Colloid Leaching Research Articles

Nutrient and colloid leaching from un-amended versus vermicompost-amended soil

This study aimed to investigate the effects of flow rate, initial soil moisture conditions and vermicompost (VC) application on total dissolved solids (TDS), nitrate (NO3), dissolved organic carbon (DOC) and colloid leaching from soil. Columns were filled with natural soil or VC-amended soil and were leached under unsaturated flow conditions using flow rates of 4.3–5.1, 2.5 and 0.4 mL/min. Two sets of experiments were conducted, one with the soils at initial air-dried conditions and the other with initial saturated conditions. Control experiments were conducted under saturated-flow conditions using flow rates of 2.7–3.8 mL/min for comparison. Results of the leaching experiments showed that EC, nitrate and DOC levels in the effluent were higher for the VC-amended soil versus the un-amended soil. Leaching under initial air-dried conditions and at the lowest flow rate (lowest water saturation) also produced higher EC, nitrate and DOC concentrations in the effluent. Conversely, effluent colloid concentrations were slightly higher for saturated-flow conditions, and elution behavior was similar between the un-amended and VC-amended soils. Statistical analysis revealed that the total leaching of TDS, nitrate, DOC and colloid under the initial air-dried conditions was not significantly different (p < 0.05) in comparison with the initial saturated conditions. Although, higher flow rates showed high amount of total leaching of TDS, nitrate, DOC and colloid, unsaturated flow rate of 2.5 mL/min leached more efficiently than flow rate of 2.7–4.3 mL/min in un-amended soil. Findings of this study would help decision makers to better manage soil salinity, drainage and groundwater pollution. Particularly, applying lower flow rates for irrigation applications to maintain unsaturated conditions can significantly decrease colloid migration and soil erosion.

Dry-wet and freeze-thaw cycles enhance PFOA leaching from subsurface soils

Subsurface soil naturally experiences dry-wet and freeze-thaw cycles, which could affect the leaching of previously adsorbed pollutants. A slow release of poly- and perfluoroalkyl substances (PFAS) from impacted subsurface soil may serve as a long-term diffuse source of PFAS to groundwater. Yet, the extent to which these weathering conditions may affect the subsurface release of PFAS is unknown. We subjected columns packed with soil pre-adsorbed with perfluorooctanoic acid (PFOA) to dry-wet and freeze-thaw cycles and observed a spike in PFOA concentration in leachate following each weathering treatment compared to no weathering treatment. Weathering conditions released a high concentration of soil colloids, which were confirmed by particle-size distribution analysis, SEM-EDS, and XRD. Fractionation of PFOA in the water sample reveals that up to 36 % of leached PFOA was associated with soil colloids. Thus, previous studies that did not account for colloids might have underestimated the leaching of PFAS from the soil. Overall, the results indicate that natural weathering conditions can enhance subsurface leaching of colloids and colloid-associated PFOA. Therefore, current conceptual site models to quantify the leaching of PFAS from source zones should account for weathering and the contribution of colloids.

Leaching of natural colloids from forest topsoils and their relevance for phosphorus mobility

The leaching of P from the upper 20cm of forest topsoils influences nutrient (re-)cycling and the redistribution of available phosphate and organic P forms. However, the effective leaching of colloids and associated P forms from forest topsoils was so far sparsely investigated. We demonstrated through irrigation experiments with undisturbed mesocosm soil columns, that significant proportions of P leached from acidic forest topsoils were associated with natural colloids. These colloids had a maximum size of 400nm. By means of Field-flow fractionation the leached soil colloids could be separated into three size fractions. The size and composition was comparable to colloids present in acidic forest streams known from literature. The composition of leached colloids of the three size classes was dominated by organic carbon. Furthermore, these colloids contained large concentrations of P which amounted between 12 and 91% of the totally leached P depending on the type of the forest soil. The fraction of other elements leached with colloids ranged between 1% and 25% (Fe: 1–25%; Corg: 3–17%; Al: <4%; Si, Ca, Mn: all <2%). The proportion of colloid–associated P decreased with increasing total P leaching. Leaching of total and colloid-associated P from the forest surface soil did not increase with increasing bulk soil P concentrations and were also not related to tree species. The present study highlighted that colloid-facilitated P leaching can be of higher relevance for the P leaching from forest surface soils than dissolved P and should not be neglected in soil water flux studies.

