Capturing Biogeochemical Details in River Corridor Models

The Science

In many streams and rivers, water is exchanged between the open channel and adjacent groundwater. This exchange enables biogeochemical reactions in the near-stream sediments to remove or transform carbon, contaminants, and nutrients. Researchers from Oak Ridge National Laboratory developed a new modeling strategy to represent these effects in watershed-scale models. The new model is equivalent to existing multiscale transport representations when there are no reactions, but, unlike those existing models, it accommodates biogeochemical reactions. In contrast to alternative representations based on diffusion, the new model is able to represent the development of sharp gradients in oxygen concentrations in sediments near the river.

The Impact

Computer models for carbon, nutrient, and contaminant transport and transformations in river corridors inadequately capture current and emerging understanding of hydrological and biogeochemical processes because of a spatial scale mismatch between those processes and the systems that they impact. A new model for river-groundwater exchanges allows those processes to be represented at the scale at which they are typically studied while remaining tractable at watershed scales, thus establishing a framework for a new generation of river network biogeochemistry models.

Summary

While solute transport occurs primarily in flowing stream channels, biogeochemical transformations of carbon, nutrients, and contaminants often occur in highly localized metabolically active regions in the hyporheic zone, the region of saturated sediments adjacent to the stream channel. Representation of stream hyporheic-zone processes is thus a central challenge in extending stream network flow models to include biogeochemistry. Multiscale models with hyporheic-zone processes represented in subgrid models that are coupled to stream flow models provide an alternative to explicit three-dimensional representations, which are not feasible at watershed scales.

A new multiscale representation of stream hyporheic processes associates a one-dimensional subgrid model for transport and reactions with each channel grid cell in a stream network flow model. Each subgrid model represents a collection of streamlines that are diverted into the biogeochemically active hyporheic zone before returning to the flowing channel. The subgrid model is written in travel-time form, with hyporheic age serving as the independent spatial variable. In contrast to previous travel-time representations, the new model accommodates multiple mobile or immobile chemical species and general nonlinear biogeochemical reactions. Unlike alternative formulations based on multirate diffusion, the new multiscale model is able to represent biogeochemically important gradients in redox conditions.

Principal Investigator

Scott Painter
Oak Ridge National Laboratory
[email protected]

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This work is funded by the Environmental System Science program of the Office of Biological and Environmental Research, within the U.S. Department of Energy (DOE) Office of Science, and is a product of the Interoperable Design of Extreme-scale Application Software (IDEAS)-Watershed project and the Critical Interfaces Science Focus Area at Oak Ridge National Laboratory.

References