CONTROLS ON EDWARDS AQUIFER GROUNDWATER CHEMISTRY IN SOUTH-CENTRAL TEXAS THROUGH SURFACE SOIL CHARACTERIZATION

The Edwards Aquifer in south central Texas is one of the most prolific aquifers serving freshwater to the inhabitants. However, the growth of San Antonio may pose threats to the water quality. The hydrological karst features create a potential “fast track” for traditional and emergent contaminants to enter the confined zone. Previous work has identified land use changes that have resulted in the releases of organic compounds, particularly fertilizers and older banned pesticides. This study aims to present the distribution of contaminants of concern through the characterization of surface soils (up to 0.61 m depth) and understanding the groundwater chemistry across three regions (Contributing, Recharge, and Confined) of the Edwards Aquifer. The regional limestone aquifer is overlain by clay/chalk topsoil followed by an increasing gravel contents until the top of the aquifer. Sampling of groundwater wells and soil samples near recharge and surface water features in the regions formed the basis for analysis. Groundwater wells were sampled across the demarked zones covering 50 to 150 m bgl to the Edwards-Trinity aquifer with year-round accessibility. Soil sample locations were chosen in proximity to the recharge or surface features, such as rivers and creeks. A total of 12 groundwater wells and 17 soil sample locations were selected, with at least 2 samples collected per location between Sep 2022 and Dec 2023. Dissolved organic matter (DOM) characterization, cation-anion chemistry, and bacterial coliform assays were performed to determine the initial parameters of soil and groundwater quality. Samples that have outlying characteristics (i.e., elevated organic matter from high fertilizer loads, presence of metals, and high septic bacteria [Enterococci] counts) were prepared for trace elements analyses, e.g., copper, arsenic, and lead through soil leachate and bulk digestion methods in order to assess the health implications. Field water quality measurements showed alkaline groundwater [pH 6-9], with conductivity [300-800 µS/cm], and dissolved oxygen [0-3 mg/L]. Preliminary cation analysis of the soil leachates indicated abundance of Mg²⁺ rich species. The ultimate goal of this study is to understand the controls that distribute contaminants of concern in the karst aquifer system.