RECONCILING DIFFERENCES BETWEEN RESIDENCE TIMES INFERRED FROM CONTROLLED-TRACER TESTS AND ENVIRONMENTAL TRACERS IN FRACTURED-ROCK AND KARST AQUIFERS

Environmental tracers are constituents entrained in ground-water recharge, the concentrations of which can be interpreted to infer the residence time of ground water. Collecting water samples for environmental tracers requires access to ground-water sampling locations. In contrast, controlled-tracer tests require a greater investment, including the injection of a known mass of a chemical constituent and continuous monitoring at sampling locations. Conducting tracer tests under ambient flow regimes may not be successful in complex geologic environments, because the tracer may move through preferential flow paths and bypass sampling locations. It is cheaper to collect and analyze water samples for environmental tracer than it is to conduct tracer tests; however, environmental tracers and tracer tests often yield conflicting results in the characterization of ground-water residence times in complex settings, such as fractured and karst aquifers.

The interpretation of the concentrations of environmental tracers is usually based on piston-flow or binary mixing models that may not reflect the geologic controls on ground-water flow in fractured and karst aquifers. Piston-flow and binary mixing models imply an extremely simplified interpretation of the ground-water velocity, whereas the connectivity of permeable features in fractured and karst aquifers is likely to yield a wide distribution of velocity. This wide distribution of velocity is usually reflected in the breakthrough curves of tracer tests. Reconciling the differences between the interpretations of environmental tracers and tracer tests requires the application of a more realistic range of ground-water velocities in computing the concentration of environmental tracers at sampling locations from the time-varying input of the environmental tracer in ground-water recharge. The velocity distribution cannot be uniquely identified solely from environmental tracer data. Tracer tests can provide evidence of the fastest velocities and a range associated with the velocity distribution. Tracer tests and the application of environmental tracers in the crystalline rock of central New Hampshire and the fractured and karst Madison limestone in western South Dakota are used as examples where it is necessary to reconcile the interpretations of environmental tracers and tracer tests.