Fate of Aldicarb in the unsaturated zone beneath a citrus grove

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Abstract
A field study was conducted in a 1.6‐ha citrus grove located in central Florida to evaluate transport and transformations of the nematicide aldicarb and its two metabolites (aldicarb sulfoxide and aldicarb sulfone) in the unsaturated zone over a 218‐day period during 1984. Bromide leaching was also monitored to provide a tracer of water movement in this well‐drained, deep, sandy soil. Aldicarb and bromide were applied in 3‐m wide treatment bands located midway between tree rows (7.62 m apart) and incorporated into the top 5 cm of soil by disking. This application method resulted in extreme variability in solute concentrations across and within the treatment bands, which persisted at all depths even as the pesticide and bromide leached during the study period. Aldicarb, its metabolites (sulfoxide and sulfone), and bromide leached readily through the soil and moved past the 7.92 m depth by the last sampling date (218 days). Aldicarb dissipated rapidly (half‐life ≤2 weeks), yielding aldicarb sulfoxide and aldicarb sulfone. Both of these compounds also dissipated but at a rate slower than that for aldicarb. The dissipation of total carbamate residues under field conditions was characterized by a half‐life of about 69 days, which was in agreement with laboratory‐derived values. On the basis of the findings of this study, recommendations are made on the protocols for designing and conducting field studies in order to characterize pesticide behavior in the crop root zone.