Enantiomers of α-Hexachlorocyclohexane as Tracers of Air−Water Gas Exchange in Lake Ontario

The technique of chiral phase capillary gas chromatography was applied to investigate the degradation and transport of the persistent chiral pesticide α-hexachlorocyclohexane (α-HCH) in the Lake Ontario environment. Chiral analysis gave the enantiomeric ratios (ERs) of α-HCH in samples taken May−October 1993 from the lake and its atmosphere. ERs of (+)α-HCH/(−)α-HCH for Lake Ontario surface and deep water samples were similar and averaged 0.85 ± 0.02 as compared with a value of 1.00 for the α-HCH standard. Higher ERs were observed in water samples from the Niagara River (0.91 ± 0.02) and from precipitation (1.00 ± 0.01). Air samples of α-HCH measured at 10 m above the lake show a seasonal variability with values near 1.00 in spring and fall and minimum values in individual samples near 0.90 in summer. A simple air−water gas transfer model demonstrates that enantiomeric ratios <1.0 in air are derived from equilibration of the air with the water during transport of the air mass over the lake. Based on the measured ERs, it is calculated that as much as 60% of the α-HCH in air above Lake Ontario was derived from the lake itself. These results strongly support the need for over-water air measurements to provide better estimates of air−water gas transfer fluxes of persistent organic pollutants.