Kaolinite exchange between a stream and streambed: Laboratory experiments and validation of a colloid transport model

Experiments were conducted in a recirculating flume to elucidate the fundam ental physical and chemical processes which control the stream-subsurface e xchange of colloids. Results are presented on the rate of exchange of collo ids (kaolinite day) and a conservative solute (lithium) from a stream to a sand streambed covered by stationary bed forms (dunes, ripples). Kaolinite and lithium were added to the recirculating stream, and their exchange with the bed was observed over time. Kaolinite was observed to be much more ext ensively trapped in the streambed than lithium owing to nonconservative pro cesses. By the end of most experiments, essentially all added kaolinite was taken up by the streambed. The observed exchange rates can be explained by analyzing the solute and particle fluxes through the stream-subsurface int erface and the physicochemical interactions between transported kaolinite a nd the bed sediment. The colloid pumping model predicts particle exchange b ased on pumping hydraulics, particle settling in the bed, and filtration by the bed sediments. Observed colloid and solute exchanges were successfully predicted by the process-based models without the use of fitting coefficie nts. Hydraulic parameters measured in the flume and particle parameters mea sured in separate experiments were used as model inputs. The successful pre diction of experimental results validates the modeling approach of combinin g a fundamental hydraulic exchange model with a physicochemical model for c olloid transport and filtration in the streambed. Further, because colloid transport behavior was interpreted in terms of basic exchange and trapping processes, the results of this study are expected to be directly applicable to the analysis of fine sediment dynamics in natural streams.