Geochemistry of the Cambrian-Ordivician aquifer system in the northern Midwest, United States

Distributions of solutes in aquifers of Cambrian and Ordovician age were studied in Minnesota, Wisconsin, Iowa, Illinois, northwestern Indiana, and northern Missouri to determine the sources of solutes and the probable chemical mechanisms that control regional variations in water quality. The most important chemical variation in the aquifer is the change in water type from calcium-sodium-sulfate-bicarbonate water to sodium-calcium-sulfate-bicarbonate and sodium-chloride waters along the longest regional flow path from northwestern Iowa to the Illinois basin. The most striking aspect of the distribution of dissolved solids and carbon isotopic content of bicarbonate is the increase in concentration and isotopic enrichment from southwestern Wisconsin, southern Minnesota, and northwestern Illinois south toward Missouri. This trend is perpendicular to the present hydraulic gradient that trends from northwestern Iowa southeastward to the Illinois basin. Distributions of dissolved trace constituents in the aquifers probably are related to the proximity to mineralogic sources as well as chemical and hydraulic mechanisms. Additional information on the chemical and mineralogical composition of the aquifer matrix and the isotopically lightest ground water is needed to evaluate the hypothesis of Pleistocene mixing before more quantiative studies can be done to evaluate the different proposed mechanisms that have controlled and modified the water chemistry over time. This study, however, indicates that the ground water in the region is thousands of years old. -from Author