Influence of Hydrologic Exchange Flows on Biogeochemical Dynamics in A Regulated River Reach: 3-D Reactive Transport Modeling in PFLOTRAN

Abstract:

Hydrologic exchange flows play an important role in modulating river corridor biogeochemical processes and ecological functions. They control the transport of nutrients, contaminants, and heat exchange across the aquatic interface, and thus the dissolved oxygen levels and microbial reaction rates. However, few studies have simulated the biogeochemical processes in river corridors with complex geomorphology and subsurface heterogeneity under highly dynamic flow conditions. Here, we have developed a 3-D, high spatial and temporal resolution, groundwater flow, heat and reactive transport model using PFLOTRAN, a massively parallel subsurface simulator. This model is used to simulate the dissolved oxygen, nitrogen and carbon dynamics in the riparian river corridor of the Hanford Reach of the Columbia River. We employ an unstructured mesh with a higher grid refinement near the aquatic interface and a coarser refinement elsewhere. This allows us to capture the fine-scale biogeochemical reactions near the river-aquifer interface while increasing computational efficiency on HPC. Our model is validated with field observation data collected in groundwater wells and aquifer tubes. A set of scenarios are also simulated to identify the dominant hydrologic and geologic factors controlling the biogeochemical processes. Our study demonstrates the importance of using 3-D, high resolution reactive transport model to capture the significant spatial and temporal variability of important nutrients and contaminants in the river corridors.