Hydrodynamic Dispersion Coefficient Research Articles

Impact of Conduit Geometry and Bed Porosity on Flow and Dispersion in Noncylindrical Sphere Packings

The three-dimensional velocity field and corresponding hydrodynamic dispersion in pressure-driven flow through fixed beds of solid (impermeable), uniformly sized, spherical particles are studied by quantitative numerical analysis for conduits with different cross-sectional geometries. Packings with average interparticle porosities (bed porosities) of 0.40 < or = epsilon < or = 0.50 were generated in conduits with circular, quadratic, rectangular, and semicircular cross sections utilizing a parallel collective-rearrangement algorithm. The lateral dimensions of the generated packings were chosen to represent typical values encountered in miniaturized liquid chromatography (LC) systems. The interparticle velocity field was calculated using the lattice-Boltzmann (LB) method, and a random-walk particle-tracking method was employed to model advective-diffusive transport of an inert tracer in the LB velocity field. We present the morphologies and corresponding flow patterns for these packings and demonstrate that the porosity distribution and velocity profiles of noncylindrical packings deviate significantly from those of conventional cylindrical packings. This deviation becomes more pronounced at higher bed porosities. Extended regions of high local porosity in the corners of noncylindrical conduits give rise to the formation of fluid channels of advanced flow velocity. The differences in the flow velocity distributions of cylindrical and noncylindrical packings are analyzed, and their impact on the axial hydrodynamic dispersion coefficient is shown. The presented data support the conclusion that LC performance depends critically on the conduit geometry and bed porosity. Our results have particular relevance for microchip-LC, where noncylindrical conduit geometries are prevalent.

The Longitudinal Dispersion Coefficient of Soils as Related to the Variability of Local Permeability

Solute (NaCl) miscible displacement experiments are performed on long disturbed soil columns to determine the hydrodynamic longitudinal dispersion coefficient and correlate it with the variability of the local permeability. The solute concentration, averaged over several cross-sections along the soil column, is monitored by measuring the electrical resistance between rod electrodes. The measured solute concentration breakthrough curves are fitted simultaneously with the one-region and two-region analytical models of the 1-D advection–dispersion equation to estimate the longitudinal dispersion coefficient, D L, as a function of Peclet number, Pe, for common groundwater flow velocities (2 < Pe < 50). Macroscopic simulations of miscible displacement in 2-D porous media described by a periodic permeability field with low, moderate and high variability are employed to evaluate the predictability of the one-region and two-region models, and the sensitivity of the dispersion coefficients and flow velocities estimated from soil column displacement tests to the variance of local permeability. When the variability of the local permeability becomes high, the one-region model fails, while the two-region model is capable of reproducing satisfactorily the breakthrough curves, and providing reliable values of dispersion coefficients. The two mean pore velocities estimated by the two-region model represent, on average, a fast and a slow mean velocity of the dispersion front, whereas their difference is a measure of the transient evolution of the width of the equi-concentration dispersion front.

Chloride Transport through Cement-Bentonite Barriers

This paper presents experimental results from a number of column tests on cement-bentonite (CB) mixtures permeated with chloride ( Cl− ) . Factors influencing the hydraulic conductivity (k) , the hydrodynamic dispersion coefficient (D) and the retardation factor ( Rd ) are investigated. The significant factors considered are the curing period, seepage velocity (v) , reference concentration ( CR ) , and mean particle size ( d50 ) . Other factors such as porosity (n) and mix design are also discussed. Results show that a longer curing period lowers all three transport parameters. Furthermore, as the seepage velocity increases, Rd decreases and D increases. The yielding velocity at which mechanical dispersion ( Dmd ) starts prevailing is observed at about 0.0001 cm∕s . Both Rd and D are seen to reduce with increasing CR . Thus, the concept of CR could be used to explain the data scattering in a CB hydrodynamic curve.

