Persistence and fate of PCBs in sediments of the Saint Lawrence River

Polychlorinated biphenyl (PCB) congener concentrations and composition were determined in sediments from three sections of the St. Lawrence River that have been exposed to high PCB contamination. In at least one sampling station in each river section (upper Lac St-François (LSF) and two sections in metropolitan Montreal, Canada), high PCB concentrations were found in the top 10 cm of sediments (respectively 16.2, 9.8 and 3.8 mg/kg d.w. of Aroclor 1248 equivalents). In the case of sediments along the south shore of LSF, both PCB concentrations (regression analysis) and composition (correspondence analysis) were related to the distance of sampling sites downstream from the local source area. In the most contaminated station of the lake, located near an industrial facility, analysis of sediment core profile dated by 137Cs revealed a relatively constant input of PCBs since ∼ 1982. The di- and tri-chlorinated congeners accounted for more than 70% of the quantified congeners in surface sediments of this station, whereas these congeners made up less than 30% of the PCBs found in less contaminated St. Lawrence River sediments. The PCB concentrations and compositions found in the river sediments suggest that local point sources, river hydrology, physicochemical processes and microbial dechlorination were major factors explaining the fate of PCB congeners in St. Lawrence River sediments.