Colloids and Colloid-Facilitated Transport of Contaminants in Soils: An Introduction

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 that The 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 possiproperties 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. Stumm, 1992; Buffle and Leppard, 1995), and include layer silicates, sesquioxides (Feand Al-oxyhydroxides), organic macromolecules, bacteria, and viruses. Because T sometimes rapid appearance of environmental of their high specific surface area, colloids have a high contaminants in groundwater reflects limitations in sorptive capacity and can be effective sorbents of low our current understanding of transport processes in solubility, strongly sorbing contaminants. Transport of soils. Traditional approaches to describe and predict the colloids through the vadose zone thus causes an inmovement of nonvolatile contaminants treat soil and creased risk of leaching of contaminants generally regroundwater as a two-phase system in which contamigarded as relatively immobile. nants partition between immobile solid constituents and the mobile water phase. Contaminants that are sparingly EVIDENCE OF COLLOIDsoluble in water and have a strong tendency to bind to FACILITATED TRANSPORT the solid phase are assumed to be retarded in the soil. Many contaminants readily adsorb to the soil solid phase Several experimental investigations have suggested and are therefore considered to present little danger that colloids may influence the transport of contaminants in risk assessments of surface water or groundwater in the vadose zone. Laboratory column studies have pollution. However, it has become evident that predicdemonstrated co-transport of contaminants sorbed to tions and results obtained in laboratory batch systems suspended colloids, or simultaneous leaching of in situ may not always be applicable to field situations. Spatial colloids and contaminants, while field studies have revariability or preferential flow phenomena have been vealed the association of contaminants with colloids in hypothesized to be responsible for some of the inconsisdrain or groundwater. Using homogeneously packed soil tencies between predictions and actual leaching of concolumns, Vinten et al. (1983) obtained evidence of vertitaminants (Jury and Flühler, 1992). In addition, colloids cal transport of DDT adsorbed to suspended montmoin the solid phase may be mobile and can therefore act rillonite colloids. Grolimund et al. (1996) demonstrated that suspended in situ mobilized colloids can provide a L.W. de Jonge and C. Kjaergaard, Danish Institute of Agricultural pathway for rapid transport of Pb. In another study, Sciences, Dep. of Agroecology, Research Centre Foulum, P.O. Box Flury et al. (2002) found that colloids mobilized in flow 50, 8830 Tjele, Denmark. Per Moldrup, Aalborg University, Dep. of experiments with packed sediments carried Cs along. Life Sciences, Environmental Engineering Section, SohngaardsholmsExperiments with undisturbed soil columns have vej 57, 9000 Aalborg, Denmark. Received 18 Feb. 2004. Special Section: Colloids and Colloid-Facilitated Transport of Contaminants in shown that suspended colloid–contaminant complexes Soils. *Corresponding author ([email protected]). may facilitate the transport of atrazine (e.g., Seta and Karathanasis, 1997b), and Cu and Zn (Karathanasis, Published in Vadose Zone Journal 3:321–325 (2004). 1999). In addition, it was demonstrated that in situ mobi Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA lized colloids were able to facilitate the transport of