Assessment of Contaminated Sediments in Slip C Duluth Harbor, Minnesota

A sediment remediation scoping project was conducted in a contaminated boat slip in theDuluth, MN Harbor. Previous sediment investigations of this boat slip, Slip C, showed elevated levels of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), DDT metabolites, toxaphene, mercury, cadmium, copper, lead, and zinc. A sediment survey was conducted in June 1997 to collect additional sediment samples to further delineate the spatial extent of PAH, PCB, lead, and mercury contamination, as well as the distribution of total organic carbon (TOC) and particle size classes. Total PAHs, lead, and mercury were found to co-vary, with the strongest linear relationship between total PAHs and lead (r2 = 0.877). The percentage composition of PAH compounds throughout the samples were fairly uniform, which may be indicative of a common source material of PAHs, such as from the combustion of fossil fuels. High levels of TOC (up to 30%) were found in the sediments in front of a compressed wood product plant that historically discharged industrial effluent into the slip until 1978. Lead, mercury, and total PAHs displayed a logarithmic correlation to TOC. Particle size proved to be an important indicator because the finer-grained sediments were more contaminated than the sandy, coarser-grained sediments. Lead had the best correlation with the different particle size classes, followed by total PAHs, mercury, and TOC. PCBs did not correlate well with any of the other parameters, possibly due to the small number of samples analyzed for PCBs, narrow range of corresponding TOC values, and highly elevated PCB concentrations in the core sections from one sample. Contaminant data were compared to several sediment quality assessment values/guidelines from other jurisdictions. The greatest exceedances of the guideline values occurred in the inner portion of the slip, which had more historical sources of contamination. The outer slip is more sandy and a portion of it is periodically dredged. Contaminant data were pooled with data collected in 1994 in order to produce contaminant isopleths for the 0-15, 15-30, and 30-45 cm depth intervals. These figures provided an effective visual picture of the distribution of contaminants. No potentially responsible parties were identified due to the historical nature of much of the contamination and current nonpoint sources, although this will be investigated further with the identification of historical business operations. Decisions on whether to remediate this site will be postponed until sediment quality objectives are developed for the St. Louis River Area of Concern (AOC) in late 1999, a sediment bioaccumulation study is completed in this slip in late 1999, and further work is done to assess groundwater and soil contamination adjacent to the slip. A sediment management plan for Slip C will be incorporated into an environmental management plan for the Duluth waterfront.