APPLICATION OF GROUNDWATER TRACERS TOWARD SUSTAINABILITY ANALYSIS OF GROUNDWATER IN MEMPHIS, TENNESSEE

Sustainability of high-quality groundwater resources for large urban centers requires understanding changes in recharge, discharge, and aquifer storage as well as pumpage from aquifer systems. Evaluating recharge in unconfined to partially confined aquifer systems is particularly challenging, especially where numerous hydraulically conductive zones or “windows” through confining units provide avenues for modern recharge. Fifteen years of tracer data, primarily from tritium and ³H -³He analysis, from production and monitoring wells in the Memphis aquifer are used to evaluate localized recharge in the partially confined portion of the aquifer beneath Memphis, Tennessee. Groundwater exploitation over the past 130 years in the Memphis area has resulted in regional depression of the potentiometric surface and significant downward gradient between the shallow and Memphis aquifers. Hydraulic windows to the Memphis aquifer are identified by anomalous depressions in water levels in the shallow aquifer as well as the presence of sandy sedimentary facies in the intervening confining unit. Flux through the confining unit is evaluated by ³H-loading and inverse geochemical modeling. The percentage of modern recharge or shallow aquifer water contributing to water pumped from production wells in the Memphis aquifer varies from <5 to75%, and increases with proximity to windows and surface water sources. In several cases, the quantity of modern recharge pumped from the Memphis aquifer can be partially equated to losses in discharge of nearby streams. Consideration of the quantity and quality of water recharging the Memphis aquifer in the urban area is critical for sustainability of regional groundwater resources.