Colloid-Facilitated Transport ofStrongly Sorbing Contaminants inNatural Porous Media: ALaboratory Column StudyD A N I E L G R O L I M U N D ,M I C H A L B O R K O V E C , *K U R T B A R M E T T L E R , A N DH A N S S T I C H E R

Institute of Terrestrial Ecology, Swiss FederalInstitute of Technology, ETH-ITO,Grabenstrasse 3, 8952 Schlieren, Switzerland

IntroductionIn a natural porous medium, a strongly sorbing substanceis mainly associated with the solid phase, and consequently,the solution concentration of this substance in the porewater will be small. Since the solid phase is commonlyassumed to be stationary, one usually surmises that suchsubstances migrate very slowly in the subsurface and thuspose no contamination risks at distant locations from thepollutant source. Recently, however, this conjecture hasbeen challenged by the proposition that mobile, colloidalparticles could act as pollutant carriers and thus providea rapid transport pathway for strongly sorbing contaminants(1-4). The anticipated mechanism is summarized in Figure1.

The existence of various processes, which are essentialfor this mechanism to be operational, is well established.It is known that mobile colloidal particles can be releasedwithin soils or aquifers and travel in the subsurface zoneover large distances and, due to size exclusion effects, ateven higher velocities than conservative tracers (5-10).

It is also clear that an increase of the complexation powerof the mobile water phase (i.e., in the presence of organicmatter, cosolvents, or colloidal particles) leads to an increaseof the travel velocity of contaminants (3, 11-16). However,whether in-situ generated colloidal particles may signifi-cantly contribute to pollutant transport still remains unclear.One may consider the existing evidence to be either indirect(17, 18) or one might regard this transport pathway to beunimportant (1, 2, 19).

The aim of this paper is to provide unambiguousevidence that, in a natural porous material, colloid-facilitated transport can indeed represent the dominanttransport pathway for a strongly sorbing contaminant. Ourevidence is based on laboratory column experiments, wherewe use a non-calcareous soil as the stationary phase andleaching solutions containing Na+ and Ca2+ as major cationsand Pb2+ as a strongly sorbing model contaminant. Weshall demonstrate that in-situ mobilized colloidal particlesmay indeed provide the pathway for rapid transport of Pb.Upon changes in the major cation composition of theinfiltrating solutions, colloidal particles are released fromthe porous matrix and are transported upstream of a

nonretarded normality front. Provided these particles arereleased from a Pb-contaminated region, they will also carrysubstantial amounts of Pb. In the present situation, aclassical transport model, which neglects the mobilizationof colloidal particles, underestimates the total solutionconcentrations by many orders of magnitude. While weonly mimic the natural conditions in our experiments,similar processes are likely to be operational in the field.

Experimental SectionColumn Experiments. Non-calcareous soil from an (E)Bhorizon of an aquic dystric Eutrochrept with silt loam texture(6 g kg-1 organic carbon, pH 4.1 in water, cation exchangecapacity∼70 µmol/g) was employed in this study. The soilmineralogy is dominated by vermiculite, illite, kaolinite,muscovite, and quartz with traces of chlorite and goethite.Soil aggregates (1.0-2.0 mm) were filled into glass columns10 mm in diameter and 100-120 mm in length. Thesolutions were prepared by dissolving the appropriateamounts of PbCl2, NaCl, and CaCl2‚6 H2O (p.a. Merck) inwater from a Barnstead Nanopure apparatus. The feedsolutions were pumped by a HPLC pump through a degasserinto the column and collected with a fraction collector.Prior to the experiments, the columns were stabilized with∼200 pore volumes of 0.5 M CaCl2 solution, and columnparameters were determined in standard fashion by pulseexperiments with conservative tracers (20). Typical porevolumes were 6-8 mL; porosities were 0.70-0.75. Thesevalues lead to 1.25 ( 0.05 g/mL of dry mass of soil per porevolume. We have used flow rates of 0.1-0.3 mL/min, whichcorresponds to pore water velocities of 2.3-6.9 m/day. Priorto the experiments, the dispersivities were ∼1.5 mm andcolumn Peclet numbers (Pe) were 50-75. However, aftercolloid mobilization experiments, the latter number hasdecreased to 5-15. The feed solutions were unbuffered;at the column outlet, we have measured pH 5.5 ( 0.5. Thecollected fractions were split in two parts. One was acidifiedwith distilled HNO3 to pH < 2, sonicated, and analyzed forNa, Ca, Si, Al, and Fe by inductively coupled plasma atomicemission spectroscopy (Varian Liberty 200 ICP-AES) coupled

* Corresponding author fax: (+41) 1 633 1118; e-mail address:borkovec@wawona.vmsmail.ethz.ch.

