SW—Soil and Water: Transport of Particulate and Colloid-sorbed Contaminants through Soil, Part 1: General Principles Review Article

Literature is reviewed describing through soil colloid-facilitated transport of contaminants such as phosphorus, pesticides and other agrochemicals, plus a range of biological microorganisms. Smaller microorganism (viruses and bacteria) are transported mainly (like chemical contaminants) by adsorption onto otherwise harmless mobile colloidal clay particles or soil organic matter. Potential impediments to movement of colloids through soil can be subdivided into straining and filtration, depending on whether a particle has a dimension similar to pores (leading to physical trapping) or much smaller. Filtration mechanisms, including interception, diffusion and sedimentation, have been compared to those described in the extensive engineering literature on deep bed filtration. Sorption processes are discussed, both those to static components of the soil matrix and onto mobile colloids. The chemical influences of ionic strength and pH to colloid transport are reviewed, as well as the double diffusion layer as a mechanism linking particles to surfaces. Numerous reported studies using column experiments to measure colloid or contaminant transport through soil have been reviewed. Many indicate the importance of macropore flow which allows rapid unrestricted transport of contaminant carrying colloids. Some experiments determine a filtration coefficient for a simple filtration equation representing straining and filtration processes. The few existing models (incorporating this filtration equation), both for column experiments and for the field situation, are reviewed as background to further development of a field-scale model representing colloid-facilitated transport of a range of chemical and microbiological contaminants.