PARTICULATE ORGANIC MATTER TRANSPORT IN EXPERIMENTAL RIVERBED SAND COLUMNS
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Abstract The hyporheic zone is a hot spot for coupled transport and biogeochemical reactions at the aquatic-terrestrial interface. The influx of particulate organic matter (POM), a biological colloid, into the near-surface riverbed plays a critical role in fueling microbial activity in the hyporheic zone. Hydrological exchange associated with natural and human-regulated river stage fluctuations is likely to influence POM transport and influx to the riverbed. Despite numerous studies of dissolved organic matter (DOM) transport and metabolism in the hyporheic zone, there is a paucity of information on the process that govern POM transport in the benthos of hydrologically dynamic riverine environments. In this study, three types of small-scale column transport experiments were conducted to examine POM transport using periphyton-derived photosynthetic biomass obtained from the Hanford Reach of the Columbia River in saturated, sandy porous media; (1) breakthrough experiments (BR) to understand the governing processes controlling POM transport, (2) velocity experiments (VE) to examine the effect of fluid flow rate on POM behavior, (3) flow reversal experiments (FR) to address the impact of flow direction on POM transport and remobilization by simulating flow reversal in the hyporheic zone. A numerical transport-reaction model was developed to quantitatively interpret the results of the laboratory column experiments. The combined experimental and modeling result reveals that POM retention was governed by colloid filtration and rate-limited reversible sorption to the sediment matrix. Furthermore, POM was remobilized when the flow direction was reversed, creating a pulse of suspended POM above the initial input concentration. These results suggest an interplay between river stage changes and the relative intensity of POM accumulation in the hyporheic zone. The POM transport-reaction model, calibrated according to the experimental results, is appropriate for use in a more complex hydrobiogeochemical models to evaluate the impact of POM transport and accumulation on riverbed biogeochemistry.