The Sankoty-Mahomet Aquifer in the Confluence Area of the Mackinaw and Mahomet Bedrock Valleys, Central Illinois

Wilson, Steven D.; Kempton, John P.; Lott, R. Brandon

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Description

Title

• The Sankoty-Mahomet Aquifer in the Confluence Area of the Mackinaw and Mahomet Bedrock Valleys, Central Illinois
• A Reassessment of Aquifer Characteristics

Author(s)

• Wilson, Steven D.
• Kempton, John P.
• Lott, R. Brandon

Issue Date

1994

Keyword(s)

• Groundwater -- Central Illinois
• Natural Resources -- Central Illinois

Geographic Coverage

• Tazewell County (IL)
• McLean County (IL)
• Logan County (IL)

Abstract

A study was undertaken to assess the characteristics of the Sankoty-Mahomet aquifer, one of Illinois' largest buried sand-and-gravel aquifers, which formed in the confluence area of the Mahomet and Mackinaw Bedrock Valleys. A test drilling program was completed in 1992 to provide a better understanding of the aquifer system. Sixteen wells were installed to monitor water-level fluctuations in this lowermost basal sand-andgravel aquifer composed predominately of Sankoty-Mahomet Sand. The borehole for each monitoring well was drilled into bedrock to assure penetrating the entire thickness of the aquifer. Water-level elevations were measured in more than 80 private wells during spring 1993, and a seven-day aquifer test was conducted on a municipal well at Hopedale, IL. The drilling results indicate the very complex geologic framework of the system. Data from the latest drilling program indicated differences in bedrock elevations of as much as 75 feet from elevations in earlier bedrock surface maps of the area. Several locations, previously thought to have substantially thick aquifer materials, had thick lacustrine deposits below a relatively thin aquifer. The Sankoty-Mahomet aquifer appears to have been deposited on the often deeply eroded surface of these finer materials. One or more less extensive sand-and-gravel units, interbedded with tills above the aquifer, may be intermittently connected with it. The conceptual model of the confluence area is notably different from earlier interpretations. The potentiometric surface map generated from measurements of private wells indicates that ground water flows both westward toward the Illinois River and northward into the Mackinaw Valley from the confluence area. Water levels rose throughout 1993 due to locally heavy rainfall. Vertical recharge is significant in the study area, as indicated by the aquifer's quick response to the rainfall as well as other research completed recently. In addition, hydrographs from the observation wells were studied to identify regions with similar water-level fluctuations, and locate at least two independent flow paths in the Mahomet portion of the aquifer: one from the Kenney Valley, the other from the main Mahomet Valley in Dewitt County. The flow from the Kenney Valley may be a significant portion of the horizontal flow entering the aquifer just west of the main part of the confluence area. These data indicate that the geology and characteristics of the aquifer system are still not well understood. Additional drilling and aquifer tests are essential to developing a more detailed understanding of the aquifer system and its segments. A separate, ongoing study of a portion of the confluence area is due to be completed in 1996 and should provide more insight about the amount of additional development that the system can maintain. Both the agricultural community and municipal planners view the Sankoty-Mahomet aquifer as the area's most promising water resource for future development.

Publisher

Illinois State Water Survey and the Illinois State Geological Survey

Series/Report Name or Number

Cooperative Resources Report 16

Type of Resource

text

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