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SPECIAL SECTION: COLLOIDS AND COLLOID-FACILITATED TRANSPORT OF CONTAMINANTS IN SOILS

Colloid Mobilization and Transport in Undisturbed Soil Columns. I. Pore Structure Characterization and Tritium Transport

^a Environmental Engineering Section, Dep. of Life Sciences, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^b Dep. of Agroecology, Danish Institute of Agricultural Sciences, P.O. Box 50, DK-8830 Tjele, Denmark
^c Currently Danish Institute of Agricultural Sciences, Department of Agroecology, P.O. Box 50, DK-8830 Tjele, Denmark
^* Corresponding author (C.Kjaergaard{at}agrsci.dk).

Received 3 July 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
While it is recognized that preferential flow may increase the transport of colloids, less is known about the actual influence of preferential flow on colloid mobilization in situ. Changes in pore structure upon soil exposure to drying and rewetting may additionally affect colloid mobilization. Information about the pore structure and the active flow volume, as well as the changes in these properties, are therefore important when investigating colloid mobilization. We investigate the pore structure characteristics and the transport of tritium (³H₂O) during steady unsaturated flow conditions. A total of 54 soil columns sampled along a natural clay gradient representing six clay contents (12, 18, 24, 28, 37, and 43% clay) were equilibrated to three different initial matric potentials (IMP), = –2.5, –100, and –15500 hPa. Pore structure characteristics were deduced from water retention characteristics and measurements of air-filled porosity and air permeability. Tracer experiments were conducted at 1 mm h^–1 and with a suction of 5 hPa. A mobile–immobile region model (MIM) and a three-region model (2MIM) with two mobile and one immobile region were used for describing the breakthrough curves (BTCs). The 2MIM model was able to fit the data well and predicted the existence of two mobile flow regions, most pronounced at higher clay content. The 12% clay soil exhibited matrix-dominated flow behavior, which is probably attributable to a large fraction of drained pores disconnecting the rapidly conducting flow system. Soils with 18% clay exhibited asymmetrical BTCs with early breakthrough and tailing and an increasing amount of immobile water, indicating preferential flow. Drying and rewetting, because of associated changes in the pore structure, significantly reduced the degree of preferential flow.

Abbreviations: BTC, breakthrough curve • CDE, convection–dispersion equation • EC, electrical conductivity • IMP, initial matric potential • MIM, mobile–immobile model • 2MIM, two-mobile–immobile model • SAR, sodium adsorption ratio

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
BOTH THE IN SITU mobilization and the subsequent transport of mobilized colloids depend on the pore size, geometry, and continuity of the actively conducting pore system. Investigations have shown that structural macropores may provide preferential flow pathways for water flow and suspended colloids (Smith et al., 1985; Camobreco et al., 1996). The flow velocities within these preferential flow paths can be orders of magnitude higher than the surrounding matrix, and they thereby constitute a risk for rapid transport of colloids and contaminants to groundwater. The volume fraction of active pores and the continuity of these pores have a major influence on water flow and transport of suspended colloids (White, 1985). Smith et al. (1985) found that when the continuity of macropores was destroyed by disturbing part or all of the column the recovery of Escherichia coli was much reduced. In contrast to the well-known role of structural macropores on colloid transport, less is known about the actual role of soil structure and preferential flow on in situ colloid mobilization. Soil structure and the changes in structure upon soil exposure to external and internal stresses such as drying and rewetting may be very important to in situ mobilization of colloids. The pore structure controls the active flow pathways of water and thereby the contact area between the infiltrating water and colloids. Changes in soil structure such as drying and rewetting phenomena may affect colloid mobilization by changing the cohesive forces between colloids, exposing new surfaces, and by changing the active flow volume.

It is generally recognized that soil structure and physical nonequilibrium flow conditions (channeling, short-circuiting, bypassing, preferential flow, or partial displacement) of water and solutes are highly related (e.g., Beven and Germann, 1982; White, 1985). Preferential flow results from different convective velocities among mobile and immobile water regions, and preferential flow has been demonstrated in aggregated packed soils (Biggar and Nielsen, 1962; van Genuchten and Wierenga, 1977) as well as undisturbed soils (Seyfried and Rao, 1987; Gaber et al., 1995; Langner et al., 1999). The immobile or stagnant water has typically been related to thin liquid films around soil particles, dead-end pores, nonmoving intraaggregate water, or relatively isolated regions associated with unsaturated flow (see review by Nielsen et al., 1986).

