AbstractFuture water‐level declines in deep sandstone wells penetrating the Cambrian‐Ordovician Aquifer, the most highly developed aquifer for large ground‐water supplies in the Chicago region, are considered. The Cambrian‐Ordovician Aquifer is encountered at an average depth of about 500 feet below the land surface at Chicago; it has an average thickness of 1000 feet and is composed chiefly of sandstones and dolomites. Recharge is received from overlying glacial deposits in areas averaging 47 miles west of Chicago and from leakage through a shale confining bed.Geohydrologic conditions are simulated by a model aquifer, i. e., a semi‐infinite rectilinear strip of sandstones and dolomites 84 miles wide and 1000 feet thick. The model aquifer is bounded by a recharge boundary 47 miles west of Chicago and by two intersecting barrier boundaries 37 miles east and 60 miles south of Chicago, and is overlain by a confining bed consisting mostly of shale averaging 200 feet thick. The hydraulic properties of the model aquifer and its confining bed, the image‐well theory, and appropriate ground‐water formulas are used to construct a mathematical model which provides a means of evaluating the practical sustained yield of the aquifer and predicting future water‐level declines. Records of past pumpage and water levels establish the validity of this mechanism as a model of the response of the aquifer to heavy pumping.Pumpage from deep sandstone wells concentrated in six pumping centers has increased from 200,000 gallons per day (gpd) in 1864 to 96.5 millions of gallons per day (mgd) in 1961. As a result of heavy pumping, water levels in deep sandstone wells declined more than 650 feet at Chicago between 1864 and 1961.The maximum amount of water that can be continually withdrawn from existing pumping centers without creating critical water levels or exceeding recharge is estimated to be about 46 mgd. Withdrawals from the Cambrian‐Ordovician Aquifer have exceeded the practical sustained yield since 1959. It is estimated that about 65 mgd could be obtained by shifting one existing pumping center toward the west and by adding 2 new pumping centers north and northwest of Chicago.Unless lake water is made available to those areas with short supply a pumpage increase from 96.5 mgd in 1961 to 243 mgd in 2010 can be expected. Using this pumpage increase and taking into consideration dewatering of portions of upper units of the aquifer, declines in nonpumping water levels that may be expected between 1963 and 2010 at existing pumping centers were computed by using the mathematical model.Pumping water levels in most pumping centers will be at critical stages a few feet above the top of the lowermost and most productive unit of th