SELECTION OF HERBICIDE ALTERNATIVES BASED ON PROBABLE LEACHING TO GROUNDWATER

Extension workers are sensing pressure to use soils information and ch emical characteristics data to guide farmers in selecting pesticides l east prone to leach into groundwater. Our objective was to estimate di fferences in herbicide migration to groundwater under conditions typic al for the Southeast Coastal Plain, and to consider how a farmer might be advised to use such knowledge in selecting herbicides. We used a s imple computer code for microcomputers to predict persistence and migr ation of 17 herbicides through a hypothetical, coarse-textured soil ty pical of the Southeast Coastal Plain. Appropriate herbicides were sele cted for several common crop-weed problems, such as sicklepod in soybe an and Palmer amaranth in com. Groundwater was assumed to be 3.15 m be low the soil surface. Herbicides selected covered a broad range of hal f-lives and organic carbon partition coefficients. Only after the firs t-order degradation rate constant was reduced by a factor of five did predicted soil water concentrations of several herbicides at the groun dwater interface reach normal detection limits. Still, predicted conce ntrations were below the level established for health effects advisory purposes. Due to the large number of uncertainties and the inability to estimate practical benefits, we conclude that data relating to soil and herbicide characteristics cannot be used at this time to override cost effectiveness, efficacy, and other factors normally considered b y farmers and Extension professionals in herbicides for weed control.