Processes of colloid mobilization and transport in macroporous soil monoliths
Post on 31-Oct-2016
.Geoderma 93 1999 3359
Processes of colloid mobilization and transport inmacroporous soil monoliths
M. Lgdsmand a,), K.G. Villholth b, M. Ullum a, K.H. Jensen aa Institute of Hydrodynamics and Water Resources, Technical Uniersity of Denmark, 2800
Lyngby, Denmarkb VKI, Agern Alle 11, 2970 Hrsholm, Denmark
Received 21 September 1998; received in revised form 28 April 1999; accepted 29 April 1999
Transport of pesticides, PAH and other hydrophobic or surface-complexing contaminants insoils may be enhanced by colloid-facilitated transport. A prerequisite for colloid-facilitatedtransport is the release and transport of colloids. The mechanisms for colloid mobilization andtransport in a macroporous Alfisol have been evaluated by measuring the amount and type ofcolloids leached in two large soil monoliths during long duration simulated rain events. The soilwas irrigated with water having a chemical composition close to natural rainwater and atintensities as expected under natural conditions. The results showed that the colloids wereprimarily mobilized and transported in the macropores and that the source of colloids was notexhausted for extended rainfall duration. The first flush of water mobilized loosely bound colloidsthat had a high organic content relative to the bulk soil. After the initial release, the high ionicstrength in the percolating water limited the mobilization. For prolonged leaching, the diffusion ofcolloids from the macropore walls appeared to rate-limit the mobilization process. During the lateleaching phase, the rate of colloid mobilization was positively correlated with flow velocity.q 1999 Elsevier Science B.V. All rights reserved.
Keywords: colloidal materials; erosion; preferential flow; macropores; organic materials; diffusion
Colloids are defined as suspended particles with a small size. The maximumsize is limited by a tendency of larger particles to sediment and is generally
) Corresponding author. Present address: Department of Crop Physiology and Soil Science,Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele,Denmark. E-mail: email@example.com
0016-7061r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. .PII: S0016-7061 99 00041-5
( )M. Lgdsmand et al.rGeoderma 93 1999 335934
below a few micrometers. The minimum size, separating colloids from dissolved .matter, is about 10 nm Ryan and Elimelech, 1996 . Colloids may, due to their
small size and depending on the physico-chemical conditions, remain suspendedin electrolytic fluids. In addition, soil colloids are relatively reactive with respectto sorption of chemical species due to the large specific surface area and thehigh number of functional surface groups. Both the ability to be suspended andthe sorption capacity make the colloids potential carriers for pollutants inflowing water of rivers, oceans and soils. Soil colloids mainly consist of clayminerals, organic matter and oxidesrhydroxides. Colloid-facilitated transport in
.soils requires that three different processes take place: 1 the pollutant must . sorb to the colloid, 2 the colloid must be mobile mobilized either before or
. .after the sorption and 3 the colloidrpollutant complex must be transportedthrough the soil. Hence, pollutants that have a low solubility in water and a highpartition coefficient between soil and water, e.g., certain pesticides, heavymetals and PAHs, may be transported at a rate beyond what is expected frompartitioning only to the stationary soil matrix. A number of studies have dealt
.with the complex issue of colloid-facilitated transport. Vinten et al. 1983 .soils; clay; DDTr paraquat showed that the transport of pesticide was depen-dent on the ionic strength of the infiltrating water and the texture of the soil. Afew reports deal with experimental evidence of colloid-facilitated transport of
. chemical species in undisturbed soil media. De Jonge et al. 1997 structured.soil; natural colloids; prochloraz observed that about 20% of the leached
pesticide was sorbed to mobilized particles )0.02 mm. Seta and Karathanasis . .1996 structured soil; dispersed colloids; metolachlor found that the presenceof colloids enhanced the transport of pesticide by 22 to 70% depending on thecolloid type and mobility.
The mobilization of colloids in soils and groundwater sediments due tochanges in flow or chemistry has been reported in several studies. Chemicalperturbation can affect the forces that keep the colloids bound to the grains and
thereby the mobilization of colloids Ryan and Gschwend, 1994a quartz sand;. .haematite ; Seaman et al., 1995 aquifer sand; natural colloids ; Kaplan et al.,
..1996 reconstructed soil; natural colloids . Diffusion from the detachment siteinto the flowing water may be the limiting factor for the rate of mobilization
when the chemical conditions are favourable for the release of colloids Ryan . and Gschwend, 1994a quartz sand; haematite ; Jacobsen et al., 1997 structured..soil; natural colloids . Dissolution of cementing agents may enhance the
. mobilization of colloids from grains. Ryan and Gschwend 1994b goethite-.coated aquifer sand; kaolinite found that the mobilization of kaolinite colloids
was increased when the goethite was dissolved due to changing redox condi-tions. Increasing the flow rate can cause enhanced mobilization Kaplan et al.,
. 1993 reconstructed soil; natural colloids ; Ryan and Gschwend, 1994a quartz. ..sand; haematite ; Govindaraju et al., 1995 sand; kaolinite . Kaplan et al.
.1993 found that the colloid concentration in the effluent from lysimeters
( )M. Lgdsmand et al.rGeoderma 93 1999 3359 35
depends on the flow velocity squared, suggesting mobilization by shear stress. .Pilgrim et al. 1978 reported that subsurface flows during storm events mobi-
lized large amounts of particles in the size range 48 mm. This was explainedby raindrop impact on the surface soil combined with macroporous flow paths.
