Solute transport and extraction by a single root in unsaturated soils: model development and experiment

A contaminant transport model was developed to simulate the fate and transport of organic compounds such as TNT (2,4,6-trinitrotoluene), using the single-root system. Onions were planted for this system with 50-ml plastic tubes. Mass in the soil, soil solution, root and leaf was monitored using 14C-TNT. Model parameters were acquired from the experiments in the single-root system and were used to simulate total TNT concentration in soil, providing the average concentrations in the rhizosphere and bulk soil as well as root and leaf compartments. Because the existing RCF (root concentration factor) and TSCF (transpiration stream concentration factor) equations based on logKow (octanol-water partition coefficient) were not correlated to TNT uptake, a new term, root uptake rate (Rur), and a new Tscf equation, based on the experimental data, were introduced in the proposed model. The results from both modeling and experimental studies showed higher concentrations of TNT in the rhizosphere than in the bulk soil, because mass transported from the surrounding soil into the rhizosphere was higher than that by root uptake.     

A numerical model for transient-hysteretic flow and solute transport in unsaturated porous media

A two-dimensional flow and transport model was developed for simulating transient water flow and nonreactive solute transport in heterogeneous, unsaturated porous media containing air and water. The model is composed of a unique combination of robust and accurate numerical algorithms for solving the Richards', Darcy flux, and advection-dispersion equations. The mixed form of Richards' equation is solved using a finite-element formulation and a modified Picard iteration scheme. Mass lumping is employed to improve solution convergence and stability behavior. The flow algorithm accounts for hysteresis in the pressure head-water content relationship. Darcy fluxes are approximated with a Galerkin and Petrov-Galerkin finite-element method developed for random heterogeneous porous media. The transport equation is solved using an Eulerian-Lagrangian method. A multi-step, fourth-order Runge-Kutta, reverse particle tracking technique and a quadratic-linear interpolation scheme are shown to be superior for determining the advective concentration. A Galerkin finite-element method is used for approximating the dispersive flux. The unsaturated flow and transport model was applied to a variety of rigorous problems and was found to produce accurate, mass-conserving solutions when compared to analytical solutions and published numerical results.     

Field investigation into unsaturated flow and transport in a fault: model analyses

Results of a fault test performed in the unsaturated zone of Yucca Mountain, Nevada, were analyzed using a three-dimensional numerical model. The fault was explicitly represented as a discrete feature and the surrounding rock was treated as a dual-continuum (fracture-matrix) system. Model calibration against seepage and water-travel-velocity data suggests that lithophysal cavities connected to fractures can considerably enhance the effective fracture porosity and therefore retard water flow in fractures. Comparisons between simulation results and tracer concentration data also indicate that matrix diffusion is an important mechanism for solute transport in unsaturated fractured rock. We found that an increased fault-matrix and fracture-matrix interface areas were needed to match the observed tracer data, which is consistent with previous studies. The study results suggest that the current site-scale model for the unsaturated zone of Yucca Mountain may underestimate radionuclide transport time within the unsaturated zone, because an increased fracture-matrix interface area and the increased effective fracture porosity arising from lithophysal cavities are not considered in the current site-scale model.     

A mountain-scale thermal-hydrologic model for simulating fluid flow and heat transfer in unsaturated fractured rock

A multidimensional, mountain-scale, thermal-hydrologic (TH) numerical model is presented for investigating unsaturated flow behavior in response to decay heat from the proposed radioactive waste repository in the Yucca Mountain unsaturated zone (UZ), The model, consisting of both two-dimensional (2-D) and three-dimensional (3-D) representations of the UZ repository system, is based on the current repository design, drift layout, thermal loading scenario, and estimated current and future climate conditions. This mountain-scale TH model evaluates the coupled TH processes related to mountain-scale UZ flow. It also simulates the impact of radioactive waste heat release on the natural hydrogeological system, including heat-driven processes occurring near and far away from the emplacement tunnels or drifts. The model simulates predict thermally perturbed liquid saturation, gas- and liquid-phase fluxes, and water and rock temperature elevations, as well as the changes in water flux driven by evaporation/condensation processes and drainage between drifts. These simulations provide insights into mountain-scale thermally perturbed flow fields under thermal loading conditions.     

