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Publication Abstract

Development of an Observation Well Network in the Mahomet Aquifer of East-central Illinois. Burch, Stephen L., 2008  Illinois State Water Survey, Champaign, IL,  ISWS DCS 2008-01    Full Text Available

An observation well network was built in east-central Illinois for the purpose of monitoring groundwater levels in the Mahomet aquifer. The aquifer is an unconsolidated sand deposit that lies in a buried bedrock valley and is as much as 150 feet thick. Groundwater in the aquifer occurs under confined conditions; therefore, groundwater levels rise above the top of the Mahomet aquifer. The top of the sand comprising the Mahomet aquifer occurs at depths of 175- 225 feet below the land surface.

More than 1,300 observations of depth-to-water were collected from 25 2-inch diameter observation wells between 1994 and 1998. As a result, an expectation of the depth-to-water has been established for each well, and seasonal variations have been documented.

The observations were converted to elevations, plotted on a map, and contoured. The result is a potentiometric map for the Mahomet aquifer in east-central Illinois. Interpretation of the map indicates that flow in the aquifer is away from a groundwater divide located near Paxton, Illinois. The highest groundwater elevations in the Mahomet aquifer have been recorded along this divide, which also coincides with the surface water divide between the Mississippi and Ohio River basins. Pumpage in the Champaign-Urbana area has caused groundwater levels (potentiometric heads) to be reduced by almost 50 feet since the 1950s, and the cone of depression has caused groundwater flow to the west of Champaign to be reversed from west to east.

Groundwater samples were collected from the observation wells in 1996. Total Dissolved Solids (TDS) averaged 396 milligrams per liter (mg/L) for much of the Mahomet aquifer, although much higher values are known to exist in Iroquois and Vermilion Counties. Calcium in the Champaign and Ford County samples averaged 66 mg/L, while magnesium and sodium averaged 30 and 31 mg/L, respectively. These values are comparable to those reported by the public water supplies in Rantoul, Champaign, and Mahomet. Bicarbonate, the dominant anion, averaged 381 mg/L in the Mahomet aquifer samples.

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