The Cretaceous aquifer in southwest Minnesota consists of discontinuous, basal sandstone beds in the Dakota Formation and the overlying Codell Sandstone Member of the Carlile Shale of the Colorado Group. These sandstone beds are not laterally or vertically persistent throughout the area and generally are separated shale beds in the Dakota Formation and in the overlying Colorado Group of Cretaceous age. Water in the Cretaceous aquifer is confined by overlying shale and by overlying till as much as 700 feet thick. Locally where the drift is permeable and thin and where the shale is missing, water in the aquifer is unconfined. Groundwater moves away from the Sioux Quartzite Ridge north toward the Minnesota River, south toward Iowa, and eastward toward the Mississippi River. Recharge to the aquifer primarily is by infiltration of precipitation that percolates through the overlying drift and underflow in the aquifer from South Dakota.
Water in the Cretaceous aquifer consists of five chemical types based on predominant ions; including calcium-magnesium bicarbonate, calcium-magnesium sulfate, sodium chloride, sodium bicarbonate, and sodium sulfate type waters. Dissolved-solids, chloride, and sulfate concentrations in the aquifer locally exceed standards recommended by the U.S. Environmental Protection Agency for public supplies, particularly in areas southwest of the Minnesota River.
