摘要:

The occurrence of gaseous pollutants in soils has stimulated many experimental activities, including forced ventilation in the field as well as laboratory transport experiments with gases. The dispersion coefficient in advective‐dispersive gas phase transport is often dominated by molecular diffusion, which leads to a large overall dispersivity γ. Under such conditions it is important to distinguish between flux and resident modes of solute injection and detection. The influence of the inlet type on the macroscopic injection mode was tested in two series of column experiments with gases at different mean flow velocities υ. First we compared infinite resident and flux injections, and second, semi‐infinite resident and flux injections. It is shown that the macroscopically apparent injection condition depends on the geometry of the inlet section. A reduction of the cross‐sectional area of the inlet relative to that of the column is very effective in excluding the diffusive solute input, thus allowing us to use the solutions for a flux injection also at rather low mean flow velocities υ. If the whole cross section of a column is exposed to a large reservoir like that of ambient air, a semi‐infinite resident injection is established, which can be distinguished from a flux injection even at relatively high velocities υ, depending on the mechanical dispersivity of the po

ISSN:0043-1397    

DOI:10.1029/95WR02719

年代:1996

数据来源: WILEY

摘要:

Vapor transport of chlorinated solvents in the unsaturated zone may be an important mechanism for the spread of contamination at spill sites and may be a significant factor controlling the extent of groundwater contamination. Two field experiments were carried out at the Canadian Forces Base Borden field site to provide detailed monitoring of the transport behavior of trichloroethylene vapors in the unsaturated zone. Experiments were conducted for both winter and summer conditions and under different surface boundary conditions. The observed results were simulated using a Fickian‐based numerical model with linear equilibrium phase partitioning. The model includes both diffusion and density‐induced advection and allows for the incorporation of spatial heterogeneities and nonisothermal conditions. Numerical sensitivity analyses were conducted to further evaluate the relative influence of various transport parameters on vapor migration. Use of measured field values as input parameters resulted in a very good match between the experimental results and numerical simulations. In both experiments, vapor plumes spread several meters from the source and downward to the capillary fringe within only a few days. Seasonal temperature variations were found to have a significant impact on the rate and total mass of vapor transport, and variations in organic carbon content, and to a lesser extent moisture content, exerted the greatest control on retardation of vapor migration. Transport was diffusion dominated, but density‐induced advection was an appreciable component of net transport under summer conditions, when vapor concentrations were higher. Geologic conditions at the site made overall transport relatively insensitive to the ground surface boundary cond

ISSN:0043-1397    

DOI:10.1029/95WR02965

年代:1996

数据来源: WILEY

摘要:

An analytical solution is developed for displacement of a nonlinearly sorbing solute in a three‐dimensional heterogeneous aquifer. Nonlinear sorption is modeled by means of different isotherm equations including the commonly used Langmuir and Freundlich isotherms, but also the Langmuir‐Freundlich, Tóth, and two‐site Langmuir isotherms. Evaluation of cleanup times, defined as the times required to reduce the concentration to a given fraction of its initial value, for different isotherms and combinations of input parameters demonstrate the effects of the properties of the isotherms. In particular, the asymptotic behavior as the aqueous concentration approaches zero is important for transport predictions, where Freundlich sorption leads to infinitely increasing cleanup times as the initial plume concentration decreases. The effect of the choice of sorption isotherm, evaluated by using “best‐fit”parameters for four different isotherms, is found to be large and dependent on heterogeneity for the particular data se

ISSN:0043-1397    

DOI:10.1029/95WR02767

年代:1996

数据来源: WILEY

摘要:

Recent laboratory and field studies indicate that colloids are capable of adsorbing and enhancing the transport of a variety of contaminants in groundwater. We examined the influence of kaolinite particles on Cs movement through sand columns. The times for transport of the main part of the137Cs pulse decreased by over a factor of 2 as the concentration of kaolinite in the column influent increased from 0 to 200 mg/L. Very early breakthrough of a relatively small amount of137Cs was observed and attributed to the existence of sorption sites on the kaolinite that were kinetically controlled. This early breakthrough was essentially unretarded, whereas the initial breakthrough in the absence of kaolinite showed a retardation of about 15 pore volumes. Comparison of experimental observations with calculations from a transport model demonstrates that kaolinite deposition onto the sand was described best by an irreversible, first‐order kinetics process and that retention of137Cs by these clay particles was described by linear equations for two‐site solute adsorption. Uptake of137Cs by the sand was represented by the Langmuir version of the two‐site equations. This work indicates the need to account for facilitated transport of contaminants by inorganic colloids and provides a basis for quantifying the pheno

ISSN:0043-1397    

DOI:10.1029/95WR03096

年代:1996

数据来源: WILEY

摘要:

