 to -53.78 ‰ (δD)). These findings will be coupled to geochemical modelling of mineral saturation states and nutrient composition to understand better understand flow and storage in the unconfined dolomite aquifer system and the redistribution of nutrients to regional surface waters and ultimately Lake Michigan. These data represent our initial results, as sampling will continue monthly until April 2023 to characterize seasonal changes in flow, storage times, and nutrient distribution.