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During the past few decades, several municipalities in northeastern Illinois have noted increases in the salinity of water from wells that tap aquifers in rocks of Cambrian and Ordovician age. The municipalities have discontinued the use of, or sealed-off sections of, those wells. The aquifers involved include the Ancell, the Ironton-Galesville, and the Elmhurst-Mt. Simon. To define the location, magnitude, and possible causes for the salinity increases' in the six northeastern counties of Illinois, 17 municipal wells and 1 deep test well were selected on the basis of their proximity to major pumping centers, the availability of water-quality data, and their documented maintenance history. Well depths ranged from about 960 to 3,475 feet. One well was finished in the middle confining unit, 2 wells were finished in the Ironton-Galesville aquifer, 4 wells were finished in the Eau Claire confining unit, and 10 wells were finished in the Elmhurst-Mt. Simon aquifer. The deep test well was finished below the Elmhurst-Mt. Simon aquifer in Precambrianage rock. Chloride concentrations in the municipal wells ranged from less than 5 to greater than 600 milligrams per liter; in the deep test well, they ranged from 13 to 37,000 milligrams per liter. Some changes in the chloride concentration in water from the studied municipal wells can be related to physical changes to the wells, including the partial filling in of. a well, bridging within a well, the cleaning out of a well, or the deepening of a well. Some changes in chloride concentration are not related to physical changes but may be caused by increased pumpage; changes in pumping rate, frequency, or duration; cessation of pumping; improper abandonment of wells; and the upconing of highly mineralized water. The data base was inadequate for a quantitative study of the changes in chloride concentration in water from individual aquifers in rocks of Cambrian and Ordovician age. INTRODUCTION In the late 1970's, the U.S. Geological Survey (USGS) initiated the Regional Aquifer-System Analysis (RASA) to study regional aquifer systems throughout the United States. The general goals of a RASA investigation are to evaluate each aquifer's water-supply potential and water quality, and, using computer models of the ground-water flow system, to provide a means for evaluating aquifer response to stresses placed on the flow system. The Northern Midwest RASA, which began in October 1978, studied the aquifers in the Cambrian and Ordovician Systems that lie beneath almost all of Iowa and parts of Minnesota, Wisconsin, Indiana, Illinois, and Missouri (fig. 1). A part of the Northern Midwest RASA study, as summarized by Steinhilber and Young (1979), pertains to the regional assessment of water quality and includes a study of the saline water to define its characteristics, occurrence, potential movement to fresh-water zones, and historical changes in the water quality as a result of aquifer development. In northern Illinois, the aquifers in rocks of Cambrian and Ordovician age (hereafter termed "Cambrian and Ordovician Systems") are a major source of water for municipal, industrial, and agricultural use. Demands are heaviest in the Chicago area where development of the aquifers began in 1864. Continued development of the aquifers has caused the potentiometric surface to decline several hundred feet and has enlarged the cone of depression to such a degree that it extends into southeastern Wisconsin. Demands on the aquifers are projected to increase, resulting in further declines in the potentiometric surface, During the past few decades, several municipalities in northeastern Illinois have noted increases in the chloride concentration in water from wells that tap the aquifers in the Cambrian and Ordovician Systems [concentrations at some wells exceeded 250 mg/L (milligrams per liter), the State limit for chloride in public drinking-water supplies (Illinois Pollution Control Board, 1984a, 1984b)]. As a result, some municipalities have discontinued the use of, or sealed-off sections of, those wells. In 1985, the USGS began a study using only readily available data to examine the occurrence and possible causes of changes in chloride concentration in water from wells that tap the aquifers in the Cambrian and Ordovician Systems in northeastern Illinois. Available data include information on well location, well construction and maintenance, water levels, and water quality. Data were available from State and Federal reports and data bases, including those of the Illinois State Water Survey (ISWS), the Illinois State Geological Survey (ISGS), and the USGS. Purpose and Scope This report describes the location, magnitude, and causes of the changes in chloride concentration in the aquifers in the Cambrian and Ordovician Systems in a six-county area of northeastern Illinois (fig. 1). The report includes stratigraphic columns, maps, and graphs that show the geology and hydrogeology of the study area and the changes in chloride concentration in I s n e A L___xT__^r.__^_

Changes in chloride concentration in water from municipal wells that tap aquifers in rocks of Cambrian and Ordovician age in northeastern Illinois, 1915–84