Issues

Previous studies have demonstrated that the vadose zone can be responsive to changes in infiltration and surface temperature. These studies have now been extended, using a new reactive transport simulator. The results suggest that a rich record of regional climate change histories may be preserved in the chemical composition of pore waters in deep vadose zone systems. Extracting those histories, however, will require resolving formidable conceptual and technical challenges.

A new physically based framework for prediction of hydraulic properties of structured porous media based on liquid–vapor interfacial configuration in idealized pore space geometry is reviewed. Modern interface science formalism was applied to modeling liquid organization and saturation in 2-D dual continuum pore space (matrix pores and structural features such as fractures and macropores). Introduction of steady hydrodynamic considerations led to analytical solutions for sample-scale unsaturated hydraulic conductivity.

The phenomenon of dynamic capillary pressure in unsaturated porous media is represented in terms of a dynamic coefficient, which in this paper is quantified using published experimental data and a pore-scale model for simulating interfacial dynamics in unsaturated porous media. A continuum-scale model of unsaturated flow incorporating dynamic capillary pressure is applied to determine the magnitude of dynamic coefficient that will cause significant effects on unsaturated flow. Results indicate that dynamic capillary pressure should be included as a general feature of continuum-scale models.

Recently developed multilayer turbulent transport methods are applied to compute distributions of strengths of scalar sources and sinks as well as turbulent fluxes within the plant–atmosphere continuum. Focus is given to “inverse methods” which estimate scalar sources and sinks from measured mean scalar concentration (or temperature) distributions within the canopy without resorting to ecophysiologically based input or first-order gradient-diffusion approximations. The methods presented are shown to reproduce the spectral properties of measured turbulent fluxes on time scales ranging from hours to a year.

Numerical simulations of multiphase fluid flow and solute transport were conducted for a high heat load nuclear waste tank buried in the Hanford vadose zone. Thermohydrologic processes were evaluated and comparisons with site data were made.

Detections of tracer applied during episodic large-scale ponded infiltration on the eastern Snake River Plain demonstrate vadose zone transport speeds greater than 10 m d⁻²¹, both vertically to the aquifer at 200 m depth and horizontally to perched-water wells as far as 1.3 km away.

Field and laboratory tracer experiments were conducted to investigate water imbibition and tracer penetration into an unsaturated, fractured rock matrix. Tracer distribution was profiled using a new sampling technique. The effects of initial matrix saturation and pore size and pore connectivity on tracer movement and wetting front characteristics were investigated.

The potential for transport of pathogenic microbes, originating from on-site waste disposal systems, into shallow soils underlain by fractured bedrock is studied with infiltration and mass transport experiments. Blue dye, bromide, and bacteriophage were used in the transport experiments. Significant movement of bacteriophage cells into the fractured bedrock was found to occur, leading to a recommendation for caution when designing on-site waste disposal systems on these types of soils.

Macroporous soil columns were exposed to variable pauses in irrigation to study the dynamics of mobilization and leaching of in situ colloids. The colloid mobilization was found to be a time-dependent, possibly diffusion-limited process. A new equivalent macropore model that includes two rate-limiting processes was capable of reproducing measured leaching rates for colloids.

The method of moments is employed to develop a stochastic model for multidimentional transient flow in variably saturated porous media. The van Genuchten–Mualem constitutive model is used to represent the porous media hydraulic properties. The moment equations are solved using a finite difference method, and the results are verified by high-resolution Monte Carlo simulations. Sensitivity analyses show the relative importance of the constitutive law parameters on the variance of flow variables.

Chloride concentration profiles reflect vadose zone stratigraphy in Palouse loess deposits. Abrupt changes in chloride concentrations reflect boundaries between stratigraphic units that display contrasting physical and morphological properties. Results illustrate that soil–paleosol sequences influence water residence times and movement in the vadose zone.

An analytical solution to the convective transport equation is proposed for calculating one-dimensional vadose zone solute transport for both downward and upward flow in the presence of vertically distributed root water and solute uptake. The solution, based on the method of characteristics, is most sensitive to the magnitude of the effective water application ratio, defined by the fraction of infiltrated water that is removed by ET. Although quite simplistic, the methodology is potentially useful for long-term and field to regional scale characterization of contaminant movement.

Pressure plates are used routinely to obtain water retention properties of soils. We show numerically and experimentally that soils may not eqilibrate if their unsaturated hydraulic conductivity or the plate conductance is limiting. Tests demonstrated that a range of soils did not equilibrate on 15-bar plates even under load or with improved plate contact, suggesting that alternative methods for measuring 1.5 MPa water may be required.

A new method is proposed to facilitate more complete description of complex soil water retention curves that cannot be described with classical methods. The method is based on flexible smooth functions, which provide good fits to water retention curves in the entire range from saturation to air-dry. Fitted functions are readily transformed to predict unsaturated hydraulic conductivity relations.

Long-term records of drainage measurements from large lined gold ore heap leaching facilities were used to quantify the potential contaminant loading rates to groundwater beneath such facilities in arid and semiarid regions of Nevada. Drainage rates were found to range from 2 to 23% of annual percipitation, with higher rates found in more arid settings. Model predictions of drainage rates using available data did not correlate well with measured rates, pointing to the need for collection of more complete site data for leaching facilities than currently required by regulatory agencies.