MULTIPLE SOURCES OF THE ENVIRONMENTAL TRACER SULFUR HEXAFLUORIDE IN GROUNDWATER IN SHELBY COUNTY, TENNESSEE

Geochemical data from 2009-2023 for production wells in Shelby County, Tennessee, were analyzed to determine the origin of discrepancies in sulfur hexafluoride (SF₆) results in relationship to other tracers and hydrogeologic characteristics of sampled wells. This study proposes three potential origins for the discrepancies: modern SF₆ contamination introduced during sampling, contributions of terrigenic SF₆ formation, and groundwater re-equilibration with modern air in the unsaturated zone. The results suggest contamination with modern air during sampling is significant in some settings. To address modern air contamination, an airtight sampling method was developed and shown to be successful in reducing introduction of modern air during sampling. The results also suggest that terrigenic SF₆ may affect samples from wells in proximity to faults and unconfined conditions may introduce modern air due to water re-equilibration. Sulfur hexafluoride sample results from urban areas should be carefully analyzed with respect to well conditions, proximity to faults, and proximity to local SF₆ sources.