Geophysical signatures of Barton Springs (Parthenia, Zenobia and Eliza) of the Edwards Aquifer, 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 ft (12–21 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 and natural potential (NP)] were performed over the first three springs (Main Barton, Old Mill and Eliza Springs). Conductivity (EM) surveys were conducted in some areas to distinguish utility lines. 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) characterize the hydraulic relation between the Main Barton, Old Mill and Eliza Springs; (3) determine the potential location of caves and active flow paths beneath the three springs; and (4) characterize the geophysical signatures of the fault crossing the Barton Springs pool. The geophysical surveys revealed three general types of anomalies. Resistivity results from the south of the Barton Springs swimming pool indicate presence of a thick, laterally extensive high conductivity layer above the pool elevation. This high conductivity layer is interpreted to be lateral clay deposits, either associated with the Del Rio clay or clay-rich alluvial deposits associated with Barton Creek. These clay layers appear to overlie the Edwards Aquifer south of the pool. Also south and east of the pool are cylindrical high conductivity anomalies that extend deeper than the elevation of the submerged cave observed in Barton Springs pool. These cylindrical high conductivity anomalies are also associated with NP anomalies, suggesting groundwater flow. One hypothesis is that the alignment of the high conductivity and NP anomalies corresponds to the Saline-Line Flow Route that is known to discharge primarily at Old Mill Springs, and is hydraulically connected to Main Barton Springs and Eliza Springs. This hypothesis is favored because the Saline-Line Flow Route carries relatively mineralized groundwater and is known to connect to Old Mill Springs and at some times Main Barton and Eliza Springs. There are likely several conduit paths to the pool from the southern part of Zilker Park. Flow paths to Barton Springs from the east may be localized within the uppermost leached and collapsed members of the Edwards Group, which is known for its extensive horizontal cave development. A third type of anomaly, generally found west and immediately adjacent to the Barton Springs Fault, southwest of Barton Springs pool are circular low conductive features associated with NP anomalies. These circular anomalies are interpreted to be groundwater flow conduits bearing less mineralized groundwater associated with the Sunset Valley and possibly Manchaca Flow Routes that are expected to cross that area. The surveys allow an opportunity to compare geophysical responses to directly observed features. The Barton Springs Fault appears to be associated with circular high- and low-resistivity anomalies. Eliza and Old Mill Springs also indicate significant resistivity and NP anomalies suggesting presence of submerged conduits and faults in the vicinity. Although it is obvious such conduits are present adjacent to major springs, the surveys allow examination of how such water-filled conduits appear using various geophysical methods. Known underground infrastructure was also included in the surveys to see how air and water-filled pipes responded. An air-filled train tunnel corresponded to a high-resistivity anomaly. Local utility lines crossing the site showed no significant resistivity anomaly but metal pipes were detectable with EM conductivity surveys.