Consistent Simulation of Pulse-Like Conservative and Reactive Stream-Tracer Experiments on Multiple Scales
Abstract
The redox-sensitive compound resazurin (Raz) has recently been introduced as reactive tracer for streams undergoing hyporheic exchange. Resazurin is converted to resorufin (Rru) in viable cells so that the Raz-Rru system acts as a probe for the metabolic activity of a stream, which is believed to be concentrated in the hyporheic zone. The reactive tracers, however, can also be sorbed. Advective-dispersive transport in the stream, potentially non-conventional transient storage in the hyporheic zone, sorption at the riverbed material, and transformation of the compounds leads to a coupled system that needs to be considered when analyzing pulse-like tracer experiments with the Raz-Rru system and a conservative tracer. We present a consistent model formulation of the reactive-transport system and apply it in the interpretation of experiments on multiple scales. (1) Column experiments with a length of about 10cm indicate that Raz and Rru undergo two-site sorption and chemical transformation during the passage through riverbed material. (2) We have analyzed breakthrough curves of the conservative and reactive tracers in piezometers within gravel bars during stream-tracer experiments. The conservative-tracer breakthrough curves in the gravel bar and in the stream directly beside it were deconvoluted to obtain the stream-to-piezometer travel-time distribution. Fitting the reactive tracer breakthrough curves, while accounting for the conservative travel-time distribution and the input signal in the stream, confirmed the two-site-sorption plus decay model of the Raz-Rru system in the hyporheic zone. (3) On the scale of entire stream reaches, we have jointly analyzed the conservative and reactive tracer breakthrough curves using a shape-free approach for the hyporheic travel-time distribution, two-site sorption of Raz and Rru in the hyporheic zone, and the Raz-to-Rru transformation. By this, we could identify the stream velocity, in-stream dispersion coefficient, the hyporheic exchange rate, the time distribution over which the water stays within the hyporheic zone, the transformation coefficient, and sorption parameters of the riverbed. Consistent data sets for multiple distances along the stream could be identified. In one application, where stream velocities were in the range of 1.4cm/s, the Raz-to-Rru transformation seems to have occurred in the open water body rather than in the hyporheic zone. This presentation focuses on the conceptual model assumptions and mathematical description of the reactive-transport system at hand.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H14B..07L
- Keywords:
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- 1830 HYDROLOGY / Groundwater/surface water interaction