Investigation of modern leakage based on numerical and geochemical modeling near a municipal well field in Memphis, Tennessee

Abstract Local leakage processes and potential migration pathways of modern water (< 60 years) from the shallow aquifer, into the underlying semiconfined Memphis aquifer, were evaluated to assess the vulnerability of groundwater in Memphis Light, Gas and Water’s (MLGW) Sheahan well field. To identify the source(s) and pathways of modern water, integrated hydrostratigraphic analysis, numerical modeling, hydrologic tracers, and geochemical modeling were utilized. The percentage of modern water present in Memphis aquifer production wells is estimated using inverse geochemical modeling, lumped parameter modeling, and solute transport modeling with Modular Transport, 3-Dimensional, Multi-Species model (MT3DMS). The mixing percentages determined from lumped parameter modeling and MT3DMS are generally in agreement, yielding maximum values of 14.3% and 15.3% young water, respectively; however, the estimate diverge at well 87 A. The mixing fraction difference at 87 A might account for a missing hydrogeologic connection in the groundwater model on the eastern part of the well field. Estimates for the apparent age of modern water derived from MT3DMS are generally consistent with those from lumped parameter models (LPM) for most wells, except for well 96. However, both MT3DMS- and LPM-derived mean ages are younger than tracer-based apparent ages, likely due to age mixing, unsaturated zone travel time, and possible helium loss during sampling. Thus, the MT3DMS model, calibrated with long-term tracer data could simulate the mean age and mixing percentage of modern water while emphasizing the importance of accurate hydrogeologic conceptualizations at the Sheahan well field. As a result, tracer data and solute transport modeling can identify vulnerabilities and ensure the long-term sustainability of the Sheahan well field.