Water flow and solute spreading in heterogeneous soils with spatially variable water content

Abstract

Although the spatial variability of water content is expected to be significant in heterogeneous, unsaturated media, its effect on solute transport has been neglected in most stochastic analyses. In this work we develop, evaluate, and numerically verify an analytical model describing the statistical moments of water content, soil water flux, soil water velocity, and solute spreading in the unsaturated zone under conditions of variable soil water content. Results are presented for a wide range of soil conditions. It is shown that soil water content variability increases with soil water tension and decreases with soil textural tortuosity parameter m. As expected, the water content variability also increases with those of soil texture and saturated hydraulic conductivity. However, at or above field capacity (near saturation) the water content variability is generally negligible. The water content variability is very large under dry conditions, even if the soil is relatively homogeneous. In the same soil, longitudinal macrodispersivity is generally smaller when including than when neglecting the variability in water content. Variable wafer content enhances longitudinal macrodispersivity only in soils with large horizontal anisotropy or soils with m < 2. In this case the enhancement is more significant in drier soils and at the early stages of plume displacement (less than 10λ(f)). With travel distance the longitudinal macrodispersivity in the presence of variable water content converges to that in the absence of it.