INSIGHTS INTO GROUNDWATER RECHARGE PROCESSES AND PATHWAYS BASED ON HYDROCHEMICAL AND TRITIUM DATA FROM MUNICIPAL WELL FIELDS IN SHELBY COUNTY, 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 confined aquifers. In the present study of the Memphis aquifer in southwestern Tennessee, water chemistry and tritium are used to assess recharge processes and pathways to municipal well fields in Shelby County, Tennessee.

The hydrochemical composition and tritium activity of ground water in the Memphis aquifer are generally more variable in the upper third of the aquifer than in the lower two thirds of the aquifer. The upper part of the aquifer contains a variety of water compositions ranging from low to moderate TDS (90 – 200 mg/L), Na-SO4-Cl-enriched mixed-cation-HCO3 water to high TDS (> 300 mg/L), Ca-Mg-HCO3 water. Tritium activity of water in the upper part of the Memphis aquifer ranges from 0 to 8.6 TU, with most values falling between 0.2 and 2 TU. The lower part of the Memphis aquifer contains water with more homogenous low to moderate TDS, Ca-Mg-HCO3 water compositions that show a systematic increase in TDS along a flow path from east to west across Shelby County. Tritium activity of water in the lower part of the Memphis aquifer is generally less than 0.2 TU.

In concert with previous information, the results suggest that modern water with a range of chemical characteristics is recharging through windows to the upper part of the Memphis aquifer throughout much of southern, central, and eastern Shelby County, whereas older water in the lower part of the Memphis aquifer represents recharge to the unconfined regions of the aquifer to the east that has flowed westward toward predevelopment discharge locations and the current pumping-induced, regional cone of depression. The primary sources of modern recharge have distinct chemical characteristics, which are modified to varying extents depending on the thickness and lithology of the confining unit through which the waters pass. The distinct chemical characteristics and recharge sources of ground water from the upper and lower parts of the Memphis aquifer are consistent with segregation of regional groundwater flow above and below fine-grained facies (Zilpha Clay equivalent) in the upper part of the Memphis Sand Formation.