The water retention characteristics have been used as a quantitative measure of soil structure since the pore-size distribution can be derived from the water retention curve. Estimates of the pore-size distribution, however, are not necessarily related to the efficiency of pores in conducting water (e.g., Beven and Germann, 1982; White, 1985). Approaches to identify the functional porosity have included dye studies (Bouma and Wösten, 1979; Seyfried and Rao, 1987) and visualizing preferential flow by X-ray computer-assisted tomography (CAT) scanning (Perret et al., 1999; Vanderborght et al., 2002) or single photon emission computed tomography (SPECT) scanning (Perret et al., 2000). In addition, measurements of soil air permeability have been identified as a valuable tool for characterizing soil structure (Ball, 1988; Blackwell et al., 1990; Granovsky and McCoy, 1997; Moldrup et al., 2001). The air permeability expresses the soil's capacity to conduct air by convection, and measuring air flow at pressure potentials where water has drained from the pores of interest allows examination of the pore network. Pore continuity and tortuosity may also be inferred from relationships among air permeability and air-filled porosity, as suggested by Groenevelt et al. (1984). Several studies have indicated that continuity of the actively conducting pores is probably more important than volume and number of macropores in controlling water flow (Ball, 1988) and tracer infiltration (Douglas et al., 1980; Allaire-Leung et al., 2000).

Several attempts have been made to account for the nonequilibrium nature of the solute breakthrough in undisturbed soils (e.g., reviews by Nielsen et al., 1986; Brusseau and Rao, 1990). A widely used approach has been to modify the convection–dispersion equation (CDE) to account for the partition of the water phase into mobile and immobile regions, where convective–dispersive transport is restricted to the mobile region and a diffusive exchange of solute exists between the mobile and immobile region (van Genuchten and Wierenga, 1976). The applicability of the MIM model to laboratory-scale transport processes has been demonstrated by van Genuchten and Wierenga (1977) with aggregated clay loam, Gaudet et al. (1977) with unsaturated sand, Gaber et al. (1995) with saturated and unsaturated undisturbed silt loam, and Langner et al. (1999) with saturated and unsaturated undisturbed silt loam. Other studies have demonstrated that the compartmentalization of soil water into only two regions was insufficient to account for the range of pore-water velocities encountered (Seyfried and Rao, 1987 [undisturbed clay loam]; Li and Ghodrati, 1995 [silt loam containing macropores]). In an attempt to account for the very broad range of pore-water velocities encountered in structured soils Morisawa et al. (1986) suggested a "three-region" model (Brusseau and Rao, 1989).

The primary objective of this study was to use undisturbed soil columns, sampled along a naturally occurring clay gradient with clay contents ranging from 12 to 43% (i) to examine the pore structure characteristics based on measurements of the pore-size distribution, air-filled porosity, and air permeability; (ii) to examine the active flow volume based on ³H₂O BTCs and parameter estimates deduced from model fit using the MIM approach or an extended 2MIM approach; and (iii) to evaluate, based on measurements conducted on soils with different initial matric potentials, the changes in pore structure characteristics and the active flow volume upon soil exposure to drainage and drying and rewetting. These results were also used to evaluate the role of soil structure and preferential flow on in situ colloid mobilization in a companion paper (Kjaergaard et al., 2004a).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Site and Soil Characteristics
Intact soil columns of topsoil were excavated in the early spring 2001 from 1-m² areas at the 10- to 18-cm depth, at six sites along a naturally occurring clay gradient from an arable field in Lerbjerg, Denmark (56°22'N, 9°59'E). The clay gradient and the relative locations of the sampling sites were described and illustrated in a previous paper (Kjaergaard et al., 2004b). The soil is developed on morainic deposits from the Weichsel Glacial Age, and the site has been under conventionally tilled winter wheat (Triticum aestivum L.) for several years. Primary minerals quartz, micas, and feldspars dominated the sand and silt fractions, while the clay fraction was dominated by the secondary minerals illite (20–30%), smectite (10–30%)—predominantly montmorillonite, and vermiculite (10–20%) (Schjønning et al., 1999).