Generally, the mobilization of colloids in homogenous sand is described inmany studies, but the mobilization of colloids in naturally structured soils is notwell described. Several studies have shown that colloidal particles applied
.externally can be transported in subsurface environments. Jacobsen et al. 1997 .undisturbed soil; illite and humus-coated illite showed that the mass recoveryof surface-applied colloids in leachate from subsoil columns was higher thanfrom topsoil columns and that the recovery increased with increasing flow rate.
. .McKay et al. 1993 fractured clay till; bacteriophages found a hundred-foldgreater retardation of conservative tracers compared to colloid tracers in a fieldexperiment and attributed it to the preferential diffusion of solutes into the
.matrix. Toran and Palumbo 1991 found that the retardation of colloids inpacked sand columns decreased when artificial macropores oriented in the flowdirection was embedded in the medium and that macropores with larger
.diameter created multiple peak breakthrough curves. Kretzschmar et al. 1995observed that the leaching of clay colloids passing through an intact saprolitewas dependent on the natural coating of the colloids with natural organic matter . .NOM . The untreated colloids with NOM resulted in blocking effects forfurther deposition due to a monolayer restriction for the continued attachment of
.colloids, while the treated colloids without NOM resulted in ripening due tomultiple layer attachment.
In the present study, the effect of macropores and low ionic strength .infiltration water corresponding to natural rain on colloid mobilization and
transport was investigated. The combination of continuous and hydraulicallyactive macropores and infiltration water with a low ionic strength can lead to anaccelerated removal of ions from the macropores and the surrounding matrix,resulting in destabilization of the aggregates at the macropore walls and therebyincreased mobilization and transport of colloids. Two undisturbed soil monolithsexcavated from a site in western Denmark and installed in the laboratory wereexposed to long duration rain events to investigate the processes of colloidmobilization under temporally changing chemical conditions and under varyingflow rates.
2. Materials and methods
2.1. Theoretical background
2.1.1. Chemical perturbationsThe chemistry of the pore water will affect the interacting forces acting on the
colloids: electrostatic forces, van der WaalsLondon forces and Born repulsion.
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The electrostatic forces are particularly sensitive to changes in the electrolyticproperties of the fluid. The electrostatic potential around a single spherical
.charged particle in an electrolytic fluid can be described by Kruyt, 1952 :
c sc eyk x 1 .e e,0
1 RT 1 3.09s s2) k 1000 I I2 N q .a eat Ts258C and s6.95=10y10C 2rJ m 2 . .
where c is the electrical potential at the surface of the colloid, c thee,0 eelectrical potential at the distance x from the colloid surface, 1rk the Debyelength, which is often interpreted as the thickness of the double layer of theparticle, the permittivity of the fluid, R the universal gas constant, T theabsolute temperature of the fluid, N Avogadros number, q the charge on thea eelectron, and I the ionic strength of the solution.
When the release of colloids from the grains is controlled by the electrolyticproperties of the fluid, the rate constant for detachment and attachment ofcolloids is proportional to an exponential function of the size of the energy
.barrier to detachment or attachment Ruckenstein and Prieve, 1976 :
Nf yf Nmax min ,1k Aexp y 3 .det /kTNf Nmaxk Aexp y 4 .att /kT
where k is the rate constant for detachment and k for attachment. f isdet att maxthe maximum of the potential energy between two colloids and f is themin,1primary minimum. This will cause the release of colloids across the energybarrier to be a first-order reversible heterogeneous reaction with the energybarriers serving as activation energy for the process. When the ionic strength is
.decreased, f is increased Ruckenstein and Prieve, 1976 and hence the ratemin,1of detachment is increased. If the process of colloid mobilization is controlledby chemical perturbations, there are two steps involved in the process. A processof detachment of colloids from the grains followed by a diffusion process fromthe grain surface into the pore stream. If the rate of detachment is higher thanthe rate of diffusion, e.g., at low ionic strength, the diffusion process will limitthe overall mobilization, and vice versa.
2.1.2. DiffusionFor the evaluation of diffusive transport of dissolved or suspended species
intrinsically present in macroporous soil, two different processes should be
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. .considered: 1 diffusion within the matrix and 2 diffusion from the walls ofthe macropores and into the main macropore stream. The matrix can be viewedas a semi-infinite medium and the macropore wall as the plane that limits it.When the initial concentration of the diffusing species throughout the matrix isC and the concentration at the macropore walls is zero, the accumulated0
. amount of the diffusing species M that is transported through the plane att. .xs0 per area at time t is Crank, 1975, p. 32 :
D tABM s2C 5 .(t 0 pwhere D is the diffusion coefficient of substance A in medium B. TheABconcentration in the macropore water is not zero due to upstream inflow of thesubstance, but compared to the much higher concentration in the matrix, hereconsidered the sole source, it can generally be neglected.
