Vadose Zone Model Research Articles

A Stochastic Assessment of Nuclear Waste Management Practices at the Hanford Site, Washington

ABSTRACTWaste management and disposal decisions at the Hanford Site, Washington, depend in part on an understanding of the risks and impacts associated with alternate disposal and remedial actions. A proof-of-principle site-wide assessment of the risks and impacts associated with all wastes that will remain at the Hanford Site following cleanup has been performed for the first time. It simulates contaminant release, migration, and fate from the initiation of site operations in 1944 forward, and, thus, illustrates historical and near-term influences on long-term risk and impact.A stochastic simulation tool capable of addressing 1000 waste discharge and disposal sites and 10 contaminants for a period of 1000 years has been created and applied. Human health and ecological risks as well as impacts to the regional economy and local cultures are estimated. The methodology developed is known as the System Assessment Capability (SAC). It is currently in a Revision 0 state corresponding to a proof-of-principle demonstration.An initial assessment based on the planning baseline of the U.S. Department of Energy (Richland Operations office and office of River Protection) has been undertaken. Preliminary results of the assessment indicate variability in predictions is most influenced by uncertainty in:- geochemical adsorption (i.e., distribution coefficients, Kd) for contaminant release, vadose zone, and groundwater models; especially for uranium and iodine which are somewhat but not greatly adsorbed,- solid waste burial ground inventories of iodine-129, and- liquid discharge site inventories of technetium-99.It is apparent that variables governing change in performance are a function of space and time. Initially, these results point to the need for related models and data to be examined, and, if necessary, augmented through future laboratory and field studies to better quantify or reduce uncertainty.

Analysis of Water Saturation, NAPL Content, Degradation Half‐Life, and Lower Boundary Conditions on VOC Transport Modeling: Implications for Closure of Soil Venting Systems

AbstractSimulations using a one‐dimensional, analytical, vadose zone, solute‐transport screening code (VFLUX) were conducted to assess the effect of water saturation, NAPL saturation, degradation half‐life, and boundary conditions at the vadose zone/ground water interface on model output. At high initial soil concentrations, model output was significantly affected by input parameters and lower boundary conditions yet still resulted in consistent decision‐making to initiate or continue venting application. At lower soil concentrations, however, typical of what is observed after prolonged venting application, differences in model input and selection of lower boundary conditions resulted in inconsistent decision‐making. Specifically, under conditions of low water saturation, use of a first‐type, time‐dependent lower boundary condition indicated that the primary direction of mass flux was from ground water to the vadose zone, suggesting little benefit from continued venting application. Use of a finite, zero‐gradient lower boundary condition, though, indicated continued mass flux from the vadose zone to ground water, suggesting a continued need for venting application. In this situation, sensitivity analysis of input parameters, selection of boundary conditions, and consideration of overall objectives in vadose zone modeling become critical in regulatory decision‐making.

Monte Carlo Vadose Zone Model for Soil Remedial Criteria

Many vadose zone models are available for environmental remediation, but few offer the procedures for verifying model predictions with field data and for dealing with uncertainties associated with model input parameters. This article presents a modified model combining a one-dimensional vadose-zone transport model and a simple groundwater mixing model with a function of Monte Carlo simulation (MCS). The modified model is applied to determine soil remedial concentrations for methyl tertiary butyl ether (MTBE). The modified model generates a distribution of MTBE ground-water concentrations at the point of compliance. This distribution can be used to estimate the risk of exceeding groundwater quality standard given soil remedial concentrations. In a case study, soil remedial concentration for MTBE is established to be 5 µg/kg, with a 95% and 10 µg/kg with a 50% probability that groundwater concentration will not exceed the water quality objective of 13 µg/L. Furthermore, this study uses MCS to investigate uncertainties of model input parameter hydraulic conductivity (K). One set of data (K1) is based on the results of hydraulic conductivity laboratory tests, and the other (K2) is based on the results of slug tests conducted in the field. As expected, the laboratory data show smaller K values than the field data. The comparison of the MCS results obtained from the two sets of K data indicates that the MTBE groundwater concentrations calculated based on K1 are generally 160 to 625% greater than those calculated based on K2 at the same percentiles of the MCS distribution. A higher soil remedial concentration of9jig/kg is then calculated based on the MCS results from K2 at 95%ile and 19 µg/kg at 50%ile.

Equilibrium analysis of groundwater–vadose zone interactions and the resulting spatial distribution of hydrologic fluxes across a Canadian Prairie

The spatial distribution and magnitude of time‐averaged surface runoff, evapotranspiration, net recharge, and groundwater divergence are constrained by mutual dependence on the shape and position of the water table. The water table position impacts the partitioning of rainfall by bounding the moisture profile at depth and creating a potential source of capillary rise to the root zone. By coupling a water table dependent vadose zone model that time averages over event‐scale surface fluxes to a regional groundwater model (MODFLOW‐96), we are able (1) to estimate the unique shape and position of the water table for which net recharge (percolation additions or capillary rise losses) is balanced by the divergence of the underlying groundwater flow field, (2) to estimate the distribution of recharge and discharge to and from an aquifer, and (3) to estimate the spatial distribution of the long‐term mean partitioning of rainfall into evapotranspiration, runoff, and infiltration. Parameters of the coupled model include soil physical properties (permeability, porosity, tension‐saturated matric potential, and Brooks and Corey pore size distribution index), basin topography, aquifer hydraulic conductivity, and statistical moments of the probability distributions of meteorological variables (storm intensity, storm duration, time between storms, and potential evaporation). The recharge and discharge areas predicted by the equilibrium model (without specifying a priori the water table shape) are in good agreement with field estimates by Toth [1966] for a Canadian prairie, indicating the importance of accounting for groundwater–vadose zone interactions and lateral groundwater redistribution in watershed modeling.

