DELINEATING THE SPRINGSHED OF CAVE WITHOUT A NAME: DYE TRACING IN THE LOWER GLEN ROSE LIMESTONE

Cave Without a Name (CWAN) in Kendall County, TX contains ~5.5 km of active stream conduits formed in the karstic Lower Glen Rose Limestone which forms part of the Trinity Aquifer System. The primarily rural Kendall County lies just northwest of San Antonio in the Texas Hill Country. The Trinity Aquifer is the primary source of freshwater for this and many other Hill Country counties, although its yields are relatively low compared to the adjacent Edwards Aquifer.

Springsheds contribute water via recharge features to a spring and are similar to watersheds, except that their boundaries are not constrained by topography. To delineate a springshed for Cave Without a Name, dye tracing is being performed by injecting dyes into recharge features. Dye tracing utilizes conservative tracers (dyes) to trace recharging waters from direct recharge sites to a point of discharge (e.g., springs). For this project, multiple traces will be performed from direct recharge sites (sinkholes and/or caves) in an attempt to perform relatively rapid traces via active conduits. Regional flow near CWAN is to the Southeast while local flow is towards springs and streams. The Guadalupe River, Spring Creek, and Sabinas Creek are assumed to act as local discharge boundaries, along which a number of known springs occur. Prior work by Veni (1994) and our preliminary field work suggests that there may be several adjacent springsheds in the area, which is near a large oxbow in the Guadalupe River, just upstream from the confluence with Spring Creek.

To-date, two traces have been performed by injecting dye into large sinkholes/caves. Each was primed with 1,000-2,000 gallons of water prior to injection and flushed with another 1,000-2,000 gallons of water. The injection sites are 1km and 2km away from the monitoring point in CWAN’s main stream. The dye injected in the closer recharge feature was visually seen by cave tour staff 4 days after injection, but receptors have not been analyzed to confirm dye presence. The locations of the next dye traces are dependent on the results of the current trace as we work to determine appropriate dye concentrations for use in adjacent regions with many private wells that could be impacted.