306-5 Hydrogeochemical characterization of Helotes Creek Watershed in south-central Texas

Abstract:

The Edwards and Trinity aquifers are crucial resources of drinking water for much of south-central Texas. This study provides a comprehensive hydrogeochemical characterization of Helotes Creek Watershed (HCW) in south-central Texas, evaluates hydraulic communication between the Upper Glen Rose (UGR) and Lower Glen Rose (LGR) units of the Trinity Aquifer, and characterizes rare earth elements (REEs) distributions. Seasonal groundwater samples from private HCW wells and comparison wells at the Edwards Aquifer Authority’s Field Research Park (FRP) are analyzed for physicochemical parameters, major ions, trace elements, total organic carbon and nitrogen (TOC/TN), dissolved organic matter (DOM), stable isotopes (δ¹⁸O, δD, δ³⁴S, δ¹⁵N), and REEs. Groundwater chemistry is dominated by carbonate and evaporite dissolution with clear hydrochemical separation between the UGR and LGR. The UGR is characterized by Ca-Mg-HCO₃ hydro-facies, lower conductivity and TDS, and stronger influence from rapid recharge. In contrast, the LGR is characterized by Ca-Mg-Cl-SO₄ hydro-facies, elevated conductivity and TDS, SO₄^2-, Ca²⁺, Mg²⁺, B, and Fe, indicating stronger water-rock interaction, evaporite dissolution, and longer residence times. δ¹⁸O and δD variations indicate recharge by direct precipitation with limited evaporative trends. TOC/TN and DOM patterns reveal strong surface-water interaction with slight influence of wastewater that is primarily driven by organic soils and fertilizers applied to the lands. Trace elements and major ion concentrations are below WHO thresholds. REE concentrations are low, but PAAS-normalized patterns provide supporting evidence for groundwater evolution. FRP contains higher absolute ΣREE, whereas HCW and especially LGR wells displayed stronger HREE and MREE enrichment. Overall, the combined geochemical, isotopic, organic, and REE geochemistry evidence indicates limited hydraulic communication between the UGR and LGR, with localized mixing possible along specific flow paths or pumping-stressed zones.

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