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DDT Persistence and Volatility as Affected by Management Practices after 23 Years

USDA-ARS, U.S. Salinity Laboratory, 450 W. Big Springs Road, Riverside, CA 92507;

W. J. Farmer

Soil and Environmental Sciences Dep., Univ. of California, Riverside, Riverside, CA 92521.

^* Corresponding author (rcook{at}ussl.ars.usda.gov).

ABSTRACT

In 1971, an experiment was conducted in a field containing high amounts of residual DDT (dichlorodiphenyltrichloroethane) to evaluate deep plowing, followed by flooding, with and without organic matter applications, as soil and water management tools to reduce total DDT residues and preferentially degrade the residual DDT to DDD [1,1-dichloro-2,2-bis (p-chlorophenyl) ethane]. The experimental site was revisited in 1994 to determine residual soil concentrations of DDT isomers and their metabolites in soil, soil dust, and the atmosphere. Also, volatilization flux measurements were made to evaluate rates of movement into the atmosphere. Soil concentrations of all DDT isomers and metabolites had decreased in all plots, with p,p'-DDE[1,1-dichloro-2,2-bis (p-chlorophenyl) ethylene] the major component of the total remaining residues (DDTR). The total DDTR residues in the surface 75 cm varied from 10 to 28% of their amounts in 1971. The highest concentrations were found in the deep plowed, unflooded plots with DDTR decreasing from 4 mg kg^–1 at 0 to15 cm to 0.3 mg kg^–1 at 60 to 75 cm. Deep plowing evidently increased DDT persistence by placing it deeper into the soil profile, which protected it from degradation and volatilization. Concentrations of all isomers were lower in the previously flooded plots. Degrading DDT under reducing conditions brought about by flooding lessened or prevented the formation of DDE in the soil thus ultimately reducing its redistribution into the environment. Significant concentrations of both o,p' and p,p'-DDE and DDT were detected in the atmosphere above the plots. Measurable volatilization fluxes were observed over 48-h periods in February and September. Irrigating the soil with 20 mm water dramatically increased the volatilization flux of all the DDT isomers and metabolites, particularly p,p'-DDE. The finding that DDT isomers continue to volatilize from the soil surface has implications for long-range transport of DDT and contaminating forage or foodstuff. The possible health implications from exposure to humans or animals through the air route is unknown.

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