Although degradation data for herbicides are essential in understanding their potential to be contaminants and are indispensable inputs in computer-based modeling of their fate in environment, most available data only concern surface soils. Soil samples, collected at two depths from four representative sites of a 31.4-ha field located in Blue Earth County, MN, USA, were used to determine acetochlor dissipation under laboratory conditions. A field study was also carried out within a 16-ha watershed in Dakota County, MN, USA, where 38 locations were sampled to obtain sample representative of the full range of soil properties found within the watershed. Acetochlor DT50 values ranged from 6.51 to 13.9 days for surface soils and from 20.3 to 26.7 days for subsurface soils. DT90 values were a factor of four times longer than for DT50 values. Field DT50 values for acetochlor dissipation were not significantly different for the 2 years, 5.7 ±2.5 and 7.7±4.5 days. Dissipation was slightly faster in the field as compared to the laboratory; however, the difference seems insignificant in view of the wide range in soil properties inMinnesota. In both studies, acetochlor would be classified as slightly persistent. For acetochlor, laboratory dissipation studies can be considered representative of field dissipation for the soils and climatic conditions in this study. Inclusion of subsoil degradation data in mathematical models used for ground water risk assessment may improve their capability of predicting potential movement of acetochlor to groundwater.
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Acknowledgments The authors would like to thank CAPES, CNPq (Brazil), and the Center of Agricultural Impacts on Water Quality, University of Minnesota for partial financial support and Brian Barber for his technical support in GC analysis.