Modeling in situ ozonation for the remediation of nonvolatile PAH-contaminated unsaturated soils

Mathematical models were developed to investigate the characteristics of gaseous ozone transport under various soil conditions and the feasibility of in situ ozone venting for the remediation of unsaturated soils contaminated with phenanthrene. On the basis of assumptions for the mass transfer and reactions of ozone, three approaches were considered: equilibrium, kinetic, and lump models. Water-saturation-dependent reactions of gaseous ozone with soil organic matter (SOM) and phenanthrene were employed. The models were solved numerically by using the finite-difference method, and the model parameters were determined by using the experimental data of Hsu [The use of gaseous ozone to remediate the organic contaminants in the unsaturated soils, PhD Thesis, Michigan State Univ., East Lansing, MI, 1995]. The transport of gas-phase ozone is significantly retarded by ozone consumption due to reactions with SOM and phenanthrene, in addition to dissolution. An operation time of 156 h was required to completely remove phenanthrene in a 5-m natural soil column. In actual situations, however, the operation time is likely to be longer than the ideal time because of unknown factors including heterogeneity of the porous medium and the distribution of SOM and contaminant. The ozone transport front length was found to be very limited (< 1 m). The sensitivity analysis indicated that SOM is the single most important factor affecting in situ ozonation for the remediation of unsaturated soil contaminated with phenanthrene. Models were found to be insensitive to the reaction mechanisms of phenathrene with either gas-phase ozone or dissolved ozone. More study is required to quantify the effect of OH* formation on the removal of contaminant and on ozone transport in the subsurface.     

A composite medium approximation for unsaturated flow in layered sediments

Saturated-unsaturated flow in strictly layered sediments proceeds via conductors in parallel in the direction parallel to bedding, and via resistors in series in the direction perpendicular to bedding. On sufficiently small scales of space and time, flow in such media will be subject to approximate capillary equilibrium locally, which provides a basis for approximating the effective hydraulic conductivity of a composite multi-layer medium in terms of the conductivities of the individual layers. Equations for the hydraulic conductivity tensor in "composite medium approximation" (COMA) are given in a coordinate system aligned with bedding. Hydraulic conductivity parallel to bedding is generally larger than in the perpendicular direction. The anisotropy depends on the spread of the conductivity distribution, and tends to increase for dryer conditions. The COMA model was implemented in a multi-phase flow simulator and tested by comparison with high-resolution simulations in which all layering heterogeneity is resolved explicitly. Under favorable conditions, COMA is found to accurately represent sub-grid scale flow and transport processes, providing a practical method for simulating field-scale flow and transport in layered media. The approximation improves when layers are thinner, and when flow rates are smaller.     

Lattice Boltzmann model for agrochemical transport in soils

Agrochemical transport in soils is complicated and involves physical, chemical and biochemical reactions; its mathematical modelling remains a challenging task. This paper presents a lattice Boltzmann model to simulate the agrochemical movement. The lattice Boltzmann model is a microscopic and process-based model, simulating the transport process by tracking chemical particles. The model presented in this paper is for one-dimensional vertical leaching and assumes that the chemical particles at the microscopic level move in five directions: one stagnant, two in vertical direction and two in an internal horizontal direction bounded by two reactive walls. Reactions at the walls are assumed to take place at two different rates, one in fast rate where the chemicals in the solution and on the wall are in an instant equilibrium, and the other in slow rate where the mass exchange rate between the chemicals in the solution and on the wall is a first-order kinetic. The reactions on both walls are assumed to occur instantly when the chemical particles moving in the internal direction hit the walls. To test the model, we measured the leaching of atrazine through soil columns in the laboratory. The results simulated with the lattice Boltzmann model are compared with the measured breakthrough curves and the non-equilibrium two-site convection-dispersion model; they all show close agreement. The transport parameters needed in the models are obtained from the measurement of adsorption isotherm of atrazine, bromide leaching in the same soil columns and calibration.     

