Duluth at the turn of the century was experiencing economic prosperity. Its industrial strength, generated from the westward population migration, continued during the shipbuilding and manufacturing efforts in support of World Wars I and II. What is today called the Saint Louis River/Interlake/Duluth Tar Site (SLRIDT) is the location where pig iron, coking plants, coal refiners, and tar manufacturers had formerly thrived. Up until the economic recession of the 1970’s, industry along Duluth’s St. Louis River had flourished. Although the steel industry died down in the 1970s, Duluth’s industrial age left an indelible mark on the environment, as contaminants and wastewater had been continually discharged into the river for decades.
The SLRIDT site is on the north bank of the St. Louis River, 4 miles upstream of Lake Superior. The 255 acre parcel of land includes a dock and two peninsulas created with industrial fill. The aquatic section of the site totals nearly 90 acres with two industrial shipping slips, wetlands areas, and Stryker Bay. Over time it became obvious that the site was contaminated. Oil was seen rising to the surface of Stryker Bay in 1981, and tar was oozing out of the ground on the two man-made peninsulas. In response to concern for nearby residents, fisheries, and natural areas, the location was declared a state and federal Superfund Site in 1983. The site contained chemical residuals of tar, polycylic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs), cyanide, naphthalene, and heavy metals, including mercury.
The Minnesota Pollution Control Agency (MPCA) identified four Potentially Responsible Parties associated with the site, of which three are now taking responsibility for the clean-up action. The MPCA split the remediation into three clean-up phases; the Tar Seeps Operable Unit, the Soil Operable Unit, and the Sediment Operable Unit. Remediation activities have been ongoing at the site since 1992 when clean-up of the tar seep wastes began. By 1997, both the Tar Seeps and Soil Operable Unit remediation activities were completed.
The MPCA then turned its attention to the Sediment Operable Unit, which includes all of the marine sections of the site. The SLRIDT site was eventually broken out into three marine areas; Stryker Bay, Boat Slip 6, and Boat Slip 7. Stryker Bay is a shallow embayment with wetlands on the north side. Boat Slip 6, used for loading and unloading ships, contains a deep water environment. Finally, Boat Slip 7 consists of emergent wetlands and a water environment that grades from shallow to deep. A slew of natural processes within the site area, such as waves, currents, freezing, thawing, ebb and flow, and seiches have kept the chemicals mixing, biodegrading, redistributing, and diffusing. Gas bubbles generated by microbes have also played a part, carrying sediment and oils to the surface and dropping them back through the water column.
All this history and natural phenomena has culminated in a multi-million dollar, multi-year pollution remediation project. The MPCA signed a record of decision for the Sediment Operable Unit in August 2004. A Remedial Design/Response Action Plan was developed to divide the work into logical groupings based on geographic location, type of work, and sequence in order to keep costs and schedules competitive. Work began on the site in 2006 and is scheduled to continue through 2010. The varied and sensitive aquatic environments required unique corrective action strategies. The project consists of a combination of dredging and in situ capping, along with the construction of an on-site contained aquatic disposal (CAD) facility to hold the contaminated dredged sediments.
During the first year of the project, activities included the construction of a 2,000 foot long temporary sheet pile containment wall around a portion of the east shoreline of Stryker Bay designed to trap the most elevated contaminants. A sub-aqueous and above-water cap of approximately 170,000 cubic yards of sand was placed within the sheet pile wall to compact and consolidate the underlying sediments. After the first 6 inches of sand were laid down, 11 acres of a non-woven, geo-textile activated carbon reactive core mat was added to the cap which acted as an additional engineered, bioactive barrier. The remaining sand cap was placed on top of the carbon mat. The year 2006 also saw the construction of an 18,800 cubic yard rock dike with a geo-synthetic clay liner, isolating Slip 6 from the river and creating the CAD.
Using Best Available Technology and Best Technology in Process and Treatment, a water filtration plant was constructed in the spring of 2007. Part of the design of the filtration plant is to prevent the CAD from overflowing into the St. Louis River. Instead, overflow is directed into the water filtration plant where it is treated before being released into the river.
