Colloid Release Research Articles

Spatial and Temporal Variability in Colloid Dispersion as a Function of Groundwater Injection Rate within Atlantic Coastal Plain Sediments

A subsurface injection experiment was conducted on the USDOE's Savannah River Site (SRS) to determine the influence of pump‐and‐treat remediation activities on the generation and transport of groundwater colloids. The impact of colloid generation on formation permeability at injection rates ranging from 19 to 132 L min−1 was monitored using a set of six sampling wells radially spaced at approximate distances of 2.0, 3.0, and 4.5 m from a central injection well. Each sampling well was further divided into three discrete sampling depths that were pumped continuously at a rate of ∼0.1 L min−1 throughout the course of the injection experiment. Discrete samples were collected for turbidity and chemical analysis. Turbidity varied greatly between sampling wells and zones within a given well, ranging from <1 to 740 NTU. The two sampling wells closest to the injection well displayed the greatest response in terms of turbidity to increases in injection rate. Transient spikes in turbidity generally corresponded to incremental increases in the injection rate that were followed by a decrease in turbidity to a stable injection rate–dependent level. Mineralogical analysis of the resulting suspensions confirmed the presence of kaolinite, goethite, and to a much lesser degree, quartz and illite, with many of the particles too large (>1 μm) to be readily mobile within the formation. Turbidity measurements taken during this study indicate that colloid mobilization induced by water injection was both spatially and temporally heterogeneous. Furthermore, colloid release did not follow simple predictions based on shear force, presumably due to the complexities encountered in real heterogeneous systems. These findings have important implications to our understanding of how colloids and the co‐contaminants are mobilized in the subsurface environment, as well as for the development of monitoring practices that minimize the creation of colloidal artifacts. Technical and logistical obstacles encountered in conducting such an extensive field experiment are also discussed.

In Situ Colloid Mobilization in Hanford Sediments under Unsaturated Transient Flow Conditions: Effect of Irrigation Pattern

Colloid transport may facilitate off-site transport of radioactive wastes at the Hanford site, Washington State. In this study, column experiments were conducted to examine the effect of irrigation schedule on releases of in situ colloids from two Hanford sediments during saturated and unsaturated transientflow and its dependence on solution ionic strength, irrigation rate, and sediment texture. Results show that transient flow mobilized more colloids than steady-state flow. The number of short-term hydrological pulses was more important than total irrigation volume for increasing the amount of mobilized colloids. This effect increased with decreasing ionic strength. At an irrigation rate equal to 5% of the saturated hydraulic conductivity, a transient multipulse flow in 100 mM NaNO3 was equivalent to a 50-fold reduction of ionic strength (from 100 mM to 2 mM) with a single-pulse flow in terms of their positive effects on colloid mobilization. Irrigation rate was more important for the initial release of colloids. In addition to water velocity, mechanical straining of colloids was partly responsible for the smaller colloid mobilization in the fine than in the coarse sands, although the fine sand contained much larger concentrations of colloids than the coarse sand.

ＤＬＶＯ理論に基づく地下水中でのモンモリロナイトゲルからの粒子の分散に関する評価

Theoretical study has been performed to clarify the ability of colloid release form the montmorillonite gel by the flowing groundwater. Evaluation of montmorillonite colloidal particles release from the bentonite buffer material is important for the performance assessment of radioactive waste disposal because the colloids may influence the radionuclide transport. In this study, the minimum groundwater flow rate required to tear off montmorillonite particles from surface of bentonite buffer was estimated from the shear stress on the gel front, which was calculated by the DLVO theory. The estimated shear force was converted to corresponding groundwater velocity by using Stoke's equation. The results indicated that groundwater velocity in a range of about 10−5 to 10−4 m/s would be necessary to release montmorillonite particles. This range is higher than the groundwater flow velocity found generally in deep geological media in Japan. This study suggests that the effect of montmorillonite particles release from the bentonite buffer on radionuclide transport is likely to be negligible in the performance assessment of high-level radioactive waste geological disposal.

