Stream transport of herbicides and metabolites in a tile-drained, agricultural watershed

The occurrence of metabolites of many commonly used herbicides in streams has not been studied extensively in tile-drained watersheds. We collected water samples throughout the Upper Embarras River watershed [92% com, Zea mays L., and soybean, Glycine max (L.) Merr.] in east-central Illinois from March 1999 through September 2000 to study the occurrence of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(methoxy-1-methylethyl) acetamide], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide], acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl) acetamide], and their metabolites. River water samples were collected from three subwatersheds of varying tile density (2.8-5.3 km tile km^-2) and from the outlet (United States Geological Survey [USGS] gage site). Near-record-low totals for stream flow occurred during the study, and nearly all flow was from tiles. Concentrations of atrazine at the USGS gage site peaked at 15 and 17 μg L^-1 in 1999 and 2000, respectively, and metolachlor at 2.7 and 3.2 μg L^-1; this was during the first significant flow event following herbicide applications. Metabolites of the chloroacetanilide herbicides were detected more often than the parent compounds (evaluated during May to July each year, when tiles were flowing), with metolachlor ethanesulfonic acid [2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoethanesulfonic acid] detected most often (>90% from all sites), and metolachlor oxanilic acid [2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoacetic acid] second (40-100% of samples at the four sites). When summed, the median concentration of the three chloroacetanilide parent compounds (acetochlor, alachlor, and metolachlor) at the USGS gage site was 3.4 μg L^-1, whereas it was 4.3 μg L^-1 for the six metabolites. These data confirm the importance of studying chloroacetanilide metabolites, along with parent compounds, in tile-drained watersheds.