Removal and degradation of atrazine and metolachlor by vegetative filter strips on clay loam soil

The effectiveness of filter strips, with and without vegetation, in removing dissolved atrazine and metolachlor in runoff was investigated using aluminum tilted beds set at 1% slope on Cullen clay loam soil. Runon containing atrazine and metolachlor was applied on the up-slope end of the simulated filter strips. Water samples from surface runoff, lateral subsurface movement, and leachates as well as filter strip soil samples were: collected and herbicide concentrations determined. The filter strips reduced the amount of dissolved atrazine and metolachlor in runoff by about 6% of the amount applied. The absence or presence of switchgrass did not affect the amount of herbicide filtered. About: 56 to 82% of the runon volume leached through the 30-cm soil depth of the filter strips. In the leachate, about 72 to 88% of the amount of applied herbicide was filtered or adsorbed to the soil. The presence of switchgrass reduced the amount of runoff volume and increased the amount of leachate volume. In total, about 53 to 73% of the amount of herbicide applied was removed by the filter strips. The primary mode of dissolved herbicide removal in applied runon was by infiltration and soil adsorption mechanisms. Soil herbicide con centrations were greatest at the 0 to 10 cm depth, decreased to less than 50 mug kg(-1) over a 7-week period. In the filter strip soil, the presence of switchgrass significantly increased the degradation rate of metolachlor, but not atrazine. Infiltration of runoff into the filter strips is key to reducing dissolved herbicides from moving offsite. The presence of surface connected macropores is important in facilitating this process on heavier textured soils.