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. This work is part of the Northern Midwest Regional Aquifer-System Analysis project, whose objective is to describe and model the regional hydrogeology of the Cambrian- Ordovician aquifer system in the study region. The data base used included more than 3,000 ground-water-quality analyses from all major aquifers, but especially from the St. Peter, Jordan, and Mount Simon Sandstones and their equivalents. Regional variations in the water chemistry of glacial drift and other sedimentary units that overlie the Cambrian-Ordovician aquifer system in recharge areas in Minnesota, Iowa, Wisconsin, and Illinois were also studied, but to a lesser degree.
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. Sodium predominance downgradient from the recharge area is probably related to mechanisms of ion exchange and shalemembrane filtration near the Illinois and Forest City basins.
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. The distribution of dissolved solids defines a "plume" of dilute water having a dissolved-solids concentration of about 500 milligrams per liter, compared with surrounding concentrations more than twice as large. Distribution of the isotopic content of oxygen (
