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Mud Management: The Island and the Superfund Site

Map of sites

If sawdust can be used as mulch and old clothes can be sewn into beautiful quilts, what sort of reuse might be done with excess sediment accumulating at the mouths of Great Lakes tributaries? One answer just happened on the St. Louis River.

Cooperative remediation involving Tallas Island and the St. Louis River/Interlake/Duluth Tar (SLRIDT) Superfund site ended sucessfully in early November. As mitigation of the superfund site sediments began in 2004, it was understood that the water habitat lost to the clean-up process needed to be gained somewhere else. The Minnesota Department of Natural Resources (MNDNR) suggested opening the sediment-clogged backwaters of Tallas Island, and a synergistic plan was born.

Tallas Island stopped being an island

Tallas Island wasn’t an island for many years. The main culprit? Sediment deposition by Knowlton Creek. Like many other creeks in developing areas, Knowlton Creek carries increasing sediment loads as trees are replaced by lawns, roads, and other structures in the watershed. Knowlton Creek watershed drains about 1,400 acres (6 km2) of land and deposited enough sediment in 41 years (1961 to 2002) to create a connection between the mainland and Tallas Island (effectively making it a peninsula instead of an island). Backwaters of the ex-island decreased in depth from 5 to 2 feet (about 1 meter) over the same 41-year period.1

Cleaning up the contaminated sediments

For nearly 70 years, Stryker Bay and the adjoining slips 6 and 7 (collectively known as SLRIDT) served as a repository for industrial byproducts from the coal and tar industries. The resultant tar seeps, contaminated soils and contaminated sediments earned SLRIDT a place on the federal Superfund Site list in 1983. The first two clean-up efforts focused on tar seeps and soils and were completed in 1994 and 1997, respectively.

Contaminated sediment clean-up is winding down. Part of the clean-up effort involves capping the contaminated sediments so that they cannot interact with the environment and placing environmental media on top of the cap to allow wetlands to grow.

Sediments from Tallas cap SLRIDT site

As part of the remediation effort, about 53,000 cubic yards (41,000 m3) of sediments that washed in from the Knowlton Creek watershed were removed with a hydraulic dredge barge and placed as environmental media at the SLRIDT site. Remediation activities resulted in the conversion of the shallow waters of slip 7 into on-shore wetlands. The loss of water habitat was recouped on the backside of Tallas Island. With the completion of the Tallas Island project, the connection between Knowlton Creek and the St. Louis River was restored and two new pools for fish habitat and sediment collection were created.

The plan also allowed for the retention of an existing wetland/upland buffer zone and the creation of a 5-foot deep 30-foot (1.5 m x 9 m) wide channel to the St. Louis River. The new backwater area behind Tallas Island, which may become a MNDNR Fisheries Aquatic Management Area (AMA), will likely serve as a spawning ground and nursery for fish like northern pike, muskies, bluegill and black crappie.2

New wetlands, courtesy of the Knowlton Creek watershed sediments, are already providing habitat at the SLRIDT area for a host of plants and birds. And, thanks to ingenuity, engineering, and hard work, Tallus is an island reborn.

For technical details about assessing sediment quality in the St. Louis River Estuary, read the results of Minnesota Sea Grant supported research published in Aquatic Ecosystem Health and Management: Evaluating methods for assessing sediment quality in a Great Lakes embayment (JR 505).

1 SLRIDT Superfund Site Sediment Cleanup Progress Report. September 2008. XIK Corp.

2 SLRIDT Superfund Site Sediment Cleanup Progress Report. January 2006. XIK Corp.

By Marte Thabes Kitson
December 2010

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