Use of tracer tests to investigate changes in flow and transport properties due to bioclogging of porous media

Tracer tests were conducted in three laboratory columns to study changes in the hydraulic properties of a porous medium due to bioclogging. About 30 breakthrough curves (BTCs) for each column were obtained. The BTCs were analyzed using analytical equilibrium and dual-porosity models, and estimates of the hydrodynamic dispersion and mass transfer coefficients were obtained by curve fitting. The change in transport properties developed in three stages: an initial phase (I) with no significant changes in transport properties, phase II with growth of biomass near the inlet of the columns causing changes in dispersivity, and phase III with added growth of micro-colonies deeper in the columns causing mass transfer of solutes from the water phase to the biophase. Tracer transport changed from being uniform to more non-uniform with increase in mass transfer of the tracer between the mobile phase and the immobile biomass. An increase in the bulk dispersivity value of up to one order of magnitude was observed. Numerical simulations suggest that local dispersivity values may be as much as 40 times higher in the more severe clogged areas inside the column. The bulk hydraulic conductivities of the columns decreased by up to three orders of magnitude. The hydraulic conductivity and dispersivity parameters were almost recovered after disinfection of the columns. Different models relating the changes of the hydraulic conductivity to the changes in the mobile porosity due to bioclogging were reviewed, and the micro-colony relation of Thullner et al. [Thullner, M., Zeyer, J., Kinzelbach, W., 2002. Influence of microbial growth on hydraulic properties of pore networks, Transport in Porous Media, 49, 99–122.] was found to best describe the relation between the bulk hydraulic parameters.

Numerical Analysis of a Short-Term Tracer Experiment in Fractured Sandstone

A short-term, pulse injection tracer experiment conducted in fractured quartzitic sandstone at Kukuan, Taiwan was analyzed. Tracer transport at the test site was dominated by advection but a specific attenuation mechanism leading to breakthrough curve (BTC) tailing also seemed to exist. Matrix diffusion was hypothesized as the transport mechanism that results in the tailing. This hypothesis was proved by comparing the field BTC with numerical simulation results obtained by the general-purpose flow/transport simulator, TOUGH2, based on a single-fracture conceptual model. Due to the lack of accuracy of estimating the interporosity flux by the conventional double porosity model (DPM), TOUGH2 was incorporated with the multiple interacting continua (MINC) scheme to simulate the transient characteristics of the interporosity flux. In MINC, rock matrix is discretized as a series of continua according to the perpendicular distance from the fracture that adjoins the matrix. The closer the rock matrix is to the fracture, the finer the rock matrix is discretized. This concept is fundamentally different from DPM in that rock matrix is no longer treated as a single continuum. Simulation results by TOUGH2-MINC have successfully reproduced the observed BTC tailing even under the dominating advection effect. Sensitivity studies showed that TOUGH2-MINC is sensitive to parameters including fracture aperture (2b), matrix porosity (nm) and effective molecular diffusion coefficient in matrix (Dm). If 2b, nm , Dm , are respectively 200 µm, 2%, 10 -11 m 2 s -1 , and if hydrodynamic dispersion coefficient

Effects of zeolite application on nitrate and ammonium retention of a loamy soil under saturated conditions

Nitrogen (N) loss from irrigated cropland, especially in rice paddies, results in low N-use efficiency and groundwater contamination. Soil conditions that increase ammonium and nitrate ion retention alleviate these problems. Clinoptilolite, a naturally occurring zeolite with high-exchange capacity, may be used to absorb ammonium and retard excess leaching of nitrate. The objectives of this research were to determine the effects of different rates of Ca–K-zeolite application (0, 2, 4, and 8 g/kg soil) on pore water velocity and leaching of ammonium and nitrate applied as ammonium nitrate fertiliser to a loam soil at a rate of 350 kg N/ha under saturated conditions similar to that of a rice paddy. The results indicate that Ca–K-zeolite applications of 4 and 8 g/kg soil increase the pore water velocity by 35% and 74%, respectively. The maximum relative concentration (c/co) for the nitrate breakthrough curve occurring at pore volume of about 0.5 was reduced by 15% with a zeolite application rate of 8 g/kg soil. When applying 40 cm of leaching water, leached nitrate was 75% and 63% of total applied nitrate at the soil surface with zeolite applications of 4 and 8 g/kg soil, respectively. Due to the high ion exchange capacity of zeolite, the application of zeolite at 2 g/kg soil is enough to increase the exchange sites in the soil in order to absorb the applied ammonium and prevent its leaching by the inflow water. The maximum ammonium concentration in the breakthrough curve for the zeolite application rate of 2 g/kg soil was reduced by 43% compared with the control treatment. The relationship between the hydrodynamic dispersion coefficient (D) for nitrate and pore water velocity (v) was not linear and it was correlated with squared pore water velocity. The coefficient of the relationship between D and v2 was dependent on the zeolite application rate and linearly increased with this rate.

