ASSESSING ANNUAL WATER-AIR FLUXES OF POLYCHLORINATED-BIPHENYLS IN LAKE-MICHIGAN

Air-water exchange of PCBs was determined in Lake Michigan on an event and seasonal basis in 1991-1993. Instantaneous fluxes of Sigma PCB (sum of 77 congener peaks) based on air-water concentration gradients drawn from air and water samples collected simultaneously aboard ship demonstrated net volatilization in September 1991. Air samples collected the northeastern shore of Lake Michigan (Sleeping Bear Dunes State Park) between December 1991 and July 1993 showed no seasonal trend in vapor-phase Sigma PCB concentrations and ranged from 30 to 400 pg/m(3). These air concentrations were used to calculate seasonal water-air fluxes of Sigma PCB that ranged from -18 ng m(-2) day(-1) (net deposition) to 60 ng m(-2) day(-1) (net volatilization). The seasonal variation of vapor-phase and dissolved-phase PCBs in the impacted southern quarter of the lake are unknown, thereby hindering estimation of fluxes in this region. The estimated annual net Sigma PCB flux is 12.3 mu g m(-2) yr(-1), which corresponds to 520 kg for the northern three-quarters of Lake Michigan.