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Publication Abstract

Geology, Hydrology, and Water Quality of the Cambrian and Ordovician Systems in Northern Illinois Visocky, Adrian P., Marvin G. Sherrill, and Keros Cartwright, 1985  Illinois State Water Survey, Champaign, IL,  ISWS COOP-10    Full Text Available

Cambrian and Ordovician strata provide much of the groundwater supply for approximately 250 municipalities and 150 industries in the northern half of Illinois. This report represents the cooperative effort of the Illinois State Water Survey, Illinois State Geological Survey, and U.S. Geological Survey to provide a current hydrogeologic evaluation of this water resource.

The Cambrian and Ordovician aquifers average approximately 1000 feet in thickness. Although numerous alternating layers of sandstones, limestone, and dolomites impart a heterogeneous character to them, these units are hydraulically interconnected and behave as a single aquifer. Hydraulic properties within the aquifers are generally affected by local or regional changes in thickness of the Ancell and Ironton-Galesville aquifers. Recharge occurs principally by vertical percolation of precipitation in areas where the Galena-Platteville Unit is the uppermost bedrock. Additional recharge in heavily pumped areas occurs through leakage across the Maquoketa Confining Unit.

This report introduces formal hydrostratigraphic names in describing major aquifers as divided into three aquisystems, each of which is subdivided into aquigroups. The three aquisystems and their aquigroups are: 1) Non- Indurated Rock Aquisystem, consisting of the Prairie Aquigroup - local and intermediate flow systems in alluvium, glacial drift, and Cretaceous and Tertiary sediments; 2) Indurated Rock Aquisystem, divided into a) Upper Bedrock Aquigroup - local and intermediate flow systems with connection to the Prairie Aquigroup, b) Mississippi Valley Bedrock Aquigroup - intermediate and regional flow systems in indurated rock that are always confined by indurated rock aquitards and whose base is the top of the Maquoketa Shale Group, c) Midwest Bedrock Aquigroup - intermediate and regional flow systts and whose bottom is at the top of the Eau Claire Formation or stratigraphicallyems whose top is the top of the Maquoketa Shale Group or other confining uni higher, and d) Basal Bedrock Aquigroup - intermediate and regional flow systems below the shale units of the Eau Claire Formation and above the crystalline basement rocks; and 3) Crystalline Rock Aquisystem, in which there are no significant aquifers in Illinois. The Midwest Bedrock Aquigroup is basically the same as the "Cambrian-Ordovician Aquifer" in northeastern Illinois that has been previously described. It is the chief topic of concern in this report.

The practical sustained yield of the Midwest Bedrock Aquigroup was estimated to be 65 mgd (million gallons per day) in northeastern Illinois. Pumpage has exceeded this amount since about 1958. In north-central and northwestern Illinois, development has not exceeded the practical sustained yield, which is probably at least as great as that for northeastern Illinois. The practical sustained yield in western Illinois is limited by the amount of lateral inflow that can be induced and also by poor water quality.

Individual wells in the Midwest Bedrock Aquigroup in northern Illinois usually exceed 500 gpm (gallons per minute) in yield, but in western Illinois typical well yields are smaller. Total withdrawals from deep wells between 1971 and 1980 averaged 278.6 mgd. The largest withdrawals occur in Cook, DuPage, Kane, and Will Counties. Withdrawals have caused water levels to decline as much as 900 feet in parts of Cook, DuPage, and Will Counties. Dewatering of the upper aquifers in the Midwest Bedrock Aquigroup has begun in portions of the above four counties.

Groundwater in the "unconfined" area of the Midwest Bedrock Aquigroup is chemically homogeneous and low in total dissolved solids. However, significant vertical and areal changes in the chemical character of groundwater occur in the confined area. As water moves away from the recharge area to the east and south, it increases in total dissolved solids.

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