Modelling of colloid leaching from unsaturated, aggregated soil

SummaryThe migration of colloids in soils can enhance the leaching of strongly sorbing contaminants. We present a model for the simulation of colloid leaching from unsaturated, aggregated soil media under stationary flow. Transport in the intra‐aggregate pores is simulated by convection–dispersion, and transport in the interaggregate pores, and a stagnant layer of water surrounding the aggregates, is simulated by diffusion. The model describes the release of colloids from soil aggregates, sorption and desorption processes at the air–water interfaces, and flocculation and subsequent straining from the flowing water. All three processes were simulated as functions of ionic strength. Transport of ions in intra‐aggregate pores was simulated by Fickian diffusion. The model was calibrated against experimental results of colloid leaching from columns packed with natural soil aggregates. The aggregates were of two soils differing in organic matter content. On each soil a single calibrated parameter set could describe the experiments with the three ionic strengths. The parameters for release of colloids from the aggregate surface and the sorption properties of the air–water interface were different for the two soils. The key parameters for leaching were the thickness of the stagnant layer of water surrounding the aggregates, the mechanical dispersion, the maximum concentration of colloids at the surface of the aggregates, the sorption capacity and rate coefficient of the colloids at the air–water interface, and the colloid diffusion coefficient. Simulations were also done with two additional irrigation intensities at one ionic strength. Simulated leaching was greater than measured leaching at both irrigation intensities, but the diffusion‐controlled release of colloids from the aggregates was simulated correctly.

Effects of Manure Application and Plowing on Transport of Colloids and Phosphorus to Tile Drains

Preferential flow and particle‐facilitated transport may be largely responsible for observed leaching patterns of strongly sorbing contaminants such as phosphorus. A series of field experiments was performed to investigate the effects of slurry application and plowing on the subsurface transport of colloids and P. Two 25‐m2 plots at a structured sandy loam site were irrigated on six occasions during 1 yr. Effluent sampled in tile drains below the plots was analyzed for turbidity and fractions of dissolved (<0.24 μm) and particulate inorganic and organic P. The observed flow conditions indicated macropore flow. The particle concentration in the effluent was initially high, peaking before flow peak, and later gradually decreased with flow rate. The colloid leaching pattern was attributed to an initial depletion of high colloid concentrations in the pore water and an eventual diffusion‐limited release of colloids from immobile intra‐aggregate water to mobile water. Seasonal variability and management practices caused significant variations in the leaching of P forms. After slurry application dissolved P dominated P loss to the drains. At the events in autumn and winter, particle‐facilitated transport of P came to dominate over dissolved P transport, reaching a maximum of 80% of P loss. Results suggested that plowing increases the risk of particle‐facilitated and dissolved P leaching in rainstorms shortly after the inversion of the soil. We observed an almost fourfold increase in the leaching of P per volume of leachate when comparing irrigation experiments before and after plowing. Amounts of P associated with particulate matter in drain water were constant within events, but varying between storms. For Danish structured clay soils, P density in leached particles was found to range between a maximum of 6 mg P g−1 for soils having recently been fertilized and an approximate minimum of 3 mg P g−1 for soils not recently fertilized.

Batch Leaching Tests: Colloid Release and PAH Leachability

The Toxicity Characteristic Leaching Procedure (TCLP) was developed by the U.S. Environmental Protection Agency to assess leaching potential of contaminants from waste, and to provide a test to classify hazardous waste. It is a batch leaching test where a waste (such as contaminated soil) and an extraction fluid are agitated for a predetermined time. Since TCLP employs an aggressive mixing technique, it is possible that hydrophobic contaminant-laden colloidal fractions may appear as “dissolved” constituents. In this study, TCLP was employed to determine the leachability of PAH contamination from a coal tar contaminated site. Generated colloids and the apparent aqueous concentrations of naphthalene and phenanthrene were measured at various mixing times in the extraction fluid. A mathematical model was developed that predicted the apparent aqueous contaminant concentration in the filtrate. This model accounted for the presence of colloids in the filtrate, and quantified contaminant desorption from colloids. The fraction of colloid-bound contaminant was predicted to be negligible for naphthalene. However, phenanthrene was predicted to have a significant fraction of the total contaminant in the colloidal phase, while naphthalene was primarily dissolved. The desorption model and PAH desorption data are presented here to determine the extent of colloid-facilitated desorption during leaching tests.

