Determining the sources of water for conduit 'sandboil' springs at the Nature Conservancy's Nachusa Grasslands Preserve, Franklin Grove, Illinois.

摘要

The Nachusa grasslands conservation area overlies a variety of heterogeneous structural and hydrogeologic systems, resulting from the close proximity of several major structural features, including the Sandwich Fault Zone and the Kankakee and LaSalle Anticlines. The resulting regional uplift and deformation produced a series of sandstone outcrops and conduit springs, spanning various sections of the, approximately 5 km2 study area. The "sandboils" are nicknamed for the extremely pure quartz sand the springs bring to the surface. The purpose of this research is to determine the source of water generating the Nachusa Grasslands "sandboil" springs by utilizing geochemical and stable isotope groundwater and surface water analyses, as well as determine the presence of mixing between hydrologic systems. It has long been assumed that the source of the springs originates from the infiltration and interflow of meteoric water through sandstone outcrops; however, it is the goal of the study to provide sufficient evidence showing the spring water originates from a deeper groundwater source. Residential well logs, geophysical studies, and sediment core analysis taken adjacent to the sandboil study sites show the geology beneath the study area to consist of fractured St. Peter Sandstone overlying the Shakopee and New Richmond Limestone-Dolostone formations. Major ion analyses show the spring water is dominated by a calclium-bicarbonate and magnesium hydrochemical facies consistent with water originating from a limestone aquifer unit. Water samples collected from monitoring wells and surface water sites near the sandboil springs also show a strong connection to a calcareous hydrologic unit, with some meteoric influence. Groundwater samples collected from the Shakopee-New Richmond formation yielded high concentrations of nitrates and nitrites, indicating agricultural contamination and a rapid connection between the surface and subsurface hydrologic systems. Stable Isotope analysis for delta2 H and delta18O also showed a well-mixed system between two hydrologic units both of which are dominated by young waters.