average ER values of heptachlor were 1.01 Æ 0.05 (n ¼ 6) at Point Petre and 0.98 Æ
0.02 (n ¼ 4) in Muscle Shoals, and they were not significantly different from the
standard. Enantiomeric ratios of heptachlor in two air samples from Columbia were
0.99 and 1.02. These ER values are consistent with a study in which racemic
heptachlor (ER ¼ 1.03) was found in Norwegian and US air (Buser et al. 1992;
Bidleman and Leone 2004; Bidleman et al. 2013a, b).
These results suggest that heptachlor exoepoxide in ambient air does not mainly
arise from heptachlor photolysis, although some racemic portion may come from this
source. Release of heptachlor exoepoxide from soils is a more plausible explanation.
ER values of this heptachlor metabolite averaged 1.30 Æ 0.08 in soils of four British
Columbia vegetable farms (Finizio et al. 1998), 2.87 Æ 1.52 (n ¼ 14) in agricultural
soils from the Corn Belt states of Ohio, Illinois and Indiana (Aigner et al. 1998), and
2.71–3.19 in an agricultural and cemetery soil from Alabama (Wiberg et al. 2001a, b).
The heptachlor exoepoxide in air samples collected 5–140 cm above the soil at the
British Columbia farm showed the same ER as in the soil, suggesting soil to air
transfer. Thus, this metabolite is likely produced in soil by enantioselective epoxidation of heptachlor and subsequently volatilised. In the Great Lakes region, it is also
possible that volatilisation from water contributes some heptachlor exoepoxide to the
atmosphere. However, non-racemic heptachlor exoepoxide was also found in Columbia, SC, and Muscle Shoals, AL, well away from large bodies of water.
In this context, Bidleman et al. raised the question, how the presence of racemic
heptachlor in ambient air can be explained (Bidleman et al. 1998a, b, c, d). If (+)heptachlor in soil undergoes preferential epoxidation to (+)-heptachlor exoepoxide,
the residual heptachlor should be non-racemic and have an ER < 1. However, this
was not observed in British Columbia soils, where the ER of heptachlor was 1.08
despite the selective formation of (+)-heptachlor exoepoxide (ER ¼ 1.30). Heptachlor
transformation in soil proceeds by at least two routes. Following its incorporation into
the soil of an experimental field in Ohio, heptachlor was dissipated with half-life of
0.91 year. Two degradation products were identified, 1-hydroxychlordene and the
more persistent metabolite heptachlor exoepoxide. After 4.5 years, heptachlor was
not detectable and heptachlor exoepoxide accounted for about 20% of the original
heptachlor applied. Heptachlor is more volatile than its epoxide and would be
expected to dissipate more quickly by evaporation. Perhaps only a small proportion
of the heptachlor in soil is converted to the epoxide, and the rest is volatilised or
transformed by processes that are not enantioselective or have different preference for
the heptachlor enantiomers. Another possibility discussed by Bidleman et al.
(1998a, b, c, d) is that racemic heptachlor is released from buildings that were
protected against termites with heptachlor or technical chlordane. Although the
enantiomeric composition of heptachlor in home air has not been determined, the
cis and trans-isomers of chlordane in the air of eight homes in Columbia and Muscle
shoals were racemic, suggesting that the heptachlor in this air would also be racemic.
Atmospheric transport of recently applied heptachlor from outside the United States
or Canada could also contribute racemic heptachlor to ambient air.
Lewis et al. (1999) showed in field and laboratory experiments that environmental changes in soils can alter the preferential transformation of chiral environmental
pollutants. The authors intended to assess the persistence of different pollutant
8.2 Transformation/Accumulation of Chiral Xenobiotics in Higher Organisms
191
0.02 (n ¼ 4) in Muscle Shoals, and they were not significantly different from the
standard. Enantiomeric ratios of heptachlor in two air samples from Columbia were
0.99 and 1.02. These ER values are consistent with a study in which racemic
heptachlor (ER ¼ 1.03) was found in Norwegian and US air (Buser et al. 1992;
Bidleman and Leone 2004; Bidleman et al. 2013a, b).
These results suggest that heptachlor exoepoxide in ambient air does not mainly
arise from heptachlor photolysis, although some racemic portion may come from this
source. Release of heptachlor exoepoxide from soils is a more plausible explanation.
ER values of this heptachlor metabolite averaged 1.30 Æ 0.08 in soils of four British
Columbia vegetable farms (Finizio et al. 1998), 2.87 Æ 1.52 (n ¼ 14) in agricultural
soils from the Corn Belt states of Ohio, Illinois and Indiana (Aigner et al. 1998), and
2.71–3.19 in an agricultural and cemetery soil from Alabama (Wiberg et al. 2001a, b).
The heptachlor exoepoxide in air samples collected 5–140 cm above the soil at the
British Columbia farm showed the same ER as in the soil, suggesting soil to air
transfer. Thus, this metabolite is likely produced in soil by enantioselective epoxidation of heptachlor and subsequently volatilised. In the Great Lakes region, it is also
possible that volatilisation from water contributes some heptachlor exoepoxide to the
atmosphere. However, non-racemic heptachlor exoepoxide was also found in Columbia, SC, and Muscle Shoals, AL, well away from large bodies of water.
In this context, Bidleman et al. raised the question, how the presence of racemic
heptachlor in ambient air can be explained (Bidleman et al. 1998a, b, c, d). If (+)heptachlor in soil undergoes preferential epoxidation to (+)-heptachlor exoepoxide,
the residual heptachlor should be non-racemic and have an ER < 1. However, this
was not observed in British Columbia soils, where the ER of heptachlor was 1.08
despite the selective formation of (+)-heptachlor exoepoxide (ER ¼ 1.30). Heptachlor
transformation in soil proceeds by at least two routes. Following its incorporation into
the soil of an experimental field in Ohio, heptachlor was dissipated with half-life of
0.91 year. Two degradation products were identified, 1-hydroxychlordene and the
more persistent metabolite heptachlor exoepoxide. After 4.5 years, heptachlor was
not detectable and heptachlor exoepoxide accounted for about 20% of the original
heptachlor applied. Heptachlor is more volatile than its epoxide and would be
expected to dissipate more quickly by evaporation. Perhaps only a small proportion
of the heptachlor in soil is converted to the epoxide, and the rest is volatilised or
transformed by processes that are not enantioselective or have different preference for
the heptachlor enantiomers. Another possibility discussed by Bidleman et al.
(1998a, b, c, d) is that racemic heptachlor is released from buildings that were
protected against termites with heptachlor or technical chlordane. Although the
enantiomeric composition of heptachlor in home air has not been determined, the
cis and trans-isomers of chlordane in the air of eight homes in Columbia and Muscle
shoals were racemic, suggesting that the heptachlor in this air would also be racemic.
Atmospheric transport of recently applied heptachlor from outside the United States
or Canada could also contribute racemic heptachlor to ambient air.
Lewis et al. (1999) showed in field and laboratory experiments that environmental changes in soils can alter the preferential transformation of chiral environmental
pollutants. The authors intended to assess the persistence of different pollutant
8.2 Transformation/Accumulation of Chiral Xenobiotics in Higher Organisms
191