For the initial stages of a diffusion process out of a plane sheet of thickness lwith uniform initial concentration in the sheet and constant zero surfaceconcentration, a similar equation can be obtained for the accumulated loss of
.diffusing substance M out of the sheet Crank, 1975, p. 244 :t
D tABM s4M l 6 .(t pwhere M is the accumulated loss of diffusing substance out of the sheet for tapproaching infinity. This equation has been applied to indicate the diffusion-controlled process of non-equilibrium sorption or desorption of various sorbates
in soil in well-mixed batch or packed flow systems Pavlatov and Polyzopoulos,.1988; Kookana et al., 1992 . For a macroporous flow system, an analogy can be
made as a semi-stagnant sheet of water along the macropore wall develops .through which the substances need to diffuse. Jacobsen et al. 1997 observed
linearity of cumulative mass of colloids leached from undisturbed soil columnsvs. square root of time. Consequently, when a process is controlled by diffusion .either in the matrix or from the walls of the macropores , a plot of cumulativemass of diffusing substance vs. square root of time will produce a straight line .at least in the initial stages of the diffusion process .
The linear relation of cumulative mass vs. square root of time does not provethat diffusion controls the process, but if diffusion controls the process therelation will be linear. For a more stringent evaluation of substance mobility thatincludes radial diffusion from the macropore wall as well as the verticaladvective transport in the macropores, the soil is equivalated to a model of animpermeable matrix with equally sized, vertical cylindrical macropores. Themacropores are considered to be full-flowing and the flow is assumed to beequivalent to laminar Poiseuille flow throughout the tubes. The validity of this
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assumption requires that L the depth from the soil surface where the Poiseuillee.flow is fully developed is small. For steady-state conditions, the advectiondif-
.fusion equation can then be simplified to Clark, 1996 :ECA
E r /EC 1 ErAu sD 7 .z ABEz r Er
where z is the depth from the soil surface, r is the distance from the center ofthe tube, u is the advective velocity in the macropores and C is thez Aconcentration of diffusing substance A as a function of r and z. If the inletconcentration at the soil surface is zero and the concentration at the macropore
.surface C is constant the following solution is obtained Clark, 1996 :A,sC G zraA ,ave n 2s1y8 exp yl 8 .n2 /C l Re ScA ,s n
where C is the averaged concentration of substance A over the tube crossA,avesection at depth z, a is the radius of the tube, Re is Reynolds number, Sc isSchmidts number, G is an infinite series of constants and l are the eigenval-n n
..ues G and l are given by Clark 1996 . This model does not adequatelyn ndescribe the diffusion of species in a macropore as the flow in the actualmacropores probably does not fulfill the assumption of full-flowing macroporesat all times, but for evaluation of the trends it is useful. When Eq. 7 and Eq. 8
.apply, the outflow concentration C will be negatively correlated to theA,ave .flow velocity u as increased dilution of the diffusing substance occurs withz
increased flow. Hence, when evaluating the possible effect of flow rate onparticle mobilization, an assessment of the concentration at the macropore wall .through Eq. 8 , rather than in the effluent, as a function of flow rate isappropriate.
2.1.3. Physical perturbationsWhen evaluating the potential for mobilization by shear stress, a torque
balance between the adhesive forces a combination of double layer interactions. .and Van der Waals force , the lift force due to turbulence near the wall and the
.drag force due to hydrodynamic stress is to be considered. Increasing particlesize, flow velocity and ionic strength promote the mobilization of particles by
.shear stress Sharma et al., 1991 .
2.2. Field site and monolith sampling
Undisturbed soil samples were taken at the Rgen field station, near Arhus, . .Denmark latitude 568 . The soil type is an Alfisol Typic Hapludalf and is
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developed from moraine deposits from the Weichselian ice period. In the .monoliths, the A horizon approximately the upper 0.25 m of the column and
.part of the B horizon approximately the lower 0.1 m were present. In Table 1,the properties of the soil are listed for the two horizons. Two monoliths column
.A and B were collected in the following way: Grass on the surface was cutshort and a stainless steel cylinder inner diameter 290 mm, height 400 mm,
.thickness 2 mm with sharpened edges was pushed into the ground by ahydraulic press to a depth of 0.35 m and excavated by shovel. The columnswere collected only a few meters apart. The soil-containing cylinders were thensealed with plastic covers on both ends of the steel cylinders and with adhesivetape on the holes for insertion of instruments and was stored at 58C. Due to thisprocedure, it was assumed that the soil samples had the same water content as inthe field when the experiments were initiated.
2.3. Laboratory methods
In the laboratory, the soil protruding the lower end of the columns was cut off .gently and the cylinder was placed on a screen of stainless steel 2=2 mm grid
.that was placed on a cylindrical container diameter 300 mm and height 10 mm .holding glass beads 2 mm diameter . In order to average any local heterogene-
ity across the column cross-section, the outflow was collected simultaneouslythrough four outlets on the perimeter of the container holding the glass beads.The outflow solution was withdrawn with a peristaltic pump at a rate exceedingthe rate of outflow from the monoliths, ensuring instantaneous sampling and
.minimizing accumulation within the chamber with glass beads Fig. 1 . Toprevent air entry into the on-line flow cell of the turbidimeter the water table in
Table 1Physical and chemical properties of A and B horizons of the Alfisol profile
A B . .00.25m depth 0.250.35m depth
.clay -2 mm wrw% 16.0 21.1 .silt 220 mm wrw% 15.3 13.3
.coarse silt 2063 mm wrw% 12.2 11.7 .sand 63125 mm wrw% 17.2 15.6 .sand 125200 mm wrw% 13.6 14 .sand 200500 mm wrw% 17.1 17.2
.sand )500 mm wrw% 6.3 6.7 .humus wrw% 2.6 0.6
.TOC wrw% 1.5 0.3 .CEC mmolrkg 156.6 112.5
.pH 0.01 M CaCl 6.17 5.892
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Fig. 1. Sketch of the experimental setup.