FIGURE 1. Schematic representation of colloid-facilitated con-taminant transport within a pore of a water-saturated porous mediumin the subsurface. Contaminants (b) are either dissolved in the liquidphase or adsorbed to the surfaces of the solid phase. The entireparticular phase is commonly assumed to be at rest, but it is possiblethat colloidal particles disperse in the liquid phase and provide arapid transport pathway for the contaminant.

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to ultrasonic nebularization (Cetac U-5000AT) and for Pbby graphite furnace atomic absorption spectroscopy (VarianSpectraAA 400). These measurements yield total concen-trations (dissolved and particulate) as verified by aciddigestion of the samples. The elements Si, Al, and Fe wereused as indicators of suspended colloidal particles; theirconcentrations were always proportional to each other.These elements were used to estimate particle concentra-tions by means of a calibration curve made with dry samplesof mobile colloidal particles (see below). The second partwas centrifuged for 2 h at 2 × 105 g (40 000 rpm), and thesupernatant was analyzed in a similar fashion. Thismeasurement gives the elemental concentrations of dis-solved species since these samples were effectively free ofsuspended particles (<0.3 mg/L) as deduced from measuredSi, Al, and Fe concentrations.

Particle Characterization. Dry samples of colloidalparticles were isolated as follows. Mobile colloidal particleswere isolated from the column effluents. The effulents werecollected, flocculated in 0.5 M CaCl2, dialyzed to removethe excess salt, and freeze-dried. Fine clay (<0.2 µm) wasisolated from dispersions of Na-saturated soil aggregatesin deionized water by sedimentation and subsequentcentrifugation for 9 min at 4 × 103 g (4000 rpm). Thesupernatant was collected; the colloidal particles wereconcentrated by coagulation in ∼0.5 M NaCl, washed saltfree, and freeze-dried.

Particle size of colloidal suspensions was analyzed bydynamic light scattering (ALV goniometer, Coherent Annovakrypton laser) at concentrations of 0.5-5 mg/L. Secondcumulant fits of the correlation functions were used toestimate the apparent hydrodynamic diameter that wasextrapolated to zero scattering angle (21, 22). The specificsurface area of the Na-saturated samples was measured bynitrogen gas adsorption (BET method, Micromeritics,Gemini 2360). Total elemental contents were obtained bydirect analysis of particle dispersions of known particleconcentrations for Si, Al, and Fe as discussed above andverified by acid digestion of the samples. Such measure-ments were also used to determine total concentrations ofmobile colloidal particles in the column effluents. Mineralcomposition was analyzed by X-ray powder diffractionanalysis (Scintag XRD 2000). Samples were saturated witheither K+ or Mg2+ and X-rayed either untreated, heated to550 °C, or treated with ethylene glycol. The appearance ofcolloidal particles was also investigated by transmissionelectron microscopy.

ResultsColloid-Facilitated Transport Out of a ContaminatedZone. Consider the following scenario, which could berealistic for a municipal dump site. A rather concentratedsolution (0.01-1 M) of a monovalent electrolyte, whichcontains a strongly sorbing contaminant, infiltrates intothe subsurface (dump site leachate) (23, 24). After sometime, the site is remediated and the source of the con-tamination is eliminated, but in the mean time thecontaminants have already migrated into the subsurfacezone. The composition of the pore water then graduallyreturns to a natural composition of the subsurface water,which is typically of low ionic concentration (10-4-10-2

M) and dominated by divalent cations.We mimic this scenario by the following column

experiment (see left column in Figure 2, experiment I). Acolumn, saturated with 50 mM NaCl solution, is infiltrated

with a 44 µM Pb2+ solution in the same backgroundelectrolyte. This “contamination” leads to a solid con-centration of ∼3.5 mg of Pb/g of soil, which correspondsto a partition coefficient of KD = 200 L/kg. In ourexperiment, the elimination of the contaminant source ismodeled by changing the feed to the original NaCl solution.The gradual infiltration with groundwater is imitated byfeeding the column with a 20 mM NaCl solution and laterwith a 0.15 mM CaCl2 solution.

