A FIELD-STUDY TO EVALUATE LEACHING OF ALDICARB, METOLACHLOR, AND BROMIDE IN A SANDY LOAM SOIL

Transport and transformation of pesticides used in agricultural situat ions are subject to variability from several sources that are impossib le to simulate in laboratory settings. This study was conducted to cha racterize pesticide leaching behavior under conventional cropping cond itions and to determine the impacts of related sources of variability. An agricultural field site (Ardilla fine sandy loam, Clarendon loamy sand, Tifton loamy sand, and Lucy loamy sand) located within the Dough erty Plain region of southwest Georgia was used to study pesticide mov ement in unsaturated and saturated soil zones from 1984 through 1988. A granular formulation of aldicarb [2-methyl-2-(methylthio) propionald ehyde O-(methylcarbamoyl) oxime), an emulsifiable concentrate of metol achlor 2-ethyl-6-methylphenyl)N-(2-methoxy-1-methylethyl) acetamide], and a bromide (Br-) tracer were applied on peanut (Arachis hypogea L.) crops under modified conventional tillage practices. Postapplication vertical movement was monitored for periods of up to 111 d for pestici des and 1307 d for Br-. Application distributions for aldicarb showed measured coefficients of variation ranging from 42 to 72, and for meto lachlor from 23 to 44. Spatial variability analyses indicated the exis tence of small-scale correlation for both pesticides upon initial appl ication, and some large-scale trending was observed for metolachlor. F ield-based transformation rates of aldicarb were up to three times hig her than laboratory-based values. There was no evidence of migration o f any of the pesticides into the saturated zone during the study, alth ough Br- was observed at less than 1 mg kg-1 in well-water samples. Al dicarb degraded almost completely within 90 d each year, and it was no t observed below 1.2 m. Metolachlor profiles always showed concentrati ons decreasing with increasing depth, and there was no significant mov ement below 0.3 m. Bromide was observed in soil samples as deep as 3 m only on the 1307th d, and the profiles showed peak concentrations no lower than 1.5 m with relative vertical dispersion increasing continuo usly.