Issues

This paper presents a new stochastic inversion approach for the quantitative interpretation of GPR Cross-Hole Zero-Offset Profiles in terms of sharp dielectric properties contrasts. The approach is tested successfully on synthetic data, and applied to two different case studies where the results are supported by site stratigraphy information.

Unstable wetting fronts developed during infiltration into dry water repellent soil, and wettable soil at moisture contents near field capacity. Instability occurred in the water repellent soil due to drastically reduced intrinsic permeability, while in wettable soils wetting front instability resulted from air entrapment in initially moist soil.

The electromagnetic-induction technique was tested to explore tree-soil relationships in tropical forests, and was shown to be a promising method to depict spatial variation in soil texture in forests. The approach also detected the impact of soil water repellency on wetting patterns in native tropical forest.

A method to quantify the biochemical conversion of the strong greenhouse gas N₂O into the environmentally benign N2 was developed. It is based on N₂O gas diffusing through a soil core under specified laboratory conditions. Soil moisture level in the soil significantly influenced the N2O reduction potential of the soil.

This paper presents a comprehensive treatment of radionuclide transport behavior for the unsaturated zone at Yucca Mountain, Nevada. A transport model is developed, informed by in situ tests and field observations. Flow processes, heterogeneities, fast pathways, and radionuclide interactions with the host rock and colloids are included in the model.

The effectiveness of a water harvesting technique (WHT) was evaluated using a three-dimensional hydrologic model for coupled surface-subsurface flows calibrated with field measurements. Water harvesting efficiency was found to be significant for high rainfall intensity events, but not for the typical climatic conditions of the study area.

This paper describes a particle-tracking technique for dual-permeability media suitable for simulation of radionuclide transport through unsaturated, fractured rock. The model is used to conduct sensitivity analyses of radionuclide transport travel times for the Yucca Mountain unsaturated zone, from the repository horizon to the water table.

A procedure for desiccating vadose zones by injection of dry gas was field tested in a contaminated vadose zone at the DOE Hanford site. The results show the potential for reducing the flux of contaminants to underlying groundwater by removing moisture and decreasing the water relative permeability.

Long-term fate of nitrate in well-characterized, deep, unsaturated heterogeneous alluvial sediments is simulated with several Richards’ equation-based flow and transport models (deterministic and stochastic). Results consistently overestimate field-measured nitrate calling into question the appropriateness of these models.

The quantification of recharge and characterization of flow and transport in fractured chalk vadose zones is not an easy task, and considerable uncertainty remains regarding the dominant flow regime. The authors report a fusion of field monitoring and modeling techniques to characterize flow processes in unsaturated chalk.

Evaluating predictive performance of regression confidence intervals and Bayesian credible intervals is important for uncertainty quantification in model-data fusion. In this study numerical analyses showed that Bayesian intervals have better predictive performance and that MCMC simulation is computationally more efficient than regression analysis.

This work aims at bridging root density profiles (i.e., root length per unit of soil volume versus soil depth) with models of the root system architecture. A sensitivity analysis was used to study the influence of root development model parameters on profile shapes and evaluate the estimation of these parameters from root density profiles.

We examined to what extent time-lapse crosshole ground-penetrating radar traveltimes, measured during a forced infiltration experiment at the Arreneas field site in Denmark, could help to quantify vadose zone hydraulic properties and their corresponding uncertainties using a Bayesian Markov-chain-Monte-Carlo inversion approach with different priors.

Low water content sediments were treated with ammonia gas to evaluate its influence on U mobility and its potential to serve as a vadose zone remediation method for U contamination. Ammonia treatment greatly decreased aqueous and adsorbed U by incorporation into precipitates and decreased Na-boltwoodite mobility, probably by precipitate coatings.

Geophysical monitoring of foam injection and transformation was studied in column experiments. Our results demonstrated the sensitivity of electrical and time-domain reflectometry signals to foam injection and transformation in porous media and suggest the potential of these methods for monitoring the response of the subsurface to foam-based remediation treatments.

A numerical procedure for estimating effective block parameters (conductivity, van Genuchten parameters) of unsaturated porous media from fundamental fracture properties such as spacing and aperture is developed. These effective parameters are independent of pressure head or saturation and can be assigned to grid nodes in field-scale simulations.

Infiltration of low-concentration surfactant solutions in step-by-step mode can effectively remove LNAPL from sediments under vadose zone conditions. Very low surfactant concentration is needed to sufficiently reduce the LNAPL–water interfacial tension to mobilize trapped LNAPL under these conditions.

During experiments investigating porous media desiccation it was observed that injected dry gas migrated predominantly in the higher permeability layers and delayed water removal from the lower permeability zones. Acceptable numerical matches of data were obtained with an extension of the water retention relation below the residual water saturation.

Simultaneous determination of soil hydraulic properties and a rooting depth parameter was studied by inverse modeling of unsaturated water flow with root water uptake in weighable lysimeters under transient atmospheric boundary conditions. Inclusion of pressure head data in the objective function yielded unique parameter estimates even for layered profiles.

A new unsaturated Monte Carlo sampling strategy, the retention curve subsampling algorithm, can reduce the number of Latin hypercube samples required for convergence to the mean and 5 and 95% confidence bounds of particle breakthrough by sampling across one-dimensional retention curve space instead of blindly combining five dimensional van Genuchten parameters.

