Persistence of ddt and lindane in tropical soils
The persistence of radiolabeled ^C-DDT and ^C-lindane (gamma-HCH) in tropical soils of Kenya under field conditions was investigated. For each insecticide two experiments were carried out at two sites of fairly different climatic conditions and types of soils. Surface applications of the compounds were done on columns of soil isolated by driving hollow stainless steel cylinders into the soil. The amounts of pesticides’ residues were determined by utilizing the liquid scintillation counting technique. Both insecticides dissipated more rapidly under the Kenyan tropical climate than reported in temperate regions. The time for 50% dissipation from the soil for ^C-DDT residues was 88 (+5) days and 98(+17) days for Mombasa and Nairobi experiments respectively. The Mombasa experiment lasted for 122 days and that of Nairobi for 217 days. The amounts of pesticide residues detected in the soils at the end of the experiments were equivalent to 38.4% and 30.4% of activity of pesticide initially applied in Mombasa and Nairobi experiments respectively. Dissipation of ^C-lindane showed two distinct phases in both Mombasa and Nairobi experiments. In both experiments, dissipation was most rapid during the initial 10 days. The time for 50% loss of pesticides residues during the initial phase was 5 days and 4 days for the Mombasa and Nairobi experiments respectively. The corresponding values of time for the final 30 days were 8 and 48 days. At the end of the experiments , the amounts of pesticide residues left in the soil were equivalent to 4.1% and 13.0% of the activity of the pesticide xiii initially applied at Mombasa and Nairobi. Binding of residues of both pesticides was shown to occur in soils of both experimental sites. Dissipation of both pesticides was attributed largely to volatilisation.