Preliminary Report on Ground-Water Resources of the Chicago Region, Illinois

The purpose of this study was to make an evaluation of the ground-water resources of the Chicago region on the basis of available data. Such an evaluation is particularly urgent at this time due to the progressively increasing demands for water supplies and the continuing decline of water levels in some aquifers. Ground-water resources in the Chicago region of Illinois are developed from four wateryielding units: 1) glacial drift aquifers; 2) shallow dolomite aquifers; 3) Cambrian-Ordovician Aquifer; and 4) the Mt. Simon Aquifer. The Cambrian-Ordovician Aquifer has been the most highly developed source of large ground-water supplies. I ts estimated yield in 1958 of more than 43 million gallons a day (mgd) approaches the amount that can be withdrawn without dewatering the IrontonGalesville Sandstone, the lowermost and most productive formation in the aquifer. Artesian pressure in the Cambrian-Ordovician Aquifer at Chicago has declined as much as 660 feet since 1864 as a result of pumpage. The glacial drift and shallow dolomite aquifers yielded more than half of the 127.9 mgd of ground water pumped in the region in 1957. This withdrawal resulted in no general decline in nonpumping water levels, indicating that the potential yield is considerably larger than present withdrawal. Future ground-water supplies should be taken from the shallow aquifers wherever possible.