Sorption, desorption and extraction of uranium from some sands under dynamic conditions

Sorption, desorption and extraction behavior of uranium in various fluvial sands of domestic origin were investigated in continuous dynamic column experiments. For the sorption of U(VI) an aqueous 10−4M UO2(NO3)2 solution was used at a flow rate of about 0,3 cm3/min. Desorption was carried out with demineralized water, and the extraction with 10−2M Na2CO3 solution following desorption. The retardation coefficients (R) and hydrodynamic dispersion coefficients (Dd), were determined using an ADE equation. From the experimentally determined values of R, bulk density and porosity, the distribution coefficients (Kd) of the UO2 2 species have been calculated for the respective processes. The extent of U sorption in sands, as well as the proportion of desorbed and extracted U from these sands, was also calculated.

Hydrodynamic interactions in the induced-charge electrophoresis of colloidal rod dispersions

The behaviour of a dispersion of infinitely polarizable slender rods in an electric field is described using theory and numerical simulations. The polarization of the rods results in the formation of dipolar charge clouds around the particle surfaces, which in turn drive nonlinear electrophoretic flows under the action of the applied field. This induced-charge electrophoresis causes no net migration for uncharged particles with fore–aft symmetry, but can lead to rotations and to relative motions as a result of hydrodynamic interactions. A slender-body formulation is derived that accounts for induced-charge electrophoresis based on a thin double layer approximation, and shows that the effects of the electric field on a single rod can be modelled by a linear slip velocity along the rod axis, which causes particle alignment and drives a stresslet flow in the surrounding fluid. Based on this slender-body model, the hydrodynamic interactions between a pair of aligned rods are studied, and we identify domains of attraction and repulsion, which suggest that particle pairing may occur. An efficient method is implemented for the simulation of dispersions of many Brownian rods undergoing induced-charge electrophoresis, that accounts for far-field hydrodynamic interactions up to the stresslet term, as well as near-field lubrication and contact forces. Simulations with negligible Brownian motion show that particle pairing indeed occurs in the suspension, as demonstrated by sharp peaks in the pair distribution function. The superposition of all the electrophoretic flows driven on the rod surfaces is observed to result in a diffusive motion at long times, and hydrodynamic dispersion coefficients are calculated. Results are also presented for colloidal suspensions, in which Brownian fluctuations are found to hinder particle pairing and alignment. Orientation distributions are obtained for various electric field strengths, and are compared to an analytical solution of the Fokker–Planck equation for the orientation probabilities in the limit of infinite dilution.

Determination of transport parameters for heavy metal in residual compacted soil using two methodologies

Values of the hydrodynamic dispersion coefficient and retardation factor obtained using the traditional and cumulative mass methods of column test analysis for zinc, manganese, and cadmium in a compacted soil are compared. The soil under study is from the B horizon of a residual gneissic tropical soil used for construction of the liner for the sanitary landfill in the District of Visconde do Rio Branco, Minas Gerais (MG), Brazil. To evaluate the behavior of landfill leachate heavy metals through the soil, soil column tests were performed on samples of compacted soil. A computational program that uses an optimization procedure to generate values of the hydrodynamic dispersion coefficient and retardation factor was developed to facilitate interpretation of the results obtained by the cumulative mass method. Values of the retardation factor and hydrodynamic dispersion coefficient were not influenced by the method of determination, even when a reduced number of effluent samples was used by the cumulative mass method. The use of the cumulative mass method, based on a reduced number of pore volumes, reduces the time and cost involved in the tests.Key words: heavy metals, column test, cumulative mass method.

Modelling of Advection-Dominated Transport in a Porous Column with a Time-Decaying Flow Field

This paper examines the problem of the advective-dispersive movement of a non-decaying, inert chemical dye solution through the pore space of a fluid saturated porous column. The objective of the paper is to present a complete study of the one-dimensional advective-dispersive transport problem by considering certain analytical solutions, experimental results and their comparisons with specific computational simulations. Dye concentrations obtained by means of an image processing method are used in conjunction with an analytical solution to identify the hydrodynamic dispersion coefficient that governs the advective-dispersive transport problem. The experimental results and identified parameters are also used to assess the computational estimates derived from several stabilized computational schemes available in the literature, for examining advection-dominated transport processes in porous media.