At the sampling sites the top 10 cm of soil and vegetation was removed, and the soil cores were excavated by manually pressing a steel cylinder (10 cm in diameter, 8 cm long) into the soil and removing surrounding soil. The intact soil cores were gently cut planar at each end, sealed, transported to the lab in lined plastic boxes, and stored at 2°C at field-moist conditions (water content close to field capacity, –100 hPa). Bulk samples from the sampling area were air dried, sieved at 2 mm, and used for analysis of soil characteristics (Table 1). Soil texture was determined using a combination of wet sieving and the hydrometer method. Total C was determined on a LECO Carbon Analyzer (St. Joseph, MI) coupled to an infrared CO₂ detector. Soil pH was determined in 0.01 M CaCl₂ with 1:2.5 (w/w) soil/electrolyte suspension. The content of calcite was measured gas-volumetrically.

View larger version (26K): [in this window] [in a new window]   Fig. 1. Illustration of pretreatment procedures and tracer experiment.

View larger version (43K): [in this window] [in a new window]   Fig. 2. Experimental setup consisting of soil column (10 cm in diameter, 8 cm long), irrigation device connected to a precision pump, column base with a nylon monofilament screening fabric (31 µm), resting on a 2-mm stainless-steel grid, collection vacuum chamber with outlet connected to a Mariotte device controlling the suction, rubber tube with two-piece pressure-controlled open and close device, and fraction collector.

Immediately after the leaching experiment, column weights were measured and the columns allowed draining to –100 hPa on tension tables and reweighed again. Column dry weight was determined after oven drying at 105°C. Sample volume was measured with a calliper. Bulk density, _b, was estimated from sample volume and the dry weight. Total porosity was calculated from the bulk density, using values of particle density between 2.64 to 2.68 measured by Schjønning et al. (2003).

Pore Structure Characterization
Air permeability (k_a) and air-filled porosity (_a) was measured directly on the soil cores at = –2.5, –100, and –15500 hPa immediately before the leaching experiment; at –5 hPa immediately after the leaching experiment; and after final drainage to –100 hPa (Fig. 1). Air permeability was measured with an air permeameter (Iversen et al., 2001) using the steady-state method of Grover (1955). Air-filled porosity was measured using an air pycnometer (Vomocil, 1965). From preliminary investigations we found that soil columns with no measurable air permeability at –100 hPa did not allow any flux of water. On the basis of these results, air permeability was measured at all columns at field-moist conditions (–100 hPa), and columns with no measurable air permeability were disregarded. Among the 25 soil columns sampled at each of the six 1-m² areas, no columns were disregarded at 12% clay, while a maximum of 12 columns were disregarded at 43% clay.

Measurements of air permeability, air-filled porosity, and estimates of gas diffusivity (D/D₀) allowed calculation of some soil structure characterizing parameters. Schjønning et al. (1999)(2002), and Moldrup et al. (2001) used the equivalent pore diameter (d) of the drained pores, as derived from the parallel tube fluid flow model by Millington and Quirk (1964) and Ball (1981), as a soil structure characterizing parameter:

[1]

where D is the gas diffusion coefficient in soil (cm³ soil air cm^–1 soil h^–1), and D₀ is the gas diffusion coefficient in air (cm² h^–1). Moldrup et al. (2000) found a highly significant relationship (r² = 0.98) for D/D₀ as a function of at –100 hPa soil matric potential:

[2]

where D₁₀₀ is the gas diffusion coefficient at –100 hPa, and _a,100 is the air-filled porosity at –100 hPa. Inserting Eq. [2] into Eq. [1] allows estimation of the equivalent pore diameter at –100 hPa (d₁₀₀) from measurements of only _a,100 and k_a,100. Measurements of _a,100 and k_a,100 and estimates of D/D₀ additionally allowed calculation of the number of air-filled pores (n₁₀₀) in a soil transect (Ball, 1981; Schjønning et al. (1999)( 2002):

[3]

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Pore Structure Characteristics
The total porosity of the soils increased with increasing clay content (Table 1). The water retention characteristics reflected the differences in the pore-size distribution (Fig. 3a) , with an increase in the fraction of pores <0.2 µm from 26% at 12% clay to 51% at 43% clay. The fraction of pores between 0.2 to 30 µm deviated less among the soils, constituting from 41 to 53% of the total porosity. The volume of pores 30 µm significantly decreased with increasing clay content, constituting 33% of the total pore volume at 12% clay to 3% at 45% clay. The fraction of pores >600 µm, which represents the pore volume drained at the –5 hPa suction applied during the leaching experiment, constituted 12% of the total porosity at 12% clay, declining to 7% at 18% clay, and 1% at 24% clay. In the higher clay soils (28%) it was not possible to identify the existence of pores >600 µm on the basis of the soil water retention characteristics, indicating that the possible volume of this pore-size fraction is very small.

