Application of Numerical Groundwater Model to Determine Spatial Configuration of Confining Unit Breaches near a Municipal Well Field in Memphis, Tennessee

Abstract

Thinning or localized absence (a breach) of an aquitard warrant concern because this limits the protection it affords water-supply aquifers beneath. The objective of this study was to assess potential spatial configurations of breaches within the aquitard overlying a water-supply aquifer in an urban well field. A three-dimensional groundwater flow model was utilized to investigate leakage pathways through the aquitard and simulate five potential breach configurations with three different hydraulic conductivity values. Through particle tracking analysis, estimates for the modern water percentage and apparent age of the modern water extracted by the production wells at the well field were obtained and compared to published age-dating data. Breach configurations resembling a broad paleochannel, which could originate through erosion of clay and silt within the aquitard, match the extent and proportion of modern water in the water-supply aquifer at the well field. This methodology has utility in evaluating the vulnerability of water-supply aquifers that are partially confined and susceptible to contamination, while assessing the likelihood of potential zones of increased vulnerability and offering targets for further investigations.