SEASONALITY OF AIR-WATER FLUXES OF HEXACHLOROCYCLOHEXANES IN LAKE-ONTARIO

We investigated the seasonality of the air-water gas transfer of hexac hlorocyclohexanes (HCH) in Lake Ontario. These organochlorine pesticid es were measured in air and water samples collected on weekly cruises on Lake Ontario from May to October 1993. Air concentrations ranged fr om 51 to 200 pg m(-3) for alpha-HCH and from 8 to 133 pg m(-3) for gam ma-HCH. Surface water concentrations of alpha-HCH showed a small seaso nal variation: mean values decreased from 922 +/- 73 pg L(-1) in May t o 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 differe nces and averaged 357 +/- 25 in surface waters. Air-water fugacity gra dients of alpha- and gamma-HCH were used to predict the direction of a ir-water gas exchange of these isomers. Reversals in exchange directio n 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 return ed to depositional for both HCH isomers in October. Net fluxes of alph a-HCH calculated using a two-film gas exchange model ranged from -50 ( deposition) to 25 ng m(-2) day(-1) (volatilization). Net fluxes of gam ma-HCH ranged from -63 to 7 ng m(-2) day(-1). We estimate that 15 kg o f alpha-HCH was removed and 37 kg of gamma-HCH was added to take Ontar io by air-water gas transfer from May to October 1993.