Evaluation and Evolution of a Groundwater Contaminant Plume at the Former Shelby County Landfill, Memphis, Tennessee

The unlined former Shelby County Landfill in Memphis, Tennessee, lies in the flood plain of the Wolf River and is known to be the source of low-level contamination in the underlying alluvial (shallow) and Memphis aquifers. Prior to closure, discovery of a hydrogeologic window in the upper Claiborne confining unit overlying the Memphis aquifer 0.2 km north of the landfill led to several groundwater investigations by the U.S. Geological Survey to evaluate the threat posed to the Memphis aquifer, the regional source for municipal water supplies. As the landfill nears its 30th year of post-closure monitoring, this study examines the evolution of groundwater quality to determine if conditions have improved or degraded with time as well as determine the usefulness of surface resistivity and induced polarization (IP) to identify preferential pathways of migration in subsurface sediments of the Gulf Coastal Plain. A 2011 comprehensive groundwater sampling event re-established the extent of impacted groundwater which were compared to historical groundwater quality data and used to plan a subsequent resistivity survey in 2012. Leachate impacted groundwater had not diminished along the northern landfill boundary in the shallow aquifer and continues to enter the Memphis aquifer immediately north of the landfill further degrading water quality at depth. The current post-closure sampling regime at the former Shelby County Landfill is inadequate to properly address the water quality problems at the landfill and should be reassessed. Resistivity and IP demonstrated the ability to identify impacted versus non-impacted groundwater, as well as the water-table boundary, landfill waste cells, sewer lines, and clay-rich sediments. Future research should continue to focus on areas north of the landfill to determine the contact relationship between the shallow and Memphis aquifers and further downgradient in the Memphis aquifer to determine the true extent of contamination from the former Shelby County Landfill