Seasonality of air-water fluxes of hexachlorocyclohexanes in Lake Ontario

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 alpha-HCH and from 8 to 133 pg m(-3) for gamma-HCH. Surface water concentrations of alpha-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 gamma-HCH did not show seasonal differences and averaged 357 +/- 25 in surface waters. Air-water fugacity gradients of alpha- and gamma-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 alpha-HCH calculated using a two-film gas exchange model ranged from -50 (deposition) to 25 ng m(-2) day(-1) (volatilization). Net fluxes of gamma-HCH ranged from -63 to 7 ng m(-2) day(-1). We estimate that 15 kg of alpha-HCH was removed and 37 kg of gamma-HCH was added to take Ontario by air-water gas transfer from May to October 1993.