Fine particle filtration in riverbeds and its impact in hyporheic exchanges.

Understanding hyporheic exchange dynamics has proven to be of immense value for taking advantage of riparian services, e.g., nitrification/denitrification and carbon storage. However, a great deal of interactions are not well understood - one of them being the role of fine particle (FP) deposition in hyporheic exchanges. It has been shown that FP low concentrations in the surface water are an important driver in clogging pores in the bedforms and in hindering hyporheic exchanges. Hence, we have built a numerical model able to represent the spatial distribution of FP accumulation in riverbeds, thus, able to assess the spatial variability of permeability and the decrease of hyporheic exchange that this entails. The model consists of a set of transport equations for describing the FP movement and filtration within the porous media. They are coupled with Darcy's law for describing flow patterns, connecting then the spatial variation of deposited clay with the consequential spatial variation in permeability. The basis for setting up the numerical model was the experiments reported by Fox et al. (2018) where measurements of kaolinite deposition on immobile sand dunes were performed. Their results were used to estimate uncertain parameters of the model and a good fit was achieved. Although the proposed model is a non-linear system where both flow and fine particle deposition depend on each other's evolution, a steady state can be reached when the pores in the shallowest layers are completely clogged. At this point, transport to deeper regions is completely hindered and the connection between surface water and groundwater is basically lost. The importance of considering the spatial variability of permeability becomes clearer when comparing our model results with previous attempts of clogging models where spatially variable permeability is not considered, but rather constant values are used.