Simulating Streams as Biogeochemical Reactors

Water quality is an emergent property of solute interactions with process domains, such as stream corridors, aquifers, and soils. Here, we explore the utility of a "storage-exchange reference frame" that partitions a process domain as an "environmental reactor" having both a nonreactive transport compartment and a reactive storage zone. We developed a simulation model of stream corridors with a storage-exchange reference frame and varied the exchange of water and solutes between the channel (transport compartment) and hyporheic zone (reactive storage), and the first-order reaction rate constant governing reaction-rate kinetics in the reactive storage. We subsequently calculated Damköhler indices to quantitatively describe controls on environmental reactive transport. Whole stream Damköhler indices were strongly governed by the interaction of the biogeochemical reaction-rate constant with the hydraulic exchange of water and advective exchange of solutes between the channel and hyporheic zone. Thus, the storage-exchange reference frame is useful for incorporating the effects of linked channel-hyporheic processes in stream corridors and shows promise for extension to other process domains.