Partitioning of Butachlor at Air–Water Interface and Transport in Unsaturated Sandy Soils

Several experiments, including adsorption isotherm and flow column tests, were performed to investigate the effect of the air–water interface on the migration of butachlor (C₁₇H₂₆CINO₂) in unsaturated soils. To obtain a better simulation of unsaturated flow from the column tests, a tension infiltrometer was employed to exert a suction head on the upper end of the column. The lower end of the column was open to the atmosphere so that the water flow was driven by gravity. It was found that butachlor adsorbed on the air–water interface as well as on the soil surfaces. The Langmuir adsorption isotherm was observed for the air–water interface at concentration ranges up to 2 mg/L. The areas of the soil surface and air–water interface were estimated to quantify the retardation and facilitation effects caused by the soil surface and air–water interface. The preferences of butachlor for adsorption on the air–water interface and soil surface with organic substances were nearly identical. This preference was about twice that for organic-free soil surface. The migration velocity of the air–water interface was estimated to be about 40% of that of the average bulk water phase, which led to the conclusion that butachlor transport was retarded by the adsorption on the soil surface, and was facilitated by the existence of the air–water interface. This effect, therefore, should be considered in the evaluation of the mobility of hydrophobic pesticides in unsaturated soils.