Quality of water in the St. Peter aquifer is generally acceptable for most uses. Sulfate concentrations increase toward the southwestern part of the aquifer because of highly mineralized leakage from overlying Cretaceous deposits. Concentrations of sodium, magnesium, and sulfate generally increase to the west, and those of calcium, bicarbonate, and chloride generally increase toward the margins of the aquifer. Calcium magnesium bicarbonate water is the most common type in the aquifer. The aquifer is well protected from surface sources of Contaminants by the overlying Decorah-Platteville-Glenwood confining bed, which overlies about 80 percent of the St. Peter aquifer. Thin drift, which overlies the east side of the aquifer, provides much less protection than the confining bed. Water in the aquifer generally flows toward the Mississippi River and its tributaries. Some water flows southward into Iowa from a ground-water divide in Dodge and Steele Counties, Well yields from the aquifer, which are generally between 100 and 250 gallons per minute, are generally adequate for most uses.

This report is one of a series on the hydrogeology and water quality of the 14 principal aquifers in Minnesota prepared by the U.S. Geological Survey. The U. S. Environmental Protection Agency requested these studies because of the need for information to develop its Underground Injection Control Program.