RESPONSE OF ELECTRICAL RESISTIVITY AND INDUCED POLARIZATION AT AN UNLINED LANDFILL IN GULF COASTAL PLAIN SEDIMENTS, MEMPHIS, TENNESSEE

Landfills pose a long-term threat to underlying groundwater systems. Assessment and monitoring of landfill-leachate impacts on groundwater can be costly, especially when evaluating during the federally required 30-year post-closure care period. As the former Shelby County Landfill in Memphis, Tennessee nears the end of its post-closure care period, we examine the usefulness of electrical resistivity (ER) and induced polarization (IP) to identify the extent of leachate-impacted groundwater at the site and preferential pathways of leachate migration in the underlying Gulf Coastal Plain sediments. The ER and IP methods proved useful in identifying the water table boundary, landfill cell, clay-rich sediments, and potential subsurface impacts from a sewer line. Resistivity data from surface ER correlated well to specific conductance of water from well sampling. Based on this, ER was used to differentiate leachate-impacted and non-impacted groundwater. Agreement between groundwater quality and geophysical data shows leachate-impacted groundwater enters the underlying shallow groundwater system along the northern landfill boundary and migrates downward to the Memphis aquifer, the regional drinking water source. The results confirm that the landfill serves as a continuous source of leachate and the depth of leachate impact in the Memphis aquifer is deeper than currently sampled in the post-closure monitoring wells at the site. Limited external controls hamper the ability to constrain some of the ER and IP interpretation, particularly in areas where ER data are more complex, and these areas are targets for future investigations.