Degradation and movement of aldicarb residues in Florida citrus soils

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AbstractThe movement and degradation of the pesticide aldicarb was monitored in two Florida citrus groves. One of these was a bedded grove near Oviedo and the other was located in the central ridge near Lake Hamilton. Soil core samples were collected at 0.3 or 0.6 m depth increments six times over an eight‐month period and were analyzed for aldicarb and its two oxidation products aldicarb suljoxide and aldicarb suljone. These analyses showed a relatively rapid decrease of aldicarb accompanied by an increase in aldicarb sulfoxide and aldicarb suljone; the levels of these carbamate metabolites peaked after about 50 days and then began to decline. The dissipation rate of aldicarb carbamate residues in surface soils corresponded to a half‐life of approximately 20 days at both jield sites. The degradation rate in subsoils appeared to be significantly longer (half‐life greater than 60 days), especially at the Lake Hamilton site.Aldicarb carbamate residue concentrations measured in replicate soil analyses were highly variable, with coeflcients of variation ranging up to 300% for samples collected at the same time and depth. These data point out the need for carefirl design of soil‐sampling strategies (including numbers, timing, and location of samples) in order to obtain statistically valid results from unsaturated zone pesticide monitoring studies.