Geophysical anomalies in the vicinity of Barton Springs and determination of groundwater flow path into the Barton Springs swimming pool, Austin, Texas

Abstract Barton Springs is a major discharge site for the Barton Springs segment of the Edwards Aquifer and is in Zilker Park in Austin, Texas. There are three other smaller springs that are called Upper, Eliza, and Old Mills Springs. Barton Springs discharges into the Barton Springs pool from the Barton Springs fault and several outlets along the fault and from a cave on the floor of the pool. Dye-tracing studies conducted over several decades have revealed general groundwater flow paths to Barton Springs. However, the discharge paths near the pool are poorly known. Previously published geophysical data, including electrical resistivity tomography (ERT), self-potential (SP), and seismic refraction tomography (SRT), indicated significant anomalies to the south of Barton Springs. ERT profiles displayed low-resistivity anomalies, where cospatial SP profiles indicated positive anomalies. In general, these integrated results indicated the locations of the conduits and revealed the inner near-surface structure of the study area. However, the locations of the conduits had never been tested by drilling and dye tracing. During December 2018, Barton Springs started discharging sediment plumes into the pool. The source was attributed to a geothermal drilling site in a residential neighborhood, which was located approximately 4000 ft to the southwest of the pool. In addition, a newly drilled Barton Springs Edwards Aquifer Multiport well near the pool provided new insights into the geology of the Edwards Aquifer. In this paper, geophysical results were integrated with sediment-plume data, subsurface borehole logs, and the newly discovered monocline folding structure to characterize the preferential groundwater flow direction near Barton Springs and to ground-truth the geophysical data. The results demonstrated the efficacy of geoelectric methods (ERT and SP) for mapping the locations of dissolution cavities and preferential groundwater flow paths in the Barton Springs segment of the Edwards Aquifer.