The effects of partly irreversible solute exchange: comparison between conservative and sorptive transport in streams

Contaminant transport in streams and rivers is often affected by the solute exchange between the main stream and various adjacent storage zones as well as chemical and biological processes. This paper focuses on the difference between the effects of conservative and reactive solute exchange, respectively, in terms of an analytical transport model that takes into account main stream advection, solute exchange with storage zones, instantaneous equilibrium sorption and first order reactions. The effects of longitudinal variation of water flow and hydraulic radius as well as transversal variation of streambed porosity were included in the analysis, which was conducted on a reach by reach basis. To be able to use a temporally variable upstream concentration as boundary concentration, the measured breakthroughs were divided into 30–150 segments of constant concentration and the solutions were obtained as the sum of solutions for all segments. An experiment in Uppland County in Sweden, constitutes the basis for this study. Results from a simultaneous injection of the tracers tritium, 3H 2O, and chromium, 51Cr(III), made it possible to distinguish between conservative and reactive transport along a 30 km reach of the Säva Stream. The peak concentration of the reactive chromium decreased faster with distance, than that of tritium. The main hypothesis is that the chromium exchange is affected by both rapid sorption processes and slower reactions in the subsurface storage zones. In this framework, the slow processes appear as irreversible, which is true at least on the time scale represented by the data. The irreversibility is probably caused by the net effect of various slow chemical processes, e.g. solute precipitation/co-precipitation and irreversible sorption, which decreases the washout rate after the main bulk of the solute has passed by in the stream. A fully reversible exchange description, with instantaneous equilibrium partitioning, gives a clearly overestimated tail for chromium, which is not the case with the present concept. Results obtained by independently evaluating seven subreaches of the stream give mean Reaction Damköhler numbers for the storage zone equalling 2.63 for the transport of 51Cr and 0 for 3H 2O.