Implications of sampling frequency to herbicide conservation effects assessment

Herbicide losses from agriculture represent potential human health hazards, and are one focus of the Conservation Effects Assessment Project. Since frequent herbicide sampling can be rigorous and expensive, it is desirable to determine expected uncertainties associated with reduced sampling frequencies. Atrazine, simazine, alachlor, acetochlor, metolachlor, and glyphosate were monitored in tile-fed drainage ditches. Water samples were collected during the 2004 to 2007 cropping seasons at eight monitoring sites located at the outlets of sub basins ranging in size from 298 to 19,341 ha (736 to 47,793 ac). Herbicide data were analyzed based upon daily sampling, then for 7 possible weekly sampling scenarios, and 14 possible biweekly sampling scenarios. In addition, the value of sampling more intensively during runoff events was evaluated. Statistical analyses indicate the need for management practices to reduce atrazine and metolachlor loading to drainage water can best be assessed in these drainage networks using daily sampling in conjunction with a more intensive sampling regime during storm events, while sampling frequency had little impact on observed levels of other herbicides. This indicates that biweekly sampling may be sufficient for monitoring of some herbicides, allowing for reduced analytical costs.