View larger version (27K): [in this window] [in a new window]   Fig. 3. Response of matric potential on (a) water retention characteristics, and (b) air permeability. Equivalent pore diameter of (a) water occupied, and (b) air-filled pores corresponding to the applied pressure potential is illustrated at the second x axis. Error bars: ±SE.

Changes in soil structure following drainage and drying–rewetting are shown in Fig. 4 . For soils kept at –2.5 hPa before the experiment, only final measurement at –100 hPa was achieved, as these soils were not at –100 hPa before the tracer experiments. Both the fraction of pores >30 µm (_a,100) (Fig. 4a, 4b, 4c), and the air permeability (Fig. 4d, 4e, 4f) decreased with increasing clay content. Generally, drying to –15500 hPa and rewetting increased the fraction of pores >30 µm (Fig. 4c) and the air permeability of this pore fraction (Fig. 4f). This agreed with the results of Reeve et al. (1980) showing a close correlation between total clay content and shrinkage. Increasing porosity is generally associated with drying and rewetting (e.g., Pardini et al., 1996; Czarnes et al., 2000).

View larger version (28K): [in this window] [in a new window]   Fig. 4. Pore structure characteristics determined at –100 hPa as a function of clay content and initial matric potential (IMP) before (initial, filled symbols) and after (final, open symbols) the tracer experiment: (a–c) measured air-filled porosity, a,100; (d–f) measured air permeability, ka,100; (g–i) estimated equivalent pore diameter, d100; and (j–l) estimated number of pores 30 µm, n100. Error bars: ±SE correspond to measurements on three replicate columns for each clay soil.

Tritium Breakthrough Characteristics
The transport parameters (mean values) for each combination of clay and IMP are summarized in Table 2. The soil water content () at steady outflow increased with clay content, because of the differences in the pore-size distribution, resulting in a relative saturation ranging from 88 at 12% clay to around 100% at 24% clay. This indicates a very low (not measurable) fraction of pores >600 µm in the high clay soils. There seemed to be no effect of IMP on the relative saturation. The average Darcy flux velocity (q) among all soil columns was rather constant at 2.5 cm d^–1.

View larger version (35K): [in this window] [in a new window]   Fig. 5. Tritium (3H2O) breakthrough curves plotted as relative concentration (C/C0) against number of eluted pore volumes (V/V0) as a function of clay content and initial matric potential (IMP) of –2.5, –100, and –15500 hPa. Dotted line marks one pore volume. Replicates corresponding to measurements on three replicate columns are represented by different symbols.

The effect of drainage or drying and rewetting (IMP) depended on soil clay content, with no variations at 12% clay. At higher clay contents (18%) significant changes in the shape and position of the BTCs were generally observed after drying and rewetting (IMP –15500 hPa), while only minor effects of IMP –100 hPa were observed. At 18% clay the BTCs at IMP –15500 hPa adopted almost symmetrical behavior (Fig. 5f), and this was also the case at 24% clay (data not shown). At higher clay content the nonequilibrium behavior was still evident, but the position of the BTCs generally changed, with significant later peak arrival. With decreasing IMP the differences among the clay contents were significantly reduced. Estimating the number of pore volumes eluted at the 12.5 and 50% ³H₂O displacement (Table 2) revealed that increasing clay content and high IMP both reduced the number of pore volumes eluted at 12.5 and 50% ³H₂O-displacement, indicating a higher degree of preferential transport.

