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

The transport of nonreactive solutes through a coupled, two-dimensional, ra ndomly heterogeneous vadose-saturated zone system subject to temporally ran dom rainfall is predicted by Monte Carlo simulation and compared with previ ously published analytic results for three different rainfall patterns. The relative contributions of the uncertain inputs (i.e., rainfall and saturat ed conductivity) to the prediction uncertainty of solute transport are quan tified in terms of the statistical moments of the pore water velocity, the plume spatial moments, and solute flux breakthrough curves at downstream co ntrol 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 relationship s developed by Rubin and Bellin [1994]. As a result, the mean plume displac ement, estimated by the trajectory of the mean plume center of mass, was fo und to be nearly identical for these cases. In the temporally random rainfa ll case, the saturated zone mean plume experienced more spread in the direc tion of mean flow at early times. However, the asymptotic rates of spatial spreading of the mean solute plumes were found to be approximately equivale nt for the uniform and random rainfall cases and well predicted by the appr oximate expressions for longitudinal macrodispersivity in nonuniform flow p roposed by Destouni and Graham [1995]. Random rainfall and random soil prop erties increased prediction uncertainty of the solute plume behavior in the vadose zone by an order of magnitude when compared with the uniform rain a nd random soil case. This effect was reduced considerably when the solute e ntered the saturated zone, where random rainfall produced only slightly lar ger prediction uncertainty than the uniform rainfall case. The analytic mod el developed by Destouni and Graham [1995] accurately predicted the tempora l breakthrough of the mean solute plume at saturated zone control planes fo r all cases, if transport through the unsaturated zone accounted for the ef fects of temporally random rainfall using the methodology developed by Fous sereau et al. [2000a, 2000b]. Results of this work indicate that for the hu mid climates studied here, uncertain rainfall patterns dominate transport p rediction uncertainty in the shallow unsaturated zone, while uncertain solu te breakthrough to the saturated zone and uncertain hydraulic conductivity dominate prediction uncertainty in the saturated zone.