Session: Reactive Transport Modeling: A Cutting-Edge Tool for Investigating Coupled Processes:

The hyporheic zone is a hot spot for coupled transport and biogeochemical reactions at the aquatic-terrestrial interface. 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. The frequent inflow of river water into riverbed creates conditions conducive to the influx and transport of fine-grained POM, a biological colloid, 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 and its potential impact on dissolved oxygen distributions as a function of changing river stage and fluid flow rate and direction, with important implications for riverbed biogeochemistry and nutrient exchange in hyporheic corridor. 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. 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, carbon, N and Fe cycling) as influenced by the relative intensity of POM input and the availability of oxygen and other electron acceptors for microbial metabolism. The model can be applied to assess the potential impact of future environmental conditions or energy management strategies on watershed systems.

Steven Loheide III

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