Geochemical evolution of groundwater along a flowpath determined from oxygen isotope analysis in a carbonate free, silicate aquifer

Chemical reactions controlling the chemistry of groundwater were investigated in a carbonate-free, silicate aquifer of an isthmus lying between Crystal Lake and Big Muskellunge Lake, northern Wisconsin. Sandy, glacial drifts of recent glacial age occur as a major aquifer in the study area. Shallow groundwater in the isthmus area is recharged by both precipitation through the isthmus soil and seepage from nearby Crystal Lake. Stable isotopic contrasts (δ18O) between the source waters allow identification of a unique flowpath along which the chemical composition of the groundwater evolves. The groundwater chemistry along the flowpath shows small temporal variation. Groundwater in the isthmus is highly undersaturated with respect to the most of minerals commonly observed in soils of silicate terrains such as plagioclase, pyroxene, amphibole, biotite, etc. Only waters in the downgradient show slight oversaturation against K-feldspar. Alkalinity, calcium and magnesium show continuously increasing trend toward the downgradient reflecting the important role of mineral dissolution reactions. The uniform increase rates of alkalinity in the sandy aquifer may suggest that the total dissolved minerals within a time are consistent in the sandy aquifer. However, the concentrations of monovalent cations (sodium and potassium) rarely increase above certain levels. This chemical behavior could not be explained with only mineral dissolution, and may indicate other reactions and/or processes such as cation exchange in regulating groundwater chemistry of the area.