Carpenter, Philip J.
M.S. (Master of Science)
Department of Geology
Mississippian limestone underlies most o f Monroe and St. Clair counties in southwest Illinois and forms a mature karst terrain. This is evident in the hundreds of sinkholes, a well-developed epikarst surface, and by the presence o f caves and springs in the area. Groundwater probably moves primarily through conduits and fissures in the bedrock in a complex groundwater flow system. The City of Columbia Municipal Landfill is located in, or on the edge of, this karst area. The landfill, primarily containing construction waste, is being closed, and monitoring wells need to intercept any landfill leachate. The area around the landfill was initially investigated by mapping fractures from rock exposures, sinkhole orientations, and lineaments. Major fractures trend approximately N60°E and N40°W. Electrical geophysical methods were then employed to locate subsurface fractures that might be good locations for monitoring wells. Electromagnetic (EM) profiling with horizontal loops (vertical dipole) and a 33 ft (10 m) coil spacing recorded conductivity anomalies o f about 5 to 15 mS/m elongated in approximately N60°E or N40°W directions. Using coils oriented vertically (horizontal dipole) with a 33 ft (10 m) spacing (which “ sees” only about half as deep) these northeast- and northwest-trending anomalies do not show up. This suggests the vertical dipole anomalies are from enlarged fractures at, or just below the bedrock surface, and not from shallow soil structure or waste outside the landfill. An electrical resistivity 2-D cross-section shows an undulating bedrock surface between the low resistivity soil (40-50 Qm) and the high resistivity limestone (over 1000 fim) with increased depth to bedrock in the conductive zones mapped with EM methods. These local bedrock lows probably lie above enlarged fractures that would be suitable targets for monitoring wells.
Schneider, James C., "Geophysical identification of karst fissures near a landfill in southwestern Illinois" (1998). Graduate Research Theses & Dissertations. 3284.
ix, 86 pages
Northern Illinois University
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