Dual-permeability Model Research Articles

Single and dual-permeability models of chlorotoluron transport in the soil profile

This study presents the transport of chlorotoluron in the soil profile under field conditions. The herbicide Syncuran was applied on a plot (4 m˛) using an application rate of 2.5 kg/ha of active ingredient. Soil samples were taken after 119 days to study the residual chlorotoluron distribution in the soil profile. The single and dual-permeability models in HYDRUS-1D (Šimůnek et al. 2003) were used to simulate water movement and herbicide transport in the soil profile. Soil hydraulic properties and their variability were previously studied by Kutílek et al. (1989). The solute transport parameters, such as the adsorption isotherm and the degradation rate, were determined in the laboratory. Since the solute transport in the field was probably affected by preferential flow, the chlorotoluron distribution in the soil profile calculated using the single-permeability model had a different character than observed chlorotoluron concentrations. The chlorotoluron distribution within depth calculated using the dual-permeability model was closer to the observed behavior of chlorotoluron. While the herbicide did not reach a depth of 8 cm for the single-porosity system, in the case of the dual-permeability model the solute moved to the depth of 60 cm. The dual-permeability model significantly improved correspondence between calculated and observed herbicide concentrations.

Stochastic Modeling of Diffuse Pesticide Losses from a Small Agricultural Catchment

The objective of this study was to identify the main sources of variation in pesticide losses at field and catchment scales using the dual permeability model MACRO. Stochastic simulations of the leaching of the herbicide MCPA (4-chloro-2-methylphenoxyacetic acid) were compared with seven years of measured concentrations in a stream draining a small agricultural catchment and one year of measured concentrations at the outlet of a field located within the catchment. MACRO was parameterized from measured probability distributions accounting for spatial variability of soil properties and local pedotransfer functions derived from information gathered in field- and catchment-scale soil surveys. At the field scale, a single deterministic simulation using the means of the input distributions was also performed. The deterministic run failed to reproduce the summer outflows when most leaching occurred, and greatly underestimated pesticide leaching. In contrast, the stochastic simulations successfully predicted the hydrologic response of the field and catchment and there was a good resemblance between the simulations and measured MCPA concentrations at the field outlet. At the catchment scale, the stochastic approach underestimated the concentrations of MCPA in the stream, probably mostly due to point sources, but perhaps also because the distributions used for the input variables did not accurately reflect conditions in the catchment. Sensitivity analyses showed that the most important factors affecting MACRO modeled diffuse MCPA losses from this catchment were soil properties controlling macropore flow, precipitation following application, and organic carbon content.

Identification of Interpretation Model and DST Data Analysis Obtained from Fractured Basement Reservoir

DST analysis has been conducted for three wells of HY-1, HY-2 and HY-3. To identify the optimum interpretation model for the fractured basement reservoir, DST data were pre-analyzed extensively. Integration of the acquired DST data and identified interpretation model indicated that the hydraulic fracture model, dual-porosity model, and dual-permeability model were adequate as reservoir models for the three wells analyzed. Therefore, we can verify the connectivity and distribution of fracture of the reservoir at the vicinity of wells with the commercial well test analysis model of PanSystem TM .

Simulating Solute Transport in a Structured Field Soil: Uncertainty in Parameter Identification and Predictions

Dual-permeability models have been developed to account for the significant effects of macropore flow on contaminant transport, but their use is hampered by difficulties in estimating the additional parameters required. Therefore, our objective was to evaluate data requirements for parameter identification for predictive modeling with the dual-permeability model MACRO. Two different approaches were compared: sequential uncertainty fitting (SUFI) and generalized likelihood uncertainty estimation (GLUE). We investigated six parameters controlling macropore flow and pesticide sorption and degradation, applying MACRO to a comprehensive field data set of bromide andbentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one-2,2dioxide] transport in a structured soil. The GLUE analyses of parameter conditioning for different combinations of observations showed that both resident and flux concentrations were needed to obtain highly conditioned and unbiased parameters and that observations of tracer transport generally improved the conditioning of macropore flow parameters. The GLUE "behavioral" parameter sets covered wider parameter ranges than the SUFI posterior uncertainty domains. Nevertheless, estimation uncertainty ranges defined by the 5th and 95th percentiles were similar and many simulations randomly sampled from the SUFI posterior uncertainty domains had negative model efficiencies (minimum of -3.2). This is because parameter correlations are neglected in SUFI and the posterior uncertainty domains were not always determined correctly. For the same reasons, uncertainty ranges for predictions of bentazone losses through drainflow for good agricultural practice in southern Sweden were 27% larger for SUFI compared with GLUE. Although SUFI proved to be an efficient parameter estimation tool, GLUE seems better suited as a method of uncertainty estimation for predictions.

