RECHARGE TO THE MEMPHIS AQUIFER THROUGH A WINDOW IN THE UPPER CLAIBORNE CONFINING UNIT NEAR A CLOSED MUNICIPAL LANDFILL, SHELBY COUNTY, TENNESSEE, USA

Water supplies from confined aquifers are typically less vulnerable to contaminated recharge than are supplies from unconfined aquifers. Zones of hydraulic connection or “windows” through confining units can provide avenues for modern recharge to impact confined aquifers. In this study, movement of landfill leachate through a window in the upper Claiborne confining unit to the Memphis aquifer was investigated to better understand leachate migration processes and recharge mechanisms. Landfill leachate infiltrating from a closed landfill has locally contaminated water within the shallow aquifer, confining unit, and Memphis aquifer. The hydrochemical composition of background water in the vicinity of the landfill is a dilute, mixed-cation, bicarbonate water. Leachate influenced waters have higher chloride, fluoride, and major and trace metal concentrations than background waters. The leachate-influenced waters also show variable sulfate/sulfide, nitrate/ammonium, iron, and manganese concentrations that are consistent with a descending plume of chemically reduced groundwater. Mixing models using Ba and Sr suggest as much as a 60% leachate contribution to recharge waters descending through one or more windows in the confining unit to the Memphis aquifer. However, waters within wells in the Memphis aquifer as much as 1 km downgradient from the window have leachate contributions of 20% or less; the decrease is presumably a result of hydrodynamic dispersion, but possibly complemented by sorption or precipitation of Ba, Sr, and other metals within the aquifer. Tritium-3He data and comparison to reconstructed tritium loadings indicate increasing age of water descending through the window and the mixing of older water with the modern, leachate-influenced water along flow paths in the upper part of the Memphis aquifer. Mixing of the two waters in the upper part of the Memphis aquifer appears to be more prominent in the fall than the spring, presumably due to increased recharge of young water to the Memphis aquifer during the spring. The results demonstrate flow of leachate-influenced waters within the shallow aquifer toward one or more windows in the confining unit. The leachate-influenced waters descend through the windows largely by plug flow, but readily mix and disperse as a descending plume within the Memphis aquifer.