Environ. Sci. Technol. 1994, 28, 1491-1501
Seasonal Variations in Air-Water Exchange of Polychlorinated Biphenyls in Lake Superior Kerl C. Hornbuckle,t Jeff D. Jeremlason,t Clyde W. Sweet,* and Steven J. Elsenreich'gt
Gray Freshwater Biological Institute and Department of Civil Engineering, University of Minnesota, P.O. Box 100, Navarre, Minnesota 55392,and Illinois State Water Survey, Champaign, Illinois 6 1820 Instantaneous and seasonal fluxes of polychlorinated biphenyls (PCBs) across the Lake Superior air-water interface were predicted using a modified two-film gasexchange model. Instantaneous fluxes of CPCB (sum of 80 congeners) were calculated from air and water samples collected simultaneously during short cruises in 1988,1990, and 1992. Volatilization fluxes dominated in late summer in warmer waters (15-22 "C) while CPCBs were deposited into cool springtime waters (-2 "C). Seasonal fluxes were calculated using air samples collected at 12-day intervals over 2 years (1991-1992). Gas-phase CPCB exhibited two concentration maxima: in May and in August. The monthly average homolog concentrations were modeled as the sum of two Lorentzian functions and applied to the two-film gas-exchange model. The largest volatilization fluxes were calculated for fall, when water temperatures were relatively warm (7-10 "C) and vapor PCB concentrations were low (-65 pg/m3). Vapor deposition to Lake Superior is indicated from late April through May, into 0-3 "C water when vapor CPCB concentrations peak at -200 pg/m3. The annual CPCB flux from Lake Superior for 1992 is 250 kg/yr.
Introduction Vapor exchange of semivolatile organic compounds (SOCs) across the air-water interface is an important process in the natural environment (1-5). A repetitive cycle of vapor deposition to and volatilization from oceans, seas, and large lakes contributes to SOC distributions far from their original sources (6-11). Vapor deposition into previously uncontaminated regions is often a major source of the chemicals to the region (3,6-8). Once natural waters are contaminated by SOCs, usually by direct input to the water, volatilization may impact the local atmosphere and rival land-based sources as the major source of atmospheric pollution of some SOCs (9-11). Polychlorinated biphenyls (PCBs) are a class of persistent SOCs that degrade slowly and tend to bioaccumulate (12-14). Some PCB congeners interfere with bird and mammal reproduction, are suspected carcinogens, and may cause developmental disorders in humans (15-18). Due to large-scale production and use of PCBs in the Great Lakes basin, PCBs are chemicals of concern to state and federal regulatory agencies that oversee Great Lakes water quality (19,20). For these agencies, quantitative estimation of the atmospheric component of the sources and sinks in the Great Lakes is vital to predicting chemical residence time, ecosystem exposure, and the atmospheric component of whole-lake mass balances. Although atmospheric deposition is thought to be responsible for 90% of Lake Superior's PCB burden,
* Author to whom correspondence should be addressed; e-mail address:
[email protected]. f University of Minnesota. Illinois State Water Survey. 0013-936X/94/0928-1491$04.50/0
@ 1994 American Chemical Society
volatilization is a major loss process for PCBs from the water column (8, 21, 22). Whether volatilization or deposition occurs in Lake Superior is dependent on season (23-25). Seasonal variation in PCB flux is expected because the system is close to chemical equilibrium, seasonal water temperature variations will modify the chemical equilibrium and mass transfer rates, and recent studies have indicated that atmospheric PCBs exhibit strong seasonal variations (25-29). The objectiveof this study was to quantify instantaneous fluxes and seasonal trends in PCB congener and homolog flux to and from Lake Superior. Our strategy was to measure instantaneous fluxes using 15 pairs of air and water samples collected from research ships in early spring and late summer. PCB concentrations from these samples are applied to a modified two-film model that depends on the following parameters: water temperature, wind speed, and PCB physical and chemical properties. The seasonal trend in the direction and magnitude of PCB flux is similarly calculated from the same model and a separate set of vapor-phase samples. Variations in vapor-phase PCB concentrations used for the seasonal fluxes were modeled from vapor-phase samples collected over 2 years on the southern shore of Lake Superior. The modeled vapor-phase concentrations, together with seasonal wind speeds and water temperatures, contribute to the prediction of annual air-water exchange of PCBs in Lake Superior.
Experimental Methods Sampling Strategy. Air and water sampling in the Great Lakes were performed aboard the R/V Seward Johnson (Harbor Branch OceanographicInstitute, Harbor Branch, FL) in July 1988 and August 1990and aboard the U.S. EPA R/V Lake Guardian (Bay City, MI) in May 1992. Air samples were collected from the bow (19901992) or flybridge (1988) with a high volume air sampler (GPS1 Graseby/GMW, Cleves, OH) equipped with glass or quartz fiber filter and polyurethane foam. The air sampler operated only when the wind was
