158-4 Geochemical Analysis and Rare Earth Element Characterization of Karstic Groundwaters Impacted by Anthropogenic Activities: Implications for the Edwards Aquifer

Session: Redox-Driven Nutrient and Contaminant Dynamics in Terrestrial Systems

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Abstract:

Contaminated drinking water is often the root cause of diseases such as mild to severe gastrointestinal illnesses and chronic carcinogenicity. The Edwards Aquifer system satisfies half of San Antonio’s drinking water demands and is juxtaposed with the underlying Trinity Aquifer. The Field Research Park (FRP) and the Helotes Creek watershed (HCW) are in the Glen Rose formation of the Trinity Aquifer. Water chemistry of the adjoining carbonate Trinity and Edwards aquifers in central Texas has been analyzed to evaluate the potential impact of anthropogenic activities on the quality of these aquifers. Water samples collected from 12 HCW and 4 FRP wells were analyzed for common cations (Ca²⁺, Li⁺, Mg²⁺, etc.), anions (F^-, Cl^-, Br^2-, etc.), trace elements (Fe, As, Cu, etc.), and stable isotopes (O and H, δN and δS). Dissolved organic matter (DOM), total organic carbon (TOC), and dissolved inorganic carbon (DIC) were measured to help characterize the evolution of Trinity Aquifer groundwater before it reaches the Edwards Aquifer. The effectiveness of rare earth elements (REEs) as tracers of water-rock interactions was also analyzed by comparing carbonate fractionation patterns identified from literature with the spatial variations of REEs in the groundwater from these sites. Based on Piper diagram analysis the dominant groundwater type was Ca-Mg-HCO₃ for both HCW and FRP, indicative of the karstic nature of the regional Edwards and Trinity Aquifers. The groundwater from HCW showed pH ranging from 6.79 – 7.54, with specific conductivity of 700 – 2393 µS/cm and TOC concentrations between 8.2 – 17.4 mg/L. The FRP showed pH ranges of 7.16 – 7.67, specific conductivity between 477 – 810 µS/cm and TOC values of 7.2 – 11.4 mg/L. Variation between HCW and FRP TOC values indicate higher potential contamination due to anthropogenic activity. One FRP well displayed a continued decrease in all cation and anion concentrations throughout the year. This may be indicative of extreme drought conditions in the area for that period leading to reduced recharge and dilution effects within the aquifer. Overall, these results underline the need to understand the influence of anthropogenic activities on the Edwards Aquifer and an imminent need for continued monitoring of groundwater quality in the area to better protect public health for those reliant on groundwater.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6368

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