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Publication Abstract

The New Chicago Model: a Reassessment of the Impacts of Lake Michigan Allocations on the Cambrian-Ordovician Aquifer System in Northeastern Illinois Burch, Stephen L., 1991  Illinois State Water Survey, Champaign, IL,  ISWS RR-119    Full Text Available
This study reports the effects of substituting water from Lake Michigan for ground-water withdrawals in northeastern Illinois. It describes the use of a digital computer model to predict future ground-water levels based on anticipated pumping schedules. The model focuses primarily on the "Chicago region," which consists of Cook, DuPage, Kane, Kendall, Lake, McHenry, and a portion of Will and Grundy Counties.

The effort made during this project departed so far from the original Chicago model, that the code used in this report is referred to as the "New Chicago Model." The source code for the new model was translated from Fortran to QuickC, although most of the variable names used by Prickett and Lonnquist (1971) were preserved, particularly in calculations of head, storage, and recharge at each node. The new version was developed and tested in the era of the Intel 80286 processor, and several runs were made on the faster 80386-based machines.

Six geologic surfaces were used in the New Chicago Model to define the five-layer Cambrian-Ordovician aquifer system. Each layer varies in hydraulic conductivity and thickness, and therefore in its ability to transmit water. The transmissivities of each layer, when summed at each node, describe the aquifer system in greater detail than has been done previously. Distinctive stratigraphic controls exerted by the Prairie du Chien Group in Illinois and the Mt. Simon in Wisconsin have been included to help incorporate regional differences into the New Chicago Model.

The pumpage data set contains information on 1,150 individual wells. A distance-weighting program was developed to distribute a proportional amount of an individual well's historical pumpage to each of the surrounding four comers of the model grid. Demand forecasts were developed on the basis of trends at each facility utilizing the Cambrian-Ordovician aquifer. Future well locations were determined simply by averaging the Lambert coordinates for each well at each of the 289 Illinois facilities.

The model predicts that Chicago's regional pumping cone will first become shallower without becoming significantly smaller in areal extent. Ground-water levels will rise throughout much of northeastern Illinois between 1985 and 1990, particularly in Cook County, since it was the first to switch to Lake Michigan water. The model predicts that by 2010, water levels will rise in some places by 350 feet or more throughout DuPage and much of western Cook Counties, and by almost 650 feet around Elmhurst. Water levels will rise by 50 feet or more as far away as Belvidere, DeKalb, Morris, and Kankakee. The actions taken in Illinois will even cause water levels to rise in southeastern Wisconsin and northwestern Indiana.



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