Processes of colloid mobilization and transport in macroporous soil monoliths

Geoderma 93 Ž1999. 33–59

Processes of colloid mobilization and transport in macroporous soil monoliths M. Lægdsmand a

a,)

, K.G. Villholth b, M. Ullum a , K.H. Jensen

a

Institute of Hydrodynamics and Water Resources, Technical UniÕersity of Denmark, 2800 Lyngby, Denmark b VKI, Agern Alle´ 11, 2970 Hørsholm, Denmark

Received 21 September 1998; received in revised form 28 April 1999; accepted 29 April 1999

Abstract Transport of pesticides, PAH and other hydrophobic or surface-complexing contaminants in soils may be enhanced by colloid-facilitated transport. A prerequisite for colloid-facilitated transport is the release and transport of colloids. The mechanisms for colloid mobilization and transport in a macroporous Alfisol have been evaluated by measuring the amount and type of colloids leached in two large soil monoliths during long duration simulated rain events. The soil was irrigated with water having a chemical composition close to natural rainwater and at intensities as expected under natural conditions. The results showed that the colloids were primarily mobilized and transported in the macropores and that the source of colloids was not exhausted for extended rainfall duration. The first flush of water mobilized loosely bound colloids that had a high organic content relative to the bulk soil. After the initial release, the high ionic strength in the percolating water limited the mobilization. For prolonged leaching, the diffusion of colloids from the macropore walls appeared to rate-limit the mobilization process. During the late leaching 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

1. Introduction Colloids are defined as suspended particles with a small size. The maximum size 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 protected] 0016-7061r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 6 - 7 0 6 1 Ž 9 9 . 0 0 0 4 1 - 5

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below a few micrometers. The minimum size, separating colloids from dissolved matter, is about 10 nm ŽRyan and Elimelech, 1996. . Colloids may, due to their small size and depending on the physico-chemical conditions, remain suspended in electrolytic fluids. In addition, soil colloids are relatively reactive with respect to sorption of chemical species due to the large specific surface area and the high number of functional surface groups. Both the ability to be suspended and the sorption capacity make the colloids potential ‘carriers’ for pollutants in flowing water of rivers, oceans and soils. Soil colloids mainly consist of clay minerals, organic matter and oxidesrhydroxides. Colloid-facilitated transport in soils requires that three different processes take place: Ž 1. the pollutant must sorb to the colloid, Ž2. the colloid must be mobile Ž mobilized either before or after the sorption. and Ž3. the colloidrpollutant complex must be transported through the soil. Hence, pollutants that have a low solubility in water and a high partition coefficient between soil and water, e.g., certain pesticides, heavy metals and PAHs, may be transported at a rate beyond what is expected from partitioning only to the stationary soil matrix. A number of studies have dealt with the complex issue of colloid-facilitated transport. Vinten et al. Ž 1983. Žsoils; clay; DDTr paraquat. showed that the transport of pesticide was dependent on the ionic strength of the infiltrating water and the texture of the soil. A few reports deal with experimental evidence of colloid-facilitated transport of chemical species in undisturbed soil media. De Jonge et al. Ž 1997. Ž structured soil; natural colloids; prochloraz. observed that about 20% of the leached pesticide was sorbed to mobilized particles ) 0.02 mm. Seta and Karathanasis Ž1996. Žstructured soil; dispersed colloids; metolachlor. found that the presence of colloids enhanced the transport of pesticide by 22 to 70% depending on the colloid type and mobility. The mobilization of colloids in soils and groundwater sediments due to changes in flow or chemistry has been reported in several studies. Chemical perturbation can affect the forces that keep the colloids bound to the grains and thereby the mobilization of colloids Ž Ryan and Gschwend, 1994a Ž quartz sand; haematite.; Seaman et al., 1995 Ž aquifer sand; natural colloids. ; Kaplan et al., 1996 Ž reconstructed soil; natural colloids.. . Diffusion from the detachment site into the flowing water may be the limiting factor for the rate of mobilization when the chemical conditions are favourable for the release of colloids Ž Ryan and Gschwend, 1994a Žquartz sand; haematite. ; Jacobsen et al., 1997 Ž structured soil; natural colloids.. . Dissolution of cementing agents may enhance the mobilization of colloids from grains. Ryan and Gschwend Ž1994b. Ž goethitecoated aquifer sand; kaolinite. found that the mobilization of kaolinite colloids was increased when the goethite was dissolved due to changing redox conditions. Increasing the flow rate can cause enhanced mobilization Ž Kaplan et al., 1993 Ž reconstructed soil; natural colloids. ; Ryan and Gschwend, 1994a Ž quartz sand; haematite.; Govindaraju et al., 1995 Ž sand; kaolinite.. . Kaplan et al. Ž1993. found that the colloid concentration in the effluent from lysimeters

