MODERN AQUIFER CHEMISTRY AS A FUNCTION OF WATER-ROCK INTERACTION: A CASE EXAMPLE FROM EASTERN WISCONSIN

Paleozoic rocks in eastern Wisconsin preserve a complex diagenetic history that strongly influences modern aquifer chemistry. The most significant event was a progression of hydrothermal dolomitization and associated MVT mineralization, followed by later calcite cementation. Additional modifications, including localized faulting and fracturing, as well as pervasive stylolitization, have significantly altered these rocks from their original depositional composition and texture. Early fluid-rock interaction involved heated brines (~100°C) moving out of the Michigan basin during the Paleozoic Era, and perhaps later. In addition to dolomitization, a complex array of trace sulfides, sulfates, carbonates, and fluorite mineralization is heterogeneously distributed throughout the rocks. As a result, this suite of minerals has a strong control on modern aquifer chemistry, which depends upon groundwater sources and flow paths, redox potential, and saturation indices of various minerals.

Whole-rock chemistry, geologic mapping, and chemical and isotopic analysis have revealed a clearer picture of the water-rock interaction processes responsible for today’s aquifer chemistry. Lead and Sr isotopes indicate some interaction between hydrothermal fluids and Precambrian basement. All Paleozoic rocks in the region were influenced to some degree, and the Cambrian-Ordovician portion of the section retains much of the original mineral suite. In contrast, overlying Silurian strata have had more “flushing” of original brines and a far more advanced MVT mineral decomposition during development of the karst aquifer.

Several significant water quality issues relate to younger fluid-rock interaction. Along the western boundary of the confined aquifer, and in some Pleistocene glacial sediments, sulfide oxidation and associated reactions liberate As, Ni, Co, and other heavy metals. This is exacerbated by aquifer drawdown and other anthropogenic processes. Farther east along flow paths, dissolution of celestine and fluorite results in elevated dissolved Sr and F. Radium is prevalent in parts of the aquifer, with concentrations increasing eastward, but the source remains elusive. Elevated Li, B, Na, and Cl are related to dilution of Michigan basin brines, rather than water-rock interaction.