A risk index for characterising flow pattern in soils using dye tracer distributions

A novel risk index for the vulnerability of groundwater by pollutants is defined as the form parameter of the Pareto distribution and estimated from dye tracer experiments. The Pareto distribution appears as the limit distribution of the extreme value theory, which has been applied to an idealized model of drops that run along a path. The properties of the risk index are investigated by a Monte Carlo study, where the paths are modelled by means of Gaussian random fields. The method is applied to three profiles obtained from Brilliant Blue tracer experiments of the soil physics group at ETH Zurich. It is shown that a single profile can be rather well characterised by the risk index. However, due to the high variability of the dye tracer profiles, an estimated number of at least 15 profile pictures are necessary to characterise a soil.     

Groundwater flow, multicomponent transport and biogeochemistry: development and application of a coupled process model

A research tool for modeling the reactive flow and transport of groundwater contaminants in multiple dimensions is presented. Arbitrarily complex coupled kinetic–equilibrium heterogeneous reaction networks, automatic code generation, transfer-function based solutions, parameter estimation, high-resolution methods for advection, and robust solvers for the mixed kinetic–equilibrium chemistry are some of the features of reactive flow and transport (RAFT) that make it a versatile research tool in the modeling of a wide variety of laboratory and field experiments. The treatment of reactions is quite general so that RAFT can be used to model biological, adsorption/desorption, complexation, and mineral dissolution/precipitation reactions among others. The integrated framework involving automated code generation and parameter estimation allows for the development, characterization, and evaluation of mechanistic process models. The model is described and used to solve a problem in competitive adsorption that illustrates some of these features. The model is also used to study the development of an in situ Fe(II)-zone by encouraging the growth of an iron-reducing bacterium with lactate as the electron donor. Such redox barriers are effective in sequestering groundwater contaminants such as chromate and TCE.     

Using illumination and shadow to model aerodynamic resistance and flow separation: An isotropic study

The momentum extracted from fluid flow by the underlying surface roughness is important for understanding processes of entrainment, transportation and deposition of sediments. The parameter z₀ is a length scale that characterizes the loss of wind momentum attributable to the roughness elements. However, it is very difficult to estimate accurately and precisely even under carefully controlled conditions in wind tunnels. This limits the use of the parameter over large areas and in particular across scales of roughness, e.g., grain to form scale. This is problematic for studies of wind erosion and dust dispersion which require estimates of aerodynamic resistance over very large areas.A new concept is proposed with the potential to unify the estimates of fluid flow resistance along the continuum of sparse to tightly packed object spacing and across multiple scales. It is based on the creation of shadows by the illumination of roughness elements and the assumption that flow separation is created behind roughness elements on a plane surface as a function of free-stream wind velocity and obstacle height. The concept was implemented using a computer program and validated against a wind tunnel study that estimated z₀ for configurations of spheroids. Various spheroid coverages used in the wind tunnel study were reconstructed using a digital elevation model of the surface simulated by the computer. A strong relationship was established (R²=0.91) over two orders of magnitude between the shadow area ratio (SAR) and z₀.Fluid drag was shown to be dependent on the arrangement of roughness elements at the surface. The configurations of spheroids were replaced by cylinders of the same basal area and computer simulations of shadow area were repeated. Object shape was evidently important to the overlap of shadow with downstream adjacent obstacles and hence aerodynamic resistance was dependent on object shape. These findings appear to contradict empirical evidence of previous studies.Illumination and shadow of objects on a plane surface appears to adequately represent z₀. Shadow appears to approximate the flow separation behind an obstacle and to represent a wake. The overlap on to downstream adjacent objects of the shadow cast from an upstream object appears to mimic the interference of wakes caused by fluid flow moving around stationary objects with close spacing. There is a compelling argument for the use of SAR as a unifying measure of aerodynamic resistance over the continuum between isolated and tightly packed objects. Furthermore, given elevation data of objects on a plane surface the results show that shadow length is a point-based measure that may be integrated for all points evaluated to provide SAR. The demonstrated angular relationship between illumination and drag (shadow and flow separation) has considerable potential for estimating aerodynamic resistance over multiple scales and for significant investigations of (i) the anisotropic nature of aerodynamic resistance and (ii) its estimation using directional measurements of reflectance and bidirectional reflectance models.     

