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First‐order and numerical analyses of flow and transport in heterogeneous bimodal variably saturated formations

Addressed here are bimodal, heterogeneous, variably saturated formations, viewed as mixtures of two populations (background soil and embedded soil) of differing spatial structures. Two distinct cases are considered: In the first case, the texture of the embedded soil is finer than that of the background soil; in the second case, the reverse is true. First‐order, Lagrangian‐stochastic analysis of vadose‐zone transport was used to investigate the combined effect of the texture of the embedded soil and the mean pressure head on solute spread and breakthrough in these formations. The main results of the first‐order analysis suggest that features of solute transport associated with the two different formations exhibit a crossover behavior and that the mean pressure head associated with the crossover may be estimated from the asymptotic macrodispersivities associated with the two formations. The applicability of the results of the first‐order analysis to more realistic conditions was tested with a series of detailed numerical simulations of the flow and transport in three‐dimensional, heterogeneous, bimodal, variably saturated formations; relatively simple, steady state flow and more complicated, transient, nonmonotone flow originating from periodic influx and water uptake by plant roots were considered. For the steady state flows, results of the simulations were in qualitative agreement with the results of the first‐order analysis. For the more realistic flow regime, the results of the simulations suggested that the difference between the responses of the two different bimodal formations might decrease substantially, similar to the situation in steady state flow associated with mean pressure head at which a crossover occurs.