Hydrogeologic and water-quality characteristics of the Upper Carbonate aquifer, Southeast Minnesota

The Upper Carbonate aquifer is part of a sequence of sedimentary bedrock units deposited in Paleozoic seas that occupied a depression known as the Hollandale embayment. The aquifer is comprised of four formations, which, in ascending order, are the Galena Dolomite, Dubuque Formation, Maquoketa Shale, and Cedar Valley Limestone. Total thickness of the aquifer is as much as 650 feet. Yields from wells in the aquifer range from about 100 to 500 gallons per minute. Water flows mostly through fractures and solution channels toward the periphery of the aquifer and, locally, toward river and bedrock valleys.

The quality of water in the Upper Carbonate aquifer is suitable for most uses. However, the water is susceptible to contamination in karst areas because sinkholes and disappearing streams provide direct passageways for entry of contaminants into the aquifer. Calcium magnesium bicarbonate type waters are most common. Concentrations of dissolved solids and some major ions, specifically magnesium, sodium, bicarbonate, and sulfate, are generally highest in the southwestern part of the study area.