Three-Region Model for Tritium Transport
In this study, several BTCs exhibited distinct double peaks. Even though MIM has be used to describe the existence of double peaks (e.g., Mortensen, 2001), the second peak being a consequence of diffusive exchange between the mobile and immobile region, we found that MIM was not able to describe the existence of double peaks well (Fig. 6b–6f) . To describe the transport of tritium in these soils we assumed that the water phase conceptually consisted of three separate physical pore regions: (i) a region with rapid water flow (e.g., in the larger interaggregate pores), _m1 (m³ m^–3); (ii) a region with moderate water flow (in the smaller interaggregate pores), _m2 (m³ m^–3); and (iii) a region with immobile water (in the intraaggregate pores), _im (m³ m^–3). The two mobile regions are assumed to have independent pore water velocities (v₁, v₂; cm h^–1) and dispersion coefficients (D₁, D₂; cm² h^–1). The dispersion coefficients are calculated using

[4a]

[4b]

where _m1 and _m2 are the independent dispersivity coefficients in the mobile regions (cm). If the input flux of water per cross-sectional area of column (the Darcy velocity, q; cm h^–1) is known the value of _m2 is given as

[5]

View larger version (37K): [in this window] [in a new window]   Fig. 6. Selected tritium (3H2O) breakthrough curves plotted as relative concentration (C/C0) against number of eluted pore volumes (V/V0) with calculated best fit using either (a) the mobile–immobile model (MIM), or (b–f) the two mobile–one immobile model (2MIM) and the mobile–immobile model (MIM). Note that two different x and y axes are used.

[6a]

[6b]

where (cm^–1) is a constant assumed identical for both mobile regions. The overall governing differential equations for ³H₂O transport are

[7a]

[7b]

[7c]

where R is the retardation factor assuming linear equilibrium sorption (R = 1 + _bK_d/) of tritium and C denotes the activity of tritium in the three regions. It is assumed that R has the same value in all regions. The equations were solved in Microsoft Excel using a forward time backward space finite difference scheme corrected for second-order numerical errors (Moldrup et al., 1994). A time step of 0.025 h and a spatial step of 0.0058 m were used.

The curve-fitted parameter values for each combination of clay and IMP are summarized in Table 3. With and q measured during transport experiments, parameters _m1, v₁, v₂, ₁, ₂, , and R are fitted from the BTCs by minimizing the sum of squared deviations (SSD) between measured and predicted ³H₂O concentrations. The fitting procedure was done by manually selecting initial values for the parameters to be fitted and then using Microsoft Excel to optimize the parameter values while minimizing the SSD. The optimal values of the parameters _m2 and _im can be calculated once optimal values for the fitted parameters are found. Based on the observed retardation of tritium, the retardation factor (R) was estimated from the 12% clay showing the latest breakthrough. The tritium BTC in the 12% clays was fitted using only one mobile region (MIM model; e.g., following the approach by Parker and van Genuchten [1984] in their CXTFIT model, or by Gaber et al. [1995]). The retardation factor was fitted from the BTC allowing the values of the other input parameters (_m1, v, , ) to assume optimal values. From the curve fitting a retardation factor of 1.2 was found, and it was assumed that R had the same value in all regions in all clay soils. Two mobile regions (2MIM model) were considered for all soils with 18% clay. With a fixed R value at 1.2, six parameters needed to be fitted for the 2MIM model.

The model results revealed that the fraction of immobile water generally increased with clay content and decreased with decreasing IMP (Table 3). This agrees with the interpretations based on the shapes of the BTCs, with the lowest values of _im obtained for those soil columns showing the highest degree of symmetry in the BTCs. The matrix-dominated flow behavior of the BTCs at 12% clay is surprising, since these soils cores actually contained a very large fraction of large pores. However, the active flow volume is determined by the effective pore continuity, the pore water velocity, and the steady-state soil water content. The lower relative saturation and higher volume of large pores, which may have resulted in a disconnection of the rapidly conducting pore network in the 12% clay soils, and the relative low convective velocity resulted in negligible amounts of immobile water. This agrees with the results of Seyfried and Rao (1987), who reported increasingly symmetrical shapes of tritium BTCs when soil water tension increased from 0 to 2 kPa in an undisturbed clay loam. Langner et al. (1999) reported increasing equilibrium behavior when increasing soil water tension from 0 to 10 hPa. In contrast, studies performed on packed sand columns demonstrated that tailing in BTC increased with increasing soil water tension (Nielsen and Biggar, 1961; Gaudet et al., 1977; DeSmedt and Wierenga, 1984). However, as noted by Brusseau and Rao (1990), homogeneous porous media with a narrow pore-size distribution may be more susceptible to isolation of immobile regions during decreases in soil water content because of the lack of continuous pores with smaller diameter.