The significance of flow in the matrix of the Chalk unsaturated zone

The significance of flow in the matrix of the Chalk unsaturated zone, in comparison with flow in fractures, has been the subject of much debate. In this article, important elements of the literature are discussed in detail and several simple modelling analyses based on steady-state flow are presented. A study of the sensitivity of solute spreading to fracture spacing in models that ignore matrix flow shows that this latter assumption is generally incompatible with observed solute profiles, unless unrealistically small fracture spacings are assumed. The effect of air phase continuities (e.g. bedding planes) on matrix flow has also been examined. These discontinuities are frequently interrupted by points of connectivity between matrix blocks. An issue therefore is the relationship between connectivity and its effect on inter-block conductance. A simple analysis of the Laplace equation shows that just 1% connectivity represents an effective pathway equivalent to 18% of the local rock hydraulic conductivity. Obviously, when there is no fracture flow, solute spreading is significantly reduced. However, dual permeability model simulations show that matrix flow reduces solute spreading in the presence of persistent fracture flow. All of the above studies suggest that flow in the matrix of the Chalk unsaturated zone is significant and that ignoring it may result in a serious misunderstanding of the system.

Dual-permeability modeling of preferential bromide leaching from a tile-drained glacial till agricultural field

A tile-drained agricultural field can be regarded as a ‘field-scale’ lysimeter that may be used to study soil water and chemical transport under relatively natural conditions. Tile discharge and effluent bromide concentrations measured in a previous field tracer experiment for a structured clayey loam at Bokhorst, Northern Germany, indicated strong preferential flow. Simulation using single domain HYDRUS numerical flow and transport model could nevertheless explain water outflow, however, completely failed to describe tile-drain leaching patterns of the conservative tracer. The objective of this paper was to analyze whether the nonequilibrium-type dual-permeability model concept could better capture soil structure related principle mechanisms of preferential leaching in the unsaturated soil at that study site. The dual-permeability model (DUAL) describes for soil matrix and fracture pore systems Darcian flow with coupled Richards' equations and convective–dispersive (CD) solute transport with coupled CD equations. The hydraulic parameters of the dual-permeability model were obtained from standard soil hydraulic measurements by adopting a bimodal fitting procedure, whereas transport parameters were inferred from soil column tracer experiments and geometrical transfer term parameters were derived using qualitative soil structure descriptions. The hydraulic conductivity K a in the inter-domain water transfer term and the effective diffusion coefficient D a in the solute mass transfer term were calibrated by comparing simulated with measured tile flow and effluent concentrations. The DUAL approach described water flow similarly well as the single-domain model. Bromide concentrations in the tile effluent could be approximated with DUAL when decreasing the K a and D a values by three orders of magnitude compared with the values of the soil matrix domain. The dual-permeability approach seems to reflect nonequilibrium transport mechanisms at this structured soil since it not only predicts concentration peaks in initial leaching phases during early time events of the experiment but also the observed reverse reaction of decreased effluent concentrations in initial phases of leaching events during later times of the experiment. The latter occurs when bromide-free infiltrating water is diluting bromide concentrations in the fracture domain such that less concentrated water is preferentially draining from tiles in initial phases of infiltration. Bromide effluent concentrations gradually increase in later phases since vertical flow velocities decrease while ‘diffusive’ solute transfer from the soil matrix continues. Mass transfer restrictions (e.g. due to soil aggregate coatings) may effectively be controlling preferential leaching at this site.