Long-term Effects of Organic Waste Fertilizers on Soil Structure, Tracer Transport, and Leaching of Colloids.

Organic waste fertilizers have previously been observed to significantly affect soil organic carbon (SOC) content and soil structure. However, the effect of organic waste fertilizers on colloid dispersibility and leaching of colloids from topsoil has not yet been studied extensively. We investigated how the repeated application of different types of agricultural (liquid cattle slurry and solid cattle manure) and urban waste fertilizers (sewage sludge and composted organic household waste) affected soil physical properties, colloid dispersion from aggregates, tracer transport, and colloid leaching from intact soil cores. Total porosity was positively correlated with SOC content. Yearly applications of sewage sludge increased absolute microporosity (pores <30 μm) and decreased relative macroporosity (pores >30 μm) compared with the unfertilized control, whereas organic household waste compost fertilization increased both total porosity and the absolute porosity in all pore size classes (though not significant for 100-600 μm). Treatments receiving large amounts of organic fertilizers exhibited significantly lower levels of dispersible colloids compared with an unfertilized control and a treatment that had received moderate applications of cattle slurry. The content of water-dispersible colloids could not be explained by a single factor, but differences in SOC content, electrical conductivity, and sodium adsorption ratio were important factors. Moreover, we found that the fertilizer treatments did not significantly affect the solute transport properties of the topsoil. Finally, we found that the leaching of soil colloids was significantly decreased in treatments that had received large amounts of organic waste fertilizers, and we ascribe this primarily to treatment-induced differences in effluent electrical conductivity during leaching.

Heavy Metal Leaching as Affected by Long-Time Organic Waste Fertilizer Application.

The recycling of urban waste products as fertilizers in agriculture may introduce contaminants such as heavy metals into soil that may leach and contaminate groundwater. In the present study, we investigated the leaching of heavy metals from intact soil cores collected in the long-term agricultural field trial CRUCIAL. At the time of sampling, the equivalent of >100 yr of urban waste fertilizers following Danish legislation had been applied. The leaching of Cu was significantly increased in the treatments receiving organic waste products compared with the unfertilized control but remained below the permissible level following Danish drinking water guidelines. The leaching of Cu was controlled primarily by the topsoil Cu content and by the leaching of dissolved organic carbon (DOC) but at the same time significantly correlated with leaching of colloids in soils that had not received fertilizer or had received an organic fertilizer with a low concentration of Cu. The leaching of Zn, Cd, and Co was not significantly increased in urban waste-fertilized treatments. The leaching of Mo was elevated in accelerated waste treatments (both agricultural and urban), and the leaching of Mo was linked to the leaching of DOC. Since leaching of Cr and Pb was strongly linked to the level of colloid leaching, leaching of these metals was reduced in the urban waste treatments. Overall, the results presented should not raise concern regarding the agricultural use of urban waste products in agriculture as long as the relevant guidelines are followed.

Tracer, Dissolved Organic Carbon, and Colloid Leaching from Erosion‐Affected Arable Hillslope Soils