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depends on the flow velocity squared, suggesting mobilization by shear stress. Pilgrim et al. Ž1978. reported that subsurface flows during storm events mobilized large amounts of particles in the size range 4–8 mm. This was explained by raindrop impact on the surface soil combined with macroporous flow paths. Generally, the mobilization of colloids in homogenous sand is described in many studies, but the mobilization of colloids in naturally structured soils is not well described. Several studies have shown that colloidal particles applied externally can be transported in subsurface environments. Jacobsen et al. Ž 1997. Žundisturbed soil; illite and humus-coated illite. showed that the mass recovery of surface-applied colloids in leachate from subsoil columns was higher than from topsoil columns and that the recovery increased with increasing flow rate. McKay et al. Ž 1993. Žfractured clay till; bacteriophages. found a hundred-fold greater retardation of conservative tracers compared to colloid tracers in a field experiment and attributed it to the preferential diffusion of solutes into the matrix. Toran and Palumbo Ž1991. found that the retardation of colloids in packed sand columns decreased when artificial macropores oriented in the flow direction was embedded in the medium and that macropores with larger diameter created multiple peak breakthrough curves. Kretzschmar et al. Ž 1995. observed that the leaching of clay colloids passing through an intact saprolite was dependent on the natural coating of the colloids with natural organic matter ŽNOM.. The untreated colloids Ž with NOM. resulted in blocking effects for further deposition due to a monolayer restriction for the continued attachment of colloids, while the treated colloids Ž without NOM. resulted in ripening due to multiple layer attachment. In the present study, the effect of macropores and low ionic strength infiltration water Žcorresponding to natural rain. on colloid mobilization and transport was investigated. The combination of continuous and hydraulically active macropores and infiltration water with a low ionic strength can lead to an accelerated removal of ions from the macropores and the surrounding matrix, resulting in destabilization of the aggregates at the macropore walls and thereby increased mobilization and transport of colloids. Two undisturbed soil monoliths excavated from a site in western Denmark and installed in the laboratory were exposed to long duration rain events to investigate the processes of colloid mobilization under temporally changing chemical conditions and under varying flow rates. 2. Materials and methods 2.1. Theoretical background 2.1.1. Chemical perturbations The chemistry of the pore water will affect the interacting forces acting on the colloids: electrostatic forces, van der Waals–London forces and Born repulsion.

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The electrostatic forces are particularly sensitive to changes in the electrolytic properties of the fluid. The electrostatic potential around a single spherical charged particle in an electrolytic fluid can be described by ŽKruyt, 1952. :

ce s ce,0 eyk x 1

k

s

Ž at

)

Ž1.

´ RT

1

3.09 s

2

2 Ž Na qe . 1000 I

'I

T s 258C and ´ s 6.95 = 10y10 C 2rJ m .

Ž2.

where ce,0 is the electrical potential at the surface of the colloid, ce the electrical potential at the distance x from the colloid surface, 1rk the Debye length, which is often interpreted as the thickness of the double layer of the particle, ´ the permittivity of the fluid, R the universal gas constant, T the absolute temperature of the fluid, Na Avogadro’s number, qe the charge on the electron, and I the ionic strength of the solution. When the release of colloids from the grains is controlled by the electrolytic properties of the fluid, the rate constant for detachment and attachment of colloids is proportional to an exponential function of the size of the energy barrier to detachment or attachment Ž Ruckenstein and Prieve, 1976. :

ž ž

k det A exp y k att A exp y

N fmax y fmin ,1 N kT N fmax N kT

/

/

Ž3. Ž4.

where k det is the rate constant for detachment and k att for attachment. fmax is the maximum of the potential energy between two colloids and fmin,1 is the primary minimum. This will cause the release of colloids across the energy barrier to be a first-order reversible heterogeneous reaction with the energy barriers serving as activation energy for the process. When the ionic strength is decreased, fmin,1 is increased ŽRuckenstein and Prieve, 1976. and hence the rate of detachment is increased. If the process of colloid mobilization is controlled by chemical perturbations, there are two steps involved in the process. A process of detachment of colloids from the grains followed by a diffusion process from the grain surface into the pore stream. If the rate of detachment is higher than the rate of diffusion, e.g., at low ionic strength, the diffusion process will limit the overall mobilization, and vice versa. 2.1.2. Diffusion For 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: Ž1. diffusion within the matrix and Ž 2. diffusion from the walls of the macropores and into the main macropore stream. The matrix can be viewed as 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 is C0 and the concentration at the macropore walls is zero, the accumulated amount of the diffusing species Ž Mt . that is transported through the plane Ž at x s 0. per area at time t is ŽCrank, 1975, p. 32.: Mt s 2C0

(

DAB t

p

Ž5.

