MULTIDOMAIN MODEL FOR PORE-SIZE DEPENDENT TRANSPORT OF SOLUTES IN SOILS

A multi-domain model for the transport of chemicals in soils is develo ped. The solute flux is related to the microscopic water flux, which i s modelled using concepts to estimate the hydraulic conductivity of po rous media. The pore space of the soil is divided into an arbitrarily large number of domains each representing an equivalent pore radius. T he domains are arranged on a structural coordinate, perpendicular to t he direction of mean water flow. Transport in the flow direction takes place in each domain by convection and diffusion with pore-size speci fic velocities. Solute mixing between the domains is simulated as conv ective-dispersive transport along the structural coordinate. The model is solved numerically for one-dimensional steady-state water flux und er unit-gradient conditions. Required input parameters are the unsatur ated conductivity function of a soil and a pore interaction coefficien t which characterizes the solute exchange between the pore domains, Si mulations show a gradual change from convection dominated transport (i solated tube model) to convective-dispersive transport. The length sca le where this change takes place depends on the lateral mixing intensi ty, pore-size distribution of the medium, and saturation degree.