Vadose Zone Research Research Articles

Vadose zone perspectives in global arsenic research: A review and future opportunities

AbstractFew contaminants have been linked to more devastating human health and environmental impacts than carcinogenic arsenic (As). Geogenic As contamination of groundwater used as a drinking water source continues to threaten hundreds of millions of lives worldwide, with the As crisis in South and Southeast Asia often called “the largest mass poisoning in history.” In addition, anthropogenic As pollution derived from industrial activities (e.g., mining and smelting processes, wood preservation, and historic pesticide use) has created large sites of intensely contaminated soils and water bodies that urgently require remediation. Because of its profound negative impacts on environmental quality, As has also been the focus of considerable scientific research. In particular, vadose zone research, which aims to understand fluid flow and contaminant transport in variably saturated porous media, has been critical to identify sources of As contamination, predict the fate of As in natural and engineered systems, and help guide regulatory agencies, policymakers, and practitioners to minimize As impacts. In this work, we review several key topics in global As research that have been advanced by vadose zone knowledge. These topics include the release of geogenic As to groundwater, the remediation of anthropogenic As contamination, and the design and operation of As treatment systems. We end this review by highlighting urgent and important knowledge gaps in As research that can benefit from a more rigorous understanding of vadose zone processes.

Stable Isotope Approaches in Vadose Zone Research

Core Ideas This special section focuses on stable isotopes approaches in the vadose zone. Recent advances and challenges in stable isotope approaches are summarized. Isotopes are used to study flow, transport, geochemical, and ecohydrological processes. The articles present key findings in analytical and modeling developments. Results can improve understanding of water flow and contaminant transport in the critical zone. Recent developments in the measurement, modeling, and application of stable isotope tools to provide insights on water flow, transport, biogeochemical, and ecological processes in the vadose zone have motivated a special section in Vadose Zone Journal. We give an overview of recent advances and challenges in stable isotope approaches in the vadose zone and present the key findings from the five manuscripts in this special section. These five manuscripts advance our understanding of (i) analytical developments in water stable isotope analysis, (ii) modeling of water and transport within the soil matrix using water stable isotopes, and (iii) using compound‐specific isotope analysis for identifying the fate of nonaqueous‐phase liquids in the unsaturated zone.

SoilJ: An ImageJ Plugin for the Semiautomatic Processing of Three‐Dimensional X‐ray Images of Soils

Core Ideas Three‐dimensional X‐ray imaging is a valuable tool for vadose zone research. Quantitative 3‐D X‐ray image analyses require a large amount of time and expertise. SoilJ is an X‐ray image processing tool for the automatized analyses of X‐ray images. SoilJ lowers the amount of time and expertise needed to evaluate 3‐D X‐ray images. Noninvasive three‐ and four‐dimensional X‐ray imaging approaches have proved to be valuable analysis tools for vadose zone research. One of the main bottlenecks for applying X‐ray imaging to data sets with a large number of soil samples is the relatively large amount of time and expertise needed to extract quantitative data from the respective images. SoilJ is a plugin for the free and open imaging software ImageJ that aims at automating the corresponding processing steps for cylindrical soil columns. It includes modules for automatic column outline recognition, correction of image intensity bias, image segmentation, extraction of particulate organic matter and roots, soil surface topography detection, as well as morphology and percolation analyses. In this study, the functionality and precision of some key SoilJ features were demonstrated on five different image data sets of soils. SoilJ has proved to be useful for strongly decreasing the amount of time required for image processing of large image data sets. At the same time, it allows researchers with little experience in image processing to make use of X‐ray imaging methods. The SoilJ source code is freely available and may be modified and extended at will by its users. It is intended to stimulate further community‐driven development of this software.

Application and Validation of an Upscaling Method for Unsaturated Water Flow Processes in Heterogeneous Soils

Upscaling of soil water flow processes and soil hydraulic properties is one of the most important issues in vadose zone research. The difficulties in such upscaling come from the heterogeneity and nonlinearity of the soil properties. Sharing some common element with the multiscale methods, an upscaling method was previously proposed for a class of nonlinear parabolic equations including the Richards equation, which is often used for modeling flow processes in soils. To verify its applicability in more realistic and varied flow scenarios, in this study we applied the method to one‐ and two‐dimensional simulations of unsaturated water flow through heterogeneous soil profiles under different boundary conditions including infiltration, evaporation, and drainage. The Gardner–Basha model and the Mualem–van Genuchten model were used as the constitutive relations to close the Richards equation. Results show that the upscaling method can effectively capture the large‐scale structure of fine‐scale behavior in these realistic and varied scenarios. Note that in this study we pre‐calculated the effective hydraulic functions by solving the local problems outside the coarse‐scale simulation to avoid the expensive computation of upscaling these functions, which need to be updated at each time step on each coarse block. In our examples, we observed that, compared with the upscaling method that calculates the effective hydraulic functions by solving the fine‐scale problems during the coarse‐scale simulation, the upscaling method with pre‐calculation saved more than 83 and 90% of the central processing unit time in the one‐ and two‐dimensional simulations, respectively, without compromising accuracy.