where DAB is the diffusion coefficient of substance A in medium B. The concentration in the macropore water is not zero due to upstream inflow of the substance, but compared to the much higher concentration in the matrix, here considered the sole source, it can generally be neglected. For the initial stages of a diffusion process out of a plane sheet of thickness l with uniform initial concentration in the sheet and constant zero surface concentration, a similar equation can be obtained for the accumulated loss of diffusing substance Mt out of the sheet ŽCrank, 1975, p. 244. :

(

Mt s 4 M` l

DAB t

p

Ž6.

where M` is the accumulated loss of diffusing substance out of the sheet for t approaching infinity. This equation has been applied to indicate the diffusioncontrolled process of non-equilibrium sorption or desorption of various sorbates in soil in well-mixed batch or packed flow systems Ž 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. Ž 1997. observed linearity of cumulative mass of colloids leached from undisturbed soil columns vs. square root of time. Consequently, when a process is controlled by diffusion Žeither in the matrix or from the walls of the macropores., a plot of cumulative mass of diffusing substance vs. square root of time will produce a straight line Žat least in the initial stages of the diffusion process. . The linear relation of cumulative mass vs. square root of time does not prove that diffusion controls the process, but if diffusion controls the process the relation will be linear. For a more stringent evaluation of substance mobility that includes radial diffusion from the macropore wall as well as the vertical advective transport in the macropores, the soil is equivalated to a model of an impermeable matrix with equally sized, vertical cylindrical macropores. The macropores are considered to be full-flowing and the flow is assumed to be equivalent to laminar Poiseuille flow throughout the tubes. The validity of this

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assumption requires that Le Žthe depth from the soil surface where the Poiseuille flow is fully developed. is small. For steady-state conditions, the advection–diffusion equation can then be simplified to ŽClark, 1996. :

uz

ECA Ez

s DAB

ECA E r 1 Er r Er

ž /

Ž7.

where z is the depth from the soil surface, r is the distance from the center of the tube, u z is the advective velocity in the macropores and CA is the concentration of diffusing substance A as a function of r and z. If the inlet concentration at the soil surface is zero and the concentration at the macropore surface CA,s is constant the following solution is obtained ŽClark, 1996. : CA ,ave CA ,s

s 1 y 8Ý

Gn

l

2 n

exp yl2n

ž

zra Re Sc

/

Ž8.

where CA,ave is the averaged concentration of substance A over the tube cross section at depth z, a is the radius of the tube, Re is Reynolds number, Sc is Schmidt’s number, Gn is an infinite series of constants and l n are the eigenvalues Ž Gn and l n are given by Clark Ž1996... This model does not adequately describe the diffusion of species in a macropore as the flow in the actual macropores probably does not fulfill the assumption of full-flowing macropores at all times, but for evaluation of the trends it is useful. When Eq. 7 and Eq. 8 apply, the outflow concentration Ž CA,ave . will be negatively correlated to the flow velocity Ž u z . as increased dilution of the diffusing substance occurs with increased flow. Hence, when evaluating the possible effect of flow rate on particle mobilization, an assessment of the concentration at the macropore wall Žthrough Eq. 8., rather than in the effluent, as a function of flow rate is appropriate. 2.1.3. Physical perturbations When evaluating the potential for mobilization by shear stress, a torque balance between the adhesive forces Ž a combination of double layer interactions and Van der Waals force., the lift force Ž due to turbulence near the wall. and the drag force Ždue to hydrodynamic stress. is to be considered. Increasing particle size, flow velocity and ionic strength promote the mobilization of particles by shear stress ŽSharma et al., 1991. . 2.2. Field site and monolith sampling

˚ Undisturbed soil samples were taken at the Røgen field station, near Arhus, Denmark Žlatitude 568.. The soil type is an Alfisol ŽTypic Hapludalf. and is

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developed from moraine deposits from the Weichselian ice period. In the monoliths, the A horizon Žapproximately the upper 0.25 m of the column. and part of the B horizon Žapproximately the lower 0.1 m. were present. In Table 1, the properties of the soil are listed for the two horizons. Two monoliths Ž column A and B. were collected in the following way: Grass on the surface was cut short and a stainless steel cylinder Žinner diameter 290 mm, height 400 mm, thickness 2 mm. with sharpened edges was pushed into the ground by a hydraulic press to a depth of 0.35 m and excavated by shovel. The columns were collected only a few meters apart. The soil-containing cylinders were then sealed with plastic covers on both ends of the steel cylinders and with adhesive tape on the holes for insertion of instruments and was stored at 58C. Due to this procedure, it was assumed that the soil samples had the same water content as in the 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 Ž 2 = 2 mm grid. that was placed on a cylindrical container Ždiameter 300 mm and height 10 mm. holding glass beads Ž 2 mm diameter. . In order to average any local heterogeneity across the column cross-section, the outflow was collected simultaneously through four outlets on the perimeter of the container holding the glass beads. The outflow solution was withdrawn with a peristaltic pump at a rate exceeding the rate of outflow from the monoliths, ensuring instantaneous sampling and minimizing accumulation within the chamber with glass beads Ž Fig. 1.. To prevent air entry into the on-line flow cell of the turbidimeter the water table in

