Selected Analytical Methods for Well and Aquifer Evaluation

200,000 gpd (Walker and Walton, 1961). A water supplyequal to the practical sustained yield can be obtained bypumping production wells 1 and 3 at 57 gpm and productionwell 2 at 25 gpm. Partial penetration, well loss, andvariations in hydraulic properties were considered in computationsof drawdowns due to pumping the productionwells themselves.The rate of ground-water withdrawal increased from115,000 gpd in 1957 to 146,000 gpd in 1959. If pumpagecontinues to increase in the future at this rate, it is estimatedthat the practical sustained yield of the existing 3-wellsystem will be exceeded in 1963.The Arcola case history is suggestive of how analyticalmethods can be utilized to evaluate wells and aquifers sothat available ground-water resources can be properlymanaged. By checking computed performance of wells andaquifers with records of past pumpage and water levels, thehydrologist is assured of reasonably accurate solutions.Evaluation of Several Aquifers in IllinoisTo date the principles outlined above have been appliedto aquifer conditions in five areas in Illinois in additionto the Arcola area. The practical sustained yield of wellfields and aquifers in the Chicago region in northeasternIllinois and in the Taylorville, Tallula, Assumption, andPekin areas in central Illinois have been evaluated (Waltonand Walker, 1961). Model aquifers and mathematicalmodels for these areas are illustrated.Chicago RegionThe Cambrian-Ordovician Aquifer is the most highlydeveloped aquifer for large ground-water supplies in theChicago region. The Cambrian-Ordovician Aquifer is encounteredat an average depth of about 500 feet below theland surface at Chicago; it has an average thickness of 1000feet and is composed chiefly of sandstones and dolomites.The Maquoketa Formation consisting largely of shale overliesthe Cambrian-Ordovician Aquifer and confines thewater in the deep aquifer under leaky artesian conditions.The Cambrian-Ordovician Aquifer receives water fromoverlying glacial deposits in areas averaging 47 miles westof Chicago where the Maquoketa Formation is absent.Based on the results of 63 aquifer tests and other studies,the coefficients of transmissibility and storage of the Cambrian-OrdovicianAquifer and the coefficient of verticalpermeability of the Maquoketa Formation are fairly uniformthroughout large areas in northeastern Illinois andaverage 17,000 gpd/ft, 0.00035, and 0.00005 gpd/sq ft,respectively (Suter et al, 1959; and Walton, 1960a). Thecoefficient of transmissibility decreases rapidly south andeast of Chicago.The results of geologic and hydrologic studies indicatethat it is possible to simulate the Cambrian-OrdovicianAquifer with an idealized model aquifer as shown in figure65C. The model aquifer is a semi-infinite rectilinear stripof sandstones and dolomites 84 miles wide and 1000 feetthick. The model aquifer is bounded by a recharge boundary47 miles west of Chicago and by two intersecting barrierboundaries 37 miles east and 60 miles south of Chicago,and is overlain by a confining bed consisting mostly of shaleaveraging 200 feet thick. The mathematical model for theCambrian-Ordovician Aquifer is shown in figure 65D.Pumpage of ground water from the Cambrian-OrdovicianAquifer increased gradually from 200,000 gpd in 1864 to50 mgd in 1959 (Walton et al, 1960). Pumpage is concentratedin six centers as shown in figure 65D, the Chicago,Joliet, Elmhurst, Des Plaines, Aurora, and Elgin areas. As aresult of heavy pumping, artesian pressure in deep sandstonewells declined more than 600 feet at Chicago between 1864and 1958.Studies made with the mathematical model show thatthe practical sustained yield of the Cambrian-OrdovicianAquifer is about 46 mgd (Suter et al, 1959) and is largelylimited by the rate at which water can move eastwardthrough the aquifer from recharge areas. The practicalsustained yield of the aquifer is here defined as the maximumamount of water that can be continuously withdrawnwith the present distribution of pumping centers withouteventually dewatering the most productive and basal wateryieldingformation of the Cambrian-Ordovician Aquifer.The practical sustained yield of the aquifer was exceededin 1959, and in a sense ground-water users in the Chicagoregion started to mine water and to borrow water fromfuture generations.Declines in nonpumping water levels that may be expectedbetween 1958 and 1980 at pumping centers werecomputed by using the mathematical model and assumingthat the distribution of pumpage remains the same as it wasin 1958 and pumpage increases in the future as it has inthe past. Computed declines ranged from 300 feet in theChicago area to 190 feet at Elgin and averaged 250 feet(Suter et al, 1959).Taylorville AreaMunicipal and industrial water supplies at Taylorvilleare obtained from wells in glacial deposits chiefly Illinoianin age that range in thickness from 50 to 180 feet (Walkerand Walton, 1961). The glacial deposits occur in a buriedvalley cut into relatively impermeable bedrock of Pennsylvanianage and are a complex of ice-laid till, water-laid siltsand-and-gravel outwash, and wind-deposited silt and finsand (loess). The bedrock valley trending northeast tosouthwest through Taylorville was at one time in the pastfilled with glacial till. The outwash sand and gravel aquiferwhich yields water in large quantities to wells occurs as afill in a narrow valley cut into the upper part of the tilldeposits. The outwash deposits range in width from one-halfto one mile and range in thickness from a few feet to113 feet.Water occurs in the aquifer under water-table conditions59