Chemical Heterogeneity of Organic Soil Colloids Investigated by Scanning Transmission X-ray Microscopy and C-1s NEXAFS Microspectroscopy

Colloid release and deposition in soils and sorption of inorganic and organic pollutants to soil colloids are strongly influenced by the composition and chemical heterogeneity of colloidal soil particles. To investigate the chemical heterogeneity of organic soil colloids at the particle scale, we used synchrotron scanning transmission X-ray microscopy (STXM) and C-1s near-edge X-ray absorption fine structure (NEXAFS) spectroscopy on 49 individual particles isolated from the surface horizons of three forest soils. Stacks of 130 images of each particle were collected at different X-ray energies between 280 and 310 eV. From these image arrays, NEXAFS spectra were obtained for each pixel and analyzed by principle component analysis and cluster analysis (PCA-CA) to characterize the intraparticle heterogeneity of the organic components. The results demonstrate that the organic matter associated with water-dispersible soil colloids is chemically heterogeneous at the single-particle scale. PCA-CA identified at least two distinct regions within single particles. However, the spectral variations between these regions were much smaller than the variations of averaged NEXAFS spectra representing different particles from the same soil horizon, implying that interparticle heterogeneity is much larger than intraparticle heterogeneity. Especially the contents of aromatic and carboxyl carbon exhibited a large variability. Overall, the NEXAFS spectra of water-dispersible soil colloids were similar to the NEXAFS spectrum of the humic acid fraction, but differed clearly from the fulvic acid and dissolved organic matter fractions extracted from the same soil horizon using conventional techniques.

Role of Organic Acids in Promoting Colloidal Transport of Mercury from Mine Tailings

A number of factors affect the transport of dissolved and particulate mercury (Hg) from inoperative Hg mines, including the presence of organic acids in the rooting zone of vegetated mine waste. We examined the role of the two most common organic acids in soils (oxalic and citric acid) on Hg transport from such waste by pumping a mixed organic acid solution (pH 5.7) at 1 mL/min through Hg mine tailings columns. For the two total organic acid concentrations investigated (20 microM and 1 mM), particle-associated Hg was mobilized, with the onset of particulate Hg transport occurring later for the lower organic acid concentration. Chemical analyses of column effluent indicate that 98 wt % of Hg mobilized from the column was particulate. Hg speciation was determined using extended X-ray absorption fine structure spectroscopy and transmission electron microscopy, showing that HgS minerals are dominant in the mobilized particles. Hg adsorbed to colloids is another likely mode of transport due to the abundance of Fe-(oxyhydr)oxides, Fe-sulfides, alunite, and jarosite in the tailings to which Hg(II) adsorbs. Organic acids produced by plants are likely to enhance the transport of colloid-associated Hg from vegetated Hg mine tailings by dissolving cements to enable colloid release.

Disturbance of Water‐Extractable Phosphorus Determination by Colloidal Particles in a Heavy Clay Soil from the Netherlands

Water extraction methods are widely used to extract phosphorus (P) from soils for both agronomic and environmental purposes. Both the presence of soil colloids in soil water filtrates, and the contribution of colloidal P to the molybdate-reactive phosphorus (MRP) concentration measured in these filtrates, are well documented. However, relatively little attention has been given to the direct disturbance by colloids of MRP measurement. The objective of this paper is to show this influence found for water extracts with a soil to solution ratio of 1:60 (v/v) (P(w)), obtained from a heavy clay soil in the Netherlands. Colloidal particles, which passed a 0.45-mum filter, caused a large overestimation of MRP. The low ionic strength of the P(w) filtrates (on average 0.64 mmol(c) L(-1)) probably caused soil dispersion and increased detachment of colloids from soil during extraction. After NaCl addition, followed by 0.45-mum filtration, MRP was on average 93% lower. This can be ascribed to flocculation of colloids and removal by filtration. A low ionic strength can thus lead to the direct disturbance by colloidal particles of MRP measurement in waters from soils sensitive to release of colloids.

