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