Innovative remediation and restoration at one of the largest sediment Superfund sites in Minnesota
The 255-acre St. Louis River/Interlake/Duluth Tar (SLRIDT) Superfund site in Duluth represents one of the first hybrid dredging and capping sediment remediation projects completed in the United States. Seven decades of industrial discharges created two peninsulas and three water bodies containing polycyclic aromatic hydrocarbons and other contaminants in a Lake Superior estuary that is a Great Lakes Area of Concern. Tar seeps and soils were cleaned up in the 1990s, but the 90 acres of contaminated sediment proved most challenging.
In 1999, the Minnesota Pollution Control Agency (MPCA) decided that all contaminated sediment must be dredged and placed off site—a remedy that would have cost $140 million and raised concerns about air and water impacts in the surrounding neighborhood.
Working with the responsible parties, the MPCA, a peer-review team, natural resource managers, and numerous other interested groups, Barr developed and designed an innovative remedy involving an onsite contained aquatic disposal (CAD) facility and 18 different types of caps. One of the innovations included a surcharge cap to avoid dredging the highest PAH concentration sediment in a residential bay while restoring the pre-remediation bathymetry within two years of placement. Another innovation involved the first commercial use of an activated carbon mat to prevent porewater under the surcharge cap and in the CAD from impacting the bioactive zone of the caps.
The SLRIDT remedy involved several examples of beneficial use of materials, including the CAD, which involved converting a contaminated former shipping slip into a shallow bay by placing dredged contaminated sediment behind a lined rock dike at the mouth and then reintegrating it with the river after capping. Also, organic sediment dredged to restore a nearby river channel as part of the project was placed over the site’s caps to accelerate ecosystem recovery with local aquatic flora and fauna.
The project resulted in savings of $90 million compared to an all-dredging approach and restored 106 acres of aquatic and riparian habitat for fish, wildlife, and the community of Duluth. The site is currently in the second decade of a 30-year long-term monitoring and maintenance (LTM&M) program, which has shown the caps are performing as designed and the ecological recovery has been successful.