Colloid-Facilitated Transport of Strongly Sorbing Contaminants in Natural Porous Media: Mathematical Modeling and Laboratory Column Experiments

Mobile colloidal particles may act as carriers of strongly sorbing contaminants in subsurface materials. Such colloid-facilitated transport can be induced by changes in salinity, similar to freshwater intrusion to a contaminated aquifer saturated with saltwater, or groundwater penetration into a contaminated site saturated with a dumpsite leachate. This process is studied for noncalcareous soil material with laboratory column experiments with sodium and calcium as major cations and with lead as a strongly sorbing model contaminant. The measured breakthrough curves of these elements were described with a mathematical transport model, which invokes release and deposition kinetics of the colloids, together with adsorption and desorption of the relevant ions to the solid matrix as well as to the suspended colloids. In particular, the specific coupling between colloid and solute transport is considered. The crux of a successful description of such colloidal transport processes is to capture the inhibition of the particle release by adsorbed divalent ions properly and explicitly to considerthe dependence of colloid release on the solution chemistry and the chemical conditions at the solid-liquid interface. Experiments and modeling address colloid-facilitated transport of lead out of a contaminated zone and through a noncontaminated zone, including effects of flow velocity and length of the noncontaminated zone. We finally show that colloid-facilitated transport can be suppressed by the injection of a suitably chosen solution of a calcium salt.

Transport and Retention Mechanisms of Colloids in Partially Saturated Porous Media

The transport, retention, and release of hydrophobic and hydrophilic polystyrene latex microsphere colloids were examined in 0.5-cm-thick, 26-cm-long slab chambers filled with either regular (hydrophilic) or weakly water-repellent sand. The water-repellent sand consisted of a mixture of 0.4% strongly water-repellent grains with unmodified regular sand for the remainder. The concentration of colloids in the outflow water was measured at the same time as the pore-scale distribution of colloids was recorded in still and video images. Although the type of sand affected the flow pattern in the top of the chamber, it did not affect the breakthrough for the same type of colloids. More hydrophilic colloids were eluted in the drainage water than hydrophobic colloids. Images showed that there was a greater retention of the hydrophobic colloids due to strongly attractive hydrophobic interaction forces between colloids and subsequent filtering of colloidal aggregates in the narrow passages between grains. Once filtered, these aggregates then served as preferred sites for attachment of other hydrophobic colloids. The hydrophilic colloids were retained primarily in a thin film of water at the edge of the menisci, the air–water–solid (AWS) interface. Centrifugal motion within the pendular rings observed in the videos contributed to movement of the colloids toward the AWS interface, where colloids were retained due to both low laminar flow velocities near the grain surface and straining in the thin water film at the edge of the meniscus. Except near the solid interface, sorption at the air–water (AW) interface was not observed and appeared unimportant to the retention of colloids. The findings form an essential link between colloid retention and transport processes at the interfacial, pore, and Darcy scales.

Relation of Relative Colloid Stability Ratio and Colloid Release in Two Lead-Contaminated Soils

Potential release of soil colloids into groundwater has attracted considerable attention because of their association with heavy metals. Much effort has been focused on using colloid-deposition-based parameters (batch experiment) to predict colloid release in soils (column experiment), though they are two separate processes. In this paper, relative colloid stability ratio was estimated and used to relate to colloid release in soils. Changes in colloid release with incubation times were measured in two lead-contaminated soils. Colloid stability ratio (W) is defined as the ratio of the colloid flocculation rate of a suspension under normal observation conditions (slow flocculation) to the rate under rapid flocculation conditions. Assuming that colloid flocculation rate is related to changes in light absorbency of a solution, relative colloid stability ratio (RW) was estimated by dividing the rate of absorbency change of soil suspension observed in 0.06 M NaCl (slow flocculation) by that in 0.01 M CaCl2 (rapid flocculation) electrolyte solutions. Both colloid release and R W changed randomly with incubation times in both soils. However, colloid release generally increased as RW increased with correlation coefficients of 0.67–0.89. This result suggests that the overall colloid release from these soils was related to RW, even though they are different conceptually. However, research is needed to further develop the concept of using relative colloid stability ratio to predict colloid release in soils.

