Revisiting the Hydrologic Divide Between the San Antonio and Barton Springs Segments of the Edwards Aquifer: Insights from Recent Studies

Brian A. Smith
Brian B. Hunt
Steve B. Johnson

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Groundwater divides delineate the boundaries of aquifer systems and influence not only the local aquifer hydrodynamics but also the groundwater budget. The groundwater divide separating the San Antonio and Barton Springs segments of the Edwards Aquifer in Texas has historically been drawn along topographic or surface water divides between the Blanco River and Onion Creek in the recharge zone, and along potentiometric highs in the confined zone between the cities of Kyle and Buda in Hays County. The purpose of this study is to review the results of previous studies about the groundwater divide and to evaluate recently collected data that pertain to the groundwater divide. Studies have been conducted over the past five years using detailed potentiometric data, tracing techniques, and groundwater flow modeling to better characterize the Edwards Aquifer and to understand the relationship between the Barton Springs segment and the portion of the San Antonio segment north of San Marcos Springs. These studies reveal that during wet conditions the groundwater divide is located generally along Onion Creek in the recharge zone, extending easterly along a potentiometric ridge between the cities of Kyle and Buda toward the saline zone boundary. During dry conditions the hydrologic divide moves south and is located along the Blanco River in the recharge zone, extending southeasterly to San Marcos Springs. The groundwater divide is a hydrodynamic feature dependent upon the hydrologic conditions (wet versus dry) and the resulting hydraulic heads between Onion Creek and the Blanco River. The major influences on the position of the divides are groundwater mounds beneath the Blanco River and Onion Creek as the amount of recharge changes between wet and dry periods. During wet periods more water is recharged along Onion Creek because it has a greater potential for recharge through numerous recharge features. During dry periods, the Blanco River continues to flow and recharge the aquifer long after flow has ceased in Onion Creek and