Table 1 Physical and chemical properties of A and B horizons of the Alfisol profile

clay - 2 mm Žwrw%. silt 2–20 mm Žwrw%. coarse silt 20–63 mm Žwrw%. sand 63–125 mm Žwrw%. sand 125–200 mm Žwrw%. sand 200–500 mm Žwrw%. sand ) 500 mm Žwrw%. humus Žwrw%. TOC Žwrw%. CEC Žmmolrkg. pH Ž0.01 M CaCl 2 .

A Ž0–0.25m depth.

B Ž0.25–0.35m depth.

16.0 15.3 12.2 17.2 13.6 17.1 6.3 2.6 1.5 156.6 6.17

21.1 13.3 11.7 15.6 14 17.2 6.7 0.6 0.3 112.5 5.89

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Fig. 1. Sketch of the experimental setup.

the constant-level cell upstream Ž Fig. 1. was kept 0.3 m above the inflow of the turbidimeter by a magnetic valve controlled by a photoelectrical sensor in the constant level cell. A layer Žapproximately 0.01 m on average. of acid-washed sand Ž 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 drop impact. The drainage from the monoliths was gravitational. The irrigation water was applied from a rain simulator with 69 hypodermic needles Ž 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 ŽTable 2. were added to deionized water ŽDW. and the solution was aerated to ensure equilibrium with CO 2 of the atmosphere. During this process, the pH was adjusted to 4.6 with 0.1 M HCl. The ionic strength was approximately 0.3 mM and the electrical conductivity ŽEC. was about 3 mSrm. The artificial rainwater was led to the rain simulator by a peristaltic pump. Three different rain intensities were used: low Ž1.6 mmrh., medium Ž3.2 mmrh. and high Ž6.5 mmrh.. The experiments were conducted at room temperature Ž about 208C.. Soil water suction was measured at four depths using tensiometers connected to transduc-

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Table 2 Chemical composition of the artificial rainwater Salts

Concentration ŽmM.

Concentration Žgrm3 .

NaNO 3 NaCl KCl CaCl 2 2H 2 O MgSO47H 2 O ŽNH 4 . 2 SO4

0.048 0.056 0.0047 0.0112 0.0121 0.030

4.07 3.24 0.35 1.65 2.98 3.41

ers, and water content was measured at three depths using TDR-probes Ž 70 mm rods with 5 mm spacing.. A sequence of two different flow experiments were carried out: Ž1. application of constant high rain intensity and Ž 2. application of a sequence of decreasing and subsequently increasing rain intensities. Each experiment consisted 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 3–4 days to allow drainage and restitution of the soil. The experiments with decreasing and increasing rain intensities were performed with high–medium– low intensity for one day each and then low–medium–high for one day each. The experiment with constant high rain intensity was repeated four times on column A Ždenoted exp. 1A to 4A. and twice on column B Žexps. 1B and 2B.. The amount of effluent leached from the columns before the start of the different constant high flow experiments is shown in Table 3. Note that exps. 4A and 2B were exposed to approximately the same amount of leaching before the experiment was initiated. Two tracer experiments with 25 kgrm3 chloride were performed on each column after the irrigation experiments: one at low and one

Table 3 Accumulated leached effluent at the start of the experiments and accumulated mass of colloids leached after 10 and 100 mm of outflow for the experiments with constant high irrigation rate Experiment no.

Cumulative outflow Žmm.

Cumulative mass of colloids after 10 mm Žmg.

Cumulative mass of colloids after 100 mm Žmg.