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.

Deposition and mobilization of clay colloids in unsaturated porous media

We report results on the effects of porewater pH and transients in porewater flow on the deposition and mobilization of colloid‐sized clay particles within unsaturated sand columns. The deposition rates of illite under steady‐flow conditions were essentially independent of pH, while the deposition rates of kaolinite nearly doubled as the pH decreased from 7.4 to 4.6. Mobilization of kaolinite colloids was slow or negligible under steady‐flow conditions; however, transients in porewater flow induced rapid colloid release. A model that accounts for rate‐limited deposition reactions and that links colloid mobilization to variations in moisture content and porewater velocity describes the effluent colloid concentrations measured during the steady‐flow and transient‐flow stages of the column experiments. On the basis of these results we infer that the effects of pH on clay‐colloid deposition depend on the mineralogy of the clay colloids and that perturbations in flow are critical in mobilizing clay colloids within the vadose zone.

Transport of colloids in unsaturated porous media: A pore‐scale observation of processes during the dissolution of air‐water interface

We present results from pore‐scale observations of colloid transport in an unsaturated physical micromodel. The experiments were conducted separately using three different sizes of carboxylate polystyrene latex spheres and Bacteriophage MS2 virus. The main focus was to investigate the pore‐scale transport processes of colloids as they interact with the air‐water interface (AWI) of trapped air bubbles in unsaturated porous media, as well as the release of colloids during imbibition. The colloids travel through the water phase but are attracted to the AWI by either collision or attractive forces and are accumulated at the AWI almost irreversibly, until the dissolution of the air bubble reduces or eliminates the AWI. Once the air bubbles are near the end of the dissolution process, the colloids can be transported by advective liquid flow, as colloidal clusters. The clusters can then attach to other AWI down‐gradient or be trapped in pore throats that would have allowed them to pass through individually. We also observed small air bubbles with attached colloids that traveled through the porous medium during the gas dissolution process. We used Derjaguin‐Landau‐Verwey‐Overbeek (DLVO) theory to help explain the observed results. The strength of the force that holds the colloids at the AWI was estimated, assuming that the capillary force is the major force that holds the colloids at the AWI. Our calculations indicate that the forces that hold the colloids at the AWI are larger than the energy barrier between the colloids. Therefore it is quite likely that the clusters of colloids are formed by the colloids attached at the AWI as they move closer at the end of the bubble dissolution process. Coagulation at the AWI may increase the overall filtration for colloids transported through the vadose zone. Just as important, colloids trapped in the AWI might be quite mobile when the air bubbles are released at the end of the dissolution process, resulting in increased breakthrough. These pore‐scale mechanisms are likely to play a significant role in the macroscopic transport of colloids in unsaturated porous media.

Analytical solutions for monodisperse and polydisperse colloid transport in uniform fractures

Analytical solutions are derived describing the transport of suspensions of monodisperse as well as polydisperse colloid plumes of neutral buoyancy within a fracture with uniform aperture. Various initial and boundary conditions are considered. It is shown that both the finite colloid size and the characteristics of the colloid diameter distribution significantly affect the shape of colloid concentration breakthrough curves. Furthermore, increasing the standard deviation of the colloid diameter enhances colloid spreading and increases the number of attached colloids when colloid–wall interactions are taken into account. Excellent agreement between available experimental data and the analytical solution for the case of an instantaneous release of monodisperse colloids in a natural fracture is observed.

Colloid mobilization in water-saturated porous media under transient chemical conditions.

