Numerical analysis of flow and transport in variably saturated bimodal heterogeneous porous media

Numerical simulations of flow and transport of a tracer solute were used to investigate solute spreading and breakthrough in three-dimensional, hetero geneous, partially saturated porous media. Two different media were conside red, the properties of which were modeled as a bimodal and as an "equivalen t" (same mean and variance but not the same two-point covariance), unimodal spatially correlated random functions. The bimodal medium consisted of a s patially distributed background soil and randomly dispersed low-permeabilit y clay lenses which occupied 10% of the media volume. Both time-invariant a nd periodic influx (rain/irrigation) rates at the soil surface were conside red in this investigation. Under steady state flow conditions originating f rom a time-invariant influx rate at the soil surface, when the soil is rela tively wet, the difference between the hydraulic responses of the bimodal m edium and the unimodal medium increased with increasing influx rate. For a given influx regime at the soil surface the embedded clay lenses associated with the bimodal medium are shown to enhance both solute spreading and the skewing of solute breakthrough curves considerably. Transient flow, origin ating from a periodic influx at the soil surface, which, in turn, is charac terized by substantial redistribution periods with diminishing water satura tion, may considerably decrease the difference between the responses of the two media. The latter result stems from the fact that in the bimodal mediu m considered in this investigation, for the range of water saturations pert inent to the redistribution periods, the difference between the conductivit ies of the background soil and the embedded clay lenses diminishes substant ially.