AbstractThe persistence of carbofuran (2, 3‐dihydro‐2, 2‐dimethylbenzofuran‐7‐yl methylcarbamate) and the β‐ and γ‐isomers of HCH (hexachlorocyclohexane) applied to surface (oxidised) and sub‐surface (reduced) layers of a flooded soil was studied using radiolabelled insecticides. In one experiment, these compounds were placed in the surface (2–5 mm) and sub‐surface (10–15 cm) layers of 10‐day flooded soil columns. HCH isomers were unstable under flooded soil conditions irrespective of their placement, but disappeared slightly faster when applied to the sub‐surface layer, possibly due to the more reduced conditions prevailing. In contrast, sub‐surface‐applied carbofuran was more stable than surface‐applied carbofuran. The decreased stability of surface‐applied carbofuran may be due to a relatively higher pH in the surface layer and in the flood water which was in immediate contact with the surface layer. In another experiment, surface and sub‐surface soil samples were collected from a rice field which had been flooded for 30 days. These soils were then again flooded under laboratory conditions prior to addition of carbofuran and β‐HCH. Upon submergence, both surface and sub‐surface soil samples attained almost equally reduced conditions. In flooded surface soil samples, more rapid degradation not only of carbofuran but also of β‐HCH occurred, compared with
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