The technique of chiral phase capillary gas chromatography was applied to investigate the degradation and transport of the persistent chiral pesticide R-hexachlorocyclohexane (R-HCH) in the Lake Ontario environment. Chiral analysis gave the enantiomeric ratios (ERs) of R-HCH in samples taken May-October 1993 from the lake and its atmosphere. ERs of (+)R-HCH/(-)R-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 R-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 R-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 R-HCH in air above Lake Ontario was derived from the lake itself. These results strongly support the need for overwater air measurements to provide better estimates of airwater gas transfer fluxes of persistent organic pollutants.
We investigated the seasonality of the air-water gas transfer of hexachlorocyclohexanes (HCH) in Lake Ontario. These organochlorine pesticides were measured in air and water samples collected on weekly cruises on Lake Ontario from May to October 1993. Air concentrations ranged from 51 to 200 pg m -3 for R-HCH and from 8 to 133 pg m -3 for γ-HCH. Surface water concentrations of R-HCH showed a small seasonal variation: mean values decreased from 922 ( 73 pg L -1 in May to 679 ( 47 pg L -1 in August and gradually increased to 897 ( 76 pg L -1 by October. Levels of γ-HCH did not show seasonal differences and averaged 357 ( 25 in surface waters. Air-water fugacity gradients of R-and γ-HCH were used to predict the direction of air-water gas exchange of these isomers. Reversals in exchange direction were observed for both compounds on time scales of days, weeks, and months. On average, air-water gas transfer of HCH was depositional in May and early June, reversed to volatilizational by August, and returned to depositional for both HCH isomers in October. Net fluxes of R-HCH calculated using a two-film gas exchange model ranged from -50 (deposition) to 25 ng m -2 day -1 (volatilization). Net fluxes of γ-HCH ranged from -63 to 7 ng m -2 day -1 . We estimate that 15 kg of R-HCH was removed and 37 kg of γ-HCH was added to Lake Ontario by air-water gas transfer from May to October 1993.
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