PCB DECLINE IN SETTLING PARTICLES AND BENTHIC RECYCLING OF PCBS AND PAHS IN LAKE SUPERIOR

Sediment traps were deployed at five sites in Lake Superior at multipl e depths during lake stratification in 1987 and 1991. Mass, organic ca rbon, PCB, and PAH fluxes were determined. PCB concentrations on settl ing solids declined from 1984 to 1991 with a first-order rate constant of 0.26 yr(-1) similar to reported water column concentration decreas es (0.20 yr(-1)). Total PCB settling fluxes from the upper 35 m of wat er averaged 121 +/- 40 ng/m(2).d in 1987 and 48 +/- 23 ng/m(2).d in 19 91. Settling fluxes are greater than reported wet and dry deposition f luxes (2.8 ng/m(2).d) and demonstrate the intense recycling of PCBs wi thin the lake. A large fraction (>50%) of the total Lake Superior wate r PCB burden is transported each year by settling particles to within 35 m of the lake bottom, but only 2-5% of settling PCBs accumulate in bottom sediments. Thus, most of the PCBs are recycled in the benthic r egion, possibly representing a major entry point for PCBs into higher trophic levels through the benthic food web. Benthic recycling of PAH compounds with three and four rings occurred, but a larger fraction of these settling PAHs accumulated in bottom sediments (8-33%). No consi stent temporal trends were observed in PAH concentrations on settling particles from 1984 to 1991.