.the constant-level cell upstream Fig. 1 was kept 0.3 m above the inflow of theturbidimeter by a magnetic valve controlled by a photoelectrical sensor in the
.constant level cell. A layer approximately 0.01 m on average of acid-washed .sand 0.5 mm mean grain size was spread over the soil surface in order to cover
the irregularities of the soil and to prevent mobilization of colloids by dropimpact. The drainage from the monoliths was gravitational. The irrigation water
was applied from a rain simulator with 69 hypodermic needles 0.4 mm in.diameter and 20 mm long . The irrigation water was produced to match the
.chemistry of the rainwater at the experimental site. The salts Table 2 were .added to deionized water DW and the solution was aerated to ensure equilib-
rium with CO of the atmosphere. During this process, the pH was adjusted to24.6 with 0.1 M HCl. The ionic strength was approximately 0.3 mM and the
.electrical conductivity EC was about 3 mSrm. The artificial rainwater was ledto the rain simulator by a peristaltic pump. Three different rain intensities were
. . .used: low 1.6 mmrh , medium 3.2 mmrh and high 6.5 mmrh . The .experiments were conducted at room temperature about 208C . Soil water
suction was measured at four depths using tensiometers connected to transduc-
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Table 2Chemical composition of the artificial rainwaterSalts Concentration Concentration
3 . .mM grmNaNO 0.048 4.073NaCl 0.056 3.24KCl 0.0047 0.35CaCl 2H O 0.0112 1.652 2MgSO 7H O 0.0121 2.984 2 .NH SO 0.030 3.414 2 4
ers, and water content was measured at three depths using TDR-probes 70 mm.rods with 5 mm spacing .
.A sequence of two different flow experiments were carried out: 1 applica- .tion of constant high rain intensity and 2 application of a sequence of
decreasing and subsequently increasing rain intensities. Each experiment con-sisted of the continuous application of rain lasting between 1.4 and 3.5 days.Experiments on the same column was separated by a break in the rain for 34days to allow drainage and restitution of the soil. The experiments withdecreasing and increasing rain intensities were performed with highmediumlow intensity for one day each and then lowmediumhigh for one day each.The experiment with constant high rain intensity was repeated four times on
. .column A denoted exp. 1A to 4A and twice on column B exps. 1B and 2B .The amount of effluent leached from the columns before the start of the differentconstant high flow experiments is shown in Table 3. Note that exps. 4A and 2Bwere exposed to approximately the same amount of leaching before the experi-ment was initiated. Two tracer experiments with 25 kgrm3 chloride wereperformed on each column after the irrigation experiments: one at low and one
Table 3Accumulated leached effluent at the start of the experiments and accumulated mass of colloidsleached after 10 and 100 mm of outflow for the experiments with constant high irrigation rateExperiment Cumulative Cumulative Cumulative
.no. outflow mm mass of colloids mass of colloids . .after 10 mm mg after 100 mm mg
1A 0 2 402A 229 8 1403A 792 25 2504A 1367 20 2401B 0 5 1002B 1198 30 220
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at medium flow rate. Before each tracer experiment, the flow in the soilmonolith was at steady state. Then the rain was intermittently stopped and 100ml KCl solution was added by pipette at the surface during 15 min in themedium flow experiment and during 30 min in the low flow experiment,whereupon the rain application was resumed. After the flow and tracer experi-ments, a 500 ppm Rhodamine B solution was applied to the columns. The dyesolution was applied through the rain simulator with medium irrigation rate for12 h. Upon completion of the dye experiment, 0.02 m horizontal slices wereincrementally cut off the lower end of the column and the visibly dyed
.macropores )1 mm on the cleared soil surface were registered with respect to .number and size radius of roughly circular pore cross-sections . The visibly
dyed pores represent the active macropores. The total cross-sectional area ofactive pores in each layer was calculated from the radius and number of dyedpores and assumed to represent the active macroporosity in the layer.
