NON-PARAMETRIC COMPARISON OF RESIDENCE TIME DISTRIBUTIONS DERIVED FROM CONSTANT-RATE AND INSTANTANEOUS STREAM TRACER EXPERIMENTS

Artificial tracers are frequently employed to characterize solute residence times in stream systems and infer the nature of stream water interaction with groundwater. When the duration of tracer application is different between experiments, the resulting tracer breakthrough curves at downstream locations are difficult to compare directly due to the inherent differences caused by application technique. We applied a recently developed numerical deconvolution method to derive stream residence time distributions (RTDs) from the results of constant-rate release experiments, allowing a novel, non-parametric comparison to normalized data from instantaneous release experiments. The non-parametric, direct comparison can be graphically represented by plotting the deconvolved and normalized RTDs on the same scale. We use this approach to demonstrate whether field tests of different duration provided the same information regarding hydrologic storage in our study reaches. Paired short- and long-duration field experiments were performed in each reach using instantaneous and constant-rate tracer releases, respectively. The experiments were conducted in two study reaches that were morphologically distinct in channel structure and substrate size. RTDs were derived by normalization and deconvolution of instantaneous and constant-rate results, respectively. For both reaches, comparisons of RTDs between instantaneous and constant-rate release experiments demonstrated nearly identical hydrologic retention characteristics inferred from short- and long-term tracer tests. Because non-parametric RTD analysis does not presume any shape of the distribution, it is useful for comparisons across tracer experiments with variable inputs and for validations of fundamental transport model assumptions.