Groundwater Tracing Study of the Barton Springs Segment of the Edwards Aquifer

Nico M. Hauwert
James W. Sansom Jr.
David A. Johns

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Groundwater tracing conducted in the Barton Springs segment of the Edwards Aquifer from 1996 through 2000, has provided new insight on groundwater flow directions and velocities. A groundwater tracing study performed by the Barton Springs/Edwards Aquifer Conservation District and the City of Austin has measured groundwater velocities and destinations from 17 natural recharge features and one well within the Barton Springs segment. A total of 20 traces were conducted in all the major contributing watersheds supplying water to Barton Springs, including Barton, Williamson, Slaughter, Bear, Little Bear, Onion Creeks, as well as the Blanco River. This study accomplished its data quality objectives by detecting 85% of the traces in at least one monitoring site. To conduct the traces, up to five distinct organic tracers were used: fluorescein, rhodamine WT, eosine, sulforhodamine B, and pyranine. The tracers were injected into caves, sinkholes, and other recharge features and were generally flushed into the aquifer with about 10,000 gallons of water. In one trace, dye was poured into a well that was flushed with creekwater flowing adjacent to the well. Activated charcoal receptors were placed at wells and springs where tracer arrival was possible, in order to adsorb cumulative concentrations over the placement duration. Grab samples were also collected to measure tracer concentrations at specific times. Potentiometric surface, geological, and cave maps were used to estimate groundwater-flow paths in between the tracer detection sites. The results of this study demonstrate that groundwater recharging the Barton Springs segment in the Barton and Williamson Creek watersheds travels either north or northeast towards either Barton or Cold Springs. Portions of the upper Recharge Zone of Barton Creek (from the Mount Bonnell Fault down to the Loop 360 crossing) and Williamson Creek (from Highway 290 down to the Brush Country Road crossing) contribute flow to Cold Springs and other springs on the south bank of the Colo