Issues

To supplement manuscripts in this issue's special section addressing colloids and colloid-facilitated transport, a short introduction to the topic of mobilization and transport of colloids in the vadose zone is given. Additionally, the authors briefly introduce the papers in the special section.

This paper introduces this special section on colloid transport with a general perspective on the importance of colloids in subsurface systems. A number of studies are discussed to illustrate the important knowledge gaps related to colloid dynamics in both saturated groundwater and the vadose zone.

This review surveys experimental studies of colloid deposition and mobilization and recent efforts in the modeling of colloid transport and mass transfer in the vadose zone.

Natural organic colloids affect flow of water and transport of solutes in soils. Systematic and data driven numerical case studies are used to derive environmental conditions and analyze scenarios resulting in enhanced or reduced contaminant mobility due to cosorption or cumulative sorption.

Interactions between microbial processes and flow and transport processes in the subsurface have important consequences for a diverse range of applications. This paper reviews the status of research activity on microbial and transport process interactions in soils and highlights areas for additional research.

Experimental and modeling studies were conducted to examine mechanisms of colloid transport and retention in water-saturated physically heterogeneous systems. The processes of straining and attachment of colloids were examined experimentally along with the effects of pore velocity variability, flow bypassing, and textural interfaces.

Column experiments were conducted to compare transport behavior and retention of Hanford colloids and kaolinite through Hanford sediments and silica Accusand. Parameters examined included flow velocity, solution ionic strength, and properties of colloids and transport media.

A new methodology is presented that may be more indicative of in situ colloid mobilization than conventional methods measuring water-dispersible colloids (WDC). Results demonstrate that low-energy WDC are negatively correlated with soil clay content and influenced by initial matric potential and wetting rate.

Soil structure, active flow volume and structure stability are key properties affecting colloid mobilization. Analyses are presented of pore structure characteristics, tritium transport during unsaturated conditions, and changes in active flow volume upon drying-rewetting on undisturbed soil columns representing a natural gradient in clay content.

Experimental investigations are presented of colloid mobilization from undisturbed soil columns during unsaturated conditions. Results demonstrate that colloid dispersibility is a key parameter reducing colloid mobilization at high clay contents upon drying. Preferential flow may limit colloid mobilization due to a low displacement of ions.

Colloid transport is of environmental importance and is usually assessed by looking at the transport of an ensemble of colloids. This study presents an approach where the transport of single colloids can be measured and modeled, an approach that should improve the accuracy and precision of transport mechanism studies.

The question of what actually controls the fate of colloids in the unsaturated subsurface environment is still largely unanswered. A real-time pore-scale visualization method was used to show the trapping of microspheres within the thin film where the water meniscus attaches to the grain surface.

We investigated the sorption of pyrene to mobile colloids in soils. Experiments were performed to assess the influence of colloid source release process, indigenous organic matter, and solution chemistry on the sorption of pyrene and hereby the potential for colloid-assisted transport of pyrene.

Controlled leaching experiments were performed on 42 soil columns sampled from a large plot of an agricultural field to determine the importance of a variety of soil physical or chemical parameters on particle-facilitated phosphorus transport. Soil structure was found to play a dominant role in this transport.

The influence of citrate on leaching of iron-cyanide complexes from an acidic soil developed from gas purifier waste was investigated. Without citrate, CN concentrations in leachate were below drinking water standards, but high concentrations of citrate additions led to significant increases in CN leaching.

A special section of this issue is devoted to the emerging topic of uncertainty in vadose zone flow and transport prediction. The guest editors introduce the included papers in the context of a classic modeling approach, then outline critical research needs.

The use of neutron logging of water content in unsaturated basalt is made difficult by chemically bound hydrogen in minerals. This research examines the difficulty of deriving a valid correlation between neutron log response and water content in basalt for both saturated and unsaturated conditions.

Air-water and DNAPL-water capillary pressure-saturation relations were determined for a large, undisturbed column of weathered rock (saprolite). The scaled air-water relation successfully predicted the DNAPL-water relation following correction for the variation in capillary pressure with height within the column.

Reconstruction of paleohydrologic conditions at a desert site was performed using various present day vadose zone profiles of matric potential and chemistry in concert with flow and transport models. The authors evaluate the effect of uncertainties in boundary conditions on prediction of past conditions.

