Herbicide Losses in the Saint Joseph River Watershed: Impacts of Hydrology and Land Management.

Herbicide losses from row crop agriculture represent potential human health hazards, and are a major focus of the Conservation Effects Assessment Project (CEAP).  Atrazine, simazine, alachlor, acetochlor, metolachlor, and glyphosate were monitored in tile-fed drainage ditches draining to a drinking water source.  Water samples were collected daily at 8 monitoring sites located at the outlets of sub basins (298-19,341 ha (736-47,793 ac)) during the 2004-2007 cropping seasons (April–November). The primary objectives of this study were to evaluate levels of these herbicides in a set of surface drainage ditches under the current conservation practices, and to correlate herbicide losses with hydrology and land management.  During the cropping seasons of 2004 and 2006, rainfall and runoff timing were such that major runoff events occurred soon after the time of most planting activities and typical herbicide application.  During these years, higher overall levels of atrazine and metolachlor were observed, with maximum flow-weighted seasonal concentrations of 22 and 10 ug/L, repectively.  Most losses occurred during the first few runoff–producing rainfall events following the period of spring herbicide applications.  During the years of 2005 and 2007, when major runoff events did not occur until later in the season, losses were still associated with the first runoff event following planting and application, but magnitudes of losses were lower.  Simazine, acetochlor, alachlor, and glyphosate levels were not significantly affected by temporal changes in hydrology.