A reaction-transport model for calcite precipitation and evaluation of infiltration fluxes in unsaturated fractured rock

The percolation flux in the unsaturated zone (UZ) is an important parameter addressed in site characterization and flow and transport modeling of the potential nuclear-waste repository at Yucca Mountain, NV, USA. The US Geological Survey (USGS) has documented hydrogenic calcite abundances in fractures and lithophysal cavities at Yucca Mountain to provide constraints on percolation fluxes in the UZ. The purpose of this study was to investigate the relationship between percolation flux and measured calcite abundances using reactive transport modeling. Our model considers the following essential factors affecting calcite precipitation: (1) infiltration, (2) the ambient geothermal gradient, (3) gaseous CO(2) diffusive transport and partitioning in liquid and gas phases, (4) fracture-matrix interaction for water flow and chemical constituents, and (5) water-rock interaction. Over a bounding range of 2-20 mm/year infiltration rate, the simulated calcite distributions capture the trend in calcite abundances measured in a deep borehole (WT-24) by the USGS. The calcite is found predominantly in fractures in the welded tuffs, which is also captured by the model simulations. Simulations showed that from about 2 to 6 mm/year, the amount of calcite precipitated in the welded Topopah Spring tuff is sensitive to the infiltration rate. This dependence decreases at higher infiltration rates owing to a modification of the geothermal gradient from the increased percolation flux. The model also confirms the conceptual model for higher percolation fluxes in the fractures compared to the matrix in the welded units, and the significant contribution of Ca from water-rock interaction. This study indicates that reactive transport modeling of calcite deposition can yield important constraints on the unsaturated zone infiltration-percolation flux and provide useful insight into processes such as fracture-matrix interaction as well as conditions and parameters controlling calcite deposition.     

Distribution patterns of lead accumulation in roadside soils: a case study from Erzurum, Turkey

Surface soils along roadsides or near to industrial areas may contain high concentrations of lead that adversely affect plant production. Lead contamination of roadside soils is generally attributed to the exhaust emissions of automobiles operating with leaded gasoline. The objectives of this study were to determine the relationships between lead accumulation in soils and the distance from the road edge, predominant wind direction and soil depth, and to define vertical and horizontal distribution patterns of lead accumulation within the study area. The lead content of soil decreased rapidly with the distance from the road, and the relationship was described by a power function. The contamination was more dependent on the predominant wind direction. Our results indicated that lead that had accumulated within the top few centimetres of soil had been mixed throughout the ploughing layer in cultivated lands. Soils within 40 m of the motorway had at least 2 to 6 times higher amounts of lead than the background level.     

One-dimensional model for biogeochemical interactions and permeability reduction in soils during leachate permeation

This paper uses the findings from a column study to develop a reactive model for exploring the interactions occurring in leachate-contaminated soils. The changes occurring in the concentrations of acetic acid, sulphate, suspended and attached biomass, Fe(II), Mn(II), calcium, carbonate ions, and pH in the column are assessed. The mathematical model considers geochemical equilibrium, kinetic biodegradation, precipitation-dissolution reactions, bacterial and substrate transport, and permeability reduction arising from bacterial growth and gas production. A two-step sequential operator splitting method is used to solve the coupled transport and biogeochemical reaction equations. The model gives satisfactory fits to experimental data and the simulations show that the transport of metals in soil is controlled by multiple competing biotic and abiotic reactions. These findings suggest that bioaccumulation and gas formation, compared to chemical precipitation, have a larger influence on hydraulic conductivity reduction.     

Phytoextraction of cadmium and zinc from arable soils amended with sewage sludge using Thlaspi caerulescens: development of a predictive model

The objectives of this study were to assess the potential for using Thlaspi caerulescens as a phytoextraction plant and develop a user-advice model, which can predict the frequency of phytoextraction operation required under prescribed conditions. Pot and field trials were conducted using soil collected from a dedicated sewage sludge disposal facility. Soil amendments (sulphuric acid, potassium chloride and EDTA) intended to increase Cd solubility were also tested. Predictive models of Cd and Zn uptake were developed which were able to reproduce the observed pH-dependence of Cd uptake with an apparent maximum around pH 6. Chemical treatments did not significantly increase the uptake of Cd. The periodic use of phytoextraction with T. caerulescens to maintain soils below statutory metal concentration limits, when modern sewage sludges are repeatedly applied, seems very attractive given the non-intrusive and cost-effective nature of the process. The major limitations lie with the large-scale husbandry of T. caerulescens.     

