SOIL AS A SOURCE OF ATMOSPHERIC HEPTACHLOR EPOXIDE

The chiral pesticide heptachlor can undergo photolysis to yield the ra cemic products heptachlor-exo-epoxide (HEPX) and photoheptachlor. Hept achlor is also metabolized to nonracemic HEPX in biological systems an d soils. HEPX in ambient air samples from the southern United States a nd Lakes Ontario and Superior was nonracemic and enriched in the (+) e nantiomer. Average enantiomer ratios (ER = areas of (+)/(-) HEPX) in t hese locations ranged from 1.51 to 2.02, and were similar to ER values of HEPX reported for agricultural soils. Airborne heptachlor was race mic, with ERs of 0.98-1.02. These results suggest that the main source of HEPX in ambient air is not photolysis of heptachlor, hut rather me tabolism of heptachlor in soils followed by volatilization of HEPX. Th e study exemplifies the use of chiral analysis for investigating the e nvironmental fate of pesticides.