HYDROSTRATIGRAPHY OF A WINDOW THROUGH THE UPPER CLAIBORNE CONFINING UNIT, MEMPHIS, TENNESSEE

Localized areas of vertical hydraulic communication, termed windows, between aquifers that are separated regionally by a confining unit are important features in influencing recharge and potential contamination of confined aquifers. At Shelby Farms in Memphis, Tennessee, previous research indicates downward migration of water from the shallow, water-table aquifer through a window in the underlying upper Claiborne confining unit to the Memphis aquifer, the primary source of drinking water for the city of Memphis. In this study, data were collected from new and existing boreholes drilled at the Shelby Farms site. Grain-size analyses were performed on approximately 64 samples collected from various depths in several new boreholes at both confined and window locations. Stratigraphic cross-sections through the window area were prepared and intergranular hydraulic conductivity was calculated (using a modified Hazen method) for selected depth intervals at both confined and window locations. Bulk mineralogy, clay mineralogy, and total organic carbon analyses also were performed on selected samples. In 13 boreholes at confined locations, the shallow aquifer, confining unit, and Memphis aquifer are present. At two confined locations, the confining unit consists of two clay-rich facies whose maximum possible intergranular hydraulic conductivity is 5.3 m/day. Within the window area, the shallow aquifer directly overlies the Memphis aquifer, and the mean hydraulic conductivity of cores from both aquifers is 30 m/day, which indicates a medium that is capable of conducting a relatively significant downward flux of water. Stratigraphic cross-sections through the area show that the base of the shallow aquifer is at a similar elevation for all locations. In contrast, the confining unit thins from 28 m to zero at the window. Therefore, the origin of the window at this site is interpreted as attributed more to facies variations in the confining unit and Memphis aquifer than to late Cenozoic fluvial incision. The results indicate how facies variations may influence window geometry and hydraulic characteristics in settings comprising unconsolidated coastal-plain sediments.