1A 2A 3A 4A 1B 2B

0 229 792 1367 0 1198

2 8 25 20 5 30

40 140 250 240 100 220

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at medium flow rate. Before each tracer experiment, the flow in the soil monolith was at steady state. Then the rain was intermittently stopped and 100 ml KCl solution was added by pipette at the surface during 15 min in the medium flow experiment and during 30 min in the low flow experiment, whereupon the rain application was resumed. After the flow and tracer experiments, a 500 ppm Rhodamine B solution was applied to the columns. The dye solution was applied through the rain simulator with medium irrigation rate for 12 h. Upon completion of the dye experiment, 0.02 m horizontal slices were incrementally cut off the lower end of the column and the visibly dyed macropores Ž) 1 mm. on the cleared soil surface were registered with respect to number and size Ž radius of roughly circular pore cross-sections. . The visibly dyed pores represent the active macropores. The total cross-sectional area of active pores in each layer was calculated from the radius and number of dyed pores and assumed to represent the active macroporosity in the layer. During the experiments pH, electrical conductivity Ž EC. , mass concentration of colloids Ž Ccoll ., turbidity, and total and dissolved organic carbon Ž C TOC and C DOC . were measured in the effluent. Total accumulated outflow was registered by weighing of incremental samples. In one experiment, the colloid-size distribution was measured. The samples for measuring pH, EC and Ccoll and flow rate were taken manually from an effluent container when the volume exceeded 250 ml, while the samples for measurements of colloid size and number and TOC were taken with a 20-ml syringe directly from the flow line Ž Fig. 1.. The samples for colloid-size analyses were diluted Ž1:100 with DW. immediately after sampling to prevent flocculation of the colloids and the TOC samples were conserved with 12.5 mlrl 10% HNO 3; both sample types were stored at 58C until analyses. Turbidity was measured on-line using a Hach turbidimeter 2100 equipped with a flow cell with an approximate volume of 28 ml. Turbidity, suction and water content was recorded every 5 min using a computerized logging system. The measurements of Ccoll were made by filtering about 250 ml sample on an Advantec Toyo glass filter with a pore size of 1 mm. The filter was dried at 1058C for 1 h prior to the filtration and for 2 h after and the weight increase was noted. The samples were stored at least 1 week at 58C before the filtration to ensure good flocculation of the colloids and to obtain a more accurate determination of the mass concentration of the colloids. In the first four experiments on column A, Ccoll was not measured with this method and instead a linear relation between turbidity and Ccoll was developed and used to estimate Ccoll : Ccoll s 0.60

ppmrNTUP turbidity

Ž9.

This relation was estimated from turbidity measurements on suspensions of naturally occurring colloids from the experimental site using a fraction of the soil containing only colloids - 5 mm. The coefficient of determination Ž R 2 . was 0.99 for this particular fraction. A similar calibration method was used by

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Jacobsen et al. Ž1997. but for measurements of light extinction. Even though this method may underestimate the total mass of colloids, if the colloids are substantially larger than 5 mm and overestimate it when the colloids are substantially smaller than 5 mm, this uncertainty was only found to affect the results in the initial and final stages of the leaching of colloids. For a comparison of the measured and estimated values in column B, see Fig. 6. From the EC of the effluent, the amount of total dissolved solids Ž TDS. can be estimated using a linear relation given by Tchobanoglous and Schroeder Ž1985.: TDS s k EC

Ž 10.

where k ranges from 5.5–7.0 ppm P mrmS. In this soil, a k of 7.0 was found most dequate Ž R 2 s 0.95. from a regression of values obtained by Eq. 10 Ž using EC 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 Ž) 1 mm. effluent.. The analysis of TOC was performed on an O.I. Analytical 700 TOC-analyzer with an auto sampler. The standard deviation associated with the analysis was 0.1–2.0%. On samples from exps. 1B and 2B, DOC was measured as well. Here, the samples were filtered through an acid-washed Sartorius Minisart disposable filtering unit with a pore size of 0.2 mm and the filtrate was analyzed for TOC. From the values of TOC and DOC, the particulate organic carbon POC Ž C POC s C TOC y C DOC . and thereby the average f oc of the colloids ) 0.2 mm Ž f oc s C POCrCcoll . was calculated. Particle-size distribution and concentration by number were measured on effluent samples from exp. 2B using a single-particle counter Ž SPC. Ž Degueldre et al., 1996. . Each measurement consisted of three runs per sample where particle size classes ) 100 nm, ) 200 nm 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 two columns differ with respect to the macropore system. The breakthrough in column A was more rapid than in column B and the tailing was more distinct in column B. The peak of chloride is achieved after 0.03–0.06 pore volumes in column A, and after 0.10–0.18 pore volumes in column B. The dye experiments showed that the water flow took place primarily in macropores. The active macropores were mainly wormholes Ž diameter: 2–8 mm. and root channels Ždiameter: 1–2 mm. . There were no visible signs of

44

M. Lægdsmand et al.r Geoderma 93 (1999) 33–59

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 Ž 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 in column A Ž 1.51 m2rm3 vs. 1.24 m2rm3 .. 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 Ž 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. Ž 1994.. The retention time in column A was relatively small due to a smaller active macroporosity thus leading to higher flow velocities. In contrast, a larger contact area between macropores and matrix was present in column B leading to a higher potential for diffusive exchange between the two domains. 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 Ž ; 0.27. than that of all the other experiments Ž; 0.30.. Therefore, the first flow experiment on both columns represents the response behavior of soil when a rain event follows a dry period. Fig. 3 shows the turbidity, outflow rate and EC at the beginning of the