Analysis of a mesoscale infiltration and water seepage test in unsaturated fractured rock: Spatial variabilities and discrete fracture patterns

A mesoscale (21 m in flow distance) infiltration and seepage test was recently conducted in a deep, unsaturated fractured rock system at the crossover point of two underground tunnels. Water was released from a 3 mx4 m infiltration plot on the floor of an alcove in the upper tunnel, and seepage was collected from the ceiling of a niche in the lower tunnel. Significant temporal and (particularly) spatial variabilities were observed in both measured infiltration and seepage rates. To analyze the test results, a three-dimensional unsaturated flow model was used. A column-based scheme was developed to capture heterogeneous hydraulic properties reflected by these spatial variabilities observed. Fracture permeability and van Genuchten alpha parameter [van Genuchten, M.T., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892-898] were calibrated for each rock column in the upper and lower hydrogeologic units in the test bed. The calibrated fracture properties for the infiltration and seepage zone enabled a good match between simulated and measured (spatially varying) seepage rates. The numerical model was also able to capture the general trend of the highly transient seepage processes through a discrete fracture network. The calibrated properties and measured infiltration/seepage rates were further compared with mapped discrete fracture patterns at the top and bottom boundaries. The measured infiltration rates and calibrated fracture permeability of the upper unit were found to be partially controlled by the fracture patterns on the infiltration plot (as indicated by their positive correlations with fracture density). However, no correlation could be established between measured seepage rates and density of fractures mapped on the niche ceiling. This lack of correlation indicates the complexity of (preferential) unsaturated flow within the discrete fracture network. This also indicates that continuum-based modeling of unsaturated flow in fractured rock at mesoscale or a larger scale is not necessarily conditional explicitly on discrete fracture patterns.     

An empirical map of critical loads of acidity for soils in Great Britain

The method used to produce a critical load map of acidity for soils in Great Britain is described. Critical loads were assigned to the dominant soil in each 1 km grid square of the UK national grid. Mineral soils were assigned a critical load based on mineralogy and chemistry, using approaches appropriate to UK conditions. Critical loads for peat soils are based primarily on a maximum acceptable reduction of peat pH, and results from laboratory equilibration studies. The map shows that soils with small critical loads (<0.5 kmol_c ha⁻¹ year⁻¹) i.e. highly sensitive to acidic deposition, dominate in the north and west of Britain; the south and east are dominated by soils with large critical loads, with small areas of more sensitive soils associated with sandy soil-forming materials. A modified critical load map illustrates the potential impact of agricultural liming on soil critical loads.     

Dissipation of 2,4-D in soils of the Humid Pampa region, Argentina: a microcosm study

Phenoxy herbicides like 2,4-dichlorophenoxyacetic acid (2,4-D) are widely used in agricultural practices. Although its half life in soil is 7-14d, the herbicide itself and its first metabolite 2,4-dichlorophenol (2,4-DCP) could remain in the soil for longer periods, as a consequence of its intensive use. Microcosms assays were conducted to study the influence of indigenous microflora and plants (alfalfa) on the dissipation of 2,4-D from soils of the Humid Pampa region, Argentina, with previous history of phenoxy herbicides application. Results showed that 2,4-D was rapidly degraded, and the permanence of 2,4-DCP in soil depended on the presence of plants and soil microorganisms. Regarding soil microbial community, the presence of 2,4-D degrading bacteria was detected even in basal conditions in this soil, possibly due to the adaptation of the microflora to the herbicide. There was an increment of two orders of magnitude in herbicide degraders after 15d from 2,4-D addition, both in planted and unplanted microcosms. Total heterotrophic bacteria numbers were about 1x10(8) CFUg(-1) dry soil and no significant differences were found between different treatments. Overall, the information provided by this work indicates that the soil under study has an important intrinsic degradation capacity, given by a microbial community adapted to the presence of phenoxy herbicides.