J. Great Lakes Res. 26(2):196–208 Internat. Assoc. Great Lakes Res., 2000 PCBs and PAHs in Southern Lake Michigan in 1994 and 1995: Urban Atmospheric Influences and Long-Term Declines John H. Offenberg † and Joel E. Baker* Chesapeake Biological Laboratory The University of Maryland Center for Environmental Science Solomons, Maryland 20688 ABSTRACT. Surface water samples were collected in southern Lake Michigan off Chicago, IL to deter- mine if elevated levels of urban atmospheric pollutants influence concentrations of organic contaminants in nearshore surface waters. Water was sampled every 12 hours for 2 to 12 days during May 1994, July 1994, and January 1995 sampling intensives. Total (dissolved + particulate; sum of 85 congeners) poly- chlorinated biphenyl (Σ-PCB) concentrations ranged from 80 pg/L (26 July 1994) to 350 pg/L (18 Janu- ary 1995), while total polycyclic aromatic hydrocarbon (Σ 24 -PAH) concentrations during the same period ranged from 7 ng/L (18 May 1994) to 19 ng/L (17 January 1995). PCB concentrations in surface waters were higher when winds were from the S/SW, suggesting that the rate of direct atmospheric deposition of urban PCBs to the surface water may be fast relative to aquatic horizontal mixing and removal mecha- nisms. Additionally, PCB concentrations were higher during the winter than in the spring or summer, but PAH concentrations were not significantly different throughout the year. Lipid normalized PCB concen- trations in zooplankton (> 202 μm) appear to be higher during the winter (780 vs. 14,200 ng Σ-PCBs/g- lipid; July vs. January), but PAH levels in zooplankton were invariant with season (21,000 vs. 21,200 ng Σ-PAH/g lipid; July vs. January). The seasonal trend in surface water concentrations suggests relatively constant, elevated deposition of urban PAHs throughout the year, but net PCB exchange that varies sea- sonally. In spite of these elevated atmospheric deposition fluxes, the concentration of Σ-PCB in Lake Michigan surface waters has declined ten fold over the past 14 years. The resulting pseudo-first order loss rate constant for Lake Michigan is 0.17 ± 0.3/yr (t 1/2 = 4.0 years), which closely matches those found for the atmosphere and biota of Lake Michigan. INDEX WORDS: Atmospheric deposition, surface water, zooplankton, AEOLOS, PCB, PAH, Lake Michigan, winter. 196 *Corresponding author. E-mail: baker@cbl.umces.edu † Current address: Statoil Research Centre, 7005 Trondheim, Norway INTRODUCTION Inputs of persistent organic pollutants, such as polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs), have severely im- pacted the Great Lakes water quality and fisheries (DeVault et al. 1996). Atmospheric deposition of these contaminants is an important, if not dominant source of PCBs and PAHs to the northern Great Lakes relative to all other sources (Strachan and Eisenreich 1988, Hoff et al. 1996, Hillary et al. 1998). Large urban/industrial areas are a significant source of these hazardous air pollutants to the adja- cent coastal atmosphere (Simcik et al. 1997, Harner and Bidleman 1997, Pirrone et al. 1995, Cotham and Bidleman 1995, Holson et al. 1991). Recent studies suggest that elevated atmospheric concen- trations of PCB and PAHs in the greater Chicago area enhance loadings to southern Lake Michigan (Offenberg and Baker 1997; Simcik et al. 1996, 1997, 1998, 1999a; Pirrone et al. 1995; Holson et al. 1991; Franz et al. 1998; Zhang et al. 1999). The elevated atmospheric concentrations in Chicago lead to large dry deposition fluxes of 0.2 μg/m 2 /day from PCBs bound to extremely large (super-mi- crometer) aerosols (Holson et al. 1991). Further- more, PCBs from the urban atmosphere of Chicago are effectively delivered to the surface of the lake by precipitation (Offenberg and Baker 1997). The objective of this research was to quantify the concentrations and speciation (dissolved and associ- ated with suspended particles and macrozooplank- ton) of PCBs and PAHs in the surface waters of