EVALUATING MODERN RECHARGE TO THE MEMPHIS AQUIFER AT THE LICHTERMAN WELL FIELD, MEMPHIS, TN

The Lichterman well field is a municipal water plant in south-central Shelby County, Tennessee, that is vulnerable to vertical seepage of modern (<60 years) groundwater into the underlying semiconfined Memphis aquifer. Regional recharge of the Memphis aquifer originates to the east of Shelby County and these waters are hundreds to thousands of years in age. Near the Lichterman well field, local recharge of modern water to the shallow aquifer occurs through infiltration and loss of Nonconnah Creek stream water into the shallow aquifer. Once in the shallow aquifer, modern water is suspected of leaking through hydrologic windows in the upper Claiborne confining unit to the Memphis aquifer, which is unconfined at the Lichterman well field. In order to identify likely recharge pathways and sources of modern water well field water from 11 production wells and 1 shallow monitoring well were sampled for major solute chemistry, ³H, ³He, SF₆, and noble gases. Geochemical inverse modeling estimated mixing proportions of modern water in production well waters of as much as 12%. Lumped parameter modeling (LPM) of ³H/³He and SF₆ data best fit an exponential piston flow model for regional recharge and a dispersion model for modern recharge from suspected hydrologic windows nearby. LPMs of production wells estimated mixing proportions of modern water from 5 to 41%. Hydrostratigraphic cross sections of the study area, developed from driller’s and geophysical logs, reveal potential pathways of modern water leakage to the Memphis aquifer adjacent to the well field. The new conceptual hydrogeologic model for groundwater conditions in the Lichterman well field area suggest vertical seepage of modern water into the Memphis aquifer may be limited by the unconfined conditions in the Memphis aquifer as well as limited saturation of the overlying shallow aquifer.