The precise succession of infiltrating solutions is givenin Figure 2a (left). Below, the measured concentrations inthe column outflow are shown. We report the concentrationof the dissolved ions together with the total concentrationsof the corresponding elements (dissolved and particulate).In the first part of the experiment, one observes a stronglyretarded, self-sharpening breakthrough front of dissolvedPb2+ with a retardation coefficient around 500. Soon afterthe infiltration with the pure NaCl solution, the self-broadening front starts to form. This front interacts witha normality front, which originates from the decrease ofNaCl concentration in the input solution, and travels withthe velocity of a conservative tracer. With the change ofthe input to the CaCl2 solution of lower concentration,colloid-facilitated transport of Pb sets in. This effect issignaled by the occurrence of the narrow peak in the totalPb concentration with its maximum at ∼150 µM at 2810pore volumes (see Figure 2b, left). This peak is accompaniedby a similar peak in the colloid concentration (as indicatedby Al concentration in Figure 2c, left). While not too obviousin Figure 2b (left), let us stress that the concentration ofdissolved Pb2+ drops to very low concentrations after thenormality front at 2810 pore volumes. In this regime,essentially all mobile Pb is associated with colloidal particles;after centrifugation, we measure <0.1 µM Pb in thesupernatant. The dissolved concentration of Pb2+ isprobably much lower, however, as one is extremely sensitiveto small traces of colloidal particles.

While the peak at 2810 pore volumes appears to be verynarrow in Figure 2b (left), the inset in Figure 2b (left) showsthat colloid-facilitated transport is in fact dominant for atleast 50 pore volumes. One also observes that the totalconcentrations of the elements associated with the colloidalfraction are all proportional to each other; the curves shownin the inset are all parallel. The colloid concentrationreaches∼4 g/L at the peak maximum but drops quickly atfirst and later very slowly to remain almost constant around140 mg/L. Release of colloidal particles is terminated bythe arrival of the self-sharpening ion-exchange front; afterthis front, the colloid concentration becomes negligiblysmall.

Colloid-facilitated transport occurs since the decreaseof salt concentration at 2810 pore volumes generates twochromatographic fronts. In the normality front at 2810pore volumes, concentrations of all dissolved ions decrease,while after the second retarded (self-sharpening) front theion exchange between Na+ and Ca2+ is completed. Betweenthese two fronts, the porous medium is essentially saturatedwith monovalent cations and the matrix is in contact witha solution of low ionic concentration; this situationrepresents conditions that are favorable for particle release.

What is the nature of the colloidal particles responsiblefor this transport pathway? Some properties of the mobilecolloidal particles, fine clay fraction, and the whole soil aresummarized in Table 1. Particle size, elemental composi-tion, and mineralogy of the mobile colloidal particles are

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approximately constant throughout the experiment. Fromelectron micrographs, one observes that the mobile colloidalparticles are very polydisperse with the largest particlediameter around 1 µm. This value is well comparable withthe dynamic light scattering results; note that this tech-niques weights the large particles much more strongly (22).The particle size distribution of the mobile colloidal particles

can be estimated from dynamic light scattering and specificsurface area data. Assuming a log-normal mass distribu-tion, we find a mean particle diameter of ∼10 nm and acoefficient of variation of ∼1.3 (number-weighted quanti-ties). The other properties of the mobile colloidal particles(e.g., elemental content, mineral composition, specificsurface area) are rather similar to the fine clay fraction; we

FIGURE 2. Two column experiments demonstrating facilitated transport of Pb by in-situ mobilized colloids. (a) Succession of infiltratingsolutions mimicking possible contamination-remediation scenarios. Concentrations of various elements in the column effluent; totalconcentrations (dissolved and particulate, b) and concentrations of dissolved species (O). Solid lines are calculations based on the two-siteion-exchange transport model (see Table 2). (b) Pb models a strongly sorbing contaminant. (c) Al is proportional to the total concentrationof colloidal particles; major cations are shown in (d) Na and (e) Ca. The inset shows total concentrations of Si, Al, Fe, and Pb in thecolloid-facilitated transport regime. These elements are all associated with mobile colloidal particles; their concentrations are all proportionalto each other (parallel curves). Experiment I (left column) demonstrates colloid-facilitated transport near a contaminated zone. After thebreakthrough of dissolved Pb2+, the decrease in normality of the infiltrating solutions leads to two normality fronts. After the second normalityfront around 2810 pore volumes, the narrow peaks in Pb and Al concentrations reveal colloid-facilitated transport so that Pb is entirelyassociated with mobile colloidal particles. Experiment II (right column) shows that colloid-facilitated transport is also possible throughan uncontaminated zone. After the normality front around 90 pore volumes, the narrow peaks in Pb and Al concentrations reveal colloid-facilitated transport. The classical breakthrough of dissolved Pb2+ follows much later and is evidenced as the shallow peak with its maximumaround 400 pore volumes. In both experiments, colloid-facilitated transport dominates the transport of Pb for at least 50 pore volumes (seeinset).