Rapid and Fully Automated Measurement of Water Vapor Sorption Isotherms: New Opportunities for Vadose Zone Research

Rapid and Fully Automated Measurement of Water Vapor Sorption Isotherms: New Opportunities for Vadose Zone Research

Rapid and Fully Automated Measurement of Water Vapor Sorption Isotherms: New Opportunities for Vadose Zone Research

Eminent environmental challenges such as remediation of contaminated sites, the establishment and maintenance of nuclear waste repositories, or the design of surface landfill covers all require accurate quantification of the soil water characteristic (SWC) at low water contents. Furthermore, several essential but difficult‐to‐measure soil properties, including clay content and specific surface area, are intimately related to water vapor sorption. Until recently, it was a major challenge to measure detailed water vapor sorption isotherms accurately within a reasonable time frame. This priority communication illustrates potential applications of a new, fully automated, and rapid Vapor Sorption Analyzer (VSA) to pertinent issues in vadose zone research. Detailed vapor sorption isotherms for 25 variably textured soils were measured with the VSA within 1 to 3 d. Links between generated isotherms and pesticide volatilization, toxic organic vapor sorption kinetics, and soil water repellency are illustrated. Several methods to quantify hysteresis effects and to derive soil clay content and specific surface area from VSA‐measured isotherms are presented. Besides above mentioned applications, potential relationships to percolation threshold for solute diffusion in unsaturated soil and to soil cation exchange capacity are discussed to stimulate new and much‐needed vadose zone research.

Hydropedology—A Perspective on Current Research

In this special section entitled “Frontiers of Hydropedology in Vadose Zone Research,” a set of 17 peer-reviewed papers were chosen that have been developed based on the presentations given at the second International Conference on Hydropedology held in Leipzig, Germany in July 2012. The papers

Vadose Zone Journal: A Decade of Multidisciplinary Research

Ten years ago we welcomed you to the first issue of Vadose Zone Journal (VZJ). As the first two editors of VZJ, we very much remember the enormous excitement about finally having a dedicated outlet designed to bring much-needed direction to vadose zone research ([van Genuchten, 2002][1]; [Hopmans et

Conception of vadose zone research in the area of Goczałkowice reservoir.

Abstract Goczałkowice reservoir is one of the main source of drinking water for Upper Silesia Region. In reference to Water Frame Directive matter since 2010 the strategic research project: „Integrated system supporting management and protection of dammed reservoir (ZiZoZap)”, which is being conducted on Goczałkowice reservoir, has been pursued. In the framework of this project complex groundwater monitoring is carried on. One aspect is vadose zone research, conducted to obtain information about changes in chemical composition of infiltrating water and mass transport within this zone. Based on historical data and the structural model of direct catchment of Goczałkowice reservoir location of the vadose zone research site was selected. At the end of November 2012 specially designed lysimeter was installed with 10 MacroRhizon samplers at each lithological variation in unsaturated zone. This lysimeter, together with nested observation wells, located in the direct proximity, create the vadose zone research site which main aim is specifying the amount of nitrate transport in the vertical profile.

Special Section: Progress in Modeling and Characterization of Frozen Soil Processes

Freezing and thawing affect a multitude of physical, chemical, and biological processes in the vadose zone. Concerns over potential impacts of global warming on the spatial extent of permafrost and poorly understood hydrological and ecological processes in frozen soils have prompted renewed interest in this subject. The vadose zone research community should become more engaged in addressing contemporary challenges concerning frozen soil. This special section presents 12 papers addressing a range of topics ranging from measurement techniques and theoretical developments to field studies of seasonal freeze–thaw cycles and associated thermal and hydrological regimes in permafrost and rock glacier. It is hoped that the studies reported here will generate interest in addressing some of the fascinating challenges associated with rapid changes in the frozen vadose zone over large areas in the cold regions that may alter the hydrological and biogeochemical cycles, including greenhouse gas emissions.

Soil Architecture and Physicochemical Functions: An Introduction

Soils function as Earth's life support system, a thin layer full of life covering most of the terrestrial surfaces. Soils form the foundation of society. Norman Borlaug stated in his Nobel laureate lecture that “the first essential component of social justice is adequate food for all mankind.” If we are to provide this component while sustaining environmental quality in the midst of a growing population and rapidly diminishing resources, it is imperative to study and obtain a deeper level of understanding of soil functions using state‐of‐the‐art technologies as well as provide the next generation of environmentalists, soil scientists, and environmental engineers with the best education possible. The 16 papers in this special section on soil architecture and physicochemical functions in the Vadose Zone Journal contribute to these goals by improving and linking measurement, visualization, and modeling of soil structure (architecture) and physical, chemical, and biological processes in different porous media systems and at different scales. Several studies in this special section also outline and discuss emerging and exciting interdisciplinary challenges for the rapidly growing vadose zone research community, including the need for enhanced public awareness of the soil's essential life‐support functions, putting value on soil ecosystem services (“capital of soil”), and design of optimal soil‐based growth media for long‐term missions in space.

