Pesticide transport to subsurface tile drains in humid regions of North America

This report reviews and summarizes findings from more than 30 North American studies of pesticide transport into subsurface agricultural drains. Background information about subsurface drainage use and its importance for crop production and environmental protection is also presented. The presence of a subsurface drainage system generally increases the volume of infiltration and consequently decreases the volume of surface runoff water and sediment compared with similar soils where subsurface drainage systems are not installed. Therefore, in general, the presence of subsurface drainage decreases surface runoff losses of sorbed compounds such as pesticides, both because of lower runoff volumes and often also because of lower concentrations in the runoff resulting from the delayed initiation of runoff. Pesticide concentrations and mass losses are usually much lower in subsurface drainage than in surface runoff, often by an order of magnitude. In the medium- and fine-textured soils where subsurface drainage is common, the dominant mechanism for pesticide transport to the drain tiles is most likely preferential flow during rainfall/drainage events occurring soon after pesticide application. Concentrations and mass losses in subsurface drains are highly variable from year to year, depending on weather patterns.