Scale-dependent Spatial Variabilities of Hydrological Exchange Flows and Transit Time in a Large Regulated River

Abstract Hydrological exchange flows (HEFs) across the river-aquifer interface and the associated transit time of river water in the aquifer have important implications for contaminant plume migration and biogeochemical processes in the river corridor. HEFs and transit time are influenced by both subsurface physical features and hydrologic forcing related to the transport process, which can exhibit complex spatial and temporal variations. In this study, we used a massively parallel subsurface flow model and a particle-tracking model to study the influences of different control factors on spatial variability of HEFs and transit time distributions in the Hanford Reach of the Columbia River in Washington State. 100 million particles were randomly injected in time and space and then tracked in a model domain that covers a 51-km2 area (15.1million model cells). We used hourly river stages and groundwater levels to drive the model to provide dynamic velocity fields for the particle tracking in the two+-year simulation period. The groundwater flow simulation and particle-tracking results provide a comprehensive assessment of the spatial distribution of HEFs and transit time in a large, complex river corridor. Our results show that the aquifer hydrogeological structure has strong correlations with the extent and magnitude of exchange flux. The transit time 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 new insights gained through this study can be used to support the development of reduced-order models of HEFs and transit time distributions for large complex river systems.