The Role of Groundwater and River water Interactions in Modulating Land Surface and Subsurface States and Fluxes: A Local-Scale Case Study along the Columbia River Shoreline

Lateral flow and transport between groundwater and river water through the subsurface interaction zone (SIZ) is a major pathway for energy, water, solute, and gas transfer between terrestrial and aquatic systems. Groundwater - surface water exchange is significant at multiple scales, but is not adequately resolved in Earth System Models (ESMs). In this study, an integrated land surface and subsurface model enhanced with hydrologic exchange was assembled within the land component of an ESM (i.e., the Community Land Model (CLM) coupled with PFLOTRAN) to investigate how land surface and subsurface states and fluxes are influenced by the lateral flow and mixing of waters within the SIZ. The new model was applied to a domain including 400 m of the Columbia River shoreline where subsurface properties and processes have been well-characterized through sediment characterization, pump tests, tracer experiments, and field monitoring of river water intrusion events driven by river stage changes. Simulations of CLM-PFLOTRAN at multiple spatial resolutions were conducted using observed meteorological and river stage data under different climate and hydrologic conditions. The coupled model revealed the importance of interaction zone processes in regulating temporal and spatial variability in land surface and subsurface hydrological fluxes and state variables, indicating strong nonlinear coupling between hydrologic and biogeochemical processes in riparian zones. The simulations are validated against field measurements collected at the site. Our results provide a foundation for better understanding the spatial and temporal dynamics of biogeochemical cycling and biogenic gas generation in the SIZ, and their regulation by the changing water cycle and climate.