Virtual soils help explore the relationship between soil architecture and hydraulic behavior. Tillage-induced features near the soil surface govern atmospheric boundary fluxes, while soil heterogeneity effects on groundwater recharge are more complex. Upscaling becomes difficult when lateral fluxes become relevant at the scale of observation.

We use full-waveform, integrated hydrogeophysical inversion of time-lapse, off-ground GPR data to estimate the soil hydraulic properties at a fixed location over a bare agricultural field. The results suggest that the proposed method is promising for mapping the shallow soil hydraulic properties at the field scale with a high spatial resolution.

This paper reviews the current knowledge base on the temporal stability of soil water contents, highlights the interactions of the temporal stability controls, and proposes research avenues which can advance multiple applications that the temporal stability of soil water contents currently has in environmental monitoring, modeling, and management.

Plant water uptake models that use 3-D representations of root structures have been developed recently. These models require a 3-D root architecture as input. We investigated how the root architecture model RootTyp can be parameterized from observations of root arrival data at minirhizotron tube interfaces.

Experiments were conducted with ⁹⁹Tc-spiked concrete in contact with Hanford sediments to determine the diffusivity of this radionuclide under vadose zone conditions. Calculations based on the diffusion profiles in the soil half-cells after a time lapse of ~1.9 yr yielded a ⁹⁹Tc diffusion coefficient value of ~1.0 × 10⁻⁹ cm² s⁻¹.

The performance of various ERT arrays to monitor soil moisture dynamics in the field was investigated. Classical measures such as data recovery as well as spatial measures such as semivariances were used for evaluation. The arrays detected distinct patterns under different cropping systems, but variances and semivariances were underestimated.

Experiments were conducted to evaluate the effective viscosity of foams within porous media. The experiments included the factors of the liquid fraction of foam, the foam injection rate, and the permeability of the porous media. The effective viscosity is described by a mathematical expression containing these factors.

An Actively Heated Fiber Optics method is tested to estimate soil moisture profiles along 30 meters of fiber optic cable buried in a large coil in a lysimeter variably saturated with depth. A comparison with independent soil moisture measurements shows an excellent matching in wet soil, while a significant underestimation occurs in dry conditions.

The extraction of water from unsaturated porous media by applied super-absorbent polymers (SAPs) was investigated. The rate of water extraction by various types of SAPs was observed to decrease with time, this decrease being associated with the temporal reduction in water saturation and relative permeability near the SAP–porous medium interface.

Natural biodegradation in an oil contaminated aquifer is evaluated based on carbon dioxide efflux measurements at the soil surface. To evaluate the suitability of this approach, measured CO₂ effluxes are combined with analyses of vadose zone gas, including stable and radioisotopic content of CO₂. Results are integrated with a reactive transport model.

Sixteen units of a new Automated Suction Lysimeter were successfully tested in field conditions during 2007 to 2009. They were accurate, reliable and cost effective and can be used in large numbers with minimum supervision. A power efficient algorithm, programmable controller and leakage protection system were the main advantages of the system.

A novel method for scaling water flow and solute transport during soil-water redistribution was developed. The scaled results are invariant for a broad range of soil textures and initial conditions. The scaling method gives insight regarding transport processes and provides an approximate solution of the highly non-linear governing equations.

Relations between relative permeabilities, saturations, and pressure heads were evaluated to predict average nonwetting phase saturation for unstable two-phase displacements in a micromodel using a continuum-based simulator. Predictions were good as long as independently fitted relations were used.

Three cross-borehole geophysical methods are used to image water migration in the unsaturated zone after a point injection of water. Mass balance calculations and moment analysis highlight the differences in resolution between the methods. The choice of moisture content threshold value significantly influences results of the moment analysis.

The saturation behavior of partially frozen soils is affected by pore geometry, wetting interactions, and the colligative effects of solutes. The wetting interactions between pore ice and the sediment matrix also produce the net thermomolecular force that causes frost heave. For illustration, the saturation behavior and thermomolecular force are quantified using a simple, two-dimensional, model porous medium in a series of Monte Carlo simulations.

Cosmic-ray neutron sensors provide area-average soil moisture measurements over many hectares horizontally and tens of centimeters vertically. In this work, we validate the average soil moisture measurements with a distributed sensor network at a heterogeneous site in Arizona, with error estimates on the same order as other soil moisture sensors.

To overcome limitations due to spatial variability, an experimental design for field-scale leaching experiments was applied where the treatments were arranged in a repetitive pattern along a transect. This revealed the impacts of land use and irrigation characteristics on water infiltration and bromide leaching using geostatistical data analysis.

The guest editors introduce the papers that are featured in this special section and discuss how they contribute to addressing the current challenges in model–data fusion in the vadose zone.

The information flow among the tasks of framework assessment, numerical modeling, model forecasting and hind casting, and system-performance monitoring is illustrated. Results provide an understanding of artificial recharge in high-altitude desert settings where large vertical distances may separate application ponds from their target aquifers.

The guest editors introduce the papers in this issue’s special section on contaminants in the vadose zone. The 12 papers provide examples of novel approaches to the challenges and advances in the prediction, characterization, monitoring, and remediation of contaminants in deep vadose zone environments.