The fluid passing through the pores of soils and geologic materials is not just water with dissolved inorganic chemicals, but a complex mixture of organic and inorganic molecules. Large organic molecules such as humic and fulvic materials may impact the movement of contaminants. If the large organic molecules are mobile and they act like surfactant micelles, they can facilitate the movement of hydrophobic organic contaminants. Enfield and Bengtsson performed a sensitivity analysis evaluating the importance of several processes that might impact the movement of hydrophobic organic chemicals. The sensitivity shown in the analysis demonstrates the importance of a knowledge of the mobility of the organic colloids and the ability of the organic colloids to partition a contaminant and facilitate its movement as well as the amount of organic colloid present. The analysis indicates that if the colloid is not mobile it will not have an impact on chemical transport and also shows the importance of the amount of mobile carbon assuming the partition coefficient was the same for all carbon (mobile and associated with the mineral fraction of the soil). Experimental evidence of facilitated transport for hexachlorobenzene due to the presence of a polysaccharide (blue dextran) is also discussed.