Determination of Main Barton Springs Groundwater Flow Path: 2‐D Resistivity and Natural Potential Surveys at Barton Springs Swimming Pool, Austin, Texas

Abstract

The Main Barton Springs is a major discharge site for the Edwards Aquifer and is located in Zilker Park in Austin, Texas. The spring discharges into the Barton Springs pool near the diving board at an obvious fault line (Barton Springs 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 in between 40 and 70 feet. It was geologically assumed that the groundwater recharged from the Barton Spring Segment, which is located several miles to the south‐west of the Barton Springs pool area, follows the Barton Springs fault strike and empties into the pool. To test this hypothesis geophysical surveys [resistivity imaging and natural potential (NP)] were performed across the Barton Springs fault and in the southern part of the Zilker Park. Only NP surveys were allowed within the boundaries of the pool because of endangered species of Barton Springs Salamander. The purpose of the surveys was multi‐folded: 1) to locate the precise location of the Main Springs on the south banks of the Barton Springs pool; 2) to determine the potential location of caves and active flow paths beneath the spring; 3) to characterize the geophysical signature of the fault crossing the Barton Springs pool. We collected 5 N‐S and 5 E‐W resistivity and 5 NP profiles across and in the vicinity of the Barton Springs fault. Resistivity results to the east and southeast of the fault indicate a low resistivity zone caused by clay and/or clay‐filled voids or caves at a depth of 25 feet extending down to 80 feet. Results also indicate that the low resistivity zone has a lense‐shaped geometry and it is limited to the east by a significant fracture or fault. The NP results show high anomalies over the low resistivity zone indicating possible location of caves and/or groundwater flow. Geophysical results altogether, thus, suggest that significant amount of groundwater flow follows the path to the south of the fault along a fracture/fault zone that appears to coincide with the Springs location emptying into the pool within the Georgetown Formation.