.During the experiments pH, electrical conductivity EC , mass concentration . of colloids C , turbidity, and total and dissolved organic carbon C andcoll TOC
.C were measured in the effluent. Total accumulated outflow was registeredDOCby weighing of incremental samples. In one experiment, the colloid-size distri-bution was measured. The samples for measuring pH, EC and C and flow ratecollwere taken manually from an effluent container when the volume exceeded 250ml, while the samples for measurements of colloid size and number and TOC
.were taken with a 20-ml syringe directly from the flow line Fig. 1 . The .samples for colloid-size analyses were diluted 1:100 with DW immediately
after sampling to prevent flocculation of the colloids and the TOC samples wereconserved with 12.5 mlrl 10% HNO ; both sample types were stored at 58C3until analyses. Turbidity was measured on-line using a Hach turbidimeter 2100equipped with a flow cell with an approximate volume of 28 ml. Turbidity,suction and water content was recorded every 5 min using a computerizedlogging system. The measurements of C were made by filtering about 250 mlcollsample on an Advantec Toyo glass filter with a pore size of 1 mm. The filterwas dried at 1058C for 1 h prior to the filtration and for 2 h after and the weightincrease was noted. The samples were stored at least 1 week at 58C before thefiltration to ensure good flocculation of the colloids and to obtain a moreaccurate determination of the mass concentration of the colloids. In the first fourexperiments on column A, C was not measured with this method and insteadcolla linear relation between turbidity and C was developed and used to estimatecollC :coll
C s0.60 ppmrNTUP turbidity 9 .collThis relation was estimated from turbidity measurements on suspensions of
naturally occurring colloids from the experimental site using a fraction of the 2.soil containing only colloids -5 mm. The coefficient of determination R
was 0.99 for this particular fraction. A similar calibration method was used by
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.Jacobsen et al. 1997 but for measurements of light extinction. Even though thismethod may underestimate the total mass of colloids, if the colloids aresubstantially larger than 5 mm and overestimate it when the colloids aresubstantially smaller than 5 mm, this uncertainty was only found to affect theresults in the initial and final stages of the leaching of colloids. For a compari-son of the measured and estimated values in column B, see Fig. 6.
.From the EC of the effluent, the amount of total dissolved solids TDS canbe estimated using a linear relation given by Tchobanoglous and Schroeder .1985 :
TDSskEC 10 .where k ranges from 5.57.0 ppmPmrmS. In this soil, a k of 7.0 was found
2 . most dequate R s0.95 from a regression of values obtained by Eq. 10 usingEC measured in the effluent of exp. 1B and values obtained from the difference
in dry matter measured in bulk effluent and in the retenate of the filtered )1. .mm effluent .
The analysis of TOC was performed on an O.I. Analytical 700 TOC-analyzerwith an auto sampler. The standard deviation associated with the analysis was0.12.0%. On samples from exps. 1B and 2B, DOC was measured as well.Here, the samples were filtered through an acid-washed Sartorius Minisartdisposable filtering unit with a pore size of 0.2 mm and the filtrate was analyzedfor TOC. From the values of TOC and DOC, the particulate organic carbon
.POC C sC yC and thereby the average f of the colloids )0.2POC TOC DOC oc .mm f sC rC was calculated. Particle-size distribution and concentra-oc POC coll
tion by number were measured on effluent samples from exp. 2B using a . .single-particle counter SPC Degueldre et al., 1996 . Each measurement
consisted of three runs per sample where particle size classes )100 nm, )200nm and )500 nm were determined.
3. Results and discussion
3.1. Tracer breakthrough and macropores
The breakthrough of chloride is shown in Fig. 2. It is evident that the twocolumns differ with respect to the macropore system. The breakthrough incolumn A was more rapid than in column B and the tailing was more distinct incolumn B. The peak of chloride is achieved after 0.030.06 pore volumes incolumn A, and after 0.100.18 pore volumes in column B.
The dye experiments showed that the water flow took place primarily inmacropores. The active macropores were mainly wormholes diameter: 28
. .mm and root channels diameter: 12 mm . There were no visible signs of
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Fig. 2. Concentration of chloride in the effluent during the four tracer experiments.
preferential flow paths along the sides of the cylinder. The active macroporosity .was larger in column B than in column A 0.07% vs. 0.14% . Assuming vertical
cylindrical tubes between the layers, a total macropore wall area was estimated.The volumetric macropore wall area was also slightly bigger in column B than
2 3 2 3.in column A 1.51 m rm vs. 1.24 m rm .The results of the tracer and dye experiments showed that the water primarily
was lead through the macropores, supporting previous findings by Beven and .Germann 1982 , and that there is a large heterogeneity even for relatively large
soil columns collected within a short distance, supporting previous findings by .Sassner et al. 1994 . The retention time in column A was relatively small due to
a smaller active macroporosity thus leading to higher flow velocities. Incontrast, a larger contact area between macropores and matrix was present incolumn B leading to a higher potential for diffusive exchange between the twodomains.
3.2. Constant irrigation rate experiments
Because no pre-wetting of the columns was performed, the initial water .content of the first flow experiments was lower ;0.27 than that of all the
.other experiments ;0.30 . Therefore, the first flow experiment on bothcolumns represents the response behavior of soil when a rain event follows a dryperiod. Fig. 3 shows the turbidity, outflow rate and EC at the beginning of the
( )M. Lgdsmand et al.rGeoderma 93 1999 3359 45
first experiment on each column. Initial peaks in the turbidity are observed onboth columns. Similar early-time leaching of particles with flow events have
been observed in smaller-scale investigations Kaplan et al., 1993; Jacobsen et ..al. 1997 with soil from the same site as the present experiments and in field
experiments Bottcher et al., 1981; Grant et al., 1996; Ryan et al., 1998;Villholth et al., 1999 at the same site where the samples for the present
..experiments was collected . This increased mobilization by the first flow ofwater can be attributed to local creation of shear stress by the first flow of water .Ryan et al., 1998 ; a preferential sorption of colloids to the airrwater interfaceon an advancing water film in the macropores; a lower cohesive bonding of
.colloids within the initially dry soil Bottcher et al., 1981 ; or to a higher contentof organic matter in the colloids mobilized in the beginning of the experiments
.and thereby higher stability of colloids in suspension see later . The turbidity inthe initial peak decreases before the flow has reached its maximum value Fig.