The CAD was connected to Stryker Bay via sediment slurry/recirculating water pipelines, where dredging of contaminated sediments containing PAHs above 13.7 parts per billion took place. Operating from a spud barge (an excavator with a flat-cut hydraulic bucket), bay sediments are scooped up and discharged into a sediment slurry pit. After the material is screened to remove debris, it is mixed with recirculated water from the CAD, creating a slurry. This mixture is then pumped back to the CAD and deposited using a tremie diffuser. Any water displaced from the CAD flows through the filtration plant before discharging into the river. Using the precision of GPS technology to pinpoint exact locations, the dredging of approximately 122,500 cubic yards of impacted sediment occurred during a 24 hours per day/7 days per week operation. A water-filled damn/weir wall was installed to isolate the bay from the river as an engineered control to protect the environment from suspended chemicals in the water column. In addition to contaminants in the water, there was a risk of releasing PAH compounds into the air. As such, emissions and ambient air were carefully monitored at fixed stations around the site and the nearby residential neighborhood.
A portion of the remaining contaminated soils in two wetlands areas were also excavated in 2007. The top layer of removed wetland material containing organic material is being composted, while the deeper wetlands soils were disposed of into the CAD.
During 2008, dredging activities proceeded into the St. Louis River and the Federal Navigation Channel from the completed Stryker Bay dredging activities at the end of 2007. Approximately two-thirds of the impacted sediments in the river were removed and placed into the CAD. Operational means and methods for these activities mirrored those used in Stryker Bay. However, the excavator and hydraulic bucket were replaced with a crane and mechanical bucket to perform dredging to a depth of 30 feet. Post dredge sand and armoring placement continued through the construction season as well as clean cap placement activities in Slip 7.
Activities slated for 2009 included the completion of the dredging of the St. Louis River and the Federal Navigation Channel. The temporary sheet pile containment wall in Stryker Bay was removed, and a portion of the cap/surcharge sand was removed and reused to create a sub-aqueous sand cap on the CAD/Slip 6. Covering, capping, and armoring activities were completed in the dredged area of Stryker Bay. Slip 7 was also capped, restoring the bay to its pre-excavation bathymetry.
Planned 2010 Activities
Stryker Bay cap/surchage removal and CAD cap completion will take place after the installation of an activated carbon reactive core mat (RCM) in the deeper portions of the CAD (approximately 11 acres). The CAD end dike will then be modified to reintegrate the CAD with the St. Louis River. Habitat and shoreline restoration activities will be completed as well as Tallas Island environmental media (or aquatic soils) recovery. Tallas Island is a small land area that has been landlocked by sedimentation. The mitigation effort was approved by the Minnesota Department of Natural Resources to recover lost watershed from the capping and remediation activities at the SLRIDT site. Tallas will be reestablished as an island containing fisheries, wetlands, and nature conservation areas for the eventual use by Duluth residents and visitors. The dredged aquatic soils are rich in nutrients that will promote vegetation and fisheries. These soils will be transported to the project site and positioned upon the completed shallow cap and cover areas and will aide in the recovery of the area to a natural state. This innovative solution will improve not only the project site but enhance adjacent ecosystems.
The SLRIDT is a multi-million dollar remediation project that will restore the integrity and usability of a large parcel of land adjacent to the St Louis River in Duluth, Minnesota. It will also result in the restoration of the aquatic environment of the river and the re-use of Tallas Island. This project is a result of a long history of unchecked discharges by multiple industries in addition to natural phenomena contributing to the dispersion and spread of hazardous materials. As such, it required a comprehensive strategic plan of action where the project was broken down in to manageable parts and each addressed separately according to its unique requirements (Tar Seeps, Soil, and Sediment). Simultaneous efforts of dredging, water treatment, erecting containment walls, placing caps, covers, and armoring materials must all work in harmony so one effort does not compromise the other. The management of a project of this magnitude required a detailed critical path schedule, an experienced team, and the use of new technology to ensure the desired result which the citizens of Duluth and the surrounding areas can enjoy for years to come.
Hard Hat Services was selected by the Responsible Parties to Project Manage the SLRIDT efforts and particularly the Sediment Operable Unit. Hard Hat is pleased to report the project is proceeding well and is well within budget. Hard Hat’s Mr. Stuart Russell, P.E., is the Senior Project Coordinator and has ultimate responsibility for its completion. Mr. Russell can be reached at 630-364-7043 or at www.hardhatinc.com.