as a third phase, thus enhancing the transport of strongly sorbing contaminants (McCarthy and Zachara, 1989).Until some two decades ago, it was believed that only the soil liquid and gaseous phases were mobile and could facilitate the transport of chemicals and nutrients through the vadose zone. It is now generally SOURCES OF MOBILE SOIL COLLOIDS accepted that also part of the soil solid phase is mobile, and thatThe tendency of soil colloids to disperse from soil mobile organic and inorganic soil colloids may facilitate chemical transport. However, the magnitude and significance of these colloidal aggregates in response to infiltration of water is a natural transport processes are yet to be determined. It is essential to examine phenomenon, sometimes even leading to the developwhether current models for transport and fate of chemicals in soil ment of illuvial subsurface horizons with higher contents and groundwater need to be revised. The collection of papers in this of clay compared with the upper eluvial horizons. Microspecial section of Vadose Zone Journal mainly take their origin, morphological features showing deposits of clay skins but not exclusively, from an international workshop "Colloids and on ped faces and at the interface of water-conducting Colloid-Facilitated Transport of Contaminants in Soil and Sediments" pores represent evidence of such colloid translocation held at the Danish Institute of Agricultural Sciences, Denmark, 19-20 (Buol and Hole, 1961). Dispersion of colloids is also Sept. 2002. The workshop was organized to review our present knowlsuspected to be responsible for affecting soil physical edge of colloid behavior and transport in porous media and the possi-properties such as surface crusting, surface erosion, wability of colloid-bound transport of contaminants and nutrients in soil and groundwater. Here we will first give a brief introduction to the ter infiltration, and hydraulic conductivities (e.g., Miller topic of mobilization and transport of colloids in the vadose zone, and Baharuddin, 1986;Shainberg et al., 1992). The source and highlight previous evidence of colloid-facilitated transport. We of mobile colloids in the vadose zone is generally considthen introduce the review and technical papers in the special section. ered to be the in situ release of water-dispersible colWe hope that the information provided in this special section will loids. Colloids are operationally defined as particles belead to improvements in our understanding and associated conceptual tween 1 to 10 nm and 2 to 10 m in diameter (e.g., models of contaminant transport and fate in soil.

than 50% of the yearly contribution of P to Danish freshwaters (Kronvang et al., 1995). In spite of these Strongly sorbing compounds such as P, pesticides, and heavy metals significant reductions in discharge, P is still problematic can be transported through soils while being adsorbed to mobile colloidal particles. While the rapid leaching of nonadsorbing chemicals

Water repellency (WR) of soils can induce hydrological problems such as reduced water infiltration, enhanced surface runoff and erosion, and the forming of preferential flow patterns in soil. Although soil organic matter (SOM) may cause both soil aggregation and a hydrophobic‐material‐coating of aggregates, little is known about WR in aggregated soils. We investigated the degree of WR as functions of volumetric water content (θ) and pF [= log (‐ψ; soil‐water potential)] for sieved fractions of a volcanic ash soil samples from different depths with varying soil organic carbon (SOC) contents of between 1.1 and 12.3%. Water repellency was determined by the molarity of ethanol droplet (MED) test. Water repellency was observed in the samples with SOC ≥ 4.6%, and the effects of sample pretreatments (pressure chamber desorption, air‐drying at 20°C, and oven‐drying at 60°C) on the degree of WR were small. The degree of WR varied greatly with both SOC content, θ, and pF. Peaks of WR were observed when the water retained in intra‐aggregate pores was drained to a moderate extent with the corresponding pF values located in a relatively narrow range from 3.2 to 3.6. This indicates that the hydrophobicity of high‐SOC aggregate surfaces might be enhanced the most at a specific soil‐water potential. Examining relations between water repellency parameters, the integrated areas below the WR‐θ and WR‐pF curves were useful indexes for characterizing WR, and linear relationships between the integrated areas and both SOC and water contents at maximum repellency were found.

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-m 2 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 (,0.24 mm) 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 21 for soils having recently been fertilized and an approximate minimum of 3 mg P g 21 for soils not recently fertilized.

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The mobility of strongly sorbing pesticides in soils may be higher than formerly assumed, due to facilitated transport along with mobilized colloids. Our objective was to verify whether facilitated transport enhances the vertical movement of prochloraz (N‐propyl‐N‐[2‐(2,4,6‐trichlorophenoxy)ethyl] imidazole‐1‐carboxamide). Experiments were carried out using 20 cm diam. by 20 cm long undisturbed soil columns taken from the topsoil (sandy loam, Typic Hapludalf) from a field under barley/grass (Hordeum vulgare L/Lolium perenne L.) cultivation near Rogen, Denmark. Prochloraz was applied to the surface as a pulse in solution. The irrigation intensity was 10 mm h−1, and the lower boundary of the columns was at atmospheric pressure. Three treatments each with four replicates were performed to study the effect of ionic strength and pH on pesticide and particle transport. Leaching of particles and pesticide was promoted by decreasing the ionic strength of the irrigation solution, and increasing pH by ammonia application. This is in line with the theory of electrostatic interactions. Preferential flow and particle transport were the two most important factors determining the amount of pesticide leached. A large variability in amount leached was observed among the replicates (standard deviations of 38–141%) mainly due to differences in water transport in the columns. Particle‐facilitated transport was significant, but did not dominate prochloraz transport. Depending on the pH and ionic strength of the applied solution, 2.5 to 13.1% of the leached pesticide was sorbed to particles with diameter d > 0.24 µm, and 3 to 9% to particles with 0.02 < d < 0.24 µm.

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), ψ = −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.