LATE-TIME TRACER TEST METHOD FOR MEASURING TRANSIENT STORAGE RESIDENCE TIME DISTRIBUTIONS IN STREAMS

We have developed a method to analyze tracer tests that quantifies the distribution of transient storage residence times. After injection of a tracer into a stream, some of the tracer moves into transient storage, comprised of the hyporheic zone and dead zones within the stream channel. After the main pulse of tracer passes a downstream measurement point, the concentrations in the tail of the breakthrough curve provide a simple and direct measurement of the transient storage residence time distribution (RTD). Our method differs from previous, related methods in that we make no a priori assumptions about the form of the RTD. The method is theoretically able to quantify any RTD, though in practice we have only explored a few common RTDs (e.g., power-law, exponential/multi-exponential, log-normal). We outline the method, provide several examples, and highlight a computer code that has been developed to use the method (STAMMT-L, Haggerty and Reeves, 2002). Lastly, we provide an overview of the insights gained to date, including issues associated with fractal scaling of transient storage that have become apparent using this method.