APPLICATION OF GEOCHEMICAL AND MODERN-WATER TRACERS TO ASSESS AQUIFER VULNERABILITY IN A HYDROGEOLOGICALLY COMPLEX SETTING IN MEMPHIS, TENNESSEE, USA

The Memphis aquifer is a semi-confined to confined unconsolidated sand aquifer that serves as the primary water source for Shelby County, Tennessee. Downward leakage of modern-water from shallow aquifers through windows in the upper Claiborne confining unit potentially degrades water quality in the Memphis aquifer and indicates vulnerability. Previous studies at the Davis wellfield have presented a decline in water quality over time, with mixing proportions of the shallow Mississippi River Valley alluvial (MRVA) aquifer within the Memphis aquifer as large as 45%. To further refine and quantify the sources and mixing proportions of modern-water in the Memphis aquifer at the Davis wellfield, major solute and trace metal chemistry, tritium (³H), and tritiogenic helium-3 (³He) data were obtained from groundwaters (10 Memphis aquifer wells and 3 MRVA wells) and a surface water sample. Existing borehole data within the wellfield were re-evaluated with recent structural data and indicate that the wellfield lies on a north-south trending horst structure with bounding faults to the west and east. Mixing proportions of the MRVA aquifer were evaluated using the geochemical modeling program PHREEQCi and isotope (¹⁴C, ³H and ³He) data. Spatial trends of mixing percentages from west to east in the wellfield indicate a modern-water recharge source from the MRVA aquifer through a fault-plane hydrogeologic window to the Memphis aquifer. Isotope data includes He concentrations 10x atmospheric values, low R/R_a values (0.13 to 0.68), depleted δD (-30.05 to -25.76 ‰) and δ¹⁸O (-5.25 to -3.51 ‰) signatures and tritium values (0.29 to 1.48 TU). Little tritiogenic ³He indicates both deep crustal and shallow, modern water contributions, likely from vertical flow along the eastern bounding fault. These results argue that there is a greater importance of fault-related mixing resulting in inter-aquifer exchange within the Memphis aquifer than previously recognized, and additional pathways of vulnerability.