Core Ideas Pedogenetic processes are spatially related to the slope positions in arable soil landscapes. The Regosol is affected by erosion‐induced CaCO3 and reduced organic C contents. The leaching potential for colloids and DOC was reduced in the truncated Regosol soil. Soil erosion modifies transport properties in the form of the effective dispersivity. In a morainic arable soil landscape, tillage and water erosion created truncated soil profiles at exposed steep slope positions and colluvial soils in depressions. This study examined the effects of erosion‐induced pedogenetic dynamics on flow and transport properties, colloid mobilization, and dissolved organic C (DOC) leaching for an uneroded Luvisol (LV) at the plateau and a strongly eroded Regosol (RG) at the slope position. Unsaturated steady‐flow experiments were performed on intact soil columns (20‐cm diameter, 16–18 cm high) extracted from the Luvisol (Ap, E, and Bt horizons; three depths) and the Regosol (Ap and CBkl horizons; two depths) in three replicates. Together with a 3H2O tracer, the leaching of colloids and DOC and the electrical conductivity were determined in the percolate. Colloid concentration was obtained from turbidity data. Effective dispersion coefficients, Deff, were determined by fitting the convection–dispersion equation and the mobile–immobile transport model (program CXTFIT) to 3H2O breakthrough curves (BTCs). Values of Deff were lower for Luvisol (20.1 ± 0.3 cm2 d−1; mean ± standard deviation) than for Regosol columns (30.2 ± 12.6 cm2 d−1). The pore structure of the Regosol topsoil was influenced by incorporation of CaCO3 from glacial till. The plow pan in the Luvisol caused an increased heterogeneity in the BTCs. Differences in transport properties and leaching potentials for colloids (LV: 98 ± 9 mg L−1; RG: 15 ± 4 mg L−1) and DOC (LV: 13.7 ± 3.7 mg L−1; RG: 6.9 ± 1.1 mg L−1) suggested a gradual change in the solid‐phase composition and structure of these contrasting soils.

Effects of past copper contamination and soil structure on copper leaching from soil.

Copper contamination affects biological, chemical, and physical soil properties and associated ecological functions. Changes in soil pore organization as a result of Cu contamination can dramatically affect flow and contaminant transport in polluted soils. This study assessed the influence of soil structure on the movement of water and Cu in a long-term polluted soil. Undisturbed soil cores collected along a Cu gradient (from about 20 to about 3800 mg Cu kg soil) were scanned using X-ray computed tomography (CT). Leaching experiments were performed to analyze tracer transport, colloid leaching, and dissolved organic carbon (DOC) and Cu losses. The 5% arrival time () and apparent dispersivity (λ) for tracer breakthrough were calculated by fitting the experimental data to a nonparametric, double-lognormal probability density function. Soil bulk density, which did not follow the Cu gradient, was the main driver of preferential flow, while macroporosity determined by X-ray CT (for pores >180 μm) proved the best predictor of solute transport. Higher preferential flow due to the presence of well-aligned pores and small cracks controlled water movement in compacted soil. Transport of Cu was rapid during the first flush (≈1 pore volume) in association with the movement of colloid particles, followed by slower transport in association with the movement of DOC in the soil solution. The relative amount of Cu released was strongly correlated with macroporosity as determined by X-ray CT, indicating the promising potential of this visualization technique for predicting contaminant transport through soil.

Mobilization and Leaching of Natural and Water Dispersible Colloids in Aggregated Volcanic Ash Soil Columns

Colloid-facilitated transport enhances migration of strongly sorbing compounds (e.g., radionuclides, phosphorus, heavy metals) in soil and groundwater. Mobilization, transport and deposition of soil colloids are the underlying processes governing colloid-facilitated contaminant transport. Although significant progress has been made in simulating mobilization and transport/deposition of model colloids in different collector systems, it may be inadequate for the prediction of natural colloidal behavior in the subsurface. This study quantifies the leaching of natural volcanic ash soil colloids (NC) as well as the simultaneous transport of applied water dispersible soil colloids (WDC) in aggregated volcanic ash soil columns. Two water-saturated soil columns were irrigated with artificial irrigation water (AIW) at an intensity of 80 mm/hr for 60 hours. Two additional columns were irrigated at the same intensity, but a colloidal suspension of 5 mg/L was applied after 20 hours for a period of 20 hours. Effluent colloid concentrations were measured in each experiment. HYDRUS-1D was used for the simulation and estimation of colloid transport parameters. The results clearly showed different kinetics for applied colloid transport and natural colloid leaching. Transport of applied WDC followed first-order attachment kinetics, while the two-site equilibrium/kinetic model with equal fractions of equilibrium and kinetic sites best described the leaching of NC. Coupling these best model approaches well predicted the simultaneous leaching of natural and applied colloids, hereby providing a useful tool for the design of colloid-based in-situ soil remediation systems.

