Solute mass transfer through the macropore-matrix interface during preferential flow in structured soils: model development

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The objective of this project is to develop an extended two-region macroscopic scale mass transfer term in two-domain models for describing preferential transport of reactive solutes in structured soils. This term should be related to mm-scale spatial distributions of local scale physico-chemical properties at the macropore-matrix interface. The idea is to include up-scaled information on locally heterogeneous sorption sites observed at intact surfaces of macropores (e.g., biopore walls, cracks and coated soil aggregates) as effective retardation parameter in the kinetic mass transfer description of the macroscopic scale model. Simulated local scale 2D and 3D mass transfer scenarios will serve as a reference. The local scale simulations will be calibrated using dye staining experiments and X-ray CT-based information on the pore structure of coating and matrix. A macroscopic transfer term will then be evaluated by comparing the averaged locally-distributed with the macroscopic mass transfer rates for various initial and boundary conditions and for samples from three structured soils. The evaluated mass transfer term will finally be tested by comparing soil column percolation experiments for the three soils with simulation results obtained with a 1D dual-permeability model with the improved mass transfer term.We expect further improvement un understanding the relation between soil structure and preferential flow, and of effects soil manangement on the movement of solutes, in particular reactive solutes, during preferential transport.