Dispersion dependence on retardation in a real fracture geometry using lattice-gas cellular automaton

Fractures have been recently identified in potential host rock for high level nuclear waste disposal, like indurated argilite formations. These fractures appear as potential rapid pathways for radionuclides transport and hydrodynamic properties of the transport inside these systems must thus be characterized. Miscible non-sorbing and sorbing tracers displacements were performed on a 2-D model derived from a real fracture geometry observed in the Tournemire argilite formation with a lattice-gas cellular automaton (LGA). LGA was shown to easily handle the complex geometry of such a fracture. The numerical breakthrough curves obtained were inverted with the 1-D CDE and MIM transport models. Two main conclusions were drawn: (i) at the length scale of the study, the non-sorbing tracer transport in our fracture geometry was more accurately interpreted in terms of the MIM model rather than in terms of the classical CDE model; (ii) in order to correctly model the sorbing tracers migration, the hydrodynamic dispersion coefficient value was found to increase with the increase of the retardation factor. A semi-empirical relation based on the Taylor–Aris theory was then used to describe this dependency.

Determinação de parâmetros de transporte de metais pesados em Latossolo compactado

Neste trabalho são comparados os valores de coeficiente de dispersão hidrodinâmica e de fator de retardamento do zinco, do manganês e do cádmio, em Latossolo (horizonte B) compactado, obtidos através dos métodos de análise tradicional e de massa acumulada para ensaios em coluna de lixiviação. Para facilitar a utilização do método da massa acumulada, desenvolveu-se um programa computacional, baseado em procedimento de otimização. Os valores dos parâmetros de transporte obtidos não foram influenciados pelas metodologias de determinação empregadas, mesmo quando no método da massa acumulada se usou um número reduzido de volume de poros efluentes das colunas de lixiviação. Utilizando-se o método da massa acumulada e se tendo por base um número reduzido de volume de poros efluentes, consegue-se reduzir tempo e custo em ensaios para determinação de parâmetros de transporte.

Determination of the effective transport coefficients of pore networks from transient immiscible and miscible displacement experiments

Transient immiscible and miscible displacement experiments performed on artificial glass‐etched pore networks are used to determine the capillary and relative permeability curves as well as the hydrodynamic dispersion coefficients. The transient responses of the total and axial distribution of the wetting phase (WP) saturation and the pressure drop across the pore network are introduced into an inverse modeling scheme to estimate the imbibition capillary pressure and relative permeability curves of pore networks by varying the capillary number, the contact angle, and the shear‐thinning rheology of the nonwetting phase (NWP). The temporal evolution of the spatial distribution of the solute concentration profiles over selected areas of the pore network is fitted to analytical one‐dimensional solute dispersion models to estimate the longitudinal dispersion coefficient as a function of the Peclet number, in single‐ and dual‐pore networks. Under an unfavorable viscosity ratio the growth pattern is a two‐dimensional fractal object, its description with the one‐dimensional macroscopic two‐phase flow equations is questionable, a high uncertainty is introduced into the capillary pressure curve, whereas “average” relative permeability functions are estimated. The relative permeability of the NWP may be a decreasing or increasing function of the capillary number over the viscous or capillary fingering pattern, respectively. The relative permeability of the WP is an increasing function of the capillary number and increases weakly with the NWP shear‐thinning rheology strengthening. The longitudinal dispersion coefficient varies nonlinearly with Peclet number following power laws with exponents ranging from 1 to 2 and depending on the relative contribution fraction of the macrodispersion (associated with the spatial variation of pore sizes) and Taylor dispersion (associated with the flow channeling) to the entire process.

LEACHING OF COLLOIDS AND DISSOLVED ORGANIC MATTER FROM COLUMNS PACKED WITH NATURAL SOIL AGGREGATES

Transport of pollutants by colloids and dissolved organic matter (DOM) may increase the leaching of strongly sorbing pollutants (e.g., PAHs, heavy metals, radionuclides, and certain pesticides). A prerequisite for colloid-and DOM-facilitated transport is the release of colloids and DOM from soil. In the present study, the leaching of colloids and DOM from columns packed with natural soil aggregates (2-4 mm) was investigated. Aggregates with different organic matter content were used: Aggregates from Soil 1, with 3.6% organic matter content, and aggregates from Soil 2, with 2.5%. The leaching experiments showed that colloid leaching increased with higher organic matter content. Colloid leaching was strongly affected by the ionic strength of the infiltrating water but less so by the specific type of cation. However, prolonged leaching (20 h) with KCl increased the leaching of colloids, probably because of the ion exchange of naturally occurring polyvalent ions with K+. The accumulated amount of colloids leached during 20-h period was orders of magnitude lower than the amount of dispersible colloids (determined by rotation of soil water mixtures), and the organic carbon fraction (foc) of the leached colloids was 3 to 4 times higher than the foc of the dispersible colloids. The leaching of DOM from Soil 1 was greater, but, relative to the soil organic matter content, it was similar for the two soils. The leaching of DOM was not significantly affected by the chemistry of the irrigation water. Irrigation with solutions of KCl and deionized water increased the hydrodynamic dispersion coefficient on Soil 2, due to swelling of the clay minerals and closure of the soil pores. Irrigation with CaCl2 led to lower hydrodynamic dispersion, because of shrinking clay minerals. Soil 1 was less sensitive to shrinking and swelling of clay minerals because of its higher organic matter content.