Cation Exchange on Vadose Zone Research Park Subsurface Sediment, Idaho National Laboratory

The exchange of Sr2+, Mg2+, Na+, Li+, and NH4+ for Ca2+ on a sediment from the Vadose Zone Research Park at Idaho National Laboratory was characterized for use in modeling transport and remediation of the 90Sr‐contaminated vadose zone at the nearby Idaho Nuclear Technology and Engineering Center. Cation exchange was measured at ionic strengths of 0.1 and 0.005 and a range of cation compositions. The Sr–Ca exchange on this sediment is nonpreferential (Vanselow selectivity coefficient KV = 1), whereas in NH4–Ca exchange NH4 is preferentially adsorbed (KV ≫ 1). The data were modeled using the Vanselow, Rothmund–Kornfeld, and Gapon (KG) exchange coefficients. The Gapon coefficient was the least dependent on exchanger‐phase composition. The KG values were incorporated into a geochemical software program that includes aqueous‐ and solid‐phase reactions. Modeled NH4+ and Ca2+ exchanger phase and solution composition were in good agreement with the experimental data at both ionic strengths.

UZIG USGS Research: Advances through Interdisciplinary Interaction

Because vadose zone research relates to diverse disciplines, applications, and modes of research, collaboration across traditional operational and topical divisions is especially likely to yield major advances in understanding. The Unsaturated Zone Interest Group (UZIG) is an informal organization sponsored by the USGS to encourage and support interdisciplinary collaboration in vadose or unsaturated zone hydrologic research across organizational boundaries. It includes both USGS and non‐USGS scientists. Formed in 1987, the UZIG operates to promote communication, especially through periodic meetings with presentations, discussions, and field trips. The 10th meeting of the UZIG at Los Alamos, NM, in August 2007 was jointly sponsored by the USGS and Los Alamos National Laboratory. Presentations at this meeting served as the initial basis for selecting papers for this special section of Vadose Zone Journal, the purpose of which is to present noteworthy cutting‐edge unsaturated zone research promoted by, facilitated by, or presented in connection with the UZIG.

Elucidation of Flow and Transport Processes in a Variably Saturated System of Interlayered Sediment and Fractured Rock Using Tracer Tests

The objective of this work was to investigate flow and transport in a layered, variably saturated system consisting of both fractured rock and sedimentary material during focused infiltration from the surface. Two tracer tests were performed using the Vadose Zone Research Park (VZRP) at the Idaho National Laboratory (INL). The first test occurred under quasi‐steady‐state conditions and the second was initiated in a much drier system and thus provided information regarding flow and transport under transient conditions. A one‐dimensional analytical model was used to fit breakthrough curves resulting from the two tracer tests. The results of this modeling provide insight into the nature of flow in the fractured basalt, surficial alluvium, and sedimentary interbeds that comprise the vadose zone of the eastern Snake River Plain. Flow through the fractured basalt is focused and preferential in nature, and multiple flow paths arise due to numerous fractures functioning as transmissive pathways in addition to flow splitting along geologic contacts. Flow velocities were significantly higher during the test with the wetter flow domain, presumably due to increases in hydraulic conductivity associated with higher water contents of the geologic materials. Perching was observed above the alluvium–basalt contact and above the lower boundary of a locally continuous sedimentary interbed. The perching behavior between the two contacts was fundamentally different; the perched layer above the alluvium–basalt contact was neither laterally extensive nor temporally persistent in the absence of infiltration from the surface. In contrast, the perched layer along the interbed was significantly thicker and gave rise to lateral flow over distances on the order of hundreds of meters. Vertical transport is shown to occur predominantly through the main bulk of the sedimentary material of the interbed; lateral flow appears to occur primarily in the fractured basalt directly above the interbed.

Upscaling Hydraulic Properties and Soil Water Flow Processes in Heterogeneous Soils: A Review

This review covers, in a comprehensive manner, the approaches available in the literature to upscale soil water processes and hydraulic parameters in the vadose zone. We distinguish two categories of upscaling methods: forward approaches requiring information about the spatial distribution of hydraulic parameters at a small scale, and inverse modeling approaches requiring information about the spatial and temporal variation of state variables at various scales, including so‐called “soft data”. Geostatistical and scaling approaches are crucial to upscale soil water processes and to derive large‐scale effective fluxes and parameters from small‐scale information. Upscaling approaches include stochastic perturbation methods, the scaleway approach, the stream‐tube approach, the aggregation concept, inverse modeling approaches, and data fusion. With all upscaling methods, the estimated effective parameters depend not only on the properties of the heterogeneous flow field but also on boundary conditions. The use of the Richards equation at the field and watershed scale is based more on pragmatism than on a sound physical basis. There are practically no data sets presently available that provide sufficient information to extensively validate existing upscaling approaches. Use of numerical case studies has therefore been most common. More recently and still under development, hydrogeophysical methods combined with ground‐based remote sensing techniques promise significant contributions toward providing high‐quality data sets. Finally, most of the upscaling literature in vadose zone research has dealt with bare soils or deep vadose zones. There is a need to develop upscaling methods for real world soils, considering root water uptake mechanisms and other soil–plant–atmosphere interactions.