Hydrogeochemical controls on aquifers of northwestern Illinois' Driftless Area, USA

Groundwater in the Driftless Area of northwestern Illinois is a very hard Ca–Mg-bicarbonate-type water that reflects the dolomite aquifer material in this karst region. Groundwater in shallow aquifers has a short residence time and is susceptible to surface-borne contaminants. Deeper aquifers have distinct chemistry indicating longer flowpaths, most clearly indicated by low tritium values and elevated potassium and fluoride concentrations. Thirty water samples were collected from springs from across Jo Daviess County in northwestern Illinois between 2015 and 2016. The groundwater samples collected from what were initially identified as springs were found to be of two types: (1) natural springs that are stratigraphically controlled and discharging from crevices and bedding planes, especially where the underlying Galena Dolomite or overlying Silurian dolomite aquifer intersects the Maquoketa Shale; and (2) abandoned flowing-artesian wells, often with no identifiable well casing at the surface, that had been mistaken for springs. Background concentrations of eight water-quality parameters were calculated to determine pre-settlement and present-day background thresholds. The background concentrations, overall chemical compositions, tritium concentrations, and F/Cl and K/Cl ratios were used to characterize and group groundwater within the study area. The results of this investigation combined with recent work by the authors [Panno et al. (IL State Geological Survey Circular 586, 2017)] were used to determine the geologic, hydrogeologic, and geochemical controls on springs in this part of the Driftless Area. The results of this investigation revealed the susceptibility of the Galena Dolomite and Silurian dolomite aquifers to surface-borne contaminants. In addition, this work has developed a means, using ion ratios and spring location relative to bedrock geology to identify discharge from abandoned wells.