Simulating Infiltration Tests in Fractured Basalt at the Box Canyon Site, Idaho

The results of a series of ponded infiltration tests in variably saturated fractured basalt at Box Canyon, Idaho were used to build confidence in conceptual and numerical modeling approaches used to simulate infiltration in fractured rock. Specifically, we constructed a dual-permeability model using TOUGH2 to represent both the matrix and fracture continua of the upper basalt flow at the Box Canyon Site. A consistent set of hydrogeological parameters was obtained by calibrating the model to infiltration front arrival times in the fracture continuum as inferred from Br− samples collected from fracture–borehole intersections observed during the infiltrating tests. These parameters included the permeability of the fracture and matrix continua, the interfacial area between the fracture and matrix continua, and the porosity of the fracture continuum. To calibrate the model, we multiplied the fracture–matrix interfacial area by a factor between 0.1 and 0.01 to reduce imbibition of water from the fracture continuum into the matrix continuum during the infiltration tests. Furthermore, the porosity of the fracture continuum, as calculated using the fracture aperture inferred from pneumatic-test permeabilities, was increased by a factor of 50 yielding porosity values for the upper basalt flow in the range of 0.01 to 0.02. The fracture-continuum porosity was a highly sensitive parameter controlling the arrival times of the simulated infiltration fronts. Porosity values are consistent with those determined during the Large-Scale Aquifer Pumping and Infiltration Test at the Idaho National Engineering and Environmental Laboratory.

Simulating infiltration in unsaturated basalt for the Large-Scale Aquifer Pumping and Infiltration Test at INEEL

The Large-Scale Aquifer Pumping and Infiltration Test (LPIT) conducted at the Idaho National Engineering and Environmental Laboratory was modeled using TOUGH2 to simulate the highly transient water infiltration and perched-water conditions in the fractured basalt and sedimentary interbeds existing at the site. The fracture and matrix continua of the basalt were represented using a dual-permeability approach. Six perched-water hydrographs, measured during the infiltration test, were used for calibration with iTOUGH2 to estimate six parameters that controlled unsaturated flow in the fractured basalt and the dense clay sedimentary interbed underlying the basalt flow. These parameters included the interfacial area between the basalt fracture and matrix continua, the basalt fracture continuum permeability, the basalt matrix continuum permeability, the interbed matrix continuum permeability, and the interbed van Genuchten capillary pressure parameters. The intent of the calibration was to obtain large-scale properties of the lithological units affecting the field-scale LPIT test. Finally, we tested the applicability of the dual-permeability conceptual model for representing transient variably saturated flow in fractured basalt. Parameters obtained from the calibration were within the range of hydrogeological parameters measured from cores obtained in the field, substantiating the physical relevance of the calibration effort.

Modeling depth-variant and domain-specific sorption and biodegradation in dual-permeability media

A dual-permeability model (S_1D_DUAL) was developed to simulate the transport of land-applied pesticides in macroporous media. In this model, one flow domain was represented by the bulk matrix and the other by the preferential flow domain (PFD) where water and chemicals move at faster rates. The model assumed the validity of Darcian flow and the advective–dispersive solute transport in each of the two domains with inter-domain transfer of water and solutes due to pressure and concentration gradients. It was conceptualized that sorption and biodegradation rates vary with soil depth as well as in each of the two flow domains. In addition to equilibrium sorption, kinetic sorption was simulated in the PFD. Simulations were conducted to evaluate the combined effects of preferential flow, depth- and domain-variant sorption, and degradation on leaching of two pesticides: one with strong sorption potential (trifluralin) and the other with weak sorption potential (atrazine). Simulation results for a test case showed that water flux in the PFD was three times more than in the matrix for selected storm events. When equilibrium sorption was considered, the simulated profile of trifluralin in each domain was similar; however, the atrazine profile was deeper in the PFD than in the bulk matrix under episodic storm events. With an assumption of negligible sorption in the PFD, both the atrazine and the trifluralin profiles moved twice deeper into the PFD. The simulated concentrations of the chemicals were several orders higher in the PFD than in the matrix, even at deeper depths. The volume fraction of the macropores and the sorption and biodegradation properties of the chemicals could also affect the amount of pesticides leaving the root zone. For an intense storm event, slow sorption reaction rates in the PFD produced higher breakthrough concentrations of atrazine at the bottom of the simulated soil profile, thus posing the risk for breakthrough of chemicals from the root zone.

