Linking Surface Topography Variations To Subsurface Mixing And Reaction Patterns
Résumé
Fluctuations in surface topography generate nested streamline patterns in the subsurface over scales ranging from millimeters to kilometers. Because solute residence times can be very different for each streamlines, these patterns exert a strong control on biogeochemical reactions. While this effect has been quantified in reactive transport models, solute transfer across streamlines has been generally neglected. Yet, this process can lead to significant solute dilution and may trigger reactions by mixing water with different chemical compositions. Considering topography-driven subsurface flow cells of different sizes, we show that the resulting streamline structures act as shear flows, with shear rates that can vary over orders of magnitude depending on scale, permeability and hydraulic head gradient. This leads to the formation of localized layers of enhanced dilution and reaction, where mixing rates can be orders of magnitude larger than diffusion limited rates (Bandopadhyay et al. under review). We develop a theoretical model that predicts the depth and magnitude of these mixing hotspots and quantifies the resulting exports of conservative and reactive chemical species at discharge locations. We discuss consequences of these findings by applying this model at hyporheic zone, hillslope, and catchment scales.