Simulating Surface Water-groundwater Exchange using High-Performance Particle Tracking

The hydrologic exchange of surface water through subsurface domains is ubiquitous across river corridors and is enhanced by dynamic river stage conditions. These exchange flows and their biogeochemical consequences are important to watershed functions because up to 96% of ecosystem respiration occurs in zones of hydrologic exchange. In the past three years, we have conducted several studies that use high-performance particle tracking (e.g., up to 100 million numerical particles) to investigate flow path, transit time, and biogeochemical implication of river water exchange in large dynamic river corridors. These simulation results were validated against groundwater well monitoring data and then used to develop mechanistic understanding of the processes linking dynamic river stage to exchange flows and biogeochemical function. Our results revealed that multifrequency flow variations led to multimodal TTDs that varied in time and space. Dam-induced high-frequency (subweekly) flow variations increased additional hydrologic exchange flows with short (subweekly) transit times, which accounted for up to 44% of reactant consumption in the river corridor along the Hanford Reach of Columbia River. The transit time of river water in the aquifer exhibits complex patterns that are affected by all the river geomorphologic, hydrodynamic, and hydrogeologic factors and are strongly correlated with the downwelling ratio of exchange flux. The particle tracking results were also used to build surrogate models of hydrologic exchanges using machine learning techniques. In this presentation, we will summarize the aforementioned particle tracking studies and the simulation code we develop for these applications. In addition, we will also introduce a new random walk particle tracking code we recently built for unstructured finite volume groundwater simulators (e.g., PFLOTRAN and ATS).