Photochemical Degradation of Malathion

Abstract

This is the final completion report for a two-year project which began 1 November 1975. The original completion date was extended to 30 September 1977 to allow collection of samples and data through the summer of 1977. Malathion is a thiophosphate insecticide, 0,0-dimethly-S-(l,2dicarbethoxyethyl) phosphorodithioate: It is less toxic than DDT and decomposes over a much shorter period of time. With the suspension of DDT in pest control programs in 1965, use of malathion has been increasingly widespread in Alaska's interior. In spite of its low toxicity to animals, malathion is poisonous at some level. Lethal doses for domestic sheep and cattle are 150 mg/kg and 200 mg/kg. of body weight, respectively. The fatal dose of malathion for a 70 kg man has been estimated to be 60 g, with some clinical exceptions (McKee and Wolfe, 1963; Hayes, 1964). Dietary levels (ppm) producing minimal or no effect after continuous feeding for 90 days to 2 years to rats and dogs have been reported as 100-1000 and 100, respectively (Lehman, 1965). On the other hand, malathion has been identified by gas chromatography in extracts of water associated with several fish kills (Garrison, Keith, and Alford, 1972). In a study of malathion persistence in the soil near Fairbanks, Alaska, during the summer of 1967, half of the sampling sites showed the presence of malathion and its oxidation product, malaoxon, prior to aerial spraying (Holty, 1970). Since there had been no ground spraying since the summer of 1966, this would indicate that malathion was not degrading in the environment as fast as anticipated. This is important since it is then possible for the spring runoff to carry significant quantities of the pesticide and its degradation products into streams and rivers in the area. Retention of the malathion appears to depend on the amount of rainfall, and the summer of 1966 had been very dry. During the wetter summer of 1967, the post-spray soil samples showed a rapid drop in the level of malathion except at sampling sites in "mucky" soils which also increased noticeably in moisture as the amount of rainfall accumulated (Figure 1). Very little has been known about the aqueous photodecomposition of malathion and nothing was known of its vapor phase stability under atmospheric conditions and exposure to sunlight prior to this study.