PNNL

The Science
Hydrologic exchange flows (HEFs) increase the contact between river water and subsurface sediments thereby playing a critical role in biogeochemical and ecological functions along river corridors. In a recent paper led by Pin Shuai and Xingyuan Chen at Pacific Northwest National Laboratory (PNNL), researchers found the dominant factors controlling the hydrogeochemical signatures of HEFs along a dam-regulated river reach are river channel morphology and (predominantly) a river channel’s subsurface hydrogeology. These features were found to control the locations of high exchange flow rates—that is, the “hot spots.” They also found that the magnitude and timing of river stage fluctuations controlled hydrological “hot moments”—a term for the timing of high exchange rates.

The Impact
This research improves scientific understanding of hydrogeomorphic controls on HEFs at river-reach scale under high-frequency flow variations, an important issue in an era of energetic dam-building worldwide.The paper also demonstrates the influences of river water intrusion on the migration of groundwater contaminant plumes—particularly for contaminant sources located within the preferential flow path shaped by ancient, deep river remnants called paleochannels. Importantly, the paper’s modeling approach and main findings are transferrable to other river corridor systems that experience regular, periodic fluctuations.

Summary
HEFs across the interface of a river and its aquifer have important implications for biogeochemical processes and for contaminant plume migration in river corridors, including those that are increasingly regulated by dams across the world. Yet little is known about the hydrogeomorphic factors that control the dynamics of HEFs under dynamic flow conditions.

To help close that knowledge gap, this follow-up study to Song et al. 2018 expands the model domain from a 2-D transect to a simulated 3-D river corridor. In this new paper, the domain now covers the entire Hanford Reach of the Columbia River. The results demonstrate large spatial and temporal variability in exchange flow magnitude and direction in response to dynamic river flow conditions. The study also highlights the role of upstream dam operations in enhancing the exchange between river water and groundwater. In turn, that enhanced exchange posits a strong potential influence on associated biogeochemical processes and on the fate and transport of groundwater contaminant plumes in river corridors.

This is the first study to mechanistically simulate, at relatively fine resolution, reach-scale hydrologic exchange as it is influenced by dynamic river-stage variations, channel morphology, and subsurface hydrogeology. Because of complex geologic and dynamic flow boundary conditions, the authors faced a great challenge in running their large numerical model (60 x 60 km) using relatively fine model resolution.

However, they were able to develop a large groundwater model using PFLOTRAN, developed by the U.S. Department of Energy (DOE), a next-generation, massively parallel, reactive flow and transport simulator. This scheme, typically employed to simulate the migration of contaminants in groundwater, enabled researchers to use reasonably fine grids (100m horizontally and 2m vertically), while at the same time simulating the complexity of a large field setting. To perform their simulations, the researchers also employed resources from the National Energy Research Scientific Computing Center.

In all, the PNNL-led research aligns with DOE’s mission to provide next-generation science-based models of watershed systems. The next step, already underway, is to study the effect of dam operations on river corridor thermal regimes and the resulting implications for river ecology.

Contact
Xingyuan Chen, Pacific Northwest National Laboratory, Xingyuan.Chen@pnnl.gov

Funding
This research was supported by the U.S. Department of Energy (DOE), Office of Biological and Environmental Research (BER), as part of the Subsurface Biogeochemical Research Scientific Focus Area (SFA) at Pacific Northwest National Laboratory (PNNL).