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45

first experiment on each column. Initial peaks in the turbidity are observed on both columns. Similar early-time leaching of particles with flow events have been observed in smaller-scale investigations Ž Kaplan et al., 1993; Jacobsen et al. 1997 Žwith soil from the same site as the present experiments.. and in field experiments ŽBottcher et al., 1981; Grant et al., 1996; Ryan et al., 1998; Villholth et al., 1999 Ž at the same site where the samples for the present experiments was collected... This increased mobilization by the first flow of water can be attributed to local creation of shear stress by the first flow of water ŽRyan et al., 1998.; a preferential sorption of colloids to the airrwater interface on an advancing water film in the macropores; a lower cohesive bonding of colloids within the initially dry soil ŽBottcher et al., 1981. ; or to a higher content of organic matter in the colloids mobilized in the beginning of the experiments and thereby higher stability of colloids in suspension Žsee later. . The turbidity in the initial peak decreases before the flow has reached its maximum value Ž Fig. 3.. This may be interpreted as the flow rate is not the major controlling factor in the initial mobilization Ž Ryan et al., 1998. . However, in the initial phase of unsteady overall flow conditions, flow rate out of the column is not a good measure of the internal flow rate, which would be the parameter of interest, as a continuous loss of water down through the column is taking place due to wetting of the initially dry soil. After the initial peak, the turbidity and colloid concentration increase as the EC of the effluent decreases ŽFig. 3. . The initial peak of EC which reaches 110 to 120 mSrm, well above the EC of the influent Ž3.0 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 ŽFigs. 3 and 4. suggests that the leaching of salts is diffusionlimited. The turbidity in subsequent experiments fluctuate with multiple peaks Žnot shown. but with no distinct initial peaks as in the first experiment. This suggests that the particle mobilization mechanisms are highly sensitive to the initial conditions of the soil medium. The amount of colloids washed out after the first 10 and 100 mm of effluent for the various constant irrigation rate experiments is shown in Table 3. It is evident that the continued rain makes it easier to mobilize the colloids. Furthermore, the pool of detachable colloids in the columns apparently is not limited under the given experimental conditions, which is probably due to the low ionic strength of the artificial rainwater that gives rise to dispersion of the aggregates in 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 ŽFig. 4.. When the EC is above 18.0 mSrm Žand 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 ŽFig. 5.. This could indicate that the stability of the colloids are dependent on the thickness of the double layer Ž1rk . which is proportional to the reciprocal of the square root of the ionic strength ŽEq. 2. and hence that the rate of detachment is the

46 M. Lægdsmand et al.r Geoderma 93 (1999) 33–59 Fig. 3. Outflow rate, turbidity and electrical conductivity in the effluent during the initial stage of the first experiments with constant high irrigation rate, Ža. Column A, Žb. Column B. The dotted line denotes outflow rate, the solid line denotes turbidity and the crosses denote electrical conductivity.

M. Lægdsmand et al.r Geoderma 93 (1999) 33–59 Fig. 4. Colloid concentration vs. electrical conductivity in the effluent for the experiments with constant high irrigation rate, Ža. Column A, Žb. Column B.

47

48

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Fig. 5. Colloid concentration vs. the reciprocal of the square root of EC in the effluent of the two columns for the experiments with constant high irrigation rate. Only data for which the EC is above 18.0 mSrm and data after the initial flush of colloids are included. The linear regression lines 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 flow patterns and macropore wall areas in the two columns. Both the colloid concentration and the EC measured in the effluent are influenced by the diffusion and transport processes in the soil Žsee Eq. 8.. Below an EC in the effluent of approximately 18.0 mSrm which occurs after extended leaching Žapproximately 414 mm on column A and 312 mm on column B. other factors than the ionic strength seem to control the mobilization. Fig. 6 shows the cumulative mass of mobilized colloids vs. the square root of time from the time of first breakthrough of water for the six different experiments with constant high flow rates. Experiments 4A and 2B show a linear relationship of cumulative mass and square root of time indicating, but not proving, that the mobilization of colloids after prolonged leaching is controlled by 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 the end of these experiments the diffusion could be the rate-limiting step. In Table 4, the coefficient of determination Ž R 2-value. for the linear regression of the cumulative mass of colloids vs. square root of time in periods where EC ) 13.5

M. Lægdsmand et al.r Geoderma 93 (1999) 33–59 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, Ža. Column A, Žb. Column B. Results for measured Žm. and estimated Že. colloid mass are indicated.

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Table 4 Coefficients of determination Ž R 2 . 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 high irrigation rate experimentsa Experiment no.

Colloids ŽEC )13.5 mSrm.

Colloids ŽEC -13.5 mSrm.

TOC

TDS

1A 2A 3A 4A 1B 2B

Ž0.83. b Ž0.92. – – 0.88 Ž0.85. –

– Ž1.00. Ž0.99. Ž1.00. – 0.98 Ž0.98.