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have no evidence for any preferential release of particularminerals. An apparent difference from the whole soil isthat the mobile colloidal particles and the fine clay fractioncontain smaller amounts of quartz.

If the porous matrix was originally contaminated, thereleased particles will be contaminated as well. In thepresent case, the solid phase is loaded with∼2 mg of Pb/g,and thus the released particles will carry a comparableamount of Pb. Their actual Pb content is ∼7 mg of Pb/g.This higher value reflects the larger specific surface area ofthe mobile colloidal particles (see Table 1). The colloidalparticles are rather effective carriers of Pb. Integration ofthe measured Pb breakthrough curve between the normalityfront and the retarded exchange front shows that the mobilecolloidal particles have transported ∼7% of the totaladsorbed amount of Pb out of the column. We also notethat colloid-facilitated transport can be also observed forCa, but to a much smaller extent (see Figure 2e, left).

If one would attempt to forecast the outcome of thiscontamination scenario on the basis of a classical transportmodel, one would severely underestimate the contaminantmobility and the risk of contamination. In such a classicaltransport model, one neglects the colloid-facilitated trans-port pathway. To illustrate this point, we have developeda simple transport model for the present situation. Thetransport model is based on an approximation of theconvection-dispersion equation with the mixing-cell model(stirred reactors in series). This transport model is coupledto an appropriate chemical equilibrium model, whichdescribes the liquid-solid partitioning of all importantspecies involved (26, 27). The appropriateness of suchtransport models in situations where the colloidal transportpathway is negligible has been demonstrated for varioussystems and scales (25-29).

Our chemical model for the present three-componentsituation is summarized in Table 2. The chemical modelwas mainly derived from the observed breakthrough curvesand is consistent with a limited batch data set. Theadsorption process is modeled with two different exchangesites. From all possible solution complexes, only PbCl+ isrelevant (31) and is included in the calculation. In orderto incorporate effects of mass transfer kinetics (28) andchemical heterogeneity (32), we employ a larger dispersivityparameter than measured by the conservative tracerexperiments; Pe ) 6 is used in all calculations. Thecalculated breakthrough curves are shown in Figure 2 (solid

lines). In spite of the low Peclet number used in thecalculations, the widths of the self-sharpening exchangefronts are mostly predicted as too narrow. An additionalreason could have to do with the fact that the dispersivityof the column has increased due to mobilization of colloidalparticles. Nevertheless, the model is able to reproduce themajor features of the experimental breakthrough curvesfor all dissolved ions in an approximate fashion. Clearly,this model cannot reproduce those parts of the break-through curves where colloid-facilitated transport is domi-nant. In the present case, this classical modeling approachdoes underestimate the total concentration of Pb in theeffluent by 6 (!) orders of magnitude. To a first approxima-tion, the transport of lead can be considered not to beinfluenced by the protons; the lead concentrations namelyexceed the proton concentration by more than 1 order ofmagnitude. From batch experiments, we know further that,in the concentration ranges considered in the columnexperiments, the pH dependence of the lead adsorptionisotherms is rather weak.

Colloid-Facilitated Transport Through an Uncontami-nated Zone. While the previous experiment shows thatcolloid-facilitated transport can be important within thecontaminated zone, it does not clarify to what extentcontaminated colloids are able to pass an uncontaminatedzone. To investigate this question, we have performed asimilar column experiment to the one discussed above (seeright column in Figure 2, experiment II). In this experimentthe column is infiltrated with 240 µM Pb2+ solution in a 50mM NaCl background electrolyte such that the self-sharpening Pb front has penetrated about three-quartersinto the column. After this infiltration, the upper part ofthe column is “contaminated” with Pb at a solid concen-tration of 5.1 mg/g while the lower part is “uncontami-nated”. The “remediation” is now mimicked by infiltratingthe column with “ground water”, namely, a pure CaCl2

solution of lower normality. Again two fronts develop,namely, a normality front, that exits the column around 90pore volumes and a retarded ion-exchange front around220 pore volumes. Between these two fronts, the conditionsare favorable for colloids release. The mobilized colloidsare contaminated with Pb and cause a peak in the total Pbconcentration ∼140 µM at the column outlet. However,since these two fronts overtake the slowly moving Pb front,the colloids have now carried Pb through an uncontami-nated zone and reached the column outlet much soonerthan the classical breakthrough of the dissolved Pb2+

