PCB Decline in Settling Particles and Benthic Recycling of PCBs and PAHs in Lake Superior

Abstract

Sediment traps were deployed at fives sites in Lake Superior at multiple depths during lake stratification in 1987 and 1991. Mass, organic carbon, PCB, and PAH fluxes were determined. PCB concentrations on settling solids declined from 1984 to 1991 with a first-order rate constant of 0.26 yr{sup {minus}1} similar to reported water column concentration decreases. Total PCB settling fluxes from the upper 35 m of water averaged 121 {+-} 40 ng/m{sup 2}{center_dot}d in 1987 and 48 {+-} 23 ng/m{sup 2}{center_dot}d in 1991. Settling fluxes are greater than reported wet and dry deposition fluxes and demonstrate the intense recycling of PCBs within the lake. A large fraction of the total Lake Superior water PCB burden is transported each year by settling particles to within 5 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 region, 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. No consistent temporal trends were observed in PAH concentrations onmore » settling particles from 1984 to 1991.« less