We developed a sonic permeameter that can quickly measure the intrinsic permeability of dry unconsolidated porous materials in the laboratory. It consists of a control box, a loudspeaker, and an external waveguide filled with the sample. Sound wave attenuation and dispersion are measured by two microphones recessed in the inner wall of the waveguide. They receive a sound pulse that is sent down to the waveguide from the loudspeaker. The permeability of the sample is then calculated from the attenuation and dispersion spectra using our mathematical model. Test results obtained over several dry Ottawa sand and glass bead samples agree very well with the results obtained using a conventional constant‐head permeameter. The advantage of this apparatus is its rapid and automated measurement, easy operation, and potential workability in the field. Theoretical analysis is also extended to a hypothetical case where the porous material is totally saturated with wate

ISSN:0043-1397    

DOI:10.1029/95WR02731

年代:1996

数据来源: WILEY

摘要:

The Green‐Ampt and Philip infiltration models are simplified with a capillary tube representation of porous media that can predict wetting front movement during horizontal and vertical infiltration into dry or initially moist soil. The simple model captures the fundamental physics of unsaturated fluid flow and reduces the number of parameters that must be measured. The relevant parameters are the air‐liquid interfacial tension, the density and viscosity of the liquid, the initial saturation, and the permeability. Comparisons with published data show good predictions for infiltration under nonnegative source pressures but are less successful under negative source pressures. The model applies to imbibition of both water and nonaqueous phase liqu

ISSN:0043-1397    

DOI:10.1029/95WR02974

年代:1996

数据来源: WILEY

摘要:

The scales at which model parameters are measured with local field tests distributed on a sampling network are examined. Two scales are defined to characterize the problem: (1) the measurement scale associated with the resolution of a single field test, and (2) the network scale, associated with the separation between samples on a network. Using a spatial filtering approach, it is shown that a network of measurements can only resolve a larger‐scale component of a parameter field. The smaller‐scale component of the parameter field not seen by measurements, here called the subgrid component, can only vary on scales larger than the measurement scale and smaller than the network scale. These unobserved subgrid scales give rise to the so‐called closure problem and consequent modeling errors. When a significant proportion of the parameter variability is contained in the subgrid scales, not only will the closure problem be significant, but aliasing errors will also pollute the estimate of the large‐scale component of the parameter field. These concepts are illustrated by performing simple analytical and numerical calcu

ISSN:0043-1397    

DOI:10.1029/95WR02921

年代:1996

数据来源: WILEY

摘要:

Most stochastic models of solute transport assume flow to be at steady state. However, even locally uniform gradients tend to show seasonal fluctuations in magnitude and direction. Such transients affect the prediction of flow and plume migration and spread. The question is how and to what extent. We address this question by developing low‐order approximations for autocovariances and cross covariances of velocity, head, and log hydraulic conductivity under quasi‐steady state flow, then using a first‐order Lagrangian approach to examine their effect on advective transport. Our results show that whereas periodic temporal fluctuations in the magnitude of the mean velocity may either enhance or reduce dispersion, similar fluctuations in its direction always cause longitudinal dispersion to decrease and transverse dispersion to inc

ISSN:0043-1397    

DOI:10.1029/95WR02766

年代:1996

数据来源: WILEY

摘要:

An iterative stochastic approach is developed to estimate transmissivity and head distributions in heterogeneous aquifers. This approach is similar to the classical cokriging technique; it uses a linear estimator that depends on the covariances of transmissivity and hydraulic head and their cross covariance. The linear estimator is, however, improved successively by solving the governing flow equation and by updating the covariances and cross‐covariance function of transmissivity and hydraulic head fields in an iterative manner. As a result the nonlinear relationship between transmissivity and head is incorporated in the estimation, and the estimated fields are approximate conditional means. The ability of the iterative approach is tested with some deterministic and stochastic inverse problems. The results show that the estimated transmissivity and hydraulic head fields have smaller mean square errors than those obtained by classical cokriging even in the aquifer with variance of transmissivity up to

ISSN:0043-1397    

DOI:10.1029/95WR02869

年代:1996

数据来源: WILEY

摘要:

A sharp interface numerical model is developed to simulate saltwater intrusion in multilayered coastal aquifer systems. The model takes into account the flow dynamics of salt water and fresh water assuming a sharp interface between the two liquids. In contrast to previous two‐fluid flow models which were formulated using the hydraulic heads of fresh water and salt water as the dependent variables, the present model employs a mixed formulation having one fluid potential and a pseudosaturation as the dual dependent variables. Conversion of the usual sharp interface flow equations for each aquifer to an equivalent set of two‐phase flow equations leads to the definitions of pseudosaturation, capillary pressure, and constitutive relations. The desired governing equations are then obtained by connecting neighboring aquifers via vertical leakage. The proposed formulation is based on a Galerkin finite element discretization. The numerical solution incorporates upstream weighting and nonlinear algorithms with several enhanced features, including rigorous treatment of aquitard leakage and well conditions, and a robust Newton‐Raphson procedure with automatic time stepping. The present sharp interface numerical model is verified using three test problems involving unconfined, confined, and multilayered aquifer systems and consideration of steady state and transient flow situations. Comparisons of numerical and analytical solutions indicate that the numerical schemes are efficient and accurate in tracking the location, lateral movement, and upconing of the freshwater‐saltwater in

ISSN:0043-1397    

DOI:10.1029/95WR02919

年代:1996

数据来源: WILEY