SULFUR HEXAFLUORIDE ACCUMULATION IN GROUNDWATERS IN SHELBY COUNTY, TENNESSEE

Sulfur hexafluoride (SF₆) has garnered keen interest as a hydrologic tracer because of its low reactivity, low adsorption to organics and soils, and its presumed low likelihood of arising from sources other than the atmosphere. Concentrations of SF₆ in the atmosphere have been well documented over the past sixty years, though limited documentation exists before that time. As a result, SF₆ is considered an excellent tracer for detecting modern groundwater (< 40 years old). This kind of tracer is particularly valuable in Shelby County, Tennessee, as it has been used to determine mixing of modern recharge waters with groundwater and to evaluate the vulnerability of municipal groundwater supplies.

This study evaluates current sampling methods and analytical procedures, proposes a new sampling method, and suggests multiple sources of SF₆ accumulation in groundwaters in Shelby County, Tennessee. The first part of the study determines whether sampling error is a dominant component of SF₆ concentrations by sampling nine wells screened in the Memphis aquifer in Shelby County using conventional methods as well as a newly developed airtight methodology. The samples taken using both methods were analyzed at the Dissolved Gas Lab at the University of Utah and the Groundwater Dating Lab at the US Geological Survey in Reston, Virginia. Results from these geochemical analyses suggest the airtight sampling method may eliminate potential re-equilibration with ambient air and improve analytical consistency compared to the conventional sampling method.

In addition, this study also presents a historical analysis of geochemical trends using samples collected from approximately 100 production wells in Shelby County during the past 13 years. The historical analysis will be used to compare SF₆ and tritium/tritiogenic helium-3 (³H /³He) derived ages for consistency and to assess discrepancies that may have been caused by sampling and analytical error, possible terrigenic sources of SF₆, and possible water re-equilibration sources of SF₆. Samples with large statistical variance between SF₆ and ³H/³He derived ages as well as near-surface water interaction may indicate the prevalence of more than one SF₆ source (terrigenic, modern water, or ambient air) in historical samples collected in Shelby County.