Geologic Control of Water Quality in the Michigan Basin

摘要

The lower peninsula of Michigan consists of bedrock that forms a structural basin that is overlain by glacial Pleistocene age deposits of variable thickness. The overlying glacial deposits cause the present ground water flow direction in the bedrock aquifers. The ground water flow direction through the bedrock imparts a ground water quality that is reflective of the composition of the bedrock. Ground water flows in bedrock aquifers from upland areas in the northern and southern parts of the basin to lowland areas that discharge into Lakes Michigan, Huron and Erie. Areas of thick clay deposits in the lowland areas impart differences in quality of ground water by retarding ground water movement in discharge areas. The major freshwater aquifers in the Michigan basin are the Marshall sandstone of Mississippian age, the Grand River-Saginaw formation of Pennsylvanian age and the glacial deposits. Analysis of 290 ground water samples from bedrock aquifers and 180 samples from the glacial aquifer collected across the basin were used to identify ground water quality changes along flow paths. Major ion chemistry of ground water in recharge areas approximates a calciumbicarbonate type, and discharging ground water is a sodium chloride type. Ground water in areas where the two water types mix is a calcium sulfate to sodium sulfate type. The ground water is saline to brine in the deeper parts of the basin and in areas defined as regional discharge areas. In addition to major ion chemistry, isotopic analysis of carbon 14/13 and del deuterium/oxygen-18 were used to define ground water recharge, discharge and flow paths.