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

Assessing Long-Term Changes in Spring Water Chemistry Using Weighted Regression on Time, Discharge, and Season (WRTDS) at Barton Springs, Edwards Aquifer (1995–2025)

Paper number: 119-14
Program: Technical Sessions
Time: 5:00 PM – 5:15 PM

Authors

Abstract

The Barton Springs segment of the Edwards Aquifer (BSEA) serves as the primary or sole source of drinking water for up to 60,000 people, provides critical habitat for two federally endangered salamander species, and supplies water to Barton Springs Pool, a popular recreational site that attracts up to 800,000 visitors annually. Understanding long-term trends in water chemistry relative to discharge, seasonality, and hydrogeologic characteristics is essential for identifying and mitigating drivers of water quality in this vulnerable karst system. Over the past 30 years (1995–2025), the City of Austin Watershed Protection Department has collected quarterly baseflow water chemistry data from six major springs—Main Barton, Eliza, Old Mill, Upper, Cold, and Backdoor Springs—located within the BSEA near downtown Austin, Texas. We analyzed trends in water chemistry at each site using the Weighted Regression on Time, Discharge, and Season (WRTDS) approach. The BSEA recharge zone generally becomes less urbanized with distance from downtown Austin. Consequently, some springs receive recharge from more local, urbanized areas, while others receive recharge from more distant, less developed catchments. We hypothesized that springs with more local and urban recharge zones would show greater increases over time in constituents associated with urban pollution—such as chloride (Cl), sulfate (SO4), sodium (Na), fluoride (F), nitrate (NO3), and specific conductance (SpC)—especially during high discharge periods when faster groundwater times may limit attenuation. We also anticipated greater seasonal variability in these springs, particularly under high-flow conditions. Preliminary results generally support these hypotheses. Cold Spring and Backdoor Spring, the two springs with the most local and urban recharge zones, showed the largest overall increases in constituent concentrations from 1995 to 2025. In contrast, Old Mill Spring, which receives more regional recharge from less developed areas, generally exhibited smaller increases or no increases. Springs with more regional flow paths also appear to have stronger discharge-related effects, with increased concentrations during high-flow periods but less pronounced seasonal variability compared to those with more urban, local recharge. Ongoing analysis will further refine and quantify changes for individual constituents.

 

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

doi: 10.1130/abs/2025AM-413500

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