Modeling macropore flow effects on pesticide leaching: inverse parameter estimation using microlysimeters.

Macropore flow is a key factor determining pesticide fate, but models accounting for this process need parameters that cannot be easily measured. This study was conducted to investigate the use of inverse techniques to estimate parameters controlling macropore flow and pesticide fate in the dual-permeability model MACRO. Undisturbed columns were sampled at three landscape positions (hilltop, slope, hollow) with contrasting texture and organic carbon content. Transient leaching experiments were performed for an anionic tracer and the herbicide MCPA (4-chloro-2methylphenoxy acetic acid) during a 4-mo period, first under natural rainfall, and then under controlled irrigation in the laboratory. The tracer breakthrough for the liner-textured soil from the hilltop showed strong evidence of macropore flow, resulting in a rapid leaching of MCPA, while leaching was minimal from the organic-rich hollow soil, since macropore flow was weaker and adsorption stronger. The MACRO model was linked to the inverse modeling program SUFI (Sequential Uncertainty Fitting) to enable calibration of nine key model parameters. Based on calculated model efficiencies, MACRO-SUFI gave generally good predictions of water movement and tracer and pesticide transport, although some errors were attributed to difficulties in simulating the effects of soil moisture on degradation and the timing of water outflows. Even after calibration, significant uncertainties remained for some key parameters controlling macropore flow. Nevertheless, the parameter estimates were significantly different between landscape positions and could also be related to basic soil properties. The posterior uncertainty ranges could probably be reduced with a more exhaustive sampling of the parameter space and improved experimental designs.

Analysis of Inverse Procedures for Estimating Parameters Controlling Macropore Flow and Solute Transport in the Dual-Permeability Model MACRO

Because they are objective and reproducible, inverse modeling procedures are increasingly used to identify model parameters that cannot be easily measured. This study investigated the feasibility of using inverse methods to estimate parameters describing macropore flow, transport, and transformation processes in the dual-permeability model MACRO. MACRO was linked to the inverse modeling package SUFI, and we used numerically generated data representing transient leaching experiments for tracers and reactive solutes in microlysimeters (21-cm height). Attention was focused on parameter sensitivity, availability of experimental data (flux and resident concentrations), the degree of macropore flow in the system, and the significance of experimental errors. Reliable results were obtained in the case of strong macropore flow, but both resident and flux concentrations were needed. However, the uncertainty in d, the parameter describing mass exchange between microporosity and macroporosity, remained large, and the adsorption coefficient could not be estimated accurately. Response surface analysis showed that this was due to a lack of sensitivity to d and to internal correlation between adsorption and degradation parameters. In the case of equilibrium flow, the model was overparameterized, and the parameters related to macropore flow were not sensitive enough to be estimated properly. Experimental errors did not affect the feasibility of the procedure, although the uncertainty in the estimates increased. SUFI linked to MACRO appears to be a promising tool for optimization of the system parameters in soils affected by macropore flow, but the “experimental” design needs to be improved for reliable determination of the mass exchange parameter and the adsorption coefficient.

Lateral Water Diffusion in an Artificial Macroporous System: Modeling and Experimental Evidence

In two-domain schematizations of macroporous soils or fractured rock systems, lateral mass exchange between macropores and the soil matrix is generally modeled as an apparent first-order process. With respect to lateral diffusion, the system is thus characterized by a single parameter, the transfer rate coefficient, which is difficult to estimate a priori. We conducted water infiltration experiments in a laboratory column with an artificial macropore. The novel design of the experimental setup allowed us to discriminate between matrix flow and macropore flow, from which we could estimate the water exchange flux between the two domains. Most of the parameters in a dual-permeability model could be determined independently of the experimental data. In particular, a theoretical expression for the transfer rate coefficient was derived by assuming lateral water and solute diffusion to be similar processes. Numerical analysis of the water exchange process revealed that the transfer coefficient depended also on the macropore conductivity. When this dependency was taken into account, the model reproduced the experimental data reasonably well.

