264-2 When Can I Sample? Determining Post-Storm Wait Times to Ensure Baseflow Conditions for Spring Sampling: A Case Study from the Edwards Aquifer, Central Texas

Session: Understanding Karst Hydrology and Karst Aquifers Using Innovative Tracers and Other Technologies (Posters)

Poster Booth No.: 95

Presenting Author:

Tyson McKinney

Authors:

McKinney, Tyson¹, Sydow, Lindsey², Markowski, Michael³, Rice, Radmon⁴, Thomas, Dana⁵, Slocombe, Meghan⁶, Guevara, Joaquina⁷

(1) Watershed Protection Department, City of Austin, Austin, TX, USA, (2) Watershed Protection Department, City of Austin, Austin, TX, USA, (3) Watershed Protection Department, City of Austin, Austin, TX, USA, (4) Watershed Protection Department, City of Austin, Austin, TX, USA, (5) Watershed Protection Department, City of Austin, Austin, TX, USA, (6) Watershed Protection Department, City of Austin, Austin, TX, USA, (7) Watershed Protection Department, City of Austin, Austin, TX, USA,

Abstract:

When collecting geochemical samples from springs for long-term trend analyses of groundwater, it is important to ensure that baseflow conditions are met. In karst systems such as the Edwards Aquifer of Central Texas, rainfall and rapid recharge miles away from springs can alter spring geochemistry in a matter of hours to days. The City of Austin Watershed Protection Department (WPD) collects quarterly baseflow geochemical samples from six major springs (Barton, Eliza, Old Mill, Upper, Cold, and Backdoor) in the Barton Springs Segment of the Edwards Aquifer (BSEA) to satisfy federal permit requirements and to build a long-term dataset of baseflow groundwater geochemistry for the BSEA. Historically, the WPD has used post-storm sampling wait times based on best practices for surface water sampling to guide baseflow sampling of the springs. However, preliminary continuous monitoring of the six springs suggests that the surface water wait times are insufficient for some, if not all, of the springs. For this study, we use continuous (15-minute) specific conductance measurements at the springs and Vieux & Associates, Inc., Gage Adjusted Radar Rainfall (GARR) to calculate post-storm sampling wait times to ensure that baseflow sampling conditions are met. Using a merged precipitation watershed, 1-hour GARR data, and the Inter-Event Time Definition (IETD) package in R, we calculate start and end times of storm events. Then, we analyze the response at each spring for each storm event to calculate the time it takes the coefficient of variation of specific conductance, normalized to the overall change in specific conductance during the storm, to drop below a threshold value of 0.005. Preliminary results show that post-storm sampling wait times range from 0.5–1 day for Backdoor Spring, 3–8 days for Barton and Eliza Springs, and 6–9 days for Cold Spring. Monitoring at Old Mill and Upper Springs is anticipated to begin soon. So far, all data collection for this study has occurred under low discharge aquifer conditions, defined as less than 40 cubic feet per second (cfs) of cumulative flow from Main Barton, Eliza, and Old Mill Springs. Continuous monitoring of the springs will continue until at least two years each of low discharge and high discharge (> 80 cfs) data collection has occurred, and the transition from low to high discharge conditions (and vice-versa) has been captured.

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

doi: 10.1130/abs/2025AM-8139

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