DDT PERSISTENCE AND VOLATILITY AS AFFECTED BY MANAGEMENT-PRACTICES AFTER 23 YEARS

In 1971, an experiment was conducted in a field containing high amount s of residual DDT (dichlorodiphenyltrichloroethane) to evaluate deep p lowing, followed by flooding, with and without organic matter applicat ions, as soil and water management tools to reduce total DDT residues and preferentially degrade the residual DDT to DDD [1,1-dichloro-2,2-b is (p-chlorophenyl) ethane]. The experimental site was revisited in 19 94 to determine residual soil concentrations of DDT isomers and their metabolites in soil, soil dust, and the atmosphere. Also, volatilizati on 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-chloro phenyl) 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 i n the deep plowed, unflooded plots with DDTR decreasing from 4 mg kg(- 1) at 0 to 15 cm to 0.3 mg kg(-1) at 60 to 75 cm. Deep plowing evident ly increased DDT persistence by placing it deeper into the soil profil e, which protected it from degradation and volatilization. Concentrati ons of all isomers were lower in the previously hooded plots, Degradin g DDT under reducing conditions brought about by flooding lessened or prevented the formation of DDE in the soil thus ultimately reducing it s redistribution into the environment. Significant concentrations of b oth o,p' and p,p'-DDE and DDT were detected in the atmosphere above th e plots. Measurable volatilization fluxes were observed over 48-h peri ods in February and September. Irrigating the soil with 20 mm water dr amatically increased the volatilization flux of all the DDT isomers an d metabolites, particularly p,p'-DDE. The finding that DDT isomers con tinue to volatilize from the soil surface has implications for long-ra nge transport of DDT and contaminating forage or foodstuff. The possib le health implications from exposure to humans or animals through the air route is unknown.