RECHARGE TO THE MEMPHIS AQUIFER IN SOUTHWESTERN TENNESSEE, AN EXAMPLE OF A SAND-DOMINATED COASTAL PLAIN AQUIFER IN A HUMID REGION

The Memphis aquifer is the most important source of groundwater in the state of Tennessee. Although the rate of production of water from this aquifer is monitored, the rates and distribution of recharge to the Memphis aquifer are poorly constrained. Previous studies indicate that recharge takes places primarily in the outcrop and subcrop region of the Eocene Memphis Sand, an 800-ft thick Coastal plain sand unit in the northern Mississippi Embayment. Studies of recharge processes in the outcrop belt in western Tennessee indicate that infiltration follows a complicated path prior to recharging the aquifer and the travel time from the upland surfaces to the water table could be as much as 100 years. In our current studies, ground water levels, stream flow, and climatic conditions are being monitored in an upland watershed to test hypotheses regarding heterogeneous recharge pathways and rates to the Memphis aquifer. In addition, the chemistry of precipitation, soil water, and both surface and subsurface water, are being used to track the geochemical evolution of water through the infiltration and recharge process.

Stratigraphic analysis, tensiometer measurements, and lysimeter soil-water yield data suggesting that recharge is fastest and most direct along the hillslopes and upland gully systems. Upland surfaces are mantled by loess and one or more paleosols, retarding vertical recharge, whereas the hillslopes and upland gully systems have sandy sediment allowing infiltration. Preliminary results show a seasonal shift in water levels and a profound response to large rain events. Present efforts focus on using geochemistry and SF₆ to trace potential recharge from the upper soil horizons to the water table, both spatially and temporally. Monitoring of precipitation, evapo-transpiration, run-off, and soil storage capacity will be used to obtain a water balance-based estimate of recharge in the watershed.