SEASONAL VARIATIONS IN GEOCHEMICAL CHARACTERISTICS OF SURFACE SOILS CONTROLLING THE EDWARDS AQUIFER GROUNDWATER CHEMISTRY IN SOUTH-CENTRAL TEXAS

The Edwards Aquifer in south-central Texas is a crucial freshwater resource for over 2.5 million inhabitants in San Antonio city and the surrounding areas. However, exponential human development in the recharge and contributing zones of the aquifer has created an overwhelming threat to the water quality. Karst features create a potential “fast track” for traditional and emergent contaminants to enter the confined zone. This study aims to understand the controls that distribute chemical constituents in the karst aquifer system through the characterization of surface soils (up to 0.6 m depth) and understanding the link to groundwater chemistry across three regions (Contributing, Recharge, and Confined zones) of the Edwards Aquifer. The limestone aquifer is overlayed by clay and chalk topsoil with increasing gravel content until the top of the aquifer. Sampling of soil occurred near recharge zones and surface water features in the regions. A total of 10 soil sample locations were selected, with 4 samples collected per location between March 2023 and February 2024 (n=40), corresponding to the Medina River (n=2), Medina Lake (n=1), Salado Creek (n=1), Helotes Creek (n =2), Cibolo Creek (n =2), and Guadalupe River (n=2). The concentrations of major cations, anions, trace elements, total dissolved organic carbon (DOC), and chemical characterization of organic matter were monitored in the soil leachates using Ion Chromatography, Inductively Coupled Plasma Mass Spectrometry (ICP-MS), TOC analyzer, and Optical Fluorescence Spectroscopy, respectively. Both measurements of surface water adjacent to soil sample locations and soil leachates consistently showed alkaline characteristics [pH 6-9], with conductivity [300-800 µS/cm]. Seasonal variations from initial analysis show widespread increases during the summer season with all seasons showing Ca-rich [60 mg/L] species associated with karst systems. The soil leachates showed approximately a 2-fold increase in sulfate ion between winter averages [3.04 mg/L] and summer [5.70mg/L] averages for all locations. Magnesium ion concentrations showed a similar 4-fold increase from winter [1.00 mg/L] to summer [4.05mg/L]. TOC and trace metal concentrations have also shown the similar seasonal trends, including iron [5700 mg/kg to 9450 mg/kg] and sulfur.