The general geologic framework and groundwater situation throughout the Great Lakes Basin, major problems of quantity and chemical quality of groundwater, and factors to be considered in the conjunctive and beneficial use of the basin's entire water resources are described. Emphasis is on major aquifer systems because domestic-type supplies are available almost everywhere. The Great Lakes Basin is underlain almost entirely by a thick succession of sedimentary rocks. Glacial and alluvial deposits cover the bedrock. These deposits are as much as 1,100 feet thick, with the thickest deposits generally occurring in Michigan and locally in buried bedrock velleys of New York and Wisconsin. Groundwater is present everywhere throughout the basin, but in limited quantities in areas where the basement rock is at or near the surface. The most productive aquifers, with well yields as much as 2,500 gpm, occur in unconsolidated, well-sorted sand and gravel deposits, especially where natural recharge from streams or precipitation can occur readily. Unconsolidated aquifers are most widespread in western and central Michigan, northeastern Indiana, and the western part of Wisconsin. Carbonate (limestone and dolomite) aquifers constitute the most common bedrock aquifers in the basin. They occur along the northern and western shore of Lake Michigan, from Illinois to Cleveland, and along the southern shore of Lake Ontario. A thick sequence of productive sandstone units is present along the western and northern part of the Lake Michigan Basin. Chemical quality of groundwater is generally good, although saline, mineralized, or brackish water is present in deep bedrock throughout the basin, and at shallow depths in a few places. (Knapp-USGS)
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