ATRAZINE AND ALACHLOR DISSIPATION RATES FROM FIELD EXPERIMENTS

Chemical transport is being monitored in the root zone of three agricu ltural management systems at the Ohio Management Systems Evaluation Ar ea (OMSEA). Atrazine and alachlor concentration data from soil cores t aken to a depth of 0.9 m and partitioned into the increments of 0.0 to 0.15, 0.15 to 0.3, 0.45 to 0.6, and 0.75 to 0.9 m show the herbicides remained in the top 0.15 m of the profile during the 1991 and 1992 gr owing seasons. The slow movement of herbicides was partly due to below normal rainfall during the period. Since the herbicides have not been transported out of the soil profile, dissipation rates could be deter mined from the field observations. The data collected follow first-ord er kinetics in the dissipation of atrazine during the 1991 and 1992 gr owing season and of alachlor during the 1991 growing season for the tw o- to three-month period following chemical application. The computed rate constant, k, was 0.02 d(-1) and half-life, t(1/2), was 35 days fo r atrazine for both years. A rate constant of 0.04 d(-1) and half-life of 17 days were computed for alachlor. The degradation rates became s lower with residence time in the soil as a result of decreased availab ility from sorption/binding in the soil.