Abstract
Inter-aquifer water exchange between the shallow and Memphis aquifers in Shelby County, Tennessee may pose a contamination threat due to the downward migration of younger, poor quality groundwater into deeper, more pristine aquifer. Discontinuities (breaches) in the upper Claiborne confining unit (UCCU) allow for leakage into the Memphis aquifer, a sand-dominated aquifer that provides about 95% of the groundwater used in the Memphis area. This study created a multi-layered 3D groundwater model for Shelby County using the United States Geological Survey’s MODFLOW-NWT program to evaluate water exchange for a simulation period from January 2005 to December 2016. Results indicate an overall leakage through the UCCU of 61 m3/min into the Memphis aquifer in Shelby County, accounting for 10% of its water budget inflow, with localized areas experiencing as much as 20% water exchange. As young water tends to stay in the upper part of the Memphis aquifer, water budget assessment for the upper 60 m of the Memphis aquifer revealed leakage representing 29% of the zone inflow, and as much as 53% in certain areas. More localized studies must be conducted to understand the location, characteristics, and orientation of the confining unit breaches, as well as the inter-aquifer water exchange.
Highlights
Water exchange in groundwater systems comprised of alternating aquifers and leaky confining units can influence the water characteristics and may pose a contamination threat due to downward leakage of low-quality water, especially in urban areas where groundwater degradation is common [1,2,3], as is the case in Shelby County, Tennessee [4,5,6,7,8]
If only the upper 60 m of the Memphis aquifer are considered, the percent of exchanges can fluctuate between less than 1%, to more than 35% in quadrant V with its 8.6 m3 /min moving at Regarding the Fort Pillow aquifer, downward leakage from the Flour Island confining unit, of about 1 m3/min, accounts for less than 2% of the inflow to the Fort Pillow aquifer, which receives most of its inflow via lateral flow
Memphis aquifer aquifer and theare overlying shallow aquifer,oflocalized
Summary
Accepted: 14 September 2021Water exchange in groundwater systems comprised of alternating aquifers and leaky confining units can influence the water characteristics and may pose a contamination threat due to downward leakage of low-quality water, especially in urban areas where groundwater degradation is common [1,2,3], as is the case in Shelby County, Tennessee [4,5,6,7,8].Due to inter-aquifer water exchange, the Memphis aquifer is experiencing localized water quality degradation and contamination [9,10,11,12,13,14].Numerical modeling of inter-aquifer water exchange is commonly difficult because of the varying methods for characterizing the heterogeneity leading to the exchange [15,16], the disconnections between surface water and alluvial aquifer water from deeper aquifer systems [17], and changes in inter-aquifer water exchange with pumping [18,19]. Water exchange in groundwater systems comprised of alternating aquifers and leaky confining units can influence the water characteristics and may pose a contamination threat due to downward leakage of low-quality water, especially in urban areas where groundwater degradation is common [1,2,3], as is the case in Shelby County, Tennessee [4,5,6,7,8]. Due to inter-aquifer water exchange, the Memphis aquifer is experiencing localized water quality degradation and contamination [9,10,11,12,13,14].
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