USING ELECTRICAL RESISTIVITY TO REDUCE LANDFILL SITING RISKS IN SOUTH CENTRAL KENTUCKY KARST AREAS

Electrical Resistivity (ER) can play a key role as a tool to aid characterization of karst geohazard risks at proposed landfill sites. In Hart County, proximal to the famous south central Kentucky karst and Mammoth Cave National Park, siliciclastic caprock upland areas pose groundwater contamination risks due to immediately adjacent ravine or valley areas commonly floored by carbonate rocks. Furthermore, the complex stratigraphy associated with the Mississippian-Pennsylvanian Unconformity (Absaroka-Kaskaskia Sequence Boundary) of the landfill site presents challenges for demarcating requisite hydrogeological confining lithologic units typically required at the base of landfill cells. The site is capped by the Pennsylvanian-aged Caseyville Sandstone, a conglomeratic sandstone with minor coal and shale/mudstone units which unconformably overlie Mississippian (Chester Series) rocks of variable lithology ranging from sandstone/shale to limestone.

Our conceptual site model for the landfill site integrated ER surveying and field geology that both satisfactorily characterize the site and risks to groundwater resources in the area without the need for multiple costly borings. We conducted two ER profiles that traversed the site in generally northeast-southwest and northwest-southeast directions and utilized outcrops, an excavated pit and trench. ER data range from several low 10s to about 400 ohm-meter values for mudstone, claystone and shale units on the site whereas quartz-dominated sandstone or conglomeratic units or derived sandy regolith are consistent with ER values from about 500 ohm meters to several 10 tens of thousands of ohm meters. The greatest ohm-meter values are presumably in unweathered Caseyville Sandstone and exceed 32,000 ohm meters. The overall contrast between quartz-rich and clay poor strata (high-resistivity rocks) and clay-rich strata (low-resistivity rocks) provides independent data consistent with the observed stratigraphy in site exposures. The defined stratigraphy coupled with the understanding of regional bedrock dip that is opposite of the direction of karst features adjacent to the landfill site provides additional lowering of risks at this studied location.