.3 . This may be interpreted as the flow rate is not the major controlling factor in .the initial mobilization Ryan et al., 1998 . However, in the initial phase of
unsteady overall flow conditions, flow rate out of the column is not a goodmeasure of the internal flow rate, which would be the parameter of interest, as acontinuous loss of water down through the column is taking place due to wettingof the initially dry soil. After the initial peak, the turbidity and colloid
.concentration increase as the EC of the effluent decreases Fig. 3 . The initialpeak of EC which reaches 110 to 120 mSrm, well above the EC of the influent .3.0 mSrm , indicates that ionic species, mostly salts, are leached from the soil.The continuous decrease of EC after the first peak and in subsequent constant
.rate experiments Figs. 3 and 4 suggests that the leaching of salts is diffusion-limited. The turbidity in subsequent experiments fluctuate with multiple peaks .not shown but with no distinct initial peaks as in the first experiment. Thissuggests that the particle mobilization mechanisms are highly sensitive to theinitial conditions of the soil medium.
The amount of colloids washed out after the first 10 and 100 mm of effluentfor the various constant irrigation rate experiments is shown in Table 3. It isevident that the continued rain makes it easier to mobilize the colloids. Further-more, the pool of detachable colloids in the columns apparently is not limitedunder the given experimental conditions, which is probably due to the low ionicstrength of the artificial rainwater that gives rise to dispersion of the aggregatesin the macropore wall, thereby increasing the amount of detachable colloids.
Generally, the colloid concentration in the effluent is rising as the EC is . falling with time Fig. 4 . When the EC is above 18.0 mSrm and the initial
.peak of colloid leaching is excluded from the data , the concentration of colloids .is proportional to the reciprocal of the square root of EC Fig. 5 . This could
indicate that the stability of the colloids are dependent on the thickness of the .double layer 1rk which is proportional to the reciprocal of the square root of
.the ionic strength Eq. 2 and hence that the rate of detachment is the
Fig. 3. Outflow rate, turbidity and electrical conductivity in the effluent during the initial stage of the first experiments with constant high irrigation rate, . .a Column A, b Column B. The dotted line denotes outflow rate, the solid line denotes turbidity and the crosses denote electrical conductivity.
.Fig. 4. Colloid concentration vs. electrical conductivity in the effluent for the experiments with constant high irrigation rate, a Column A, .b Column B.
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Fig. 5. Colloid concentration vs. the reciprocal of the square root of EC in the effluent of the twocolumns for the experiments with constant high irrigation rate. Only data for which the EC isabove 18.0 mSrm and data after the initial flush of colloids are included. The linear regressionlines and the corresponding coefficients of determination are shown.
rate-limiting step in the colloid mobilization process at these intermediate times.Columns A and B give different trends, probably reflecting the different flowpatterns and macropore wall areas in the two columns. Both the colloidconcentration and the EC measured in the effluent are influenced by the
.diffusion and transport processes in the soil see Eq. 8 . Below an EC in theeffluent of approximately 18.0 mSrm which occurs after extended leaching .approximately 414 mm on column A and 312 mm on column B other factorsthan the ionic strength seem to control the mobilization.
Fig. 6 shows the cumulative mass of mobilized colloids vs. the square root oftime from the time of first breakthrough of water for the six different experi-ments with constant high flow rates. Experiments 4A and 2B show a linearrelationship of cumulative mass and square root of time indicating, but notproving, that the mobilization of colloids after prolonged leaching is controlledby the rate of diffusion rather than the rate of detachment. At the end of exps.2A, 3A and 1B, the relationship tends to get more linear indicating that at theend of these experiments the diffusion could be the rate-limiting step. In Table
2 .4, the coefficient of determination R -value for the linear regression of thecumulative mass of colloids vs. square root of time in periods where EC)13.5
Fig. 6. Cumulative mass of colloids in the effluent vs. the square root of time from the first breakthrough of water for the experiments with constant high . . . .irrigation rate, a Column A, b Column B. Results for measured m and estimated e colloid mass are indicated.
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Table 4 2.Coefficients of determination R from the linear regression analysis of cumulative mass of
colloids, TDS and TOC vs. square root of time from breakthrough of water for the constant highirrigation rate experimentsa
Experiment Colloids Colloids TOC TDS . .no. EC )13.5 mSrm EC -13.5 mSrm
b .1A 0.83 0.79 0.99 . .2A 0.92 1.00 0.81 1.00
.3A 0.99 0.80 0.96 .4A 1.00 0.85 0.98
.1B 0.88 0.85 0.89 0.95 .2B 0.98 0.98 0.90 0.98
aThe regression analyses was only performed if more than seven data points were available.bValues in brackets denote that calculated, rather than measured, values of colloid concentra-
tion were used to determine the cumulative mass of colloids.
mSrm and EC-13.5 mSrm is shown. When the EC is above 13.5 mSrm inthe effluent the fit is poor but when it is below 13.5 mSrm the R2-value is closeto unity, suggesting that the diffusion controls the mobilization at EC-13.5mSrm. The ionic strength at the detachment sites corresponding to 13.5 mSrmin the effluent could be the critical value that makes the energy barrier against
.detachment Nf yf N in Eq. 3 equal zero and hence making themax min,1diffusion the rate-controlling process, as proposed by Ryan and Gschwend .1994a . It should be noted that the use of the effluent EC as an indicator for thevarious mechanisms controlling the colloid mobilization, as apparent from thepresent results, most likely will not be generally operational because a host offactors will influence the overall colloid and ion leaching in various soilsystems, e.g., macropore structure, parent soil material, rainwater composition,biological activity etc. Hence, the approach of using EC basically illustrates theoverall processes taking place and the dynamics involved in the collectiveprocess of colloid leaching.
