PNNL

The Science
Hydrologic exchange fluxes (HEFs) between rivers and surrounding subsurface environments strongly influence water temperatures and biogeochemical processes. Yet, quantitative measures of their effects on the strength and direction of such exchanges in large rivers are lacking. A study reported in Hydrological Processes, led by scientists at Pacific Northwest National Laboratory (PNNL), demonstrates the efficacy of a new coupled surface and subsurface fluid dynamics model in quantifying HEFs at kilometer scales.

The Impact
In a world where dam-regulated river corridors are increasingly common, quantifying HEFs and their effects at river-reach scales is vitally important in protecting water quality and ecosystem health. Through 3D application of computational fluid dynamics (CFD) modeling, combined with uncertainty quantification tools, the new model can quantify HEFs in a large-scale river channel extending 1-km wide and 7-km long. This a dramatic improvement over traditional simulations, which (at most) model just a few hundred meters of river corridor.

Summary
HEFs are critical to shaping hydrological and biogeochemical processes along river corridors. Yet, in current research, numerical modeling studies to quantify riverine HEFs are typically confined to local-scale simulations in which the river is a few meters wide and up to a just few hundred meters long. Even then, such studies are challenging because of high computational demands and the complexity of riverine geomorphology and subsurface geology. In addition, there are limitations in field accessibility, and the physical demands of labor-intensive data collection along river shorelines.

A new model, developed by a multi-institutional team, addresses these challenges. Their recently published paperin Hydrological Processes demonstrates a new coupled surface and subsurface water flow model that can be applied at large scales.

The new model was validated against field-scale observations—including velocity measurements from an acoustic Doppler current profiler, a set of temperature profilers installed across the riverbed to measure vertical HEFs, and simulations from PFLOTRAN, a reactive transport model. Then, along a 7-km segment of the Columbia River which experiences high dam-regulated flow variations, the model was used to systematically investigate how HEFs could be influenced by surface water fluid dynamics, subsurface structures, and hydrogeological properties.

The simulations demonstrated that reach-scale HEFs are dominated by the thickness of the riverbed alluvium layer, followed by alluvium permeability, the depth of the underlying impermeable layer, and the pressure boundary condition.

These results are being used to guide the design and placement of new field sensor systems that will further enhance scientific understanding of HEFs in large dam-regulated rivers.

Contact
Jie Bao, Pacific Northwest National Laboratory, Jie.Bao@Pnnl.gov

Funding
Funding was provided by the U.S. Department of Energy (DOE) Office of Biological and Environmental Research (BER) as part of the BER’s Subsurface Biogeochemistry Research (SBR) program. This research is part of the SBR Scientific Focus Area project at PNNL.