Effect of heterogeneity in fracture permeability on the potential for liquid seepage into a heated emplacement drift of the potential repository

A numerical model was used to investigate the effect of spatial variability in fracture permeability on liquid seepage and moisture distribution in the vicinity of a waste emplacement drift in the unsaturated zone (UZ) of Yucca Mountain. The model is based on a two-dimensional, cross-sectional, dual-permeability model of the unsaturated zone at Yucca Mountain and uses a stochastic approach to investigate the effect of small-scale heterogeneous features. The studies were conducted using one uniform fracture permeability case, three realizations of stochastically generated fracture permeability, one discrete permeability feature case, and one increased ambient liquid flux case. In all cases, the models predict that completely dry drift conditions will develop above and below the drift in 10–100 years and remain dry for 1000–2000 years. During this period, the models predict no seepage into drifts, although liquid flux above the drifts and within the drift pillars may increase by up to two orders of magnitude above ambient flux. This is because the heat released by the emplaced waste is sufficient to vaporize liquid flux of one to two orders of magnitude higher than present-day ambient flux for over 1000 years. The results also show that unsaturated zone thermal–hydrological (TH) models with uniform layer permeability can adequately predict the evolution of seepage and moisture distribution in the rock mass surrounding the repository drifts. The models further show that although variability in fracture permeability may focus and enhance liquid flow in regions of enhanced liquid saturation (due to condensation above the drifts), vaporization and vapor diffusion can maintain a dry environment within the drifts for thousands of years.

Flow and transport in the drift shadow in a dual-continuum model

The current concept for high-level radioactive waste disposal at Yucca Mountain is for the waste to be placed in underground tunnels (or drifts) in the middle of a thick unsaturated zone. Flow modeling and field testing have shown that not all flow encountering a drift will seep into the drift. The underlying reason for the diversion of unsaturated flow around a drift is that capillary forces in the fractures and matrix prevent water entry into the drift unless the capillary pressure in the rock decreases sufficiently to allow for gravity forces to overcome the capillary barrier. As a result of the capillary barrier effect, flow tends to be diverted around the drift, affecting the flow pattern beneath the drift. For some distance beneath the drift, water saturation and flux are reduced. This drift shadow zone is much more pronounced in the fractures than in the matrix due to dominance of gravity over capillary forces in the fractures. Moving downward, away from the drift, the shadow zone asymptotically re-equilibrates to the undisturbed flow conditions due to capillary forces. The behavior of radionuclide transport in this zone of reduced flow is investigated here because this will affect the amount of time required for radionuclides to penetrate the unsaturated zone. The delay of radionuclide movement in the geosphere is one aspect of the potential repository system that could limit public exposure to radioactive waste. The behavior of flow and transport is calculated using a two-dimensional, drift-scale dual-permeability model extending to nine drift diameters below the potential waste emplacement drift. The flow model is first compared with an analytical model for a single continuum. Then, the dual-continuum flow model is investigated with respect to drift-scale and mountain-scale property sets. Transport calculations are performed for a wide range of flow conditions and for different aqueous radionuclides and colloids. Findings indicate that transport times for dissolved or colloidal material released from a drift without seepage are several orders of magnitude longer than if the releases occurred in the undisturbed flow field. Furthermore, the calculations indicate that the transport rate for radionuclides released in the drift shadow is relatively insensitive to flow rates in the fractures, but is sensitive to the flow rate in the matrix.

