MODERN WATER RECHARGE SOURCES AND GROUNDWATER FLOW TO THE MEMPHIS-SPARTA AQUIFER, SHAW WELLFIELD, MEMPHIS, TENNESSEE

The dynamics of modern water sources and groundwater flow paths to the semi-confined Memphis-Sparta and confined Fort Pillow aquifers were investigated to assess the vulnerability of groundwater resources within the Shaw wellfield located in Memphis, Tennessee. Geochemical and environmental tracer data (δD, δ¹⁸O, δ¹³C, and ¹⁴C) were used to assess compartmentalized groundwater flow in the Memphis and Fort Pillow aquifers, vulnerability of the Memphis aquifer to modern water recharge (<60 yrs old), and sources and pathways of modern water recharge. The Memphis and Fort Pillow aquifer stable isotope (δD and δ¹⁸O) data indicate local precipitation as the main source of recharge, with the Fort Pillow aquifer samples having a more negative signature as compared to the Memphis aquifer samples, signifying recharge during colder periods. The δ¹³C values ranged from -17.4 to -17.5 ‰ for the Fort Pillow aquifer, with an average of 20 Percent Modern Carbon (PMC), supporting recharge during the Pleistocene with a C4 plant carbon influence. The bulk of the Memphis aquifer samples averaged 62 PMC with δ¹³C values ranging from -21 to -22 ‰, suggesting recharge during the mid-to late Holocene with a C3 plant carbon influence. Elevational and PMC data reveal two distinct flow paths within the Memphis aquifer; a typical path signified by a statically significant positive correlation (R²= .90; p-value= .003) and a fast-path signified by a strong positive correlation (R²= .84; p-value= .252). Geochemical, tracer data, and hydrological boundary conditions interpreted from well borelogs suggest up to a 17% component of modern water, likely sourced from nearby surface waters, is produced in wells with shallow screen intervals (<100 m depth) in the upper Memphis aquifer. Vertical recharge of modern water is limited due to significant clay intervals (~5 to 20 m) located in the upper Memphis aquifer and distributed pumping throughout the entire aquifer within Shaw wellfield. Regional groundwater flow from unconfined regions east of the Shaw wellfield indicates waters with longer flow paths are less likely to have direct impact from urban development. This study suggests that the deeper Memphis and Fort Pillow aquifer at the Shaw wellfield are not vulnerable to modern water recharge and provide a sustainable municipal supply for Shelby County.