GROUNDWATER RECHARGE PROCESSES AND PATHWAYS BASED ON HYDROLOGIC AND TRACER DATA FROM THE MCCORD WELL FIELD, MEMPHIS, TENNESSEE, USA

Water supplies from confined aquifers are typically less influenced by modern recharge than those from unconfined aquifers. Zones of hydraulic connection or “windows” through confining units can provide avenues for modern recharge to impact otherwise confined aquifers. Windows have been known to occur in the confining unit between a shallow aquifer and the deeper Memphis aquifer in Memphis, Tennessee. The McCord municipal well field in Memphis produces water from the semi-confined Memphis aquifer, which is thought to receive part of its recharge locally from leakage through a hydraulic window. The shallow aquifer shows an anomalous depression in the water table over this window in the confining unit.

³H -³He piston-flow ages from production wells in the McCord well field range from 28 to >55 years and ³H loading estimates indicate as much as 35% modern water is present. SF₆ piston-flow ages from the production wells range from 28 to 37 years. SF₆ piston-flow ages from monitoring wells in the shallow aquifer within the area of the anomalous water-table depression range from 14 to 18 years, whereas other nearby monitoring wells have piston-flow ages of 25 to 28 years. Tritium activities in McCord production wells screened in the upper part of the Memphis aquifer decrease away (1.63 TU to <0.05 TU) from the window in the confining unit and away from the anomalous depression in the water table in the shallow aquifer. In addition, Fletcher Creek, which in part flows over the area of the water-table depression and window, is a losing stream in a region where most streams are gaining.

Considered together, the data suggest a strong linkage between stream loss in Fletcher Creek, the loss of groundwater storage in the shallow aquifer and the presence of modern water in production wells in the Memphis aquifer at the McCord well field.

Current research efforts involve quarterly discharge measurements in Fletcher Creek, additional water chemistry and tracer sampling in Memphis and shallow aquifer wells in the well field, continuous monitoring of water levels in Memphis and shallow aquifer wells, and hydrostratigraphic studies in the area of the McCord well field.

These data will be used to better quantify the amount of leakage entering the Memphis aquifer as well as the most probable flow paths.