Model ecosystem studies of the environmental fate of five herbicides used in conservation tillage

The environmental persistence of pesticides, their terminal residues and their contaminants, has played a major role in environmental regulations. To demonstrate degradation in field studies is difficult, both because of the length of time required and because of the complexity of demonstrating degradation of metabolites as well as of the pesticide itself. These difficulties can be circumvented by evaluating radiolabeled pesticides in a terrestrial aquatic laboratory model ecosystem which provides qualitative and quantitative data on persistence, bioaccumulation, and degradation both of the parent compound and of its metabolites. This microcosm was used to obtain data on the environmental degradation of pesticides with major roles in conservation tillage: paraquat, atrazine, linuron, oryzalin, alachlor, and paraquat paired with each of the other herbicides. The results suggest that alachlor, atrazine, linuron, and oryzalin do not pose serious risks of environmental accumulation when applied alone. The results of paired-herbicide eco-system evaluations compared to evaluations of each herbicide separately demonstrate that: paraquat is only minimally transported to aquatic organisms or to water; differences in herbicide application are more important in determining residue levels for these herbicides than are interactions between the herbicide and paraquat, although the latter do occur; and the addition of paraquat does not significantly alter the capacity of these herbicides for bioaccumulation or ecological magnification.