The observed changes in the BTCs after drying and rewetting (IMP –15500 hPa) is a consequence of the changes in soil structure caused by shrinkage and swelling upon drying and rewetting. Shrinkage and swelling increased the porosity and the number of pores >30 µm and decreased the equivalent pore diameter. This agrees with the results from Bouma and Wösten (1979), who concluded that the increased porosity resulting from shrinkage and swelling was composed of a larger volume of finer pores, while the large pores closed upon swelling. When the larger pores are closed the relative contribution of progressively finer pores increases and the pore-water velocity distribution is narrowed, which explains the displacement of the BTCs at IMP –15500 hPa and the apparent reduction in the values of _im (Table 3). We note that two estimates of mobile water seem to be not well estimated by the 2MIM model: _m2 at 18% clay (IMP –15500 hPa) and _m1 at 24% clay (–15500 hPa). In both these cases the drying and rewetting resulted in flow behavior approaching matrix-dominated flow (Fig. 5f), indicating only one mobile water region. A better fit would have been obtained with the simpler MIM.

Linking Pore Structural Characteristics and Preferential Flow
The amount of immobile water is conceptually related to the pore water velocity distribution and thereby the pore-size distribution of the water-filled pores. The pore-size distribution and the continuity of the water-filled pore system are probably key factors controlling the transport behavior of tritium in these soils. The water retention characteristics revealed that the 24 to 43% clay soils resembled each other with respect to a very low fraction of pores >30 µm and a fraction of pores >600 µm that cannot be measured, resulting in an effective saturation at or close to 100% at the applied pressure potential of –5 hPa. These soils additionally resembled each other with respect to (i) pore structure characteristics (a low air permeability, and a very low number of pores >30 µm having a rather high equivalent diameter), and (ii) the tracer breakthrough characteristics demonstrating a very rapid breakthrough and less pronounced tailing (12.5% ³H₂O displacement and the amount of immobile water). A high continuity of a few large pores with relatively little diffusive exchange between mobile and immobile water regions was probably responsible for the very early tritium breakthrough observed for these soils.

The soils low in clay (12 and 18% clay) differed from the higher clay soils by a larger fraction of pores >30 µm, and additionally a large fraction of pores >600 µm drained during the leaching experiment, causing a lower relative saturation (between 86 and 96%). The existence of air-filled pores probably resulted in disconnection of the rapidly conducting pore system, narrowing the pore-size distribution and the amount of immobile water, and consequently resulting in reduced degree of preferential flow. This was most pronounced for the 12% clay soil, where matrix-dominated flow behavior resulted.

The analysis of the dependence of moisture history on soil structural characteristics revealed that the drying to –15500 hPa and rewetting before the tracer experiment increased the porosity and number of pores >30 µm but decreased the equivalent pore diameter, indicating closing of the largest pores and a reduction in the pore-water velocity distribution. This change in soil structure resulted in a significant increase in the number of eluted pore volumes at 12.5% and 50% ³H₂O displacement and reduced the amount of immobile water for all clay soils except the 12% clay, which was not affected by the drying and rewetting. The changes in the pore structure characteristics following the drying and rewetting were thereby directly reflected in the tritium transport behavior.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The investigation of the influence of soil structure on the susceptibility of preferential flow revealed:

• The occurrence of preferential flow was not related to the volume fraction of large pores, since drainage of pores >600 µm at the applied suction of 5 hPa probably disconnected the rapidly conducting pore system, resulting in matrix-dominated flow behavior at 12% clay.
  
• At the higher clay contents (24% clay) a high degree of preferential flow seemed to relate to the existence of few large continuous water-filled pores, resulting in very high pore-water velocities in the rapid mobile water region and consequently rapid leaching of tritium.
  
• Drying the soils to the crop wilting point (–15500 hPa) and rewetting before the tracer experiment significantly reduced the degree of preferential flow as a result of an increased porosity dominated by pores with a minor equivalent pore diameter.
  

The extended three-region solute transport model (2MIM) was able to fit the measured tritium breakthrough curves well and supported the existence of two mobile flow domains, most pronounced for the higher clay soils and the high initial matric potentials. Future efforts should be directed toward better understanding and describing the influence of soil matric potential on the continuity of the actively conducting pore system, and toward linking model parameters with soil structural properties.

	ACKNOWLEDGMENTS

 
This research was funded by The European Doctoral School at Aalborg University, and the Danish FREJA-program (Female Researchers in Joint Action) under the Danish Research Council. The technical assistance of Stig T. Rasmussen and Michael Koppelgaard is gratefully acknowledged.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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