Acetochlor Concentration and Loss In Tile Drainage Water As Affected by Application Rate.

Acetochlor [2-chloro-N-ethoxymethyl-N-(2-ethyl-6-methylphenyl)acetamide], a commonly used herbicide for corn (Zea mays L.), has been found in some southern Minnesota rivers. Leaching of acetochlor from the soil surface to subsurface tile lines in poorly drained soils has been proposed as a potential transport mechanism. A research study was conducted from 2008 through 2010 on tile drained Webster-Canisteo clay loam soils (Typic Endoquolls) to determine the effect of acetochlor application rate on concentration and loss of acetochlor and its metabolites in tile drainage water. Three rates of acetochlor (0, 1.47, and 2.45 kg ai ha^-1) were broadcast-applied without incorporation (pre emergence) in mid-May each year. Flow-weighted and grab water samples were collected pre and post application during tile flow events. Samples were analyzed for acetochlor and its metabolites (ethanesulfonic acid and the oxanilic acid). Less-than-normal precipitation in 2008 and 2009 resulted in only 3 and 2 cm of tile drainage, respectively, and few water samples; whereas, 57 cm of drainage occurred in 2010. In 2010 acetochlor was detected (0.03 µg L^-1 detection limit) in 13 of 182 water samples during the year and 8 of 143 post application samples. The average acetochlor concentrations in detects were 0.10, 0.06 and 0.30 µg L^-1 for the 0, 1.47 and 2.45 kg ai ha^-1 treatments, respectively. Acetochlor loss was less than 0.001% of applied, when averaged across treatments. Ethanesulfonic acid was detected in 14 of 14 samples and the average concentration was 0.68 and 0.97 µg L^-1 for the 1.47 and 2.45 kg ai ha^-1 treatments, respectively. Oxanilic acid was detected in 12 of 14 samples and the average concentration was 0.36 and 0.53 µg L^-1 for the 1.47 and 2.45 kg ai ha^-1 treatments, respectively.