119-10 Multi-Isotope Characterization of Edwards and Trinity Aquifer Interactions along the Balcones Fault Zone and the Hill Country

Session: Recent Investigations of the Hydrogeology Edwards (Balcones Fault Zone) Aquifer, the Trinity (Hill Country) Aquifer, and Their Interactions, South-Central Texas

Presenting Author:

Michael Weathers

Authors:

Weathers, Michael Zachary¹, Gao, Yongli², Bertetti, F. Paul³, Nunu, Rebecca⁴, Quintanilla, Jessica⁵, Ma, Lin⁶, Edwards, Richard Lawrence⁷

(1) University of Texas at San Antonio, San Antonio, TX, USA, (2) University of Texas at San Antonio, San Antonio, TX, USA, (3) Edwards Aquifer Authority, San Antonio, TX, USA, (4) Edwards Aquifer Authority, San Antonio, tX, USA, (5) Edwards Aquifer Authority, San Antonio, TX, USA, (6) University of Texas at El Paso, Dept of Geological Sciences, El Paso, TX, USA, (7) University of Minnesota, Geology & Geophysics, Minneapolis, MN, USA,

Abstract:

Understanding the hydraulic connection between the Edwards (Balcones Fault Zone) and Trinity (Hill Country) aquifers is essential for sustainable groundwater management in South-Central Texas. This study integrates multiple isotopic tracers including strontium (⁸⁷Sr/⁸⁶Sr), boron (δ¹¹B), uranium and thorium (²³⁴U/²³⁸U and ²³²Th) to investigate inter-aquifer mixing, residence times, and recharge sources in faulted transitional zones near San Antonio.

Paired sampling of springs and streams reveals isotopic variability indicative of differing recharge regimes and lithologic interactions. While no consistent aquifer-specific trends in ⁸⁷Sr/⁸⁶Sr or δ¹¹B values have been identified across all sites, tracer patterns suggest the potential for geochemical fingerprinting when evaluated in conjunction with uranium and thorium isotope ratios.

Spring and stream samples across the Upper Cibolo Creek watershed display isotopic blending in transitional fault zones, suggest surface water and groundwater interactions within the watershed. While it is understood that some interchange between the aquifers exists, clear evidence of direct aquifer communication has yet to be demonstrated. Wastewater-impacted recharge zones also show distinct isotopic signatures. For example, results from preliminary sampling in 2020 to 2021 indicate combined variations in strontium and boron isotopes, along with ²³⁴U/²³⁸U and ²³²Th concentrations, help differentiate Edwards spring water from Trinity mixing or surface runoff, reinforcing evidence for partial hydraulic separation in specific areas.   These preliminary findings highlight the complexity of groundwater flow and recharge in the Upper Cibolo region and suggest localized interactions rather than pervasive aquifer-wide connectivity. Further sampling and integration with hydrogeologic mapping are underway to refine interpretations. This study provides a baseline for future isotopic characterization efforts and supports resource management within a rapidly developing recharge area.

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

doi: 10.1130/abs/2025AM-9800

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