Integrated geophysical investigations of Main Barton Springs, Austin, Texas, USA
Introduction
Barton Springs is the primary discharge of the Barton Springs Segment, discharging an average of 62 ft3/s (1.74 m3/s) over 36 years of continuous discharge measurement from 1978 to 2014 (Johns, 2015). Barton Springs actually consists of at least four spring clusters: Main Barton (Parthenia), Eliza, Old Mills (Zenobia) and Upper Barton springs. The federally-designated sole source aquifer provides the water supply to an estimated 60,000 people. Barton Springs is also the habitat for federally listed endangered aquatic salamanders, Eurycea sosorum and the blind Eurycea waterlooensis. The preservation of Barton Springs is sufficiently important for Austin citizens that the Save-Our-Springs water quality ordinance was petitioned and voted for in 1991, and $145 million in voter-approved bonds and grants were approved to purchase 22% of the recharge zone and 7% of the contributing zone for the Barton Springs Segment of the Edwards Aquifer (Thuesen, 2013).
The Edwards Aquifer is a highly permeable karstic limestone aquifer in Central Texas that is between 300 and 700 ft thick (90 and 200 m). It includes the Edwards Group and other associated limestone and consists of three segments: 1) The San Antonio segment of the Aquifer extends in a 160 mi (225 km) arch-shaped curve from the west to near Kyle in the northeast, and is between five and 40 mi (64 km) wide at the surface; 2) The Barton Springs segment extends from Kyle to south Austin; 3) the northern segment lies to the north of Austin (Fig. 1, taken from Musgrove and Banner, 2004).
Barton Springs Segment of the aquifer covers about 155 mi2 (235 km2) and is composed of limestone that is highly faulted, fractured, and dissolved, forming a very prolific karst aquifer ranging from 0 to 450 ft thick (0 to 137 m) (Rose, 1972). The groundwater basin that provides discharge to Barton Springs also use four hydrologic zones: Contributing, Recharge, Confined, and a Saline Zone (Fig. 2C, modified from Mahler and Lynch, 1999).
Dye-traced flow path studies show that the recharge water from Onion Creek, which is about 17 mi (56 km) to the southwest of Austin, can reach Barton Springs within 2 days (Fig. 2C; Hunt et al., 2005). This observation indicates that the ground water flows quickly through the well-connected conduits within the Edwards Aquifer (Hauwert, 2009). Groundwater tracing delineated three geochemically-distinct preferential flow paths of groundwater to Barton Springs: the Sunset Valley, Manchaca, and Saline–Water flow paths (Hauwert et al., 2004).
A reconnaissance geophysical study (2D resistivity, self-potential and conductivity) covering three of the Barton Springs (Main Barton, Eliza, and Old Mills) was conducted a few years ago (Saribudak et al., 2013). Results of this study indicated significant karst anomalies in the vicinity of the three springs. Especially in the Main Barton springs, resistivity and self-potential data hinted at a potential antithetic fault/conduit system in the south part of the Barton Springs Swimming pool, but there was not enough geophysical data to confirm it. Some of the resistivity and SP profiles from this work was included in this study.
In this current study, however, integrated geophysical surveys [2D and 3D resistivity, self-potential (SP), induced polarization (IP), seismic refraction tomography, and ground penetrating radar (GPR)] were performed in the vicinity of the Main Barton Springs and across the Barton Springs fault. The purpose of this additional field work was to: 1) determine the geophysical signature of the Barton Springs fault; and 2) define the suspected potential antithetic fault and conduit system, which could be the source for the ground water flow path for the Main Barton Springs.
Section snippets
Geology
Geological mapping of Austin by Garner et al. (1976) shows that faulting dominated the geology and physiography of the city and its environs. The Balcones escarpment, with a topographic relief as great as 300 ft (91 m) in Austin, is a fault-line scarp, marked by normal faults, which generally dip towards the east and southeast. The net fault offset is about 1100 ft (350 m; Hauwert, 2009, p.36). Thus, the structural framework of the Edwards Aquifer is controlled by the Balcones Fault Zone (BFZ), an
Geophysical methods
Integrated geophysical methods can provide new insights into the near-surface karstic features that Main Barton Springs and Barton Springs fault may contain. There have been a few geophysical studies published, which indicate the utilization of these methods across the Edwards Aquifer (Connor and Sandberg, 2001, Saribudak, 2011, Saribudak et al., 2012a, Saribudak et al., 2012b, Saribudak et al., 2013) and at other locations (e.g. Palmer, 2007, Carpenter, 1998, Ahmed and Carpenter, 2003, Dobecki
Interpretation of geophysical results
The geophysical data were interpreted in three sections: 1) Across the Barton Springs fault, 2) along the southern and northern banks of the Barton Springs swimming pool, and 3) in south Zilker Park, which is located to the south of the swimming pool.
Discussion and conclusions
The results of the geophysical surveys across the known location of the Barton Springs fault confirm the presence of the fault. Two resistivity profiles indicate the fault and its 25 ft (8 m) vertical throw. The seismic refraction data defines an irregular paleo-surface between the Georgetown and Edwards Aquifer in the vicinity of the fault. The fault throw displayed by the seismic refraction data is about 20 ft (6 m). The soil thickness overlying the Georgetown Formation is depicted as about 18 ft,
Acknowledgments
We thank Alf Hawkins and Justin Camp for their help during the fieldwork and their enthusiastic support for this project. Our appreciations go to Siegfried Rohdewald of Rayfract Software and Brad Carr for their help in processing the seismic refraction data and the induced polarization data, respectively. We appreciate Phil Carpenter's review of the manuscript, which helped improve its English and its flow greatly.
We also thank the following colleagues for their help in obtaining the permission
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