A stochastic Lagrangian approach is presented for quantifying solute transport statistics in a bounded, unsaturated domain. The effect of flow nonstationarity on the uncertainty of solute fluxes and solute breakthrough is quantified.

Two major issues of soil hydraulic properties are investigated: (1) hydraulic parameter equivalence among commonly used soil hydraulic conductivity functions, and (2) its implication to upscaling of hydraulic properties in heterogeneous landscapes. Resolution of these issues is important for hydrologic modeling of large-scale systems.

We provide a critical review of studies of flow and transport within the “capillary fringe.” We note that multiphase transport occurs in the immediate vicinity of the water table, above and below it, and therefore suggest replacing the concept of the capillary fringe with the concept of a “partially saturated fringe.”

One cause of unstable flow is the redistribution of flow following high-rate infiltration, a common occurrence in hydrology. Experimental and theoretical analyses are performed to develop the criteria for the occurrence of unstable flow caused by redistribution.

Estimates of flux deep within a vadose zone were made using matric potential measured at 34-m and 74-m depths in concert with unsaturated hydraulic conductivity measured on core samples. Extreme variability in the calculated unsaturated hydraulic conductivities led to flux estimates ranging up to four orders of magnitude.

Main issues related to rainfall-induced soil seal formation are reviewed with an emphasis on infiltration during soil surface sealing and through sealed soil profiles. Modeling the relationship between soil resistance to destruction and soil properties is essential to a generalized expression of the processes involved in seal formation.

Laboratory experiments are conducted to examine small-scale features of gravity-driven flow in unsaturated fractures. Specific features investigated include drainage of a liquid finger into a blob-rivulet structure, the rate of blob advancement, and rivulet flow.

We examine faulted and fractured nonwelded and poorly welded rhyolite Bishop Tuff in eastern California to determine deformation mechanisms. Microstructures reveal that open fractures provide connected paths through the deposit, and fault surfaces are smooth planes that cut the entire deposit.

Discrete-fracture flow paths and flow-focusing phenomena in two-dimensional fracture networks were studied with a numerical model. Using fracture data from the Yucca Mountain site as input the model was applied to assess the frequency and flux distributions of major water-bearing flow paths.

Field experiments with multiple solute and colloidal tracers showed extremely fast transport through 3.3 m of unsaturated fractured clayey till. Differences in obtained breakthrough curves indicated the importance of diffusive exchange between fractures and matrix.

The volatilization behavior of binary NAPL mixtures entrapped within unsaturated porous media was examined in column experiments. Simulations of volatilized effluent were performed using three volatilization models, a conventional equilibrium model, a rate-limited model, and a fixed concentration gradient model.

To be used as soil fumigant, propargyl bromide behavior in soil has to be known and management schemes developed. Multiple short irrigation events are shown to be a better control for volatilization. The effects of diffusive and advective fluxes on degradation, and temporal gas and liquid phase distributions are discussed.

Chemical analysis of samples of a thick saprolite layer found in the Piedmont region of Georgia showed that the layer has about six times the anion adsorption capacity of the overlying soil profile. This anion exchange capacity should be taken into account in chemical transport modeling.

Boron adsorption on a large set of soil samples was studied as a function of solution boron concentration and solution pH. A chemical surface complexation model was applied to quantify boron adsorption as a function of solution concentration and pH.

An integrative inverse approach for electrical resistivity tomography, based on a stochastic information fusion concept, was developed to estimate moisture content distribution. The method assimilates both prior information about geological and moisture content structures and sparse point measurements.

A new numerical solution for coupled heat and moisture flow in variably saturated, variably frozen soils is presented. The numerical model is tested with laboratory data involving a column freezing experiment. Numerical experiments for solution stability are examined.

Improved determination of water content from dielectric models requires more accurate input parameters. This work describes a time domain reflectometry method of determining the permittivity of the solid mineral fraction in soils. Results indicate that common clay minerals have permittivity values between 5 and 6.

Laboratory experiments were performed to measure the moisture retention and unsaturated hydraulic conductivity properties of soils mixed with straw material and repacked. Tomographic images of the packed samples were used to visualize the effect of incorporated straw on pore structure.