A Study of Vadose Zone Transport Model VLEACH

Application of vadose zone transport models has been hampered by lack of model validation. Difficulties to validate vadose zone models using field data not only come from model assumptions that are uncertain to the subsurface transport processes but also from the uncertainties associated with soil contaminants’ release time and quantity, soil sampling, sample transport, and analytical procedures. This article first conducts a test of a popularly used vadose zone transport VLEACH by comparing model results with a set of laboratory soil column infiltration and volatilization study data. The comparison shows a close agreement between the VLEACH model results and the laboratory data. Second, the sorption coefficient Kd calculated in VLEACH is compared with field data. The comparison indicates that VLEACH may overestimate the mass leached from soil to groundwater. The article also discusses the selection of the model simulation timestep, the vertical dimension increment, the Courant criterion, and the lower boundary condition using the sensitivity analysis method based on a case study of soil remediation for trichloroethylene. The procedures presented in this paper are important to practical model application and modification. This level of work should be routinely conducted for any new or modified version of vadose zone models.

Vadose zone measurement and modeling

The following treatise is a summary of some of the ongoing research activities in the soil physics program at the University of California in Davis. Each of the four listed areas win be presented at the Workshop on special topics on soil physics and crop modeling in Piracicaba at the University of Sao Paulo. We limited ourselves to a general overview of each area, but will present a more thorough discussion with examples at the Workshop.

Coupling Vadose Zone Models with GIS: Emerging Trends and Potential Bottlenecks

AbstractThe capabilities of geographic information systems (GIS) to capture, edit, manipulate, analyze, and display spatial and nonspatial data have long been recognized. Since its initial development in the 1960s, GIS technology has been used extensively to generate and spatially organize large volumes of data to support modeling of contaminant transport in the vadose zone. In the literature, however, two constantly recurring themes have been, on one hand, the opportunities that GIS provide for sophisticated analysis and visualization of spatial data, and on the other hand, their inadequacy for research and application in many disciplines, including environmental modeling. In vadose zone modeling, the gap between user demands and the functional capabilities of GIS have become something of a cliché. An intensive research effort will be needed to couple vadose zone models with GIS. The extent to which such research is advantageous depends upon the benefits to be derived and the potential problems that can be encountered. This paper examines broad issues concerning the coupling of vadose zone models with GIS and emphasizes the emerging trends and potential bottlenecks. Application examples involving the coupling of some vadose zone models of nonpoint source pollution with GIS are described. The issues addressed will help to explain how to loosely, closely, or tightly couple models with GIS, and to hopefully stimulate more discussions and future research activities.

A Laboratory Classifier for Separation of Ground Products by Terminal Velocity

Raised-bed plasticulture, an intensive production system used around the world for growing high-value crops (e.g., fresh market vegetables), faces a water-food nexus that is actually a food-water-energy-land-economic nexus. Plasticulture represents a multibillion dollar facet of the United States crop production value annually and must become more efficient to be able to produce more on less land, reduce water demands, decrease impacts on surrounding environments, and be economically-competitive. Taller and narrower futuristic beds were designed with the goal of making plasticulture more sustainable by reducing input requirements and associated wastes (e.g., water, nutrients, pesticides, costs, plastics, energy), facilitating usage of modern technologies (e.g., drip-based fumigation), improving adaptability to a changing climate (e.g., flood protection), and increasing yield per unit area.Compact low-input beds were analyzed against conventional beds for the plasticulture production of tomato (Solanum lycopersicum), an economically-important crop, using a systems approach involving field measurements, vadose-zone modeling (HYDRUS), and production analysis. Three compact bed geometries, 61 cm (width) × 25 cm (height), 45 cm × 30 cm, 41 cm × 30 cm, were designed and evaluated against a conventional 76 cm × 20 cm bed. A two-season field study was conducted for tomato in the ecologically-sensitive and productive Everglades region of Florida. Compact beds did not statistically impact yield and were found to reduce: 1) production costs by $150–$450/ha; 2) leaching losses by up to 5% (1 cm/ha water, 0.33 kg/ha total nitrogen, 0.05 kg/ha total phosphorus); 3) fumigant by up to 47% (48 kg/ha); 4) plasticulture's carbon footprint by up to 10% (1711 kg CO2-eq/ha) and plastic waste stream by up to 13% (27 kg/ha); 5) flood risks and disease pressure by increasing field's soil water storage capacity by up to 33% (≈1 cm); and 6) field runoff by 0.48–1.40 cm (51–76%) based on HYDRUS model simulations of 10-year, 2-h storm events in other major tomato production regions of California and Virginia.Re-designing the bed geometries in plasticulture production systems to be more compact is an example of win-win production optimization not only for traditional farms in rural areas but also for urban and peri-urban farms which are located closer to city centers. Compact beds could enable more plants per unit area, thus requiring less land area for the same production. Needing less area facilitates urban and peri-urban farming where land values can be high. Urban and peri-urban farming has several benefits, including reductions in transportation energy as production is closer to market and the ability for city wastewater to be reused for irrigation instead of freshwater withdrawals. Compact beds allow plasticulture to have smaller water, chemical, energy, carbon, waste, and economic footprints without impacting production. Improving agricultural systems in this way could enhance economic and environmental viability, which is essential for a sustainable food-water-energy-land-economic nexus.