The TOC of the effluent shows a decreasing trend from one experiment to the .next. Almost all of the TOC is present as DOC 8399% . In the first
experiments, the concentration of TOC stabilizes at about 1012 ppm and in thelast experiments it stabilizes at about 4 ppm. In Table 4, the coefficients ofdetermination of linear regression analyses of cumulative mass of leachedorganic carbon vs. the square root of time are shown. There is a relatively poorlinear correlation in the experiments, which means that apparently diffusion isnot the limiting step in the mobilization of organic matter. Rather, the release ofTOC could be controlled by a dissolution or a microbiological generation
.process Guggenberger and Zech, 1993 .A linear regression analyses of the cumulative mass of TDS estimated from
.Eq. 10 vs. square root of time from breakthrough of water showed that the
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2 . .coefficients of determination R -values Table 4 were very close to unity.This supports that the diffusion of ions through the matrix out into themacropore stream, after an initial period where the relation is not linear, is thelimiting step in the build-up of the ionic strength in the effluent.
In Fig. 7, f of the colloids is plotted against the accumulated amount ofoceffluent for the two experiments with constant high flow rate on column B. The
.amount of organic carbon in the colloids is fairly high at the beginning 12%but decreases until it, after a cumulative amount of effluent of 1200 mm in the
. .beginning of exp. 2B , reaches the value of f in the bulk soil 0.31.5% .oc .Kaplan et al. 1993 report that the f of the mobilized colloids in differentoc
reconstructed but unstructured soil profiles of a loamy Paleudult was higher thanor equal to the f of the colloids in the bulk soil, but more or less constantoc . .1.16"0.25% throughout a water application 102 mm, 51 mmrh . As op-
.posed to the findings of Kaplan et al. 1993 , the decreasing content of organiccarbon with water application in this study indicates that a depletion of organiccarbon-enriched colloids took place. This could be explained by preferentialmobilization of the colloids from the macropores. Worm linings are reported to
have a higher content of total organic carbon factor of 1.9 to 6.4, increasing
Fig. 7. The mass fraction of organic carbon of the mobilized colloids in the effluent from exps. 1Band 2B vs. the cumulative outflow from column B, and the mass fraction of organic carbon of thesoil from the A and B horizon of the soil profile. Another experiment, not described in this paper,was conducted between the two experiments but with the same rainwater chemistry.
( )M. Lgdsmand et al.rGeoderma 93 1999 335952
. with depth compared to the bulk soil in a Fragiaqualf A-horizon Stehouwer et.al., 1994 . Also, organic matter content or coatings on colloids generally tend to
.stabilize colloids Heil and Sposito, 1995; Kretzschmar et al., 1995 . Afterprolonged continuous leaching in the macropores, the organic-rich and looselybound colloids are depleted. This might explain why the accumulated mobiliza-
.tion is smaller in exp. 4A than in exp. 3A Table 3 .In summary, the results from the tracer and dye experiments, the fast
breakthrough of particles in the outflow, the high content of organic matter inthe colloids and the fact that the soil was covered to minimize direct particlerelease at the soil surface points to the macropores as the major source andconduits for the particles in these experiments. The maximum colloid concentra-tions reached in the initial peaks were approximately 10 ppm which can be
.compared to approximately 550 ppm found by Jacobsen et al. 1997 inshort-term small column studies of the same, but uncovered soil. Though other
factors such as ionic strength of the simulated rain 30 mSrm in Jacobsen et al.,.1997 , rain intensity, initial conditions, and column length may influence the
results the comparison suggests that the source of colloids may be important andneed further attention, especially when evaluating the possible effect of colloid-facilitated transport of surface-applied contaminants, e.g., pesticides.
Fig. 8. The flux of colloids normalized towards the estimated macropore wall area vs. outflow ratefor the experiments with decreasing and increasing irrigation rate.
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Changes in the size of the colloids during irrigation was examined in the last .experiment on column B exp. 2B . The colloid size distribution did not vary
.systematically with time. Most of the colloids measured 8588% were presentas particles with a size smaller than 0.2 mm and only 35% of the colloidsmeasured were larger than 0.5 mm. The lower limit of the measurements was0.1 mm, so the particles under 0.1 mm were not measured.
3.3. Decreasing and increasing irrigation rate experiments
The flow experiments with decreasing and increasing rate were carried outafter the constant rate irrigation rate experiments. During these late experiments,the colloid concentration generally was larger in the effluent from column Bthan from column A. In these experiments, the mass flux of colloids normalized
.towards macropore wall area r was evaluated in different sampling periods.collC Qcoll out
r s 11 .coll Aporewallwhere C is the concentration of colloids in the effluent, Q is the outflow incoll outthe sampling period and A is the total porewall area in the column. r isporewall coll
. .Fig. 9. Measured vs. calculated Eq. 8 concentration of total organic carbon TOC in the effluentfor the experiments with decreasing and increasing irrigation rate.
