Application of Numerical Tools to Investigate a Leaky Aquitard beneath Urban Well Fields

Abstract

Memphis aquifer is the primary drinking water source in Shelby County (Tennessee, USA) and supplies industrial, commercial, and residential water. Memphis aquifer is separated from the Shallow aquifer by a clayey layer known as Upper Claiborne Confining Unit (UCCU). All of the production wells in the Memphis area are screened in the Memphis aquifer or even deeper in the Fort Pillow aquifer. Traditionally, it was assumed that the UCCU could fully protect the Memphis aquifer from the contaminated Shallow aquifer groundwater. However, recent studies show that at some locations the UCCU is thin or absent which possibly leads to the contribution of Shallow aquifer to the Memphis aquifer. Accurately locating the breaches demands expensive and difficult geological or geochemical investigations, especially within an urban area. Hence, a pre-field investigation to identify the locations where the presence of breaches is likely can significantly reduce the cost of field investigations and improve the their results. In this study, to identify the locations where the presence of breaches in the UCCU is likely we use three different analyses: (1) pilot point calibration (PPC), (2) velocity and flow budget (VFB), and (3) particle tracking (PT) to post-process the developed groundwater results. These pre-field numerical investigations provide relevant and defensible explanations for groundwater flow anomalies in an aquifer system for informed decision-making and future field investigations. In this study, we identify five specific zones within the broad study area which are reasonable candidates for the future field investigations. Finally, we test the results of each analysis against other evidence for breaches to demonstrate that the results of the numerical analyses are reliable and supported by previous studies.

Determination of Contaminant Transport Parameters for a Local Aquifer by Numerical Modeling of Two Plumes: Trichloroethylene and Hexavalent Chromium

“…The range of dispersivity values obtained from this analysis provides several benefits over analytical or empirical values as numerical models can estimate all three dispersivity values using a minimal amount of observation data at different depths, provided having a well-calibrated flow model and known source concentration. The Memphis aquifer, although mostly confined, is threatened by anthropogenic contamination coming through the unconfined area or via preferential pathways (i.e., breaches) in the upper Claiborne confining unit [38,44,47,48], several of which are in the vicinity of municipal wellfields. Dispersivity values obtained in this study are crucial to simulating any existing or future contamination arriving at the Memphis aquifer through those breaches.…”

“…In Shelby County, Tennessee, the primary drinking water aquifer is the semi-confined unconsolidated Memphis aquifer [36,37]. The aquifer is threatened by the presence of localized preferential pathways (i.e., breaches) in the overlying aquitard (the upper Claiborne confining unit-UCCU), warranting concern that contaminants could easily bypass the aquitard's natural protection [38][39][40][41][42][43][44][45][46][47][48]. Very little is known about the contaminant transport properties of the Memphis aquifer.…”