ARS | Publication request: STORM FLOW EXPORT OF METOLACHLOR FROM A COASTAL PLAIN WATERSHED

Submitted to: Journal of Environmental Science and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 7, 1999
Publication Date: N/A

Interpretive Summary: To produce high quality, inexpensive crops, farmers often need to use chemicals to control weeds, insects, and diseases. Sometimes traces of these chemicals are found in water samples collected from streams. During high rainfall periods, runoff from farm fields can transport these chemicals into streams. One of the chemicals that is occasionally found in streams is metolachlor. Metolachlor is used with crops, usually corn, to control weeds and allow the corn plant greater access to water and fertilizer. One summer, a sampler automatically collected stream samples when the height of the stream reached a certain level. During this summer, a tropical storm and two hurricanes made landfall near the stream we were studying. These storms, plus three large thunderstorms, raised the stream level to the correct height for samples to be collected. We found that during these storms, traces of metolachlor could usually be found in the stream shortly after the storm started and its occurrence tapered off towards the end of the storm. During this summer, Hurricane Fran produced enough rainfall that samples were collected almost the entire month of September. Even though the chemical was detected several times during that summer, the total amount found was very small. This meant that very little was leaving the farmers' fields and getting into the stream.

Technical Abstract: The herbicide metolachlor has been identified as one of the most commonly detected pesticides in surface water monitoring projects throughout the United States. 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 new sampling procedure, focused on intently sampling 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. Storm flow exports leaving the watershed represented 0.1 g/ha or about 0.04% of active ingredient applied. .