QUANTITATIVE TRACING AS A PREDICTIVE TOOL TO ASSESS THE POTENTIAL IMPACTS OF HAZARDOUS MATERIALS TO WATER SUPPLIES AND ENVIRONMENTAL RECEPTORS

Water soluble fluorescent compounds are an important tool for groundwater tracing, especially in fractured and conduit dominated aquifers. Tracers can be used to determine groundwater velocities and flow paths and can also be used as contaminant surrogates to predict contaminant peaks and durations at water supplies and important environmental receptors such as springs and streams.

The Edwards Aquifer Authority has been performing a series of quantitative tracer tests over the last four years to determine a range of stage dependant groundwater velocities to public water supplies utilizing the Edwards Aquifer. In addition, the data has been used to help understand the hydraulic systems that feed Comal and San Marcos springs, the two largest spring complexes in the southwestern United States. These springs provide habitat for a number of threatened or endangered species.

By injecting a known quantity of a fluorescent compound into wells, caves and sinkholes, and using automatic water samplers to collect frequent samples at suspected receptors, the Authority has been able to calculate groundwater velocities, identify flow paths, measure peak concentrations and duration, and estimate conduit parameters such as diameter and Reynolds number. By using the ratio between the quantity of tracer used and the peak detection at the receptor, a reasonable estimate can be used to predict contaminant concentrations at receptors of concern. This technique was used in the area adjacent to a proposed city bypass to support the need for design of hazardous material traps. It can also be used to assess the vulnerability of a public water supply or environmental receptor.