(shallow peak with its maximum ∼5 µM around 400 porevolumes). As in the previous case, the inset in Figure 2b(right) shows that colloid-facilitated transport is dominant

TABLE 1

Characterization of Mobile Colloidal Particles, FineClay Fraction, and Whole Soil

uncontaminatedmobile particles

Pb-contaminatedmobile particles

Fine clay<0.2 µma

wholesoil

<2 mma

Sib 0.11 0.12 0.12 0.13Alb 0.07 0.07 0.07 0.04Feb 0.05 0.04 0.04 0.02Pbb <10-4 0.003-0.007 <10-4 <10-5

DLS (µm)c 0.74 0.90 0.30BET (m2/g)d 35 38 11

a Effective diameter based on sieving or centrifugation. b Totalelemental weight fraction determined by acid digestion and/or ICP-AES. The average relative error of these quantities is ∼10%. c Particlediameter determined by dynamic light scattering extrapolated to zeroscattering angle. The relative error due to extrapolation is ∼30%.d Specific surface area as measured by N2 gas adsorption (BET method).Typical relative error is ∼10%.

TABLE 2

Summary of Chemical Part of Transport Modelreactiona log Kb q (µmol/g)c

2Na+ + PbX h Pb2+ + Na2X -2.31 30Ca2+ + PbX h Pb2+ + CaX -1.492Na+ + CaY h Ca2+ + Na2Y -0.78 5.4Pb2+ + Cl- h PbCl+ 1.60a Ion-exchange reactions involve two different sites X and Y on the

solid phase with corresponding total concentrations given in the lastcolumn. The last row involves solution species only. b Chemicalequilibrium constant of the corresponding reactions. In the mass actionlaws, the activity of dissolved species is given in M (mol/L), the activityof adsorbed species approximated by the mole fraction on theexchanger. c Site concentration on the solid phase.

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for more than 50 pore volumes. Because of the presenceof the uncontaminated zone in this experiment, the Pbcontent of the mobile colloidal particles is∼4 mg/g, a valuewhich is lower than in the previous experiment. Betweenthe normality front and retarded exchange front, ∼3% ofthe total sorbed amount of Pb was transported by thecolloidal particles. In contrast to the previous experiment,the smaller amount of mobilized lead could be related todesorption of Pb from mobile colloidal particles, depositionof contaminated particles, or different Pb concentration.The same transport model as used above (see Table 2) isalso able to reproduce all major features of the breakthroughbehavior of the dissolved ions in an approximate fashion.But if colloid-facilitated transport sets in, the total con-centrations in the effluent are again severely underestimatedby this model, by 7 (!) orders of magnitude in this case.

A closer look at the inset in Figure 2b (right) reveals thatthe Pb concentration is not exactly proportional to theindicator elements of the mobile colloidal particles; the Pbconcentration decreases somewhat more rapidly. Thisobservation is a hint toward an important but complicatedphenomenon; such contaminated colloids have a finitelifetime and travel only a finite distance through anuncontaminated region. We suspect the average traveldistances of such contaminated colloids to be on the orderof meters. The flow rate dependence of these processes isequally important. All the above experiments were carriedout at a pore water velocity of 2.3 m/day, a value comparableto pore water velocities in the field (25, 29, 30). We haverepeated experiment II at a higher flow rate. At the largerpore water velocity of 6.9 m/day, the peak concentrationof Pb is∼80 µM while at 2.3 m/day the peak concentrationis∼140 µM. Decreasing the pore water velocity by a factorof 3, the peak Pb concentration increases by about a factorof 2. At first sight, this behavior may appear counterin-tuitive. The qualitative trend is readily explained. Withdecreasing flow rate, the residence time of a given volumeof the fluid phase in the porous medium increases. Sincethe particle release process is kinetically limited, withdecreasing flow rate the particle concentrations will in-crease.