On the evaluation of dispersion coefficients from visualization experiments in artificial porous media

High-resolution single source–solute transport experiments in glass-etched pore networks presented in a previous publication are used for quantifying the hydrodynamic dispersion as a function of Peclet number ( Pe). This type of artificial porous media is ideal for understanding the effects of pore-scale phenomena on solute dispersion thanks to its well characterized topology and pore geometry. The hydrodynamic dispersion coefficients are estimated by matching the spatial and temporal distributions of the solute concentration over various regions of the network with the numerical solution of the advection–dispersion equation and using a parameter space analysis to ensure well conditioning of the parameters. The source release function and assumptions on boundary conditions are found to affect significantly the reliability of the dispersion coefficient estimates. The estimated longitudinal dispersion coefficients are in close quantitative agreement with literature data for dispersion in porous media. The extracted transverse dispersivity ( α T) values indicate an apparent decrease of α T with increasing flow velocity, which is at variance of constant dispersivity approximation adopted commonly in soil hydrology models and is attributed to incomplete diffusive mixing at the pore intersections. Calculated values of an effective mixing ratio at pore junctions are in agreement with published theoretical estimates and reveal a transition from partial mixing to streamline routing as Pe increases within the range investigated here.

Chloride and atrazine transport through saturated soil columns

A possible contamination of water resources by the application of pesticides is a problem confronting many irrigated areas in arid and semi-arid areas. The best management practices have to be adopted to minimize pesticide transport and leaching under irrigated conditions. Atrazine dissipation in loam and sandy loam soils has been tested in the laboratory using disturbed soil columns under saturated flooding conditions. All the experiments were performed in replicates. The chloride transport was also studied to test its behavior as an inert tracer in both the soils. Atrazine and chloride breakthrough curves were analyzed with the parameter optimization program CXTFIT to determine transport parameters including pore-water velocity (v), retardation coefficient (R), hydrodynamic dispersion coefficient (D), and pulse duration (t o ). The pore-water velocity and pulse duration of the solute were estimated from the experimental conditions and kept constant during the optimization procedure. The results indicated that the R of chloride was not significantly different from 1, indicating that chloride is an inert tracer for the types of soil tested in this study. The average R of atrazine was 4.56 and 3.15 for sandy loam and loam soils, respectively. Results also showed that the hydrodynamic dispersion coefficient was much higher in the case of sandy loam soil compared to the loam soil for the two solutes, thus indicating non-equilibrium transport conditions. In the case of chloride, D increased from 0.4 for the loam soil to 16.2 cm2/min for the sandy loam soil. Similar results were observed in the case of atrazine in which D for the sandy loam soil was 60% higher than that for the loam soil. More atrazine leaching is expected under field conditions due to the presence of soil cracks and macropores.

Hydrodynamic and geomorphologic dispersion: scale effects in the Illinois River Basin

The objective of this work is to determine the relative effects of hydrodynamic and geomorphologic dispersion on the hydrological response of the Illinois River Basin (IRB) as scale increases. The specific hypothesis that was tested is that as basin size increases, the river network structure, as compared to channel hydrodynamic properties, plays an increasingly dominant role in determining the hydrologic response. The analysis was performed on eight of the major watersheds in the IRB in order to provide an adequate representation of basins that contain streams of order six or greater. The basins studied include the Des Plaines, Mackinaw, Vermilion, Fox, La Moine, Spoon, Kankakee, and the Sangamon, and have magnitudes ranging from order six to order eight. The geometric and hydrodynamic properties were derived from the analysis of digital elevation model data and from the hydraulic geometry equations for various subcatchments of the IRB put forth by Stall and Fok [Univ. of Ill. Water Res. Center Res. Rep. 15, 1968]. The hydrodynamic and geomorphologic dispersion coefficients were determined for each order stream of the eight basins and for constant flow frequencies, then compared. The results contradict the original hypothesis, for at small scales, geomorphologic dispersion tends to dominate, the extent of which depends upon the flow frequency, and at large scales, geomorphologic dispersion is less dominant. This occurs because of the behavior of the path lengths of a stream network, which geomorphologic dispersion depends upon. In addition, at high flow frequencies the geomorphologic dispersion dominates, and at low frequencies the hydrodynamic dispersion begins to play an increasingly important role, although the geomorphologic dispersion still dominates. This dominance suggests that the geomorphologic parameters of a watershed could be more important in characterizing the hydrologic response of a river basin than hydrodynamic parameters.

