119-3 New Insights on the Karstic Barton Springs Complex from Multiport Well Data, Austin, Texas.

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

Presenting Author:

Authors:

Abstract:

Barton Springs (BS) is a large karst spring complex located in south Austin, Texas and fed by the BS Segment of the Edwards Aquifer. While the spring system has been studied extensively at the spring outlet locations, very little subsurface well data exists in the vicinity of the spring complex, and thus little is understood about the hydraulics and water quality of the Edwards Aquifer and underlying Trinity Aquifer nearby to the BS outflow locations. In 2024, a multiport monitoring well was installed 500 feet south of the main BS outflow, penetrating the entire thickness of the Edwards Aquifer and the top of the Upper Glen Rose (UGR) formation. Downhole geophysical logs, head data, geochemistry data, and slug testing data were collected from the multiport well between February 2024 and August 2025.

Head data showed distinct differences in head elevation between zones in the Edwards and UGR formations, suggesting that the Edwards Aquifer is hydraulically separated from the deeper UGR in the vicinity of the BS complex. UGR levels varied more over the study period, showing larger declines in response to an ongoing drought. Head data showed similar head elevation across the Edwards Formation, with 0.79-1.55 feet of difference across zones. All Edwards zones had a head elevation higher than BS outflow locations.

Water quality data varied between Edwards zones, with SC between 601.38-1510.3 uS/cm. Zone 7 had the lowest SC (601.81 uS/cm) and highest pH (7.88) indicating the large voids intersecting this zone may represent a karst conduit system which transmits younger meteoric water through the aquifer. Evidence for a conduit system in Zone 7 is supported by slug testing data, which showed a hydraulic conductivity in this zone two orders of magnitude higher than other Edwards zones, and head data, which consistently showed lower head elevations in Zone 7 than other Edwards zones.  UGR zones had significantly higher SC than Edwards zones (1965.9-4587.1 uS/cm). Within the Edwards, geochemistry data from shallow zones reflected a freshwater, Ca-HCO3 geochemical facies, while deeper zones showed the influence of Na-Cl geochemical facies. This suggests that these deeper Edwards zones may be influenced by mixing with the Edwards Saline Zone.

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

doi: 10.1130/abs/2025AM-10592

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