Modeling transient water and solute transport in a biporous soil

In this study we investigate numerical simulations of one-dimensional water flow and solute transport in a soil with a nonuniform pore-size distribution, Water transport was modeled by treating the soil as one domain by applying Richards equation, while using alternatively a unimodal and a bimodal model for the hydraulic properties. The retention curves were fitted to a set of measured data; the relative conductivity functions were estimated by Mualem's [1976] model. Contrary to the unimodal case, the bimodal conductivity curve shows a steep decrease in water content theta near saturation. Simulated water regimes under transient boundary conditions differed strongly fur the two cases. The use of the bimodal functions yielded a preferential flow characteristic which was not obtained using unimodal functions. For both hydraulic regimes we modeled solute transport comparing four different variants of the convection-dispersion equation. For the classical one-region model we found that the breakthrough curve of an ideal tracer was not affected by the dynamics of the water flow. For the two-region approach, where the water-filled pore domain is divided into a mobile region theta(m) and an immobile region theta(im), three different conceptual treatments of theta(m) under transient conditions were investigated. For the case where theta(im) was kept constant, the different hydraulic regimes again caused only minor differences in solute transport. The same was true for the alternative case where the ratio theta(m)/theta was kept constant. However, for the third case, where theta(m) was treated as a dynamic variable which changes with the actual water content in a way that depends on the shape of the hydraulic conductivity function, the transport simulation based on the bimodal hydraulic model reflected enhanced preferential transport at high-infiltration rates.