Mass transport of organic contaminants through a self-sealing/self-healing mineral landfill liner

The self-sealing/self-healing (SS/SH) liner system is based on the fundamental principle that an impermeable seal is self-formed and self-healed by the pozzolanic reaction at the interface between two adjacent reactive layers. The objectives of this study were to evaluate the effect of contaminants on the performance of an SS/SH liner used as a hydraulic barrier, to understand mechanism of volatile organic compound (VOC) sorption on the SS/SH materials, and to estimate the mass transport parameters of contaminants through the SS/SH liner materials. The hydraulic conductivity of the liner material decreased continuously with time, and stabilized at less than 1 × 10−7 cm/s after approximately 15 days. It is known that the seal at the interface between two reactive layers is self-formed over time, and this contributes to the decrease in the hydraulic conductivity of such a liner system. The retardation of the seven target VOCs tested was greater in the SS/SH liner materials than in a clayey soil specimen owing to the higher sorption capacity. An analytical solution developed to test column equipment could reduce the time required to estimate the hydrodynamic dispersion coefficients of organic compounds by using the data on changes in concentration in the upper reservoir of the column.

Hydrodynamic dispersion in perfusion chromatography—a network model analysis

A network model was used to simulate the hydrodynamic dispersion, which occurs in perfusion chromatography for proteins under non-retained conditions. In order to elucidate the details of the velocity dependency of the hydrodynamic dispersion coefficient in a perfusive bed, the effects of pore-size distribution (PSD), pore inter-connectivity (PI), and solute molecular size were incorporated into the simulations. It was found that, for purification of proteins on a perfusive column, among these parameters, PI affected the dispersion results most significantly over the whole range of Peclet numbers investigated. POROS media were postulated to possibly have a very high PI, of around 4.77–5.37. PSD also played a very important role in determining the dispersion results; POROS R1 and POROS R2 behaved quite differently, although both of them possess bi-modal PSDs. At higher PI (connected-pore fraction higher than 0.8), the D L / UL curve for POROS R1 showed double plateaus, whereas that for POROS R2 gave only one plateau. Under the experimental conditions investigated, only the macropore size distribution contributed to the dispersion results in POROS R1, whereas the composite bi-modal PSD contributed to the results in POROS R2. Based on the simulation results, size exclusion was also found to affect the dispersion results, giving rise to low dispersion coefficient in the molecular diffusion controlled region and high dispersion coefficient in the convection region. However, in the case of a perfusive column, size exclusion effects were rather insignificant. It was also found that hindered diffusion resulted in sharp increase in the dispersion data at very high Peclet number ( Pe>10 4), which was more significant for POROS R2. The simulation results corroborated the experimental data very well. These indicate that the use of a network model to obtain the hydrodynamic dispersion coefficient is a more realistic representation of the column efficiency in perfusion chromatography, one which accounts directly for microscopic information, such as pore structure and solute molecular size.

Effect of Soil Stress on Cadmium Transport in Saturated Soils

Prediction of the pollutant migration in complex soil systems requires an understanding of advection, dispersion, and adsorption processes. This paper focuses on the influence of the effective stress of soil on the pollutant transport processes. One-dimensional laboratory and centrifuge column tests under identical seepage conditions but with different stress distribution were conducted. One-dimensional numerical simulation of the contaminant transport model, using the computer software, POLLUTE Ver. 6, was carried out to curve fit the two column test results and to determine the transport parameters based on the best curve fitting. The transport parameters such as hydraulic conductivity, hydrodynamic dispersion, and distribution coefficient determined from the bench-scale and centrifuge columns were compared. The possible effects of soil stresses on contaminant transport were discussed. The experimental results from the bench-scale column and centrifuge column studies apparently illustrate that the stres...