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, heterogeneous, partially saturated porous media. Two different media were considered, the properties of which were modeled as a bimodal and as an “equivalent” (same mean and variance but not the same two-point covariance), unimodal spatially correlated random functions. The bimodal medium consisted of a spatially distributed background soil and randomly dispersed low-permeability clay lenses which occupied 10% of the media volume. Both time-invariant and periodic influx (rain/irrigation) rates at the soil surface were considered in this investigation. Under steady state flow conditions originating from a time-invariant influx rate at the soil surface, when the soil is relatively wet, the difference between the hydraulic responses of the bimodal medium 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, originating from a periodic influx at the soil surface, which, in turn, is characterized by substantial redistribution periods with diminishing water saturation, may considerably decrease the difference between the responses of the two media. The latter result stems from the fact that in the bimodal medium considered in this investigation, for the range of water saturations pertinent to the redistribution periods, the difference between the conductivities of the background soil and the embedded clay lenses diminishes substantially.