Enhanced removal of hydrophobic organic contaminants by settling sediments in western Lake Superior

Sediment cores from Lake Superior were collected, dated, and analyzed for a suite of hydrophobic organic contaminants (HOCs) including polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and n-alkanes. The cores spanned an area from near Duluth/Superior in the western arm to the open waters of western Lake Superior. While accumulation rates for all HOCs were lower than in other Great Lakes, all analytes accumulated at a much higher rate in the western arm of Lake Superior closest to the Duluth/Superior area. The data indicate that HOCs in the western arm are removed from the water column more efficiently by settling particles resulting in higher accumulation rates and inventories. Atmospheric HOC loads focused via a terrestrial pathway by the St. Louis River also contribute to higher HOC loads to the western arm. A terrestrial biomarker, retene, and n-alkanes characteristic of higher terrestrial plant material exhibit a steep gradient decreasing from the western arm into the open waters of the western basin. A simple model was developed based on hydrophobicity and total suspended matter that supports the general observed pattern of PAH surface accumulation among the three coring sites. In particular, the model predicts a greater abundance of lower molecular weight PAHs in near-shore areas with higher total suspended material concentrations than in open waters. This phenomenon could explain the greater abundance of lower molecular weight PAHs observed in near-shore areas in other systems including the Black Sea and Lake Michigan. Differences in model results and measured surface accumulation rates and inventories of PAHs as well as carbon preference indices suggest that sediments from the western arm of Lake Superior are subject to local sources including uncombusted petroleum.