Because of its responsibility for allocating the Lake Michigan water which Illinois is permitted to divert from the Great Lakes Basin, the Illinois Division of Water Resources must consider alternative sources of water available to the Chicago metropolitan region. In early 1979, the Division contracted with the Illinois State Water Survey to study the shallow dolomite aquifer in DuPage County, an important alternative source, to verify its potential yield. The plan of the investigation was to analyze the effects of continually increasing pumpage, and to identify areas of existing and potential problems and areas where additional pumpage can be developed. In the summer of 1979, as part of this study, a detailed data collection program in the shallow dolomite was conducted over a 700-square-mile area, shown in figure 1. The area centered on DuPage County but also included parts of west and northwest Cook County, east Kane County, northeast Kendall County, and north Will County. It was bounded on the west by the Fox River and on the east and southeast by the Des Plaines River. Data obtained included pumpage from major water users and more than 1200 water level measurements. In addition, 282 water samples were collected for subsequent analysis.
The buried Mahomet Bedrock Valley and its major tributaries cover an area of about 3700 square miles in east-central Illinois. Large supplies of groundwater chiefly for municipal use are withdrawn from wells in permeable sands and gravels in thick deposits of glacial drift in the area. The glacial drift exceeds 400 feet in thickness in places. The largest source of groundwater consists of the sands and gravels of the Kansan deposits, called the deep aquifer, which occupy the deepest portions of the Mahomet Valley channel. Intercalated in the glacial drift above the Kansan deposits are sands and gravels of the Illinoian deposits, called the middle aquifer. The middle aquifer is a secondary source of groundwater. The coefficients of permeability and storage for the middle aquifer range from 230 to 4080 gallons per day per square foot (gpd/sq ft) and from 0.00001 to 0.083, respectively. They range from 310 to 4100 gpd/sq ft and from 0.000022 to 0.0023, respectively, for the deep aquifer. The coefficients of the vertical permeability of the confining beds above the middle and deep aquifers range from 0.0026 to 0.04 gpd/sq ft and 0.005 to 0.42 gpd/sq ft, respectively. Pumpage from wells increased from 8.5 million gallons per day (mgd) in 1890 to 46.3 mgd in 1960 and was 40.2 mgd in 1965. Of the 1965 total pumpage, 64.2 percent was for municipal supplies, 19.1 percent was for rural uses, and 16.7 percent was for industrial use. Wells in the deep aquifer accounted for 49.3 percent of the 1965 total; wells in the middle aquifer, 31.8 percent; wells in shallow unconsolidated deposits, 17.4 percent; and wells in bedrock aquifers, 1.5 percent. Major pumping centers with pumpage exceeding 1 mgd are located at Champaign-Urbana, Rantoul, Lincoln, Taylorville, and Hoopeston. As a result of heavy pumpage, water levels in the middle aquifer at Champaign-Urbana declined as much as 100 feet between 1885 and 1947. Subsequent shifting of pumpage to the deep aquifer west of Champaign resulted in water levels in the middle aquifer recovering from 30 to 55 feet. Because of increased withdrawals, water levels in the deep aquifer declined some 35 feet during 1948- 1963. Recovery of water levels in 1964 and 1965 resulted from a decline in pumpage. Similar though smaller water-level declines have occurred in many of the other pumping centers in the Mahomet Valley area. Recharge to buried aquifers in the Mahomet Valley occurs chiefly as leakage of water from a source bed in the shallow deposits across a confining layer. Potential recharge to these aquifers, considering only available head losses across the confining layers, is great. Computations for the Illinoian aquifer at Champaign-Urbana indicate a recharge rate of 115,000 gpd/sq mi in 1947. Similar computations for the Kansan aquifer west of Champaign during the period 1953 through 1965 indicated an average recharge rate of 107,000 gpd/sq mi. Total groundwater runoff for the valley is estimated to be about 740 mgd during years of normal precipitation. It is not unreasonable to assume that existing and/or future pumping centers could capture 60 percent of groundwater runoff, or 445 mgd. An electric analog computer consisting of an analog model and associated electronic equipment was constructed for the middle and deep aquifers and their confining and source beds in the vicinity of Champaign-Urbana to aid in studying the effects of groundwater pumpage on water levels in the Mahomet Valley. The accuracy of the computer was established by a study of records of past pumpage and water levels in three observation wells. The analog computer was used to determine pumping levels with a selected scheme of pumping from existing and future large capacity wells in the Kansan aquifer west of Champaign. Withdrawals with the selected pumping scheme would total 30.3 mgd from existing large capacity wells and 15 mgd from five future wells; pumping levels would be above the top of the Kansan aquifer.