Abstract
ABSTRACT Tracer testing is established as one of the best techniques for determining groundwater velocities and identifying groundwater flow directions in karstic systems. It has been employed in the Edwards (Balcones Fault Zone) Aquifer since the mid-1980s. Nontoxic, fluorescent organic dyes are most commonly used because they are comparatively inexpensive, relatively easily accessible, detectable at low concentrations, and not harmful to organisms that use or dwell in the aquifer or its springs. Tracer tests provide empirical evidence that is difficult to obtain any other way. Tracer tests have shown rapid groundwater velocities in the contributing, recharge, and artesian zones. Groundwater velocities were found to range from 915 to 9150 m/d in the Barton Springs segment of the aquifer; 1–3600 m/d in the San Marcos Springs area; 300–640 m/d near Comal Springs; 13 to >5300 m/d in San Antonio/northern Bexar County; and 1–1367 m/d in Kinney County, Texas. Tracer testing has shown: (1) preferential flow paths are conduit-dominated; (2) in places, there is a hydraulic connection with the underlying Glen Rose Formation; (3) large offsets on faults are not barriers to flow; and (4) portions of the aquifer act as separate pools.
Highlights
Four general areas of the Edwards (Balcones Fault zone) Aquifer have been evaluated since the mid-1980s using tracer tests
Tracer tests are powerful tools with which to investigate groundwater flow paths in the artesian, contributing, and recharge zones of the Edwards Aquifer, where groundwater velocities have been measured in thousands of meters per day
The tests revealed that the karstic nature of the Edwards Aquifer is characterized by high-velocity, discrete flow paths to a few large regional springs with three-dimensional hydraulic systems
Summary
Four general areas of the Edwards (Balcones Fault zone) Aquifer have been evaluated since the mid-1980s using tracer tests. Quantitative techniques are employed when groundwater conditions are fairly well known and travel times are relatively short (Alexander and Quinlan, 1996) During this testing, tracing agents, most commonly fluorescent organic dyes, were injected into sinking streams, surface streams, cave streams, and wells with direct connection to groundwater flowing in the aquifer. Not a formal tracing test, Zara Environmental (2010) described the biodiversity within the Edwards Aquifer (see Krejca and Reddell, this volume) based on the areal distributions of worms, gastropods, crustaceans, isopods, and fish that were collected from the aquifer This inventory did not provide point-to-point connection nor time-of-travel data, but it did establish the connectivity of voids sufficient in size in the aquifer to allow organisms to move between various sampling locations
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