Issues

Soil microbiology has traditionally focused on the terrestrial surface, although microorganisms are abundant throughout the vadose zone. This review synthesizes recent research related to microbes in the vadose zone environment, including distribution, diversity, and associated processes such as biodegradation. Research needs are suggested.

Water containing sequential dyes was infiltrated above a vertical face in a heterogeneous, layered, sandy-gravelly fluvial deposit. Illustrative numerical simulations show the ability of simple equivalent homogeneous media models that include either constant or moisture-dependent anisotropy to capture the behavior of the wetting front.

Two empirical drainage models, based on soil textural properties and simple estimates of climatic variables, are compared against Darcy's Law drainage estimates and measured drainage records. The empirical models when calibrated for site conditions appear to provide better drainage estimates than the Darcy's Law estimates.

Spatial variability of nitrate concentration and water content in a deep alluvial vadose zone underlying an irrigated orchard was quantified for three 12-yr fertilizer treatments. The combined evidence of highly variable hydraulic properties, soil texture, nitrate concentration, and water content is used to evaluate the potential for rapid nitrate leaching.

The performance of evapotranspirative (ET) covers for waste containment was evaluated based on water balance monitoring and modeling of ET covers at sites in Texas and New Mexico. Cover performance is strongly linked to precipitation and vegetation dynamics while water storage can be increased up to 2.5 times by an underlying capillary barrier.

Measured concentrations of chloride in drain water and soil water extracts for a bare 7.6-m-deep lysimeter were used with a chloride mass balance (CMB) model to estimate lysimeter drainage. Errors in drainage estimates are found to be attributable to low chloride concentrations in soil water extracts and possible mineral dissolution.

We studied the preferential movement of surface applied bromide towards a tile drain. Experimental results were analyzed using a two-dimensional dual-porosity model. The study focused on identifying the bromide transport directions and the effect of rainfall intensity on the extent of physical nonequilibrium transport.

An in situ flow experiment was conducted at Yucca Mountain to assess flowpaths followed by water released along a fault imbedded in fractured, welded tuff. Observations made from electrical resistance probes showed the persistence of primary flowpaths along the fault and secondary flowpaths in the adjacent fractured rock.

Bromide and Brilliant Blue food dye were used in infiltration experiments on four field plots subjected to different tillage operations. Photographs of dye distribution in detailed horizontal sections of the soil profiles were used along with core sample measurements of bromide concentration to quantify the plot-scale vertical transport.

Previous macroscale studies have suggested several mechanisms as possible explanations for observed dynamic capillary pressure effects in drainage of a sandy soil. In this paper a quantitative analysis based on X-ray microtomographic observation of the pore-scale drainage behavior is performed to evaluate the phenomenon in more detail.

Dynamic water-entry pressures were measured for two air-dried porous media having similar particle size, glass beads and sea sand. An equation for the dynamic water-entry pressures was derived and fitted to the data, and the dynamic water-entry pressures were used to estimate finger widths observed in experiments.

In contrast to the unsaturated hydraulic conductivity, the scaling of the air permeability with saturation is insensitive to the pore-size distribution. Air permeability is derived by straight percolation scaling techniques. The theory predicts experimental results on both two- and three-dimensional systems.

Quasi-saturated hydraulic conductivities of a sandy loam and a light clay soils each having a few percent of entrapped air were distinctly smaller than the saturated hydraulic conductivities. Quasi-saturated hydraulic conductivities were also smaller than unsaturated hydraulic conductivities of the soils with similar volumetric air content.

An algorithm based on continuum percolation theory is applied to predict water retention characteristics of seven drainage curves derived from the Hanford site. The algorithm is based on a prediction of a rapid drop in hydraulic conductivity at low saturation; this calls into question the implicit assumption of equilibration at low saturations.

Unsaturated hydraulic conductivity determinations commonly show high levels of uncertainty. This uncertainty was evaluated in instantaneous profile experiments. Results show this uncertainty to be larger in drier soil, and also larger for unsaturated hydraulic conductivity as a function of water content than as a function of pressure head.

Three new theoretical models for computing residual NAPL saturation in the vadose zone were tested using static pressure cell data. Two of the models were incorporated into the STOMP multiphase flow simulator and were successfully tested against transient flow column experiments.

Fiberglass wicks were tested for their suitability to representatively sample colloids from vadose zone pore water. Colloid type, flow rate, and pH were variables considered in the tests. While in some cases complete colloid recovery was observed, in certain cases considerable retention of colloids occurred inside the wicks.

The transport, retention, and release of hydrophobic and hydrophilic polystyrene latex microsphere colloids were examined under unsaturated conditions both visually in the soil pore water and measured as concentration in the outflow water. The findings form an essential link between colloid retention and transport processes at interfacial-, pore-, and Darcy-scale.

The generalized Richards equation, which includes unsaturation and compressibility effects, is validated against a well-documented aquifer test data. Among other factors, the effects of wellbore storage should not be overlooked in the analysis. The model required effective soil hydraulic parameters that differed from those based on laboratory evaluation.

A suite of numerical flow and transport models (MACRO, HYDRUS_1D, SWAP, MARTHE, and WAVE) are evaluated using analytical solutions of the flow and transport equations, including solutions for coupled transport in dual-velocity media. Differences in model predictions for a climatic boundary condition are interpreted on the basis of this evaluation.