Solute transport through a heterogeneous coupled vadose-saturated zone system with temporally random rainfall

The transport of nonreactive solutes through a coupled, two-dimensional, randomly heterogeneous vadose-saturated zone system subject to temporally random rainfall is predicted by Monte Carlo simulation and compared with previously published analytic results for three different rainfall patterns. The relative contributions of the uncertain inputs (i.e., rainfall and saturated conductivity) to the prediction uncertainty of solute transport are quantified in terms of the statistical moments of the pore water velocity, the plume spatial moments, and solute flux breakthrough curves at downstream control planes. Results show that the mean and variance of the saturated zone pore water velocity were approximately equivalent for the cases of uniform and random rainfall and were well predicted by the analytical relationships developed by Rubin and Bellin [1994]. As a result, the mean plume displacement, estimated by the trajectory of the mean plume center of mass, was found to be nearly identical for these cases. In the temporally random rainfall case, the saturated zone mean plume experienced more spread in the direction of mean flow at early times. However, the asymptotic rates of spatial spreading of the mean solute plumes were found to be approximately equivalent for the uniform and random rainfall cases and well predicted by the approximate expressions for longitudinal macrodispersivity in nonuniform flow proposed by Destouni and Graham [1995]. Random rainfall and random soil properties increased prediction uncertainty of the solute plume behavior in the vadose zone by an order of magnitude when compared with the uniform rain and random soil case. This effect was reduced considerably when the solute entered the saturated zone, where random rainfall produced only slightly larger prediction uncertainty than the uniform rainfall case. The analytic model developed by Destouni and Graham [1995] accurately predicted the temporal breakthrough of the mean solute plume at saturated zone control planes for all cases, if transport through the unsaturated zone accounted for the effects of temporally random rainfall using the methodology developed by Foussereau et al. [2000a, 2000b]. Results of this work indicate that for the humid climates studied here, uncertain rainfall patterns dominate transport prediction uncertainty in the shallow unsaturated zone, while uncertain solute breakthrough to the saturated zone and uncertain hydraulic conductivity dominate prediction uncertainty in the saturated zone.