Geophysical Identification Of Karst Fissures Near A Landfill In Southwestern Illinois

Mississippian limestone underlying most of Monroe and St. Clair counties in<br>southwestern Illinois forms a mature karst terrain. This is evident in hundreds of<br>sinkholes, a well-developed epikarst surface, and by the presence of caves and springs in<br>the area. Groundwater probably moves through conduits and fissures in the bedrock, in<br>an extremely complex groundwater flow system.<br>The City of Columbia Municipal Landfill is located in, or on the edge of, this karst<br>area. The landfill, primarily containing construction waste, is being closed, and monitoring<br>wells need to intercept any landfill leachate. Initially the area was investigated by mapping<br>fractures from rock exposures, sinkhole orientations, and lineaments. Major fracture<br>orientations are N6O”E and N40”W. Electrical geophysical methods were then employed<br>to locate subsurface karst features. Electromagnetic (EM) profiling with a 10 meter<br>vertical dipole coil spacing showed conductivity increases of about 5-15 mS/m over<br>possible karst fissures, and these anomalies are oriented at N6O”E or N40”W. Using the<br>horizontal dipole at a ten meter spacing, which “sees” only about half as deep, these<br>anomalies do not show up. This suggests that vertical dipole anomalies are from the<br>epikarst surface, and not from shallow soil structure or waste outside the landfill. An<br>electrical resistivity 2-D cross-section shows an undulating bedrock surface between the<br>low resistivity soil (40-50 Qm) and the high resistivity limestone (2000 Qm) with<br>increased depth to bedrock in the conductive zones mapped with EM methods.