Analysis of Inverse Procedures for Estimating Parameters Controlling Macropore Flow and Solute Transport in the Dual-Permeability Model MACRO

Analysis of Inverse Procedures for Estimating Parameters Controlling Macropore Flow and Solute Transport in the Dual-Permeability Model MACRO

Numerical study of fluid flow of deforming fractured rocks using dual permeability model

To investigate the transport behaviour of fractured rocks (tightly compacted granular material) under loading/unloading, a dual permeability model was applied by means of numerical simulation. The numerical samples had a size of 80 × 80 mm2 and consisted of granular material with average grains of 1.6 × 1.6 mm2 in size. The grain boundaries were strongly bonded and the samples behaved like intact rock blocks mechanically, but the grain boundaries were permeable and provided the primary permeability for the matrix. Fractures might initiate and grow along the grain boundaries where the level of stress exceeded their strength. The fractures newly created and the pre-existing ones that were embedded within the samples might be much more permeable than the passive grain boundaries, and they would dominate the transport behaviour of the samples if a connected fracture network formed. By increasing fluid pressure (constant differential stress) and increasing differential stress (constant fluid pressure), two types of loading schemes were applied to the samples. As a result, connected fracture networks developed at a certain stress state. The evolution of fractures and fracture networks greatly altered the flow patterns; main flows were being concentrated within those continuously open fracture networks. On the other hand, the change in hydraulic conduits has led to a strong modification of the fluid pressure distribution. Under low effective mean stress (high fluid pressure), secondary percolation phenomena occurred in the loading plane because of the presence of a connected network of open fractures. In this case, the permeability in the loading plane and in the direction perpendicular to the plane increased greatly. Under high effective mean stress (highly differential stress and constant fluid pressure); however, such a secondary percolation threshold was unlikely to exist, although a connected network developed at a critical stress state. This was because most of those fractures created did not open under high effective mean stress, and the deformation was characterized by grain boundary sliding and isolated, dilational jogs at the intersections of grain boundaries. In this case, the permeability in the loading plane had a slight reduction caused by lack of continuously open fractures, but the permeability in the direction perpendicular to the loading plane increased sharply and became highly localized. The deformation and permeability of the samples were examined when subjected to cycling loads. The fluid pressure alternated between the hydrostatic fluid pressure and a higher fluid pressure (supra-hydrostatic) at which the samples became unstable. During the cycling loading the fractures opened and closed periodically, and the permeability increased and decreased correspondingly. When the fluid pressure decreased from a high level to a low level (loading) in a cycling load, the permeability and deformation of the samples did not totally return to the previous state, indicating a clear plastic deformation. After a number of cycling loads, the plastic deformation and permeability increased progressively, which suggests strong path-dependent behaviour. The overall permeability of the samples increased with increasing extensional strain. For the same extensional strain, the permeability was greater under a low effective mean stress than under a high effective mean stress, and greater during continuous loading than cyclic loading.

Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone

In this paper, we review various approaches for modeling preferential and non-equilibrium flow and transport in the vadose zone. Existing approaches differ in terms of their underlying assumptions and complexity. They range from relatively simplistic models to more complex physically based dual-porosity, dual-permeability, and multi-region type models. A relatively simple dual-porosity flow model results when the Richards equation is combined with composite (double-hump type) equations for the hydraulic properties to account for both soil textural (matrix) and soil structural (fractures, macropores, peds) effects on flow. The simplest non-equilibrium flow model, a single-porosity model which distinguishes between actual and equilibrium water contents, is based on a formulation by Ross and Smettem [Soil Sci. Soc. Am. J. 64 (2000) 1926] that requires only one additional parameter to account for non-equilibrium. A more complex dual-porosity, mobile–immobile water flow model results when the Richards or kinematic wave equations are used for flow in the fractures, and immobile water is assumed to exist in the matrix. We also discuss various dual-permeability models, including the formulation of Gerke and van Genuchten [Water Resour. Res. 29 (1993a) 305] and the kinematic wave approach as used in the MACRO model of Jarvis [Technical Description and Sample Simulations, Department of Soil Science, Swedish University of Agricultural Science, Uppsala, Sweden (1994) 51]. Both of these models invoke terms accounting for the exchange of water and solutes between the matrix and the fractures. Advantages and disadvantages of the different models are discussed, and the need for inter-code comparison is stressed, especially against field data that are sufficiently comprehensive to allow calibration/validation of the more complex models and to distinguish between alternative modeling concepts. Several examples and comparisons of equilibrium and various non-equilibrium flow and transport models are also provided.

