Paleohydrogeology of a Paleozoic sandstone aquifer within an intracratonic basin: Geochemical and structural controls

The aqueous geochemistry of the St. Peter Sandstone, a major aquifer in the Illinois Basin, an intracratonic sedimentary basin, is extremely complex. There are multiple sources of water, including in situ remnant brines, leakage from overlying and underlying units, and recharge during both the Pleistocene and Holocene Epochs. In our study region, recharge to the St. Peter Sandstone has come from multiple locations, and structural features, primarily the LaSalle Anticlinal Belt but also possibly the Sandwich Fault Zone, are a major control on recharge and groundwater flow. In the northern part of the study region, Holocene recharge has displaced most of the Pleistocene recharge, but in the rest of the study region considerable amounts of Pleistocene recharge still exist. The ion chemistry in the St. Peter Sandstone cannot be accounted for by simple mixing of Pleistocene recharge and in situ brines, and the most likely mechanism that could account for the aqueous chemistry is leakage of fluids from overlying formations via fractures opened by flexural loading during glacial advances. The water chemistry in the Western Region of our study area is clearly different from the Central and Eastern regions, having a more pronounced signature of Pleistocene water and suggesting different flow paths and sources of recharge. The large concentrations of SO₄ ²⁻ and relatively high δ³⁴S values in the Western wells suggest an up-gradient source of anhydrite and/or gypsum, possibly from Mississippian evaporite deposits in Iowa.