Is Colloid‐Facilitated Phosphorus Leaching Triggered by Phosphorus Accumulation in Sandy Soils?

The leaching of colloidal phosphorus (P(coll)) contributes to P losses from agricultural soils. In an irrigation experiment with undisturbed soil columns, we investigated whether the accumulation of P in soils due to excess P additions enhances the leaching of colloids and P(coll) from sandy soils. Furthermore, we hypothesized that large concentrations of P(coll) occur at the onset of leaching events and that P(coll) mobilized from topsoils is retained in subsoils. Soil columns of different P saturation and depth (0-25 and 0-40 cm) were collected at a former disposal site for liquid manure and at the Thyrow fertilization experiment in northeastern Germany. Concentrations of total dissolved P, P(coll), Fe(coll), Al(coll), optical density, zeta potential, pH, and electrical conductivity of the leachates were determined. Colloidal P concentrations ranged from 0.46 to 10 micromol L(-1) and contributed between 1 and 37% to total P leaching. Large P(coll) concentrations leached from the P-rich soil of the manure disposal site were rather related to a large P-content of colloids than to the mobilization of additional colloids. Concentrations of colloids and P(coll) in leachates from P-poor and P-rich columns from Thyrow did not differ significantly. In contrast, accumulation of P in the Werbellin and the Thyrow soil consistently increased dissolved P concentrations to maximum values as high as 300 micromol L(-1). We observed no first-flush of colloids and P(coll) at the beginning of the leaching event. Concentrations of P(coll) leached from 40-cm soil columns were not smaller than those leached from 25-cm columns. Our results illustrate that an accumulation of P in sandy soils does not necessarily lead to an enhanced leaching of colloids and P(coll), because a multitude of factors independent from the P status of soils control the mobility of colloids. In contrast, P accumulation generally increases dissolved P concentrations in noncalcareous soils due to the saturation of the P sorption capacity. This indicates that leaching of dissolved P might be a more widespread environmental problem in areas with P-saturated sandy soils than leaching of P(coll).

Colloid and Bromide Transport in Undisturbed Soil Columns: Application of Two‐Region Model

Bromide tracer breakthrough and natural soil colloid leaching curves for undisturbed soil columns were used to characterize dissolved and suspended matter transport at the field scale. Data from 33 columns of 20‐cm diameter and 20‐cm height were used. Columns were collected in a grid of 25 by 30 m at an agricultural field. A two‐region (mobile–immobile water phase, MIM) solute transport model was fitted to data. The model was used to estimate bromide and colloid transport parameters including mobile and immobile water contents (θm, θim), bromide and colloid advective velocities (vBr, vColl), and mobile–immobile mass transfer coefficients (αbr, αcoll). Both soil physical properties and transport parameters were highly variable across the sampling field. Comparison of bromide transport parameters with basic soil physical properties revealed that vBr was proportional to soil clay content and bulk density (ρb), but αBr was inversely proportional to these parameters. Colloid transport parameters, vColl and αColl, however, showed only a weak correlation with clay content and ρb. Also, vColl was typically three to four times higher than vBr. The colloid velocity was generally higher than the bromide velocity, implying size exclusion of colloids. The spatial distributions of soil physical properties, bromide and colloid transport parameters, and leached quantities of particles were compared. The results suggested that bromide and colloid mass transfer (diffusion) were not controlled by the same soil physical conditions, and that soil structure and macropore flow are more important than the quantity of dispersible colloids in controlling colloid leaching.

