ACETOCHLOR IN THE HYDROLOGIC SYSTEM IN THE MIDWESTERN UNITED-STATES, 1994

The herbicide acetochlor ethoxymethyl)-N-(2-ethyl-6-methylphenyl)aceta mide] was given conditional registration in the United States by the U .S. Environmental Protection Agency in March 1994. This registration p rovided a rare opportunity to investigate the occurrence of a pesticid e during its first season of extensive use in the midwestern United St ates. Water samples collected and analyzed by the U.S. Geological Surv ey during 1994 documented the distribution of acetochlor in the hydrol ogic system; it was detected in 29% of the rain samples from four site s in Iowa, 17% of the stream samples from 51 sites across nine states, and 0% of the groundwater samples from 38 wells across eight states. Acetochlor exhibited concentration increases in rain and streams follo wing its application to corn in the midwestern United States, with 75% of the rainwater and 35% of the stream samples having acetochlor dete cted during this time period. Acetochlor concentrations in rain decrea sed as the growing season progressed. Based on the limited data collec ted for this study, it is anticipated that acetochlor concentrations w ill have a seasonal pattern in rain and streams similar to those of ot her acetanilide herbicides examined. Possible explanations for the abs ence of acetochlor in groundwater for this study include the rapid deg radation of acetochlor in the soil zone, insufficient time for this fi rst extensive use of acetochlor to have reached the aquifers sampled, and the possible lack of acetochlor use in the recharge areas for the wells examined.