Emission of chiral pesticides from an agricultural soil in the fraser Valley, british columbia

Abstract

Several organochlorine pesticides (OCs) are chiral compounds, manufactured as racemic mixtures of enantiomers. Selective degradation in soil and water leads to enantiomeric ratios of parent compounds and metabolites which differ from the 1:1 ratio of the applied pesticide. Volatilization of chiral OCs from soil was investigated at a farm in the Fraser Valley known to contain non-racemic pesticide residues. Air samples were collected from 5–140 cm above the soil on four days in September, 1995 to determine the vertical gradients and enantiomeric composition of the OCs. Concentrations of OCs in air were highest near the soil surface and declined with height. A model of Jury et al. (1984) was modified to examine the relationship between the relative concentrations of OCs in air (normalized to heptachlor epoxide) to those in soil. Results support using the octanol-air partition coefficient (K oa) as a descriptor of soil-air exchange. In the soil, α-hexachlorocyclohexane was depleted by 38% in the (-) enantiomer and the metabolite heptachlor epoxide was enriched by 39% in the (+) enantiomer. Non-racemic OCs were also found in the overlying air, indicating that enantiomeric signatures of chiral pesticides are promising tracers of emissions from soils to the atmosphere.