0.79 0.81 0.80 0.85 0.89 0.90

0.99 1.00 0.96 0.98 0.95 0.98

a

The regression analyses was only performed if more than seven data points were available. Values in brackets denote that calculated, rather than measured, values of colloid concentration were used to determine the cumulative mass of colloids. b

mSrm and EC - 13.5 mSrm is shown. When the EC is above 13.5 mSrm in the effluent the fit is poor but when it is below 13.5 mSrm the R 2-value is close to unity, suggesting that the diffusion controls the mobilization at EC - 13.5 mSrm. The ionic strength at the detachment sites corresponding to 13.5 mSrm in the effluent could be the critical value that makes the energy barrier against detachment ŽN fmax y fmin,1 N in Eq. 3. equal zero and hence making the diffusion the rate-controlling process, as proposed by Ryan and Gschwend Ž1994a.. It should be noted that the use of the effluent EC as an indicator for the various mechanisms controlling the colloid mobilization, as apparent from the present results, most likely will not be generally operational because a host of factors will influence the overall colloid and ion leaching in various soil systems, e.g., macropore structure, parent soil material, rainwater composition, biological activity etc. Hence, the approach of using EC basically illustrates the overall processes taking place and the dynamics involved in the collective process 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 Ž 83–99%. . In the first experiments, the concentration of TOC stabilizes at about 10–12 ppm and in the last experiments it stabilizes at about 4 ppm. In Table 4, the coefficients of determination of linear regression analyses of cumulative mass of leached organic carbon vs. the square root of time are shown. There is a relatively poor linear correlation in the experiments, which means that apparently diffusion is not the limiting step in the mobilization of organic matter. Rather, the release of TOC could be controlled by a dissolution or a microbiological generation process ŽGuggenberger and Zech, 1993.. A linear regression analyses of the cumulative mass of TDS Ž estimated from Eq. 10. vs. square root of time from breakthrough of water showed that the

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coefficients of determination Ž R 2-values. ŽTable 4. were very close to unity. This supports that the diffusion of ions through the matrix out into the macropore stream, after an initial period where the relation is not linear, is the limiting step in the build-up of the ionic strength in the effluent. In Fig. 7, f oc of the colloids is plotted against the accumulated amount of effluent 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 Ž12%. but decreases until it, after a cumulative amount of effluent of 1200 mm Žin the beginning of exp. 2B. , reaches the value of f oc in the bulk soil Ž 0.3–1.5%.. Kaplan et al. Ž 1993. report that the f oc of the mobilized colloids in different reconstructed but unstructured soil profiles of a loamy Paleudult was higher than or equal to the f oc of the colloids in the bulk soil, but more or less constant Ž1.16 " 0.25%. throughout a water application Ž102 mm, 51 mmrh.. As opposed to the findings of Kaplan et al. Ž 1993. , the decreasing content of organic carbon with water application in this study indicates that a depletion of organic carbon-enriched colloids took place. This could be explained by preferential mobilization of the colloids from the macropores. Worm linings are reported to have a higher content of total organic carbon Ž 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. 1B and 2B vs. the cumulative outflow from column B, and the mass fraction of organic carbon of the soil 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.

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with depth. compared to the bulk soil in a Fragiaqualf A-horizon Ž Stehouwer et al., 1994. . Also, organic matter content or coatings on colloids generally tend to stabilize colloids Ž Heil and Sposito, 1995; Kretzschmar et al., 1995. . After prolonged continuous leaching in the macropores, the organic-rich and loosely bound colloids are depleted. This might explain why the accumulated mobilization is smaller in exp. 4A than in exp. 3A Ž 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 in the colloids and the fact that the soil was covered to minimize direct particle release at the soil surface points to the macropores as the major source and conduits for the particles in these experiments. The maximum colloid concentrations reached in the initial peaks were approximately 10 ppm which can be compared to approximately 550 ppm found by Jacobsen et al. Ž 1997. in short-term small column studies of the same, but uncovered soil. Though other factors such as ionic strength of the simulated rain Ž 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 and need further attention, especially when evaluating the possible effect of colloidfacilitated transport of surface-applied contaminants, e.g., pesticides.

Fig. 8. The flux of colloids normalized towards the estimated macropore wall area vs. outflow rate for 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 Žexp. 2B.. The colloid size distribution did not vary systematically with time. Most of the colloids measured Ž 85–88%. were present as particles with a size smaller than 0.2 mm and only 3–5% of the colloids measured were larger than 0.5 mm. The lower limit of the measurements was 0.1 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 out after the constant rate irrigation rate experiments. During these late experiments, the colloid concentration generally was larger in the effluent from column B than from column A. In these experiments, the mass flux of colloids normalized towards macropore wall area Ž rcoll . was evaluated in different sampling periods. rcoll s

Ccoll Q out A porewall

Ž 11.

where Ccoll is the concentration of colloids in the effluent, Qout is the outflow in the sampling period and A porewall is the total porewall area in the column. rcoll is

Fig. 9. Measured vs. calculated ŽEq. 8. concentration of total organic carbon ŽTOC. in the effluent for the experiments with decreasing and increasing irrigation rate.