This research focuses on the effects of transients in porewater chemistry on colloid mobilization within water-saturated porous media. We develop a model that couples equations for solute transport with those for colloid release and transport. The model accounts for heterogeneity in the interaction energies between deposited colloids and the mineral grains by dividing the immobile-phase colloid population into a series of compartments. Each compartment releases colloids at a characteristic critical solute concentration, which is assigned on the basis of a piece-wise linear distribution function. We test this model against data from column experiments in which successive step-change reductions in porewater NaCl concentrations induced pulse-type releases of silica colloids from the surfaces of quartz sand. Comparison of experimental and computed results reveals that colloid release rates vary nonlinearly with the immobile-phase colloid concentration and depend on the chemical conditions under which the colloids were deposited on the quartz sand. Our work demonstrates that colloid mobilization kinetics can be quantified given knowledge of the spatiotemporal changes in porewater chemistry.

Physicochemical characterization of potential colloids from agricultural topsoils

A method based on sedimentation and centrifugation steps has been developed to fractionate agricultural topsoils after suspension in water. A high release of potential colloids has been observed with low Ca2+ concentration under soil solid-to-water weight ratio conditions higher than 1/3. The colloidal stability is characterized in terms of particle size and electrokinetic properties. There is no systematic effect of the colloid phase organic carbon content varying from 2 to 5% on the critical coagulation concentration (CCC) and zeta potential results. Consequently, the observed CCC was tested by the Derjaguin–Landau–Verwey–Overbeeck theory by taking into account physicochemical properties of the colloid phase mineral content.

Colloid mobilization and transport within unsaturated porous media under transient‐flow conditions

Contaminant‐scavenging colloids are mobilized in the vadose zone during infiltration events, characterized by transient flow regimes. In the research reported here we develop a mathematical model in order to investigate the influences of flow transients on colloid mobilization in unsaturated media. The model solves coupled equations for unsteady pore water flow, colloid release, advective‐dispersive colloid transport, and redeposition of pore water colloids. The immobile‐phase colloid population is discretized into a series of compartments, each of which are assigned a value of critical moisture content (Θcr) according to a piecewise linear density function. A compartment is activated when the model‐calculated moisture content exceeds the Θcr for the compartment, whereupon colloids are released at a rate proportional to the product of the pore water velocity and the colloid concentration within the compartment. We fit solutions of the model equations to data on the mobilization and transport of silica colloids within columns of unsaturated quartz sand. The model accurately reproduces measured pulse‐type colloid releases induced by successive step‐change increases in flow rate and moisture content. This work illuminates the important role of flow transients in colloid mobilization and, through the derivation of a model that couples pore water flow characteristics with colloid mass transfer kinetics, provides a means for quantifying the phenomenon.

Diffusion-Limited Mobilization and Transport of Natural Colloids in Macroporous Soil

This study examines the dynamics of colloid mobilization and leaching from macroporous soil columns by means of laboratory experiments and numerical modeling. On the basis of a previous column study involving high and low water flow rates in structured soil, we designed a novel experiment emphasizing the time-dependence of the colloid release process. Intact macroporous soil columns were exposed to variable pauses in irrigation (flow interruption for 30 min, 1 d, or 7 d) followed by resumed infiltration. The experiments showed that (i) there was a seemingly unlimited source of in situ colloids even after prolonged leaching and (ii) the peak concentration of colloids in the effluent after the flow interruption increased with increasing length of the preceding pause. The results demonstrated that colloid mobilization is not controlled by hydrodynamic shear induced by the flowing water but is a time-dependent and possibly diffusion-limited process. We developed a simple, equivalent macropore model to investigate the hypothesis that colloid release to the flowing water is governed by two diffusion processes, one in a uniform water film lining the macropore and one in the crust of the macropore. The model was capable of reproducing and explaining the characteristic results of our soil column experiments and required no recalibration of exchange process parameters to simulate the particle mobilization after a flow interruption. However, model calibration yielded unexpected results with respect to the size of the diffusion coefficient of the crust and did not warrant accepting the dual diffusion model hypothesis. Using a simpler mass transfer concept to quantify the mobilization of colloids in 21 soil columns, we found mass transfer coefficients to be about 30 times higher in the water film than in the crust.