Transport-reaction modeling of particulate organic matter dynamics in riverbed sediments

Abstract:

A one-dimensional advection-dispersion-reaction model was developed to simulate the transport and metabolism of particulate organic matter (POM) in riverbed sediments. The specific focus for the modeling is the hyporheic zone of the Columbia river in the vicinity of the Hanford 300 Area study site in eastern Washington. At this site large fluctuations in river stage take place both naturally (i.e. seasonally) and in conjunction with hydroelectric power dam operations. Such fluctuations create conditions conducive to the influx and transport of fine-grained POM within near-surface riverbed sediments. Although a great deal is known about dissolved organic matter (DOM) transport and metabolism in hyporheic zone sediments, there is a paucity of quantitative information on POM dynamics. Very few models have tried to account for infiltration of POM into riverbed sediments, with most work devoted to understanding the transport of Cryptosporidium oocysts. Our goal was to assemble a hydrobiogeochemical model capable of predicting the transport and metabolism of POM as a function of changing river stage and fluid flow rate and direction. In keeping with prior models of colloidal particle transport, the model accounts for velocity-dependent POM filtration as well as reversible sorption of POM to the sediment matrix. A standard suite of POM metabolic pathways is depicted according to modified Monod kinetics, driven by first-order decay of suspended and solid-associated POM as well as DOM produced during POM decay. POM filtration/sorption and organic matter metabolism parameters were based experimental studies with Hanford 300 area riverbed materials. Fluid flow dynamics and boundary conditions on solute concentrations and the abundance of POM subject to influx into the sediment were constrained by in situ measurements at the Hanford 300 area study site. The model was used to simulate the influence of periodic changes in the direction (either into or out of the riverbed) and rate of fluid flow within the upper 50 cm of the riverbed. Modeling results suggest a complex interplay between fluid flow rate/direction and riverbed biogeochemical dynamics (e.g. redox zonation, N cycling) as influenced by the relative intensity of POM input and the availability of oxygen and other electron acceptors for microbial metabolism.