Recent Studies of Flow and Transport in the Vadose Zone

Abstract

Vadose zone hydrology has received increased attention in recent years as a result of environmental concerns over potential groundwater pollution from buried wastes. Funding agencies have supported several comprehensive field studies designed to improve and test models that describe flow and transport of contaminants in heterogeneous field soils. Extensive data bases, containing measured hydraulic properties and observations of flow and transport, have been established and have been useful for model testing and comparison. Improvements in stochastic and deterministic modeling capabilities for the vadose zone have been made. The stochastic models provide a way to account for site variability. The improved deterministic models conserve mass and are computationally efficient. This paper discusses recent progress in the areas of model validation, stochastic and deterministic analysis, multiphase flow and transport, preferred pathway analysis, and other related topics.

Stable or unstable wetting fronts in water repellent soils – effect of antecedent soil moisture content

“…Over the last decade it has become increasingly evident that models based on Darcy's law and the general convection± dispersion equation are often not capable to simulate actual water¯ow and transport processes (Gee et al, 1991;Jury and Flu Èhler, 1992). The main reason is the occurrence of preferential¯ow.…”

Analysis of vadose zone tritium transport from an underground storage tank release using numerical modeling and geostatistics

“…van Genuchten (1991) of flow and transport in the vadose zone were reviewed by Gee et al (1991). The work summarizes our understanding of the subject by reviewing such vadose zone-related issues as unstable flow in layered soil (Hillel, 1987), macropore flow (Germann, 1985), advances in unsaturated flow modeling (Milly, 1988), and solute transport in field soils (van Genuchten and Shouse, 1989).…”

“…While the classical flow and transport models apparently perform satisfactorily for applications such as homogeneous or well-characterized heterogenous laboratory soil columns, their effectiveness when applied to heterogeneous field-scale situations is increasingly being questioned (Gee et al, 1991). E1-Kadi (1987) used a Monte Carlo analysis to show that neglecting spatial variability in estimating infiltration may lead to unacceptable errors.…”

“…Introduction A number of field studies have shown that the hydraulic properties of soils are spatially variable because the subsurface is naturally heterogeneous (Gee et al, 1991). Spatially variable soil properties include hydraulic conductivity, water retention relations and porosity.…”

“…Soil property variability is believed to have a much greater impact on solute transport in the vadose zone than in the saturated zone. Many investigators question the validity of model results obtained using effective hydraulic properties obtained by averaging over local values (Gee et al, 1991). New approaches using stochastic subsurface transport theories attempt to quantify the spatial variability due to soil heterogeneity instead of averaging which usually results in artificial smoothing (Sudicky and Huyakorn, 1991).…”