DiscussionWe have shown that colloid-facilitated transport canbecome the dominant transport pathway for a stronglysorbing contaminant. This observation is deduced fromlaboratory column experiments with a non-calcareous soiland leaching solutions containing Na+ and Ca2+ as majorcations and Pb2+ as a minor cation (contaminant). Undercertain but realistic conditions, colloidal particles can bemobilized in-situ from the porous matrix. Given that thismatrix was previously contaminated with Pb, these particlescan transport large quantities of Pb. A classical transportmodel, which neglects the colloid mobilization, can dra-matically underestimate the total effluent concentrationsof Pb and its travel velocity by several orders of magnitude.We have generated favorable conditions for colloid-facilitated transport by saturating the column with Pb2+ inthe presence of a high concentration of NaCl and infiltratingwith solutions of lower ionic concentrations. In spite ofthe presence of Ca2+, substantial amounts of Pb bound tocolloidal particles can be mobilized, and these particles areable to carry Pb through an uncontaminated zone. Variouskinetic phenomena are important in this process: (i)desorption of Pb from the mobile colloidal particles, (ii)

deposition of the contaminated colloids, (iii) release ofcontaminated colloids in the contaminated region, and (iv)release of uncontaminated colloids in the uncontaminatedregion. Due to the non-exponential character of all theseprocesses and the strong influence of the solution com-position, the quantification of all these phenomena willrepresent a rather demanding task. However, such aquantification represents an essential step toward thedevelopment of quantitative models of colloid-facilitatedtransport.

From our experiments, we conclude that colloid-facilitated transport of contaminants is likely to representthe dominant transport pathway within the followingscenario. Consider a region contaminated with stronglysorbing substances (say KD > 100 L/kg) and saturated witha solution containing high concentrations (0.01-1 M) ofmonovalent cations (e.g., dump site leachates, seawater).During the infiltration of this region by a solution withlower ionic concentrations (10-4-10-2 M) of arbitrary ioniccomposition (e.g., groundwater, rainwater), upstream ofthe unretarded normality front colloid-facilitated transportis likely to occur. If the infiltrating solution of lowernormality is dominated by divalent cations, the normalityfront is followed by an ion-exchange front usually manypore volumes upstream. In our case, this succession ofchromatographic fronts was generated by the infiltrationof a CaCl2 solution of low normality. However, the detailedionic composition of the infiltrating solution is unimportant.As indicated by our column experiments, one possibleoutcome of this scenario is a “chemical time bomb” (to putit in a melodramatic fashion). Thereby, substantial con-centrations of contaminants, which are associated with in-situ mobilized colloids, are released suddenly and willmigrate rapidly with the pore water to rather distantlocations. This scenario may be applicable to various wastedisposal sites where high salinity leachates are oftendominated by monovalent cations (23, 24) or in contami-nated sites that are in periodic contact with seawater.

One must be of course careful in translating ourlaboratory results into actual field situations; colloid-facilitated transport may not occur in all cases. Many othereffects, which were not considered in our study, could alsoplay an important role. These effects could include, forexample, macro-heterogeneity, preferential flow paths, poreclogging by mobilized colloidal particles, mineral composi-tion, presence of organic matter, changing pH, and redoxconditions. Unfortunately, there is very little known aboutthe influence of these effects on the colloid-facilitatedtransport process. On the other hand, we have demon-strated the importance of this transport pathway for astrongly sorbing contaminant, admittedly with a laboratoryexperiment, but otherwise under rather realistic conditionsusing a natural soil and plausible compositions of infiltratingsolutions. The same transport mechanism is likely to beoperational in the field. This proposition is supported byvarious observations of the necessary sequence of normalityand ion-exchange fronts at the field scale (29, 30) as wellas findings that colloidal particles can travel in thesubsurface zone over large distances (5-9).

Alternative colloid release mechanisms are also con-ceivable, for example, in the presence of surface activeagents or by means of precipitation-dissolution reactions(4). Thus the main aim of the present study is to provideadditional and direct evidence that colloid-facilitatedtransport can indeed represent an important transport

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pathway. Transport by in-situ mobilized colloids shouldbe therefore carefully considered in the risk assessment ofany subsurface contamination problem.

AcknowledgmentsThis work was supported by the ETH Zurich. We thank A.Stahel and H. Holthoff for expert experimental help and R.Kretzschmar for careful reading of the manuscript.

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Received for review March 15, 1996. Revised manuscriptreceived July 26, 1996. Accepted July 29, 1996.

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