The effectiveness of switchgrass (Panicum virgatum L.) filter strips in removing dissolved atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine) and metolachlor (2‐chloro‐N‐2‐ethyl‐6‐methylphenyl‐N‐2‐methoxy‐1‐methylethyl acetamide) in runoff was investigated using aluminum‐tilted beds set at 1% slope, filled with Emporia sandy loam soil (fine‐loamy, siliceous, thermic Typic Hapludults) and planted to switchgrass. Solution containing herbicides, followed by water alone after 2 and 4 wk were applied on the up slope of beds with and without switchgrass. Water samples from surface flow, lateral, and vertical leachates as well as soil samples were analyzed for the two herbicides using a gas chromatograph. Switchgrass filter strips reduced the mass of dissolved atrazine and metolachlor by 52 and 59% from the applied runon, respectively. The bare soil strips removed 41% of atrazine and 44% of metolachlor. Less than 0.5% of the applied herbicide was released by the two water runons 2 and 4 wk after herbicide‐solution application. The average concentrations of both herbicides in surface runoff were greater than in leachate samples. Herbicides were removed by the soil as runon moved through the soil profile. The concentration of either herbicide on the top surface (0–2.5 cm) was greater than in the soil immediately below (2.5–5 cm). Degradation of both herbicides was faster in beds with switchgrass than without. Greater amounts of both herbicides were retained in the first 67‐cm section of beds with the grass than without. Switchgrass helped to remove the herbicides by slowing runoff velocity and increasing their retention by soil.

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The half-lives, degradation rates, and metabolite formation patterns of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] were determined in an anaerobic wetland soil incubated at 24 degrees C for 112 d. At 0, 7, 14, 28, 42, 56, and 112 d, the soil and water were analyzed for atrazine and metolachlor, and their major metabolites. The soil oxidation-reduction potential reached -200 mV after 14 d. Degradation reaction rates were first-order for atrazine in anaerobic soil and for metolachlor in the aqueous phase. Zero-order reaction rates were best fit for atrazine in the aqueous phase and metolachlor in anaerobic soil. In anaerobic soil, the half-life was 38 d for atrazine and 62 d for metolachlor. In the aqueous phase above the soil, the half-life was 86 d for atrazine and 40 d for metolachlor. Metabolites detected in the anaerobic soil were hydroxyatrazine and deethylatrazine for atrazine, and relatively small amounts of ethanesulfonic acid and oxanilic acid for metolachlor. Metabolites detected in the aqueous phase above the soil were hydroxyatrazine, deethylatrazine, and deisopropylatrazine for atrazine, and ethanesulfonic acid and oxanilic acid for metolachlor. Concentrations of metabolites in the aqueous phase generally peaked within the first 25 d and then declined. Results indicate that atrazine and metolachlor can degrade under strongly reducing conditions found in wetland soils. Metolachlor metabolites, ethanesulfonic acid, and oxanilic acid are not significantly formed under anaerobic conditions.

Herbicide retention by residue mulch in vegetative filter strips could be an effective attenuation mechanism for removing herbicides from runoff. Adsorption studies were conducted to quantify the amount of atrazine and metolachlor that can be adsorbed and removed from runoff by thatch or fresh switchgrass residue. Adsorption of C 14 -atrazine and metolachlor on thatch or fresh switchgrass residue was conducted by using concentrations of 2.5, 7.5, 13.2, and 26.4 mmol L 21 and a 24-h equilibration period. Adsorption coefficients (K d ) decreased in the order, atrazine sorption on fresh switchgrass residue (81.1 L 21 kg 21 ), metolachlor sorption on fresh residue (32.9 L 21 kg 21 ), atrazine sorption on thatch residue (21.4 L 21 kg 21 ), and metolachlor sorption on thatch switchgrass residue (15.1 L 21 kg 21 ). On a volumetric basis (K dvol ), only atrazine showed a significantly greater K d-vol value on fresh residue than on the thatch residue. Absorption through cut ends of the residues (especially the fresh residue) may have produced adsorption capacities that would not be observed under field conditions. Fresh or thatch switchgrass residue in vegetative filter strips can help abate atrazine and metolachlor by intercepting and sorbing some of the herbicides.