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Effect of Temperature and Ultraviolet Radiation on the Persistence of Methyl Parathion and DDT in Soils¹

The effect of temperature and ultraviolet (uv) radiation on the persistence of methyl parathion (o,o-dimethylo, p-nitrophenyl phosphorothionate) and DDT [2,2 bis(pchlorophenyl)-l,l,l-trichloroethane] in soils was studied 0sing controlled environmental conditions. Biologically effective uv radiation (300 to 400 nm) was used. Three alkaline soil types were used in the study: Houston Black clay, Pima silty clay, and Pinal gravelly loam.

Soils in petri dishes were treated with a mixture of methyl parathion and DDT at 5, 20, or 1,000 µg/g. The 5 and 20 µg/g treatments were used to quantitate the amount of pesticide remaining in the soils at periodic intervals. The 1,000 #g/g treatment was used for qualitative studies of breakdown products. Standard samples of pesticides were irradiated with uv radiation for qualitative studies.

The results indicated that both temperature and uv radiation accelerated the loss of pesticides from soils. Temperature appeared to play a more important role in the loss of DI)T than in methyl parathion. At each temperature (30 and 50 C) the addition of uv radiation accelerated the loss of both methyl parathion and DDT. There was a 29% or greater loss of DDT (depending upon the soil type.), above that observed in the 50 treatment not receiving uv radiation.

The only identifiable breakdown product of DDT in the soil extracts was DDE [2,2 bis(p-chlorophenyl)-l,1- dichloroethylene]. There were no breakdown products for methyl parathion in the soil extracts. The breakdown products from the irradiated standard samples of DDT and DDE were numerous. For DDT they included DDE, DDD [2,2 bis (p-chlorophenyl)-l,l-dichloroethane], DBP (4,4'-dichlorobenzophenone), and DDMU [1-chloro- 2,2-bis(p-chlorophenyl)ethylene]. The oxon derivative methyl parathion was tentatively identified as a breakdown product of methyl parathion.

Key Words: Insecticides

² USPH Trainee aml Associate Professor, respectively, Departmerit of Plant Sciences, Texas A&M University, College Station, Texas 77843