NGWA Event and Education Recordings Archive

Water level monitoring, groundwater tracing, water chemistry and tritium dating were used to characterize groundwater in the environmentally sensitive Jollyville Plateau area in northwest Austin. Plateau hydrogeology consists of the Edwards formation, typically characterized as dominated by karst features with groundwater migration primarily through conduits underlain by the non-karstic Walnut and Glen Rose formations where groundwater movement is through original porosity, thin vuggy beds or fractures. Topographically, the area is highly dissected by steep canyons with numerous small ephemeral springs feeding intermittent creeks.

Water level monitoring indicates large downward hydrologic gradients which led to development of a conceptual groundwater model comprised of (1) a shallow system tens of feet thick feeding springs and creeks documented by groundwater tracing and (2) a deeper system greater than 100 ft below the surface poorly connected to the shallow system. To verify the model, a study was conducted to determine geochemical characteristics and the relative age of water in each system testing the hypothesis that the shallow system would have younger and less chemically evolved water than the deep system.  Water samples were collected from four surface water sites, 11 different springs and 17 wells ranging from >30 ft to >200 ft deep.  Geochemical signatures of surface and shallow groundwater are CaHCO3 evolving to greater enrichment in Mg and SO4 in deeper groundwater. Tritium results indicate that the springs and creeks contain modern water (from precipitation since 1950) averaging 2.3 and 2.5 TU respectively. The deeper system contains pre-modern water (recharged before 1950) averaging 0.05 TU.  The shallow groundwater system, including the Edwards, contains locations with a mix of modern and pre-modern water or surprisingly pre-modern water only. Results imply that the Jollyville Plateau Edwards contains vadose zone conduits that transmit recharge water to springs and creeks with limited connection to the water table.