Axi-symmetric simulation of soil vapor extraction influenced by soil fracturing

Fracturing, either pneumatic or hydraulic, is a method to improve the performance of soil vapor extraction (SVE) in relatively low permeability soils (<10 −5 cm/s). A two-dimensional model is presented to simulate trichloroethylene (TCE) soil vapor extraction modified by fracturing. Flow and transport is modeled using mobile macropore and micropore networks, which also have been identified in the literature as dual porosity, dual permeability, or heterogeneous flow models. In this model, fluids can flow in both the macropore and micropore networks. This represents a more general model compared to immobile micropore, mobile macropore models presented thus far in the literature for vapor flow and transport in two dimensions. The model considers pressure- and concentration-driven exchange between the macropore and micropore networks, concentration-driven exchange between the gas and sorbed phases within each network, and equilibrium exchange between the gas and water and a sorbed phase within each network. The parameters employed in an example simulation are based on field measurements made at a fractured site. Considered in the simulations were the influence of the volume percentage of fractures, the length of fractures, the relative location of the water table, and the influence of pulsed pumping. For these simulations, inter-network concentration-driven exchange most significantly affected mass removal. The volume percentage of fractures more significantly influence flow and mass removal than the length of fractures. The depth of the water table below the contamination plume only significantly influenced flow and mass removal when the water table was within 60 cm of the bottom of the contaminated soil in the vadose zone for the parameters considered in this study. Pulsed pumping was not found to increase the amount of mass removed in this study.

Estimating Hydraulic Properties of Soil Aggregate Skins from Sorptivity and Water Retention

Skins of soil aggregates often consist of clayey or clay-organic coatings which may affect preferential flow in aggregated soils. The objective was to determine hydraulic properties of samples with intact and removed (cut) skins and interior/skin hydraulic conductivity ratios for estimating mass transfer parameters in dual-permeability models. Soil aggregates from the Csd-horizon of a clay-loam glacial till soil (Stagnic Calcaric Regosol) were analyzed. A tension-imbibition apparatus was used for measuring water uptake of multiple aggregates at boundary matric potential heads of −1 and −5 cm. Sorptivities were used to calculate mean weighted water diffusivities and final water contents to fit wetting retention functions. Water retention and hydraulic conductivity functions for the skin layer were derived from differences in water contents and hydraulic resistances between intact and cut samples. Water absorption rates were generally smaller for intact than for cut aggregates. The water retention function of cut was shifted towards smaller water contents compared with intact samples. Mean water diffusivity of intact was 4.5 times smaller than that of cut samples. The interior/skin ratio in unsaturated hydraulic conductivity was about 12 in the measured matric potential head range. The ratio was up to 70 near water saturation and dropped below unity for soil water potentials smaller −1000 cm of water. Aggregate skins may be regarded as a separate porous domain whose hydraulic properties may control water transfer between inter- and intraaggregate pore domains in structured soils.

Stochastic Analysis of Reactive Solute Transport in Heterogeneous, Fractured Porous Media: A Dual-Permeability Approach

A nonlocal, first-order, Eulerian stochastic theory is developed for reactive chemical transport in a heterogeneous, fractured porous medium. A dual-permeability model is adopted to describe the flow and transport in the medium, where the solute convection and dispersion in the matrix are considered. The chemical is under linear nonequilibrium sorption and first-order degradation. The hydraulic conductivities, sorption coefficients, degradation rates in both fracture and matrix regions, and interregional mass transfer coefficient are all assumed to be random variables. The resultant theory for mean concentrations in both regions is nonlocal in space and time. Under spatial Fourier and temporal Laplace transforms, the mean concentrations are explicitly expressed. The transformed results are then numerically inverted to the real space via Fast Fourier Transform method. The theory developed in this study generalizes the stochastic studies for a reactive chemical transport in a one-domain flow field (Hu et al., 1997a) and in a mobile/immobile flow field (Huang and Hu, 2001). In comparison with one-domain transport, the dual-permeability model predicts a larger second moment in the longitudinal direction, but smaller one in the transverse direction. In addition, various simplification assumptions have been made based on the general solution. The validity of these assumptions has been tested via the spatial moments of the mean concentration in both fracture and matrix regions.