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

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

LEACHING OF COLLOIDS AND DISSOLVED ORGANIC MATTER FROM COLUMNS PACKED WITH NATURAL SOIL AGGREGATES

Transport of pollutants by colloids and dissolved organic matter (DOM) may increase the leaching of strongly sorbing pollutants (e.g., PAHs, heavy metals, radionuclides, and certain pesticides). A prerequisite for colloid-and DOM-facilitated transport is the release of colloids and DOM from soil. In the present study, the leaching of colloids and DOM from columns packed with natural soil aggregates (2-4 mm) was investigated. Aggregates with different organic matter content were used: Aggregates from Soil 1, with 3.6% organic matter content, and aggregates from Soil 2, with 2.5%. The leaching experiments showed that colloid leaching increased with higher organic matter content. Colloid leaching was strongly affected by the ionic strength of the infiltrating water but less so by the specific type of cation. However, prolonged leaching (20 h) with KCl increased the leaching of colloids, probably because of the ion exchange of naturally occurring polyvalent ions with K+. The accumulated amount of colloids leached during 20-h period was orders of magnitude lower than the amount of dispersible colloids (determined by rotation of soil water mixtures), and the organic carbon fraction (foc) of the leached colloids was 3 to 4 times higher than the foc of the dispersible colloids. The leaching of DOM from Soil 1 was greater, but, relative to the soil organic matter content, it was similar for the two soils. The leaching of DOM was not significantly affected by the chemistry of the irrigation water. Irrigation with solutions of KCl and deionized water increased the hydrodynamic dispersion coefficient on Soil 2, due to swelling of the clay minerals and closure of the soil pores. Irrigation with CaCl2 led to lower hydrodynamic dispersion, because of shrinking clay minerals. Soil 1 was less sensitive to shrinking and swelling of clay minerals because of its higher organic matter content.

Water‐Dispersible Colloids: Effects of Measurement Method, Clay Content, Initial Soil Matric Potential, and Wetting Rate

The fraction of clay that disperses in water, water‐dispersible clay (WDC), is recognized as an important property with respect to predicting soil erosion and colloid leaching. Using six mineralogically similar soils with 12, 18, 24, 28, 37, and 43% clay, we studied the influence of soil clay content, initial matric potential (IMP; ψ = −2.5, −100, and −15500 hPa), and wetting rate on WDC released in response to infiltration of low–ionic strength rainwater, using a low‐energy input measurement of WDC (LE‐WDC). These results were referenced by WDC obtained by a conventional, high‐energy input measurement based on air‐dried soil (HE‐WDC). The energy input in the dispersion procedure significantly affected the release of WDC. The amount of HE‐WDC increased with clay content, while the amount of LE‐WDC decreased with increasing clay content. The decrease in LE‐WDC was explained by an increase in cohesive strength, reflected by the increase in water‐stable aggregates (≥4 mm). A strong dependency of IMP on LE‐WDC was observed, with maximum release of LE‐WDC from soils that were at −2.5 hPa before measurement. Decreasing soil matric potential in the period before measurement reduced LE‐WDC and also reduced the dependency of soil clay content, with soils incubated at −15500 hPa releasing a low amount of LE‐WDC independent of clay content. The content of particulate organic C (POC) in the LE‐WDC decreased with increasing clay content, and increased after drying to −15500 hPa. Colloid dispersibility changed as a function of time and moisture status, with the main changes occurring during or immediately after adjustment of the moisture content. Increasing the wetting rate resulted in a doubling of the amount of LE‐WDC released from the initially dry soil (−15500 hPa), while no effect of wetting rate was observed at higher initial matric potentials.

Water-Dispersible Colloids: Effects of Measurement Method, Clay Content, Initial Soil Matric Potential, and Wetting Rate

The fraction of clay that disperses in water, water-dispersible clay (WDC), is recognized as an important property with respect to predicting soil erosion and colloid leaching. Using six mineralogically similar soils with 12, 18, 24, 28, 37, and 43% clay, we studied the influence of soil clay content, initial matric potential (IMP; ψ = −2.5, −100, and −15500 hPa), and wetting rate on WDC released in response to infiltration of low–ionic strength rainwater, using a low-energy input measurement of WDC (LE-WDC). These results were referenced by WDC obtained by a conventional, high-energy input measurement based on air-dried soil (HE-WDC). The energy input in the dispersion procedure significantly affected the release of WDC. The amount of HE-WDC increased with clay content, while the amount of LE-WDC decreased with increasing clay content. The decrease in LE-WDC was explained by an increase in cohesive strength, reflected by the increase in water-stable aggregates (≥4 mm). A strong dependency of IMP on LE-WDC was observed, with maximum release of LE-WDC from soils that were at −2.5 hPa before measurement. Decreasing soil matric potential in the period before measurement reduced LE-WDC and also reduced the dependency of soil clay content, with soils incubated at −15500 hPa releasing a low amount of LE-WDC independent of clay content. The content of particulate organic C (POC) in the LE-WDC decreased with increasing clay content, and increased after drying to −15500 hPa. Colloid dispersibility changed as a function of time and moisture status, with the main changes occurring during or immediately after adjustment of the moisture content. Increasing the wetting rate resulted in a doubling of the amount of LE-WDC released from the initially dry soil (−15500 hPa), while no effect of wetting rate was observed at higher initial matric potentials.

