Movement of 14C-carbofuran in a silt clay soil. A laboratory study

Carbofuran is used in the Syrian Arab Republic to control the sitona weevil, Sitona crinitus Herbst (S. macularius Marsch), on legume crops. It is sprayed on soil to kill the immature stages, which attack the leaves close to the soil surface and the nitrogen fixing nodes on the roots in the soil. A laboratory study was conducted to examine the movement of this pesticide into the soil. A known amount of 14C-carbofuran (specific activity of 2 μCi/mg) was applied to the top of the soil columns inside hard polyvinyl chloride (PVC) cylinders inserted into PVC tanks filled with soil of the same origin. The columns were sampled at varying intervals for up to 120 days. Each column was divided vertically from top to bottom into five zones. Each zone was 5 cm high. The radioactive residues were Soxhlet extracted for 6 hours. Extracts were concentrated and the aliquots counted for radioactivity using a liquid scintillation counter (LSC). In addition, samples of Soxhlet extracted soil were combusted in a biological oxidizer and the radioactivity was counted using LSC. The results show that recovery was good. Twenty-four hours after treatment the total radioactivity recovered was 71% of the total amount of insecticide applied. The major radioactive residues were confined to the top 5 cm of the soil column 24 hours after application. The amount of radioactive residues present in the lower layers increased gradually with time. However, this was accompanied by a relatively rapid and gradual loss in the total radioactivity recovered from the whole soil column. At the end of the experiment, most of the recovered radioactive residues (29%) were bound to the soil surface, whereas only 2% of the applied dose was extractable from the whole soil column. (author). 11 refs, 1 fig., 1 tab