Geophysical Results of Main Barton Springs (Parthenia) and Their Implications on the Groundwater Flow Pattern into Barton Springs Pool, Austin, Texas

Abstract

Barton Springs is a major discharge site for the Barton Springs Segment of the Edwards Aquifer and is located in Zilker Park in Austin, Texas. Barton Springs actually consists of four springs: 1) The Main Barton Springs discharges into the Barton Springs pool from the Barton Springs Fault and several outlets along a fault and from a cave, several fissures, and gravel‐filled solution cavities on the floor of the pool west of the fault. The thin‐bedded unit on the southwest side of the fault is the regional dense member, and the lower Georgetown Formation of the Edwards Group is exposed on the northeast side of the fault. The offset of the fault is between 40 and 70 feet (12 to 21 m). 2) Old Mill Springs is located in the sunken gardens southeast of the Barton Springs Pool and is primarily fed by relatively mineralized groundwater from the Saline‐Line Flow Route. 3) Eliza Springs is also located along the Barton Springs Fault north of Barton Springs pool. 4) The Upper Barton Springs is located upstream of the Barton Springs pool on the south bank. Surface geophysical surveys [resistivity imaging, natural potential (NP), seismic refraction, magnetic and ground penetrating radar (GPR) ] were performed in the vicinity of Main Barton Springs. The purpose of the surveys was to: 1) locate the precise location of submerged conduits carrying flow to Main Barton Springs on the north and south banks of the Barton Springs pool; 2) determine the potential location of caves and active flow paths beneath the three springs; and 3) characterize the geophysical signatures of the fault crossing the Barton Springs pool. The geophysical surveys revealed several anomalies. Resistivity results (2D and 3D) from the south of the Barton Springs swimming pool indicate thick, extensive low resistivity (high conductivity) anomalies. The NP data displays high anomalies across the low resistivity anomalies. Seismic refraction tomography (SRT) results indicate an eastward‐dipping fault with a 25‐ft throw in front of the south gate to the swimming pool. Location of the South Gate Fault is also evident from the resistivity, NP and the GPR data. However, the SRT data over the Barton Springs fault does not indicate a fault; instead it shows a low‐velocity zone sitting on a paleo‐topography of high‐velocity bedrock. The groundwater flow‐path to the Main Springs could follow the locations of low resistivity and high NP anomalies outlined in this study.