Environ. Sci. Technol. 1996, 30, 852-858
Seasonality of Air-Water Fluxes of Hexachlorocyclohexanes in Lake Ontario J E F F R E Y J . R I D A L , * ,† B R Y A N K E R M A N , † LOUISE DURHAM,† AND MICHAEL E. FOX‡ Lake Meteorology Laboratory, Air Quality Research Processes Branch, Atmospheric Environment Service, Environment Canada, 867 Lakeshore Road, Burlington, Ontario, Canada L7R 4A6, and Aquatic Ecosystem Restoration Branch, National Water Research Institute, Environment Canada, 867 Lakeshore Road, Burlington, Ontario, Canada L7R 4A6
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.
Introduction Air-water gas exchange is a major atmospheric pathway for the transport of persistent organic pollutants (POPs), * Corresponding author present address: St. Lawrence River Institute of Environmental Sciences, 709 Cotton Mill Road, Cornwall, Ontario, Canada K6H 7K7; telephone: 613-936-6620; fax: 613-9361803; e-mail address:
[email protected]. † Atmospheric Environment Service. ‡ National Water Research Institute.
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such as PCBs and the organochlorine pesticides, in the Great Lakes (1-4) and oceans (5-8). Estimates of the magnitude of air-gas exchange for these compounds can be made from simultaneous measurement of air and water fugacities (9). However, difficulties remain estimating seasonal and annual fluxes of POPs by air-water gas exchange (3), owing to the limited numbers of studies, which are often poorly resolved temporally. The purpose of this work was to investigate in greater detail than previous studies the flux of selected POPs, R- and γ-hexachlorocyclohexane (HCH), across the air-water interface during the times of thermocline formation in late spring, surface layer heating in the summer, and turnover in the fall. Technical HCH is a mix of five isomers consisting of R-HCH (60-70%), β-HCH (5-10%), γ-HCH (10-15%), δ-HCH (6-10%), and -HCH (3-5%) (10). The latter two of these compounds are not routinely found in environmental samples (5-8, 11-15). Only γ-HCH [lindane, also a component of Hexachlor (16)] has insecticidal properties, and lindane is registered for limited uses in North America (12, 16). Technical HCH was banned in Canada and the United States in the 1970s, although use of technical HCH has been reported in the past decade in Central and South America, India, Asia, and Europe (10, 12). Worldwide cumulative use since introduction of HCH products is estimated at over 106 t (12). The HCHs have been favored compounds for investigations of POP air-water gas transfer (5-8, 11, 12) because of their relatively high abundances in the environment and relative ease of measurement by gas chromatography and electron capture detection. As well, relative amounts of Rand γ-HCH bound to particulate matter in water (11) and air (13, 14) phases are low (
