Storm flow export of metolachlor from a coastal plain watershed

During an 18-month (1994-1995) survey of the surface water in an Atlantic Coastal Plain watershed, metolachlor was most frequently detected during storm flow events. Therefore, a sampling procedure, focused on storm flow, was implemented in June of 1996. During 1996, three tropical cyclones made landfall within 150 km of the watershed. These storms, as well as several summer thunderstorms, produced six distinct storm flow events within the watershed. Metolachlor was detected leaving the watershed during each event. In early September, Hurricane Fran produced the largest storm flow event and accounted for the majority of the metolachlor exports. During the storm event triggered by Hurricane Fran, the highest daily average flow (7.5 m2 s-1) and highest concentration (5.1 micrograms L-1) ever measured at the watershed outlet were recorded. Storm flow exports leaving the watershed represented 0.1 g ha-1 or about 0.04% of active ingredient applied.