( )M. Lgdsmand et al.rGeoderma 93 1999 335954
relevant to evaluate as the process controlling the mobilization of colloids inthese late experiments probably is a diffusion process from the macropore wall.
. .White 1985 states that the area of contact between mobile in pores and .immobile water in matrix is important when evaluating a diffusion process. In
Fig. 8, the r just before the shift in rate, which is believed to be close tocollsteady state, is plotted against the effluent flow rate. There seems to be acommon relation for the two columns between the flow rate and the mobiliza-tion per area of macropore wall. This suggests that the area in contact with thewater has an influence on the mobilization of colloids and supports thatmacropores are a likely source for the colloids.
The C , C and C at steady state of the experiments with increasingcoll TOC TDSand decreasing flow rates were fitted individually to the simple model ofdiffusion and advection in a number of uniform, cylindrical, vertical tubesconducting Poiseuille flow with constant concentration at the macropore wall .Eq. 8 . The number and average radius of the macropores in the model weredetermined by the macropore volume and wall area estimated from the two
.monoliths, and a linear macropore flow velocity u in Eq. 7 was determinedzby the measured volumetric flow rate. The constant macropore wall concentra-
.tion C was used as a fitting parameter. The diffusion coefficient of colloidsA,s
. .Fig. 10. Measured vs. calculated Eq. 8 concentration of total dissolved solids TDS in theeffluent for the experiments with decreasing and increasing irrigation rate.
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D s4P10y12 m2rs was calculated from StokesEinstein relation using acoll,waterparticle diameter of 0.1 mm which was found to be dominating in the late
.experiments . For the diffusion of TOC, the diffusion coefficient was set toy10 2 .D s5P10 m rs Valsaraj et al., 1996 and for TDS the diffusionTOC,water
coefficient was set to D s10y9 m2rs. For the tubular pores, LTDS,water e .distance from the surface where the Poiseuille flow is fully developed is lessthan 1 mm.
. .The predicted effluent concentrations of TOC Fig. 9 and TDS Fig. 10 ,using the optimized C , fit the experimental data well, suggesting that theA,s
.concentration of both organic matter mostly DOC and ions at the macroporewall is constant and hence not influenced by the applied changes in flow rate.When the concentration of organic matter and ions are constant at the wall of themacropores in the late experiments when the soil have been exposed to extendedleaching, the stability of the colloid aggregates in the wall may be constant aswell. Hence, the concentration of the colloids at the macropore wall is expectedto be constant. The predicted values for the average effluent colloid concentra-
.tion are, however, poorly correlated with the experimental values Fig. 11 ,indicating that the concentration of colloids at the wall are changing with flowvelocity. In Fig. 12, the colloid concentration at the macropore wall, fitted to Eq.
.Fig. 11. Measured vs. calculated Eq. 8 concentration of mobilized colloids in the effluent for theexperiments with decreasing and increasing irrigation rate.
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.Fig. 12. Calculated Eq. 8 concentrations of mobilized colloids at the macropore wall for theexperiments with decreasing and increasing irrigation rate.
8 using the observed effluent concentration, is plotted vs. outflow rate. It is seenthat the colloid concentration at the wall is positively correlated with the flowrate. The increasing concentration at the macropore wall at high flow would beconsistent with increased hydrodynamic shear close to the walls at higher flowrates.
During irrigation experiments with artificial rainwater, the source of leachablecolloids in a macroporous loamy Alfisol is not limited for long duration rainevents. However, the leaching process is highly dynamic with varying dominantsources and processes being responsible for the colloid release. In the beginning,a pulse of organic matter-rich colloids dominates the mobilized colloids. Later,the ionic strength controls the mobilization. This pattern will generally apply tonatural, short-term rain events. When the soil monoliths have been exposed tomultiple, long-term rain events and the ionic strength of the effluent decreases,the diffusion in the matrix or across the interface between macropore wall andstreaming water in the macropores tends to control the mobilization process.After prolonged leaching where the concentration of both organic carbon and
( )M. Lgdsmand et al.rGeoderma 93 1999 3359 57
ions in the water near the macropore walls seems to be constant with varyingflow rates, the colloid mobilization increases with increasing flow rates. Thiscan be explained by increased shear stress on the macropore walls. The colloidsappear to be mobilized from the macropore walls. At first from a pool ofparticles with an organic carbon content exceeding that of the bulk soil and laterfrom an organic-depleted pool of colloids maybe from the matrix adjacent to themacropores. The results suggest that the colloids leached during normal rainevents in a macroporous soil have a relatively high organic carbon content. Thepotential for colloid-facilitated transport may thus be significant as manycontaminants have a strong affinity for organic matter.
This study has been financially supported in part by The Danish Interministe-rial Research Programme on Pesticides. We acknowledge John F. McCarthy,Oak Ridge National Laboratories, for reviewing the manuscript and ClaudeDegueldre, Paul Scherrer Institute, Villingen, Switzerland for the analysis ofparticle number and size distribution, and Leif Basberg, Institute of Hydrody-namics and Water Resources, Technical University of Denmark for editing thefigures. Finally, the Department of Soil Science at the Research Centre Foulumis acknowledged for its help in retrieving the soil monoliths.
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