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relevant to evaluate as the process controlling the mobilization of colloids in these late experiments probably is a diffusion process from the macropore wall. White Ž 1985. states that the area of contact between mobile Ž in pores. and immobile water Ž in matrix. is important when evaluating a diffusion process. In Fig. 8, the rcoll just before the shift in rate, which is believed to be close to steady state, is plotted against the effluent flow rate. There seems to be a common relation for the two columns between the flow rate and the mobilization per area of macropore wall. This suggests that the area in contact with the water has an influence on the mobilization of colloids and supports that macropores are a likely source for the colloids. The Ccoll , C TOC and C TDS at steady state of the experiments with increasing and decreasing flow rates were fitted individually to the simple model of diffusion and advection in a number of uniform, cylindrical, vertical tubes conducting Poiseuille flow with constant concentration at the macropore wall ŽEq. 8. . The number and average radius of the macropores in the model were determined by the macropore volume and wall area estimated from the two monoliths, and a linear macropore flow velocity Ž u z in Eq. 7. was determined by the measured volumetric flow rate. The constant macropore wall concentration Ž CA,s . was used as a fitting parameter. The diffusion coefficient of colloids

Fig. 10. Measured vs. calculated ŽEq. 8. concentration of total dissolved solids ŽTDS. in the effluent for the experiments with decreasing and increasing irrigation rate.

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Dcoll,water s 4 P 10y12 m2rs was calculated from Stokes–Einstein relation using a particle diameter of 0.1 mm Žwhich was found to be dominating in the late experiments.. For the diffusion of TOC, the diffusion coefficient was set to D TOC,water s 5 P 10y10 m2rs ŽValsaraj et al., 1996. and for TDS the diffusion coefficient was set to D TDS,water s 10y9 m 2rs. For the tubular pores, Le Ždistance from the surface where the Poiseuille flow is fully developed. is less than 1 mm. The predicted effluent concentrations of TOC ŽFig. 9. and TDS Ž Fig. 10., using the optimized CA,s , fit the experimental data well, suggesting that the concentration of both organic matter Ž mostly DOC. and ions at the macropore wall 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 the macropores in the late experiments when the soil have been exposed to extended leaching, the stability of the colloid aggregates in the wall may be constant as well. Hence, the concentration of the colloids at the macropore wall is expected to be constant. The predicted values for the average effluent colloid concentration are, however, poorly correlated with the experimental values Ž Fig. 11. , indicating that the concentration of colloids at the wall are changing with flow velocity. In Fig. 12, the colloid concentration at the macropore wall, fitted to Eq.

Fig. 11. Measured vs. calculated ŽEq. 8. concentration of mobilized colloids in the effluent for the experiments with decreasing and increasing irrigation rate.

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Fig. 12. Calculated ŽEq. 8. concentrations of mobilized colloids at the macropore wall for the experiments with decreasing and increasing irrigation rate.

8 using the observed effluent concentration, is plotted vs. outflow rate. It is seen that the colloid concentration at the wall is positively correlated with the flow rate. The increasing concentration at the macropore wall at high flow would be consistent with increased hydrodynamic shear close to the walls at higher flow rates.

4. Conclusions During irrigation experiments with artificial rainwater, the source of leachable colloids in a macroporous loamy Alfisol is not limited for long duration rain events. However, the leaching process is highly dynamic with varying dominant sources 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 to natural, short-term rain events. When the soil monoliths have been exposed to multiple, long-term rain events and the ionic strength of the effluent decreases, the diffusion in the matrix or across the interface between macropore wall and streaming water in the macropores tends to control the mobilization process. After prolonged leaching where the concentration of both organic carbon and

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ions in the water near the macropore walls seems to be constant with varying flow rates, the colloid mobilization increases with increasing flow rates. This can be explained by increased shear stress on the macropore walls. The colloids appear to be mobilized from the macropore walls. At first from a pool of particles with an organic carbon content exceeding that of the bulk soil and later from an organic-depleted pool of colloids maybe from the matrix adjacent to the macropores. The results suggest that the colloids leached during normal rain events in a macroporous soil have a relatively high organic carbon content. The potential for colloid-facilitated transport may thus be significant as many contaminants have a strong affinity for organic matter.

Acknowledgements This study has been financially supported in part by The Danish Interministerial Research Programme on Pesticides. We acknowledge John F. McCarthy, Oak Ridge National Laboratories, for reviewing the manuscript and Claude Degueldre, Paul Scherrer Institute, Villingen, Switzerland for the analysis of particle number and size distribution, and Leif Basberg, Institute of Hydrodynamics and Water Resources, Technical University of Denmark for editing the figures. Finally, the Department of Soil Science at the Research Centre Foulum is acknowledged for its help in retrieving the soil monoliths.

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