HYDROGEOLOGY OF A WINDOW THROUGH A CONFINING UNIT ADJACENT TO A LANDFILL: REVIEW AND PRELIMINARY RESULTS

A planned expansion of a landfill in Memphis, Tennessee, 20 years ago led to the identification of a previously undiscovered hydraulic connection, or "window", between a shallow aquifer and the Memphis aquifer, the regional drinking-water aquifer. Past research focused on identifying the extent of the window, migration of landfill leachate into the shallow and Memphis aquifers, and evaluating the flux of water from the shallow aquifer through the window to the Memphis aquifer. Current studies are expanding upon these themes and further defining the hydrostratigraphy of geologic units within and near the window, evaluating hydraulic processes associated with water moving through the window, and assessing the potential for contaminant retardation in the aquifer materials. Data from three stratigraphic test boreholes and four rotasonic borehole well clusters indicate that the window is a result of stratigraphic heterogeneity of the regional Eocene-age confining unit and not a result of post-Eocene erosion and fill. Sedimentary facies of the confining unit represent a collage of delta plain, nearshore, delta front, and prodelta environments. Preliminary hydraulic testing of wells constructed in the rotasonic boreholes indicate hydraulic conductivities ranging from 0.46 to 18 m/day, with the highest value occurring in the shallow aquifer above the window. Environmental tracer and contaminant retardation studies are in progress.