A novel method for measurement and characterization of soil macroporosity

Quantitative macropore characterizations were performed in large zero-tension soil lysimeters of a Maury silt loam (fine, mixed, mesic Typic Paleudalf) and a Loradale silt loam (fine, silty, mixed, mesic Typic Argiudoll) soil in an effort to assess potential colloid transport. Steel pipe sections (50 cm diameter×100 cm length) were hydraulically driven into the soil for lysimeter establishment. A dye tracer was applied into each lysimeter under saturated conditions at eight-hour intervals using 500 mL pulse applications. After one pore volume elution, the center section of the stained lysimeters was removed, dissected, and photographed. Kubiena tins were inserted into the lysimeter's remaining soil at 15 cm intervals. Vertically oriented thin sections were prepared from each tin. Pore area, diameter, and orientation were evaluated from thin sections and photographs using commercial grade scanners and Photoshop® software. At an input scanning resolution of 60 pixels per cm, the software program was capable of identifying pores with diameters >160 μm. Thin section analysis provided more detail of smaller sized pores (<1.0 mm), while the cationic nature of the methylene blue dye restricted its presence primarily to the larger macropores. The Loradale soils tended to exhibit increased pore area and pore diameter over that of the Maury in both thin sections and photographs. The Loradale soil also showed a more elaborate and vertically oriented network of pores. Although standard soil physical measurements did not suggest the presence of a restrictive layer in these soils, morphological evidence generated from this methodology suggested otherwise. Colloid leaching experiments performed in separate lysimeters of the same soils in a fashion similar to that used for the dye tracer application showed colloid breakthrough and filtration patterns consistent with the macropore characterization observations obtained through the application of this method.

Particle transport in macropores of undisturbed soil columns

Particle-facilitated transport may be an important process in the leaching of contaminants such as pesticides, phosphorus and heavy metals. In this work particle transport in macropores through intact soil columns was quantified. Irrigation intensities corresponding to naturally occurring storm events were used. Intact soil columns (18.3 cm inner diameter, 20 cm length) were sampled at two different depths (2–22 cm and 42–62 cm) from a structured sandy loam. Infiltration experiments, consisting of leaching of naturally occurring particles and infiltration with two types of colloidal suspensions were performed on each column. The active macroporosity was estimated in a dye experiment. A significant transport of particles (especially clay and silt) through macropores was observed at both depths. The total amount of mobilized particles at a certain amount of water outflow was found to be higher at depth 42–62 cm than at depth 2–22 cm, but unaffected by irrigation intensity. The particle size in the effluent was found to decrease over time during both the leaching of naturally occurring particles and during the subsequent leaching of colloids from the infiltration with colloidal suspensions, but seemed to stabilize at a particle size <10 μm. The decreasing particle size may be explained by differences in both the adsorption capacity and in the exposure to hydraulic forces of the different particle sizes and by mechanical entrapment of large particles in the macropores. No significant differences were found between the infiltration of the two different types of colloid suspension, probably due to their relatively small differences in zeta potential and particle size. The mass recovery of the infiltrated colloids was found to be significantly larger at depth 42–62 cm than at depth 2–22 cm, probably due to the larger size of the macropores at depth 42–62 cm. A simple advection–reaction model, which takes two different particle size fractions (< and >10 μm) into account, was found to describe the leaching of natural particles well using initial particle concentrations on the macropore walls and detachment coefficients for small and large particles as calibration parameters. ©1997 Elsevier Science B.V.