The Upper Carbonate aquifer is part of a sequence of sedimentary bedrock units. It was deposited from Paleozoic seas which occupied a depression known as the Hollandale embayment. The aquifer comprises 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. Water flows mostly through fractures and solution channels. Regional groundwater movement is toward the periphery of the aquifer and locally toward river and bedrock valleys. Yields from wells in the aquifer range from about 100 to 500 gallons per minute. Water in the Upper Carbonate aquifer is of acceptable quality for most uses, but sinkholes and disappearing streams locally provide direct passageways for entry of contaminants into the aquifer. Calcium magnesium bicarbonate type water is most common in the aquifer. Concentrations of dissolved solids and some major ions, specifically magnesium, sodium, bicarbonate, and sulfate, are generally highest in the southwest part. (USGS)
Additional publication details
USGS Numbered Series
Hydrogeologic and water-quality characteristics of the Upper Carbonate aquifer, Southeast Minnesota
Water-Resources Investigations Report
13 maps on 2 sheets ; 27 x 26 cm. and smaller, sheets 88 x 102 cm., folded in envelope 31 x 24 cm.