Issues

In disordered porous media, two-phase flow can be fractal over some scales, due to capillarity, gravity and viscosity. This, with the boundary conditions, determines the flow stability. We present here a short review, and an example of upscaling, expressing a dynamic control over macroscopic pressure–saturation relationship and residual saturation.

Shrinkage cracks in soil strongly influence the distribution of moisture fluxes in the vadose zone. We present a new low-cost and accurate field instrument to monitor the temporal change in crack volume as affected by shrinkage and swelling cycles. Results show how the instrument is detecting the closing of cracks corresponding to soil swelling.

Natural carbon dioxide sequestration is assessed with a process-based reactive transport model for a mine site near Mount Keith, Australia. The simulations demonstrate that atmospheric carbon dioxide is trapped in the ultramafic mine tailings due to carbonation reactions involving the weathering of serpentine and brucite, resulting in the formation of hydromagnesite.

A new type of combined penetrometer–moisture probe (CPMP) was used for surveying the hydrological properties in a mountainous footslope. The CPMP readily obtained high-resolution soil water content and penetration resistance data. As a result, we successfully detected the presence of heterogeneous hydrological properties by CPMP measurements.

The effects of soil heterogeneity on solute transport are often investigated for infiltration. This study focuses on preferential transport under evaporation conditions. We discuss the potential limitations of continuum flow and transport models to reproduce an experimentally observed pattern of preferential upward transport and of electrical resistivity tomography to monitor it.

Good prediction of B transport for three soils with applied water of pH 6 and 9 was obtained using the UNSATCHEM model with a B constant capacitance model subroutine and prediction of model constants for each soil based on available soil properties. We obtained good predictions without needing to characterize B adsorption properties for each soil.

Remobilization of colloids occurs under transient flow conditions. Three different approaches were employed to simulate observed transient flow effects on the remobilization of attached viruses in sandy columns. The approach taking into account specific air-water interfacial area showed better agreement than the previously proposed models.

Simulation based on our previous theoretical model predicts the concentration of produced vapor as measured at 90-min intervals during 100 d of passive soil vapor extraction. A diffusion calculation utilizing the exchange diffusivity derived from the model agrees with the results of a simulated year-long operation, confirming application of the exchange diffusivity as a design tool.

Previous modeling studies of U(VI) at the Hanford 300 Area have greatly overestimated U(VI) elution into the Columbia River. This work demonstrates that even with sophisticated U(VI) sorption models, the plume dissipates too rapidly. A slow release of U(VI) from a nonlabile source is required to account for the plume longevity.

Reductive squestration of selenium, which is toxic at elevated concentration, may be enhanced within centimeter-sized soil clumps (aggregates). We investigated microbial Se reduction in idealized soil aggregate flow-through reactors, finding increasing concentration of reduced Se towards the core of aggregates.

Two biokinetic models are presented for describing processes in constructed wetlands for wastewater treatment that are implemented in the HYDRUS software. Simulation results show the use of both kinetic models for horizontal flow constructed wetlands. Additionally, the importance of considering the effects of wetland plants is shown.

Surface and subsurface soils were used to investigate copper (Cu) reactivity and mobility in calcareous soils. A linear model failed to predict Cu transport for all soil columns. In contrast, a second-order two site model provided good descriptions of Cu mobility including arrival time, concentration maxima, and release during leaching.

Vegetation exerts an enormous influence on soil water dynamics, and much of this is due to biophysical effects of plant roots on the surrounding soil. This review considers how roots influence and generate macropore flow paths in the root zone, how root exudates modify water retention in the rhizosphere, and highlights some promising new methods.

The dual-probe heat-pulse (DPHP) method is useful for measuring soil thermal properties. We present a semianalytical solution that accounts for the finite radius and finite heat capacity of the probes of a DPHP sensor. The solution is used to examine the effects of the finite radius and finite heat capacity of the probes.

A novel large diameter (1.2 m) vadose zone sampler has been developed to permit safe access to deep unconsolidated vadose zone profiles. It consists of a light-weight crane and 1.5-m-high, cylindrical steel sections with removable openings to permit examination of the profile. Its use in characterising an alluvial gravel system is described.

The sensitivity of streaming potential (SP_s) signals to subsurface water fluxes makes the self-potential method particularly suitable for vadose zone hydrology. In this laboratory study, we show that SP_s signals obtained during drainage contain enough information on vadose zone hydraulic properties. We propose the coupled hydrogeophysical inversion approach as a valuable tool for retrieving this information for rapid and accurate hydraulic characterization of the vadose zone.

The HYDRUS model was able to reflect measured time trends in sodium, magnesium, and calcium concentration of leachate from soil columns with two different bulk densities. This demonstrates the model's ability to help improve understanding of the interactions between soil chemical and physical properties and hence management of sodic clay soils.

This paper reviews the value of microwave methods to characterize canopies of agricultural crops. It highlights their use in detecting water stress related phenomena and it indicates avenues for further research. Especially, the combination of microwave methods with other remote sensing methods and new crop modeling approaches seems very promising.

The degree of inundation of rapid infiltration basin systems (RIBS) is controlled by prevailing overland flow and soil infiltration processes. The iTOUGH2 model was modified to describe coupled surface–subsurface flow in a RIBS configuration, and the effect on groundwater mounding and nitrate removal evaluated.

Remediation of Cr(VI) with Fe(II) in sediments at the DOE's Savannah River site was investigated using advection-dispersion modelling. Model parameters were derived from laboratory batch studies and literature values. Discrepancies found between simulated and predicted Cr transport behavior was attributed to the kinetics of chemical processes.

The guest editors summarize the eight studies in this issue's Special Section: Reactive Transport Modeling. The studies represent the diverse application of reactive transport modeling at a range of scales. This modeling work addresses the fate and transport of both reactive solutes and colloidal material, with potential for synthesizing diverse data, evaluating competing biogeochemical processes, identifying research gaps, and ultimately improving our understanding of biogeochemical processes.