Revisiting A 1955 Resistivity Survey Over A Buried Bedrock Valley Aquifer

Electrical earth resistivity surveying has been a part of the groundwater resources program at the Illinois State<br>Geological Survey (ISGS) since 1932. In a project conducted in 1994, we used a computer-controlled resistivity<br>system to collect vertical electrical soundings (VES) over the same area covered by a resistivity profiling survey<br>done in 1955. Similarities between the two surveys provide an opportunity to evaluate changes made in the resistivity<br>method over the past 40 years.<br>Both surveys:<br>. used the Wenner electrode configuration with similar maximum electrode separations and collected<br>data on a grid pattern with approximately ?&mile station spacings;<br>. were part of studies assessing the groundwater potential of the Ticona Buried Bedrock Valley aquifer<br>as a municipal water supply for the town of Streator in central Illinois;<br>. clearly delineated the boundaries of the aquifer; and<br>. helped demonstrate that the aquifer is not adequate to meet the water needs of Streator.<br>The Ticona Bedrock Valley (fig. 1; Willman, 1940) was a Quaternary drainageway ancestral to the modern<br>Illinois River Valley. The Ticona river was probably active during the Illinoian glacial age (about 300,000 to<br>100,000 years ago) when it eroded a valley into Pennsylvanian shale, sandstone, and limestone and possibly into the<br>underlying Ordovician dolomite. Upon the advance of the Woodfordian glaciers about 25,000 years ago, the old<br>valley was completely filled, and the drainage was diverted to a new valley 6 miles to the north. In the old valley,<br>coarse grained glacial outwash and older alluvium now constitute a sand and gravel aquifer 50 to 80 ft thick. The<br>top of the aquifer is about 530 to 550 ft above mean sea level (Randall, 1955). The aquifer is buried beneath 75 to<br>100 ft of younger glacial till, outwash, and lacustrine sediments.<br>The village of Grand Ridge (population 680) draws its water from the aquifer, as do several dozen landowners<br>in this rural area of central Illinois. Currently, about 0.75 million gallons per day (mgd) is withdrawn from the aquifer.<br>The city of Streator (population 14,800) 6 miles south of the buried bedrock valley, has twice evaluated the<br>Ticona Bedrock Valley aquifer as a water source. In 1954-56, Northern Illinois Water Corporation (NIWC), which<br>supplies water to Streator, considered replacing its 6 to 8 mgd surface water supply with groundwater. Presently,<br>NIWC is reviewing several options to control seasonally fluctuating nitrate levels in the surface water supply. One<br>option is blending up to 3 mgd of groundwater with the surface water to adjust the nitrate concentrations. The applicability<br>of the 1955 and 1994 surveys to NIWC was limited by the high cost of transporting the water from the well<br>head to production facilities in Streator.<br>The Ticona Bedrock Valley aquifer is an ideal subject for resistivity surveys because the thick sand and gravel<br>that make up the aquifer contrast sharply in resistivity with the overlying glacial till and underlying shale.