A.69 Stryker Bay, Duluth, MN
Contaminants of Concern:
PAH compounds, metals (As, Cd, Cr, Cu, Pb, Hg), coal tar NAPL
Source Control Achieved Prior to Remedy Selection?
Yes, historical industries are closed.
amended cappingTechnology which covers contaminated sediment with material to isolate the contaminants from the surrounding environment. (11 acres), dredging (22 acres)
A.69.3 Site Description
Primary source(s): Likely wastewater discharges from the water gas, coking, and tar facilities formerly located on the SLRIDT site.
Location: Stryker Bay is a shallow, flat-bottomed bay of approximately 41 acres with average water depth of approximately 3 to 5 ft. There are homes to the west and industrial land to the north and east. A wetland is located at the north end where an unnamed stream enters the bay from a steep urban watershed, and another wetland is located in the southwest corner near the mouth of the Bay.
The SLRIDT site has been used for industrial purposes since at least the 1890s. Prior to industrialization, the SLRIDT site was predominantly open water and was part of St. Louis Bay, bounded on the west by 63rd Avenue Peninsula.
Coke production began in 1904. A water (town) gas manufacturing plant operated intermittently from 1905 to 1961. Tar refining began in 1905 and operated at multiple sites until 1948. The most recent iron plant has not operated since about 1960.
Industrial byproducts were used in conjunction with re-deposited native sediment as fill to create new land, including the 59th Avenue Peninsula and the 54th Avenue Peninsula. The primary fill material is slag from on-site pig iron operations.
CSM summary: Numerous processes act on the groundwater/sediment/surface water interface in Stryker Bay including: upward advectionBulk transport of the mass of discrete chemical or biological constituents by fluid flow within a receiving water. Advection describes the mass transport due to the velocity, or flow, of the water body. It is also defined as: The process of transfer of fluids (vapors or liquid) through a geologic formation in response to a pressure gradient that may be caused by changes in barometric pressure, water table levels, wind fluctuations, or infiltration. (flow) of groundwater and downward flow of surface water into the sediment, diffusion of chemicals from the sediment to the water, new sediment deposition, bioturbation (mixing of sediment by organisms), biodegradation, mixing, and redistribution from bed shear induced by waves, prop wash, currents, and occasional anchoring. Within the bay, ice usually freezes to the bed around the perimeter and thaws in place. Some of these processes deliver PAHs to the surface; others dilute, degrade, and physically redistribute the PAHs.
A.69.4 Remedial Objectives
Remediation risks at this site include adverse effects to public health, aquatic plants and animal community from mobilization of organics and metals from sediment.
- Sediment cleanup levels:
- TPAH concentration must not exceed 13.7 mg/Kg
- Mercury must not exceed 0.3 mg/Kg
- Other metals (As, Cd, Cr, Cu, Pb, Ni, and Zn) 0.6 times the mPEC-Q based on Level 2 SQTs
- Dredged sediment water must be treated on site to meet MPCA surface water discharge standards prior to discharge to river.
- During remedy, ambient air naphthalene concentration in residential area must not exceed 2,000 μg/m3.
A.69.5 Remedial Approach
Final selected remedy: amended capping (AC Reactive Core Mat™) and dredging.
The selected remedy consists of a combination of in situ amended capping, environmental dredging, dredged sediment containment and institutional controlsNon-engineered instruments, such as administrative and legal controls, that help minimize the potential for human exposure to contamination and/or protect the integrity of the remedy.. Sheet piling separated dredge area from capA covering over material (contaminated sediment) used to isolate the contaminants from the surrounding environment. area. Dredged material was pumped to CAD for settling. Supernatant water from CAD was treated by wastewater treatment train of sand filter, organoclay media, AC media, and micron bag filter. Capping consisted of following sequence:
- Place 6” sand over sediments.
- Place AC Reactive Core Mat™.
- Place 3.5’ sand cover and 4.5-6.5’ surcharge load over RCM.
- After consolidation, remove surcharge sand to cap dredged sediment CAD.
Why the remedy was selected: Air modeling indicated that if the areas of the bay with sediment naphthalene concentrations > 1,000 mg/kg were dredged, that ambient air quality criteria in the neighboring residential area would be exceeded. So in these areas, capping was used instead of dredging. Modeling of contaminant transport upward into the in situ cap predicts that the cap would be effective in preventing contaminants from exceeding RAOs and Cleanup Levels in the BAZ and surface water in the long-term. In addition to a conventional sand cap, AC Reactive Core Mat™ was inserted in the design as an additional factor of safety.
A.69.6 Monitoring Approach
The agencies and PRPs are finalizing the details of a 5-year performance monitoring plan. Performance monitoring will consist of three elements:
- pore-water monitoring
- bulk sediment monitoring
- benthic community uptake monitoring for PAHs and mercury
RAOs/project objectives achieved? Air monitoring objectives were met during construction.
The 22 acres of dredging and the 11 acres of capping cost an estimated $32 million. The Reactive Core Mat™ active cap material cost was approximately $1 million. Relocation of Slip 6 dock to allow conversion to a CAD for dredged material disposal cost an additional $12 million.
A.69.8 Advantages and Limitations
Site Specific Challenges:
- Regulatory—Communication with numerous local, state, and federal agencies through The Metropolitan Interstate Committee’s Harbor Technical Advisory Committee.
- Technical—Air modeling indicated that if the areas of the bay with sediment naphthalene concentrations > 1,000 mg/kg were dredged that ambient air quality criteria in the neighboring residential area would be exceeded. Hybrid dredge/cap remedies were evaluated that alleviated the air quality concerns.
- Community—The community stakeholders actively participated in the process that led to the identification of new hybrid remedies involving mixes of dredging, capping and containment technologies.
Acceptance: Based on the comments received by the MPCA during the public comment period on the Proposed Plan, there was in general high support for this alternative. Reasons cited for support of this alternative included maintaining natural resources and maintaining riparian use for property owners in the Stryker Bay. Concerns for this alternative included leaving the contamination in the water, long term protection of public health, and the environment and financial assurances in the event that the remedy fails. Based on these comments, the MPCA has added O&M, monitoring, contingency action plans, and financial assurance to the final ROD.
Minnesota Pollution Control Authority, Record of Decision for the Sediment Operable Unit of the St. Louis River/ Interlake/Duluth Tar Site, Duluth, MN, August 2004. http://www.pca.state.mn.us/index.php/view-document.html?gid=3222 Accessed December 15, 2011.
Publication Date: August 2014