Simulation of solute transport in a heterogeneous vadose zone describing the hydraulic properties using a multistep stochastic approach

Abstract

The simulation of conservative solute transport in a heterogeneous unsaturated soil depends on the description of spatial variability of soil hydraulic and chemical properties. The data from the Las Cruces Trench Site were used to explore the impact of alternative ways of describing the variability of hydraulic properties. Three different approaches were considered: Miller and Miller scaling, Leverett scaling, and a multistep approach involving categorization of water retention curves. Conditional sequential geostatistical simulation was used to generate equally probable realizations of soil properties for each approach, and these realizations were then used as input to a numerical simulator to quantify the resultant uncertainty in solute transport predictions. Simulation results show that the scaling techniques seem to oversimplify the description of heterogeneity in the Las Cruces Trench, leading to very narrow spaces of uncertainty. Because the multistep approach allows the reproduction of existing patterns of continuity of soil classes and of contrasting values of hydraulic properties in the field, it led to solute plumes that split into several preferential pathways that were not observed in the simulations based on the scaling approaches, which increased the standard deviation of the solute plume's moments. The results indicate that measurements of both water retention curves and saturated hydraulic conductivity need to be collected for more realistic, conservative studies of flow in heterogeneous unsaturated soils.