Groundwater - surface water exchange is a common feature of larger river systems, especially those experiencing recharge from groundwater, variable surface water discharge, and relatively coarse riverbed and marginal sediments. Although common in the environment, the effects of groundwater - surface water interactions on river corridor hydro-chemical processes are not well understood. Here we describe the results of a long term study designed to identify factors controlling the dynamic concentration trends of dissolved uranium (U) in a persistent groundwater plume located in the river corridor of a major river system (Columbia River). The river experiences large seasonal variations in stage that cause inland river water intrusion, water table fluctuations, groundwater compositional changes, and temporal groundwater flow reversals. Comprehensive experimental and monitoring studies were conducted from the grain, to the well, and finally to the plume scale to characterize reaction networks, physical and chemical properties distributions, and seasonal hydrologic dynamics influencing key system analytes (Uaq, pH, specific conductance, Ca, and HCO3-/CO32-) over increasing observational scales. A goal was to identify process dominance at each scale, facilitated by application of reactive transport models including PFLOTRAN for field calculations. At the smallest scale (laboratory stirred reactors and columns with 1-D advection) system parameters were strongly influenced by the kinetic surface complexation of U (both adsorption and desorption) from surfaces and adsorptive, diffusion-limited intra-grain domains. At the well scale, where a small compact well field was monitored and injection experiments were performed, hydro-chemical parameters were strongly influenced by the kinetic remobilization of heterogeneously distributed, surface complexed U in the lower vadose zone during high river stage/high water table events, and mixing with intruded river water. In contrast, dynamic seasonal hydrology, and large scale hydro-geologic structures and depositional features (e.g., &gt;10 m) promoting groundwater-surface water exchange and mixing were found to be the most important factors controlling system parameters at the plume scale. Extremely coarse subsurface sediments presented only a small concentration of surface sites, in-situ, for adsorptive retardation in the saturated zone. Challenges to modeling this complex system will be described.

Surface complexation from the grain to plume scale in a gravel aquifer influenced by groundwater - surface water exchange: Considerations for system predictability

255th National Meeting (2018)

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