Sediment Remediation Scoping Project in Minnesota Slip, Duluth Harbor

A sediment remediation scoping project was conducted in Minnesota Slip in the Duluth, MN Harbor. Previous investigations of this boat slip indicated that elevated levels of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), DDT metabolites, toxaphene, mercury, cadmium, chromium, copper, lead, nickel, and zinc were present in the sediments. Minnesota Slip appears to be an orphan site since this contamination has not been attributed to any potentially responsible parties. A sediment survey was conducted in August 1998 and September 1999 to collect additional sediment samples to further delineate the vertical and horizontal distribution of a short-list of chemicals of potential concern (i.e., PAHs, PCBs, lead, mercury, and zinc), as well as the distribution of total organic carbon (TOC) and particle size classes. In addition, acid volatile sulfides (AVS), simultaneously extractable metals (SEM), ammonia, cadmium, chromium, copper, nickel, and selenium were measured in six surficial (0-5 cm) sediment samples in which matching sediment toxicity tests were run. For 10-d sediment toxicity tests run using the midge, Chironomus tentans, comparison of treatment survival to the control was made by observation since control survival was less than in any of the treatments. For 28- to 42-d sediment toxicity tests run using the amphipod, Hyalella azteca, survival in two of six samples was significantly lower than in the respective controls. This toxicity was associated with [SEM] – [AVS] > 5 in the corresponding sediment chemistry samples, although the specific cause of toxicity could not be determined. Isopleth figures of sediment quality data were assembled for various depth intervals for the following parameters: percentage of sand, silt, and clay, TOC, total PCBs, total PAHs, mercury, lead, and zinc. Contamination in the slip is heterogeneous, and several sites either exceeded the corresponding Level I or Level II sediment quality targets (SQTs). The greatest exceedances of the Level II SQTs occurred with PAHs. Mean probable effect concentration quotients (PEC-Qs) were calculated for surficial sediments in Minnesota Slip and compared to other surficial sites within the St. Louis River Area of Concern (AOC), as well as other Great Lakes AOCs. Minnesota Slip is much less contaminated than one of the Superfund sites in the Duluth Harbor, but more contaminated than other areas in the St. Louis River AOC. In addition, the Oswego River, NY AOC and the Cuyahoga River, OH AOC had similar mean PEC-Qs as Minnesota Slip. The volume of contaminated sediments was estimated in only the upper depth segments of Minnesota Slip in which sediment quality data were available for the entire slip. A short-list of sediment remediation options, including dredging, is presented for further consideration. However, before any of these remediation options can be seriously evaluated, best management practices need to be implemented to reduce contaminant inputs from five storm water outfalls in Minnesota Slip.