Evaluation of Groundwater Quality in the Edwards Aquifer at Northwest San Antionio, Texas, Using Trace Element and Rare Earth Element Geochemistry




Curry, Brian Robert

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The Edwards Aquifer (EA), located in central Texas, is a crucial source of drinking water for over 2.5 million residents in San Antonio and the surrounding areas. However, exponential human development in the recharge and contributing zones of the aquifer has created a growing threat to its quality. To gain insight into the impact of anthropogenic practices on water quality, this study focused on quantifying major ion, trace metal, and REE abundances in the groundwater of the Edwards Aquifer to better understand the recharge process, host rock compositions, and anthropogenic activities on groundwater quality under different land use patterns. From January to November 2023, 56 surface and groundwater samples were collected at ten locations crossing the contributing and recharge zones of the EA and three land use/land cover areas: commercial, residential, and agricultural areas. While the general water quality is fresh, which is consistent with prior studies, the study found significant variabilities in Ca/Na and Mg/Na molar ratios among the samples, potentially due to the weathering of silicate and carbonate rocks in the aquifer. Furthermore, rare earth element concentrations in these waters were not significantly different by zones or land uses. Overall, all samples exhibited similar shale-normalized REE patterns along the flow path, with changes detected in the positive Ce anomaly along the flow path acting as an indicator of changing redox conditions. The study suggests that a combination of solution complexation reactions, chemical weathering, adsorption, desorption, and redox processes influences REE concentrations and fractionation patterns.



Edwards Aquifer, geochemistry, groundwater, karst, rare earth elements, trace elements