Electrical-resistivity logs and water-quality data were used to delineate the fresh water/saline-water interface in a 22,000-square-mile area of the central Michigan Basin, where Mississippian and younger geologic units form a regional system of aquifers and confining units.

Pleistocene glacial deposits in the central Lower Peninsula of Michigan contain freshwater, except in a 1,600-square-mile area within the Saginaw Lowlands, where these deposits typically contain saline water. Pennsylvanian and Mississippian sandstones are freshwater bearing where they subcrop below permeable Pleistocene glacial deposits. Down regional dip from subcrop areas, salinity of ground water progressively increases in Early Pennsylvanian and Mississippian sandstones, and these units contain brine in the central part of the basin. Freshwater is present in Late Pennsylvanian sandstones in the northern and southern parts of the aquifer system. Typically, saline water is present in Pennsylvanian sandstones in the eastern and western parts of the aquifer system.

Relief on the freshwater/saline-water interface is about 500 feet. Altitudes of the interface are low (300 to 400 feet above sea level) along a north-south-trending corridor through the approximate center of the area mapped. In isolated areas in the northern and western parts of the aquifer system, the altitude of the base of freshwater is less than 400 feet, but altitude is typically more than 400 feet. In the southern and northern parts of the aquifer system where Pennsylvanian rocks are thin or absent, altitudes of the base of freshwater range from 700 to 800 feet and from 500 to 700 feet above sea level, respectively.

Geologic controls on distribution of freshwater in the regional aquifer system are (1) direct hydraulic connection of sandstone aquifers and freshwater-bearing, permeable glacial deposits, (2) impedance of upward discharge of saline water from sandstones by lodgement tills, (3) impedance of recharge of freshwater to bedrock (or discharge of saline water from bedrock) by Jurassic red beds, and (4) vertical barriers to ground-water flow within and between sandstone units.