Fox River Superfund Site PCB Sediment Removal, Phase 1 Remedial Action, Operable Units 2-5 Remediation

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Green Bay, Wisconsin
Date of Execution: July 2006

Significant Project Features:

  • Hydraulic dredging of 140,000 cubic yards of PCB-contaminated sediments from the Lower Fox River, DePere, Wisconsin.
  • Design, construct, operate, and maintain a 2,400 gpm liquids/solids separation system and wastewater treatment unit to treat filtrate from dewatering process.
  • Place 6-inch sand cap over 22 acres of river bottom utilizing a custom-built dissipating system.
  • Transportation and off-site disposal of TSCA and non-TSCA processed filter cake.
  • Dredging and dewatering remediation operations scheduled for 220 days.

History and Location of Project

Phase 1 of the Fox River PCB Sediment Removal project site is located adjacent to the U.S. Mills Paper Corporation paper mill, just upstream of the DePere Dam on the west bank of the Fox River in DePere, Wisconsin. Paper mill operations discharged polychlorinated biphenyls (PCBs) into the Fox River from 1957 through 1971, which contaminated river sediments. The 22-acre sediment dredging area extends approximately 750 feet offshore and covers approximately an 1,100-foot-wide area of the riverbed.

Sevenson was selected by NCR Corporation and U.S. Paper Mills Corporation to hydraulically dredge and mechanically dewater contaminated sediment and dispose of the processed material in an off-site landfill. Upon completion of dredging, Sevenson will install a 6-inch sand cap layer over the dredged area.

Description of Work

The sediments to be removed consist primarily of soft organic silt and overlying inter-bedded layers of firmer native clay of varying thickness and contain an estimated solids content of about 30% by weight. The sediments in the Phase 1 removal area also contain miscellaneous debris, rocks, boulders, and underwater vegetation that may be required to be removed prior to and during dredging. The PCB concentrations of the sediments within the dredged area range from nondetectable to approximately 3,000 ppm.

Sevenson received a contract in July 2006 to hydraulically dredge and mechanically dewater 140,000 cubic yards of PCB-contaminated sediment, with a goal of removing all sediments with PCB concentrations of more than 1 ppm. To meet the required completion of all the dredging and dewatering operations by November 2007, Sevenson will deploy a crew of more than 50 and work two 12-hour shifts, 5 days per week. Operations will be supplemented by maintaining surplus critical equipment (such as dredges and pumps) on site to minimize production loss caused by equipment downtime. To minimize the spread of contamination into the water column during dredging, Sevenson will install a silt curtain in the Fox River to surround the active project area. Cells have been delineated within the river and separated into working areas by the installation of additional silt curtains.

Sevenson will install, operate, and maintain a solids separation and water treatment pipeline capable of handling a dredge-to-water flow rate of 2,400 gpm. The on-site solids separation system consists of dredged slurry storage tankage and pipelines; shakers and desanders; storage mix tanks; and 10 recessed-chamber plate-and-frame filter presses. Sevenson will manage transportation and off-site disposal activities of all Toxic Substances Control Act (TSCA) and non-TSCA processed filter cake. Filtrate from the dewatering process will be treated on site by Sevenson in a system consisting of settling tanks, clarifiers, bag filters, and sand and carbon filters. Water treatment is designed for a flow rate of 2,400 gpm, and treated water will be discharged back to the Fox River.

Sevenson will place a minimum 6-inch sand cover layer in the dredged area using two lifts. The sand cover layer will be placed hydraulically in the offshore areas, and the sand will be placed mechanically in the near-shore areas. Sevenson anticipates placing the 6-inch sand layer over the entire 22-acre dredged area. Sevenson's cap placement system consists of a conveyor system, modified mix tank, make-up water and slurry pumps, slurry pipeline, floating barge structures, and a custom-designed slurry dissipater barge. Cap material will be placed into a hopper located on a conveyor system, where it will be transported at a constant rate into the modified mix tank. The modified mix tank will receive a constant flow of make-up water from the river. Agitators located inside the modified mix tank will blend the slurry mixture, which will be pumped at a constant rate from the modified mix tank to the slurry dissipater barge via the slurry pipeline. The slurry dissipater barge will discharge the slurry mixture with an even distribution and at a constant rate over the dredged area of the river.

Due to the shallow water depths along the shoreline and the barge draft of approximately 16 inches, cap material will be placed mechanically along the shoreline and in the dredged areas of the river bottom that are too shallow to be accessed by the dissipater barge. Cap material will be installed in 2- to 4-inch lifts to prevent disturbance of any underlying sediments.

Water Quality Controls

To minimize water quality impacts, the primary standard and project-specific water quality controls include installation of silt curtains surrounding the work area in the river. Turbidity monitoring will be performed daily by Sevenson during the cap installation activities to evaluate the effectiveness of placing the cap material in thin lifts and reaching overall cap-layer thickness standards with minimal disturbance to the sediments.

Health and Safety Overview

  • A Sevenson Health and Safety Officer will be on site during all working hours.
  • All personnel will be required to review and sign a Sevenson Activity Hazard Analysis (AHA) report each day before entering their work area.
  • All personnel will participate in a Sevenson Safety Observation Report (SOR) program in which crewmembers will be able to identify, discuss, and remedy potential safety hazards each day during their daily safety meeting.
  • The crew will wear earplugs for noise protection when working by the slurry set up area and on the slurry dissipator barge.
  • Life vests will be worn when on board work boats or operating the slurry dissipator barge, and all marine vessels will be equipped with all necessary safety gear.
  • The crew will work from dawn to dusk.