SLAC Floodplain Hydro-Biogeochemistry Science Focus Area: Soil-Gravel Bed Interfaces as Hydro-Biogeochemical Control Point in a Mountainous Floodplain Aquifer


Tristan Babey1,2* (, Lijing Wang2, Zach Perzan2, Brian Rogers2, Sam Pierce1, Rosemary Carroll3, Kristin Boye1 (, Kate Maher2


1SLAC National Accelerator Laboratory, Menlo Park, CA; 2Stanford University, Stanford, CA; 3Desert Research Institute, Reno, NV



Montane alluvial floodplains represent important water quantity and quality moderating ecosystems. They are characterized by heterogeneous geological architectures resulting from depositional processes. Typically, sharp (<50 cm wide) redox interfaces develop in the vicinity of lithological transitions and are associated with intense contaminant and nutrient cycling. The cumulative exchanges of water and solutes across these interfaces are challenging to assess as the direction and intensity of interfacial exchanges respond strongly to seasonal shifts in hydrology, including extreme events like flooding and drought. As climate change reshapes temperature and precipitation regimes throughout mountainous watersheds, the incorporation of hydro-biogeochemical interfaces into models could be key to predict water quality and availability in these environments.

The SLAC Floodplain Hydro-Biogeochemistry SFA developed numerical simulations of an alluvial aquifer located near Crested Butte, CO. This aquifer, which underlies the Slate River, is characterized by an extensive (3600 m2) interface between an overlaying fine-grained, predominantly anoxic soil and an underlying, mostly oxic cobble and gravel alluvium. Simulation results emphasized the strong spatiotemporal variability of exchanges at the soil and gravel bed interface, which in turn determines the biogeochemical function of the soil as a potential sink or source for redox-sensitive solutes. In particular, flooding events from the construction of a beaver dam greatly increased downward drainage from the soil and the export of reduced solutes to the gravel bed, which were then quickly transported in the down-valley direction. In contrast, drought events induced upward water movement with the gravel bed acting as a reservoir for the floodplain vegetation. Thus, alternating hydrological forcings resulted in the development of a geochemically dynamic mixing zone at the interface between the gravel bed and the soil. Sensitivity analyses highlighted that the extent of this mixing zone—and more generally of the response of similar floodplain environments to hydrological shifts—is primarily linked to the geological architecture of the aquifer, with contiguous floodplain sediment layers acting as preferential flow-paths that propagate the influence of hydrological perturbations throughout the alluvial aquifer.