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

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 [l,l-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-dichloro2,2-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-’ at 0 to15 cm to 0.3 mg kg-’ 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. D IS HIGHLY PERSISTENT in soil and even though its use has been discontinued in the USA since 1973, soil residues remain in many areas with p,p'-DDE making up the major proportion of the total DDT residue in such soil (Ware et al., 1978; Cooke and Stringer, 1982; Boul et al., 1994). Under aerobic conditions, much of the p,p'-DDT in soils is degraded to DDE. Spencer and Cliath (1972) reported that the vapor pressure of p,p'-DDE was several times greater than that of p,p'DDT, and Cliath and Spencer (1972) reported that DDE was the major DDT component found in the atmosphere over a field which previously received technical DDT. This led to the conclusion that much of the p,p'-DDT in well-aerated soils may be volatilized as p,p’-DDE (Spencer, 1975). Volatilization is a major mechanism for movement of such residues from soil to aboveground plant parts (Nash and Beall, 1970) and volatilization is a major process for movement of residues away from W.F. Spencer, G. Singh, C.D. Taylor, R.A. LeMert, and M.M. Cliath, USDA-ARS, U.S. Salinity Laboratory, 450 W. Big Springs Road, Riverside, CA 92507; and W.J. Farmer, Soil and Environmental Sciences Dep., Univ. of California, Riverside, Riverside, CA 92521. Received 7 July 1995. *Corresponding author ([email protected]). Published in J. Environ. Qual. 25:815-821 (1996). 815 treated areas resulting in potential exposure to animals and humans through the air route (Taylor and Spencer, 1990; Willett et al., 1993; Hileman, 1994; Samuel and Pillai, 1989). The presence of p,p'-DDE in the biosphere is of much concern to biologists. Metcalf et al. (1971) concluded that DDE is a highly persistent, lipid-partitioning metabolite of DDT that is responsible for the major portion of the environmental effects of concentrations and storage in animal tissues following the use of DDT. Soil and water management practices can be used to alter the pathway of DDT breakdown in soil and thereby change the relative amounts of the various compounds lost by volatilization. In laboratory studies under oxygendeficient conditions caused by flooding and organic matter treatments, DDT was degraded to DDD and other water soluble metabolites (Guenzi and Beard, 1967, 1968). In 1971, a field experiment was conducted 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, thereby changing the ratio of the various compounds potentially evaporating from the soil surface (Farmer et al., 1974; Spencer et al., 1974). The experimental site was revisited in 1994. Soil samples were taken to a depth of 75 cm, soil dust samples were vacuumed from the soil surface and measurement of air concentrations and chamber volatilization flux measurements were made to evaluate atmospheric concentrations and movement into the atmosphere from the soil surface. This paper reports the effects of the past management treatments on amounts of various DDT residues presently in the soil and their movement into the atmosphere. MATERIALS AND METHODS