Persistence of parathion and its oxidation to paraoxon on the soil surface as related to worker reentry into treated crops.

Soil and dust samples from beneath citrus trees contained relatively high concentrations of paraoxon and parathion for at least 45 days after parathion was applied at normal rates. These data indicate that parathion dripping on dry soil from sprayed crops, or sprayed directly on the soil surface, can persist for long periods and be oxidized to the highly toxic paraoxon. The residue concentrations are much higher on the loose dust particles than in the bulk soil. The data indicate that the soil or grove floor should be considered as a possible contributing source of toxicant along with foliage, or dust on foliage, to workers reentering treated fields. In any event, the data confirmed the high rates of oxidation of parathion to paraoxon on particulate matter, whether on foliage (SPEAR et al., 1974) or at the soil surface. The particulate matter appears to contribute to the accumulation of paraoxon, either by enhancing oxidation of parathion to paraoxon or by decreasing dissipation of paraoxon.