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

Recent research into the hydrogeological characteristics of the Edwards (Balcones Fault Zone) and/or Trinity (Hill Country) aquifers, south-central Texas, including their inter-aquifer connections, vulnerability, and prospects for long-term sustainability given changes in population, land use, and climate of the region.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 209

Quantifying Recharge Basin Contributions to Springflow in a Karst Aquifer Using Solute Transport Modeling

Paper number: 119-4
Program: Technical Sessions
Time: 2:25 PM – 2:40 PM

Authors

Abstract

The Edwards Balcones Fault Zone (BFZ) Aquifer in south-central Texas is a vital source that supplies water for municipal, agricultural, industrial, and ecological uses. Recharge occurs through nine distinct basins and sustains two major spring systems, Comal and San Marcos Springs, that provide habitat for diverse aquatic ecosystems and are home to several federally listed threatened and endangered species. This study applies a solute transport modeling approach to investigate two key research questions: (1) What percentage of springflow originates from each recharge basin? (2) What percentage of recharge in each basin contributes to springflow?

A solute transport model was developed from an existing groundwater flow model by assigning conservative tracers to each recharge basin. This approach allowed us to track groundwater movement through the aquifer and quantify its partitioning among well pumping, aquifer storage, and spring discharge. The approach was first validated using two synthetic test cases, confirming its ability to reliably represent mass transport and mass distribution. The model was then applied to the Edwards BFZ Aquifer to quantify recharge basin contributions to Comal and San Marcos Springs.

Preliminary results indicate that Comal Springs receives most of its springflow from the Frio, Sabinal, and Nueces Basins, while San Marcos Springs is primarily sustained by recharge from the Blanco and Comal–Dry Comal–Cibolo Basins. Estimates of transmission efficiency suggest that although multiple recharge basins contribute to both spring systems, the effectiveness of those contributions varies among source areas.

The accuracy of this solute transport approach depends on the reliability of the underlying groundwater flow model and is subject to uncertainties in model parameters and input data. Future work will incorporate additional datasets, such as groundwater chemistry and isotopic data, to further validate and refine the approach. The method developed in this study is transferable and can be applied to other hydrological systems to evaluate recharge-source contributions to springflow or other discharge points.

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

doi: 10.1130/abs/2025AM-413133

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