PNNL

The Science                                

Large dam-regulated rivers experience complex flow variations induced by both natural and human-caused events. These variations change the patterns of how river water and groundwater mix, which influences nutrient cycles and thus river water quality. Researchers at Pacific Northwest National Laboratory used numerical particle tracking to evaluate how flow variations interact with physical subsurface variations to control the spatial and temporal patterns of groundwater–river-water mixing and alter nutrient consumption rates at the Hanford Reach of the Columbia River. They found that the complex river stage variations led to complex transit-time distributions because exchange pathways change under different flow conditions. Numerical particle tracking provides an efficient method to capture the substantial variability and dynamics of water exchange in complex river corridors where field experiments are not feasible.

The Impact

In this study, hourly and daily river stage variations, controlled by upstream dam operations, increased both water exchange rates and nutrient consumption in the river corridor. These results indicate that frequent flow variations along rivers regulated by dams generally increase nutrient cycling in river corridors, especially for fast reactions such as aerobic respiration. A better understanding of the fundamental relationships between river flow, water exchange, and nutrient cycling can inform decisions regarding river regulation to maximize potential benefits or minimize potential drawbacks to river ecosystems.

Summary

Dams upstream and downstream of the Hanford Reach of the Columbia River induce frequent variations in river stage at this site. To understand how this variation affects hydrological exchange and nutrient cycling downstream, researchers first used the extensive site characterization data available for this river corridor to build a baseline groundwater flow and transport model. Then they used a forward particle-tracking method to estimate transit-time distributions for water flowing through the subsurface aquifer during a seven-year simulation window. The researchers then paired the estimated transit-time distributions with the rates of aerobic respiration and denitrification known to occur along this section of river corridor to quantify rates and amounts of nutrients being processed. Finally, the researchers evaluated the effects of dam operation on transit times and nutrient cycling rates and amounts.

The researchers found that dam-induced high-frequency variations in flow increased hydrologic exchanges between the river water and groundwater. These increases accounted for 44% of nutrient consumption in the river corridor along the Hanford Reach. The numerical particle-tracking approach developed in this study can be extended to other study sites that have robust site characterization data. This approach is also essential to extend nutrient cycling models from river reaches to larger-scale watersheds and basins.

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 BER’s Subsurface Biogeochemical Research Program (SBR). This contribution originates from the SBR Scientific Focus Area at Pacific Northwest National Laboratory. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the DOE Office of Science.