Transport of colloidal particulate organic matter in experimental riverbed sand columns

Stream and riverbed sediments are hot spots for coupled transport and biogeochemical processes at the aquatic-terrestrial interface. The influx of fresh, metabolically labile particulate organic matter (POM) into the near-surface riverbed plays a critical role in fueling microbial activity in the hyporheic zone. Despite the importance of POM in driving biogeochemical reactions, key questions remain about the physical transport of POM under transient and reversable flow conditions. This study examined the transport of fresh periphyton-derived photosynthetic biomass in laboratory columns containing sandy porous sediment from the Hanford Reach of the Columbia River. Breakthrough (BT) experiments at different fluid flow velocities, with and without reversals in flow direction, were used to assess the processes controlling POM transport. Although the timing of initial POM breakthrough was the same as the conservative tracer, extensive retention (ca. 60-80 %) of photosynthetic biomass was observed. Filtration coefficients (ca. 0.05-0.08 cm^-1) did not vary systematically with fluid flow velocity. The experiments demonstrated significant tailing on the falling limb of BT curves. Transport-reaction modeling indicated that these results could be explained by a combination of particle filtration and rate-limited attachment/detachment of colloids to/from the sediment matrix. Flow reversal experiments examined the potential impact of flow direction change on POM remobilization, a common but previously unquantified and unmodeled phenomenon. Significant remobilization of POM took place during flow reversal following initial POM influx, creating a pulse of suspended POM outflow above the initial input concentration. Incorporation of a first-order remobilization process into the transport-reaction model was able to reproduce pulses of remobilized POM associated with multiple flow reversals. The results of this study shed light on the interplay between river water-hyporheic zone fluid exchange and the direction/intensity of fresh POM transport in the near-surface riverbed.