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 accepted that also part of the soil solid phase is mobile, and that mobile organic and inorganic soil colloids may facilitate chemical transport. However, the magnitude and significance of these colloidal transport processes are yet to be determined. It is essential to examine whether current models for transport and fate of chemicals in soil and groundwater need to be revised. The collection of papers in this special section of Vadose Zone Journal mainly take their origin, but not exclusively, from an international workshop “Colloids and Colloid-Facilitated Transport of Contaminants in Soil and Sediments” held at the Danish Institute of Agricultural Sciences, Denmark, 19–20 Sept. 2002. The workshop was organized to review our present knowledge of colloid behavior and transport in porous media and the possibility of colloid-bound transport of contaminants and nutrients in soil and groundwater. Here we will first give a brief introduction to the topic of mobilization and transport of colloids in the vadose zone, and highlight previous evidence of colloid-facilitated transport. We then introduce the review and technical papers in the special section. We hope that the information provided in this special section will lead to improvements in our understanding and associated conceptual models of contaminant transport and fate in soil.

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