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ISWS Data/Case Study 2004-01PDF


Title:Groundwater Conditions of the Principal Aquifers of Lee, Whiteside, Bureau, and Henry Counties, Illinois
Author(s):Burch, Stephen L.
Subject(s):Groundwater conditions
Principal aquifers
Abstract:A large supply of groundwater occurs in a buried aquifer lying in the bedrock valley of the ancestral Mississippi River. This deposit, known as the Sankoty sand, supplies many irrigation wells and underlies more than 750 square miles (sq mi). A shallower and less extensive aquifer, the Tampico, occurs near the surface and underlies more than 480 sq mi. This study defines the regional groundwater flow patterns for these two aquifers in northwestern Illinois and reports the results of measuring groundwater levels in observation wells. The Tampico aquifer is separated from the underlying Sankoty aquifer by an intervening layer of clay or clays. Groundwater within the upper unit exists under unconfined conditions (that is, at atmospheric pressure). The saturated sands comprising the Tampico aquifer are typically 30 to 40 feet thick and are tapped by shallow wells or sandpoints. The Sankoty sand is 100 to 150 feet thick and is commonly used in irrigation wells in Illinois. Groundwater within this unit is pressurized and occurs under confined conditions. The pressure head in the aquifer declines from an elevation of about 670 feet near the town of Ohio to less than 570 feet near Albany along the Mississippi River. A steeper gradient occurs as groundwater flows toward a second outlet near Hennepin. As a result, groundwater elevations decline to levels below 450 feet where the aquifer discharges to the Illinois River. Pumpage during the summer months, largely from irrigation wells causes groundwater levels in the Sankoty aquifer to decline 11 to 13 feet. The area of greatest drawdown extends from Tampico to Walnut, Illinois. Groundwater levels in the Tampico aquifer do not decline as much. A decline of 3 to 3.5 feet is common in the aquifer’s water table. Irrigation wells annually withdraw an estimated 21,000 acre-feet of groundwater. Although the Sankoty aquifer is favored for irrigation, the actual distribution percentage for each aquifer is unknown. No significant, regional water-quality problems were detected in samples collected from either aquifer. The groundwater in both aquifers is of a calcium-bicarbonate type. The water is very hard, with an average value of 306 milligrams per liter (mg/L) in the Sankoty aquifer and 329 mg/L in the overlying Tampico aquifer. The quality of samples from the Sankoty aquifer was excellent, although they contained more iron and are more alkaline than samples from the Tampico aquifer. No discernible patterns were observed in the distribution of total dissolved solids (TDS) values for either aquifer. The average TDS value for water samples was 435 mg/L (Tampico aquifer) and 363 mg/L (Sankoty aquifer). Groundwater in the Tampico aquifer was usually of excellent quality, but it sometimes contained nitrates.
Issue Date:2004
Series/Report:ISWS Data/Case Study DCS 2004-01
Genre:Technical Report
Date Available in IDEALS:2015-04-17

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