U.S. Geological Survey ground-water studies in Illinois

Ground water is an important source of water supply in Illinois. The largest amount of ground*water withdrawal is in the northern one-third of the State where aquifers to a depth of about 1,500 feet below land surface contain large quantities of potable water. Approximately 74 percent of the public water-supply systems in Illinois use ground water to supply potable water to more than 5.5 million people. Ground-water withdrawals account for almost 25 percent of the total water withdrawn for public water supplies in Illinois. Many public water-supply systems in the Chicago area have recently changed from using ground water pumped from wells to using water delivered from Lake Michigan. The major issues related to ground water in Illinois are: Water- quality degradation or contamination from point and nonpoint sources, and Water availability, because of the lowering of ground-water levels in the bedrock aquifers in northeastern Illinois and elsewhere in the State where pumpage has exceeded aquifer recharge and the susceptibility of the limited surface-water supplies in central and southern Illinois to drought.