River Raisin Area of Concern NAPL Area Sediment Dredging and Capping


Location Monroe, Michigan
Contract Value $6,783,457
Period of Performance May 2016 – December 2016
Role Prime Contractor

Significant Project Features

  • Dredging of approximately 25,000 CY of sediments containing PCB’s and NAPL from an area adjacent to shore and extending into the navigation channel
  • Control and monitoring of sediment re-suspension during sediment removal
  • Dewatering, characterization, and conditioning of sediments for offsite disposal
  • Construction and O+M of a 100 GPM water treatment plant to process all water generated during remediation
  • Placement of an engineered sand/organoclay cap as well as multilayer armor protection upon completion of dredging
  • Site restoration

History & Location Details

River Raisin was designated an Area of Concern (AOC) under the 1987 Great Lakes Water Quality Agreement.  The agreement is a commitment between the U.S. and Canada to restore and protect the water of the Great Lakes.  The River Raisin AOC project is located in the southeastern portion of Michigan’s Lower Peninsula in Monroe County. The USEPA defined the AOC as the lower 2.6 miles of the River Raisin, beginning downstream of the low-head dam at Winchester Bridge in the City of Monroe. It extends approximately 0.5 miles into Lake Erie, within and adjacent to the federal navigation channel, and continues along the nearshore zone of Lake Erie, north and south, for approximately 1 mile. This project specifically addressed removal of TSCA level PCB-contaminated sediments within an approximately 1.45-acre portion of the AOC.

After mobilizing equipment, supplies, temporary facilities, and personnel to the site, Sevenson designed, constructed, operated and maintained a 100-GPM temporary waste water treatment system to treat construction water generated during work activities. The system treated all water generated by sediment dewatering, mechanical dredging, and decontamination operations, as well as accumulated runoff collected in the Sediment Processing Area (SPA). Sevenson obtained an Industrial/Non-Domestic User Discharge Permit form the Monroe MWPCF.

Excavation and Dredging Activities

Before commencing sediment removal operations, Sevenson conducted pre-dredge debris and hydrographic surveys. Dredging occurred in water depths ranging from less than 1 foot (adjacent to the shoreline) to approximately 37 feet (in the Navigation Channel). Sevenson successfully dredged 24,800 CY of PCB-impacted sediment; placed cover material over a 0.3-acre area and installed an engineered cap over 1.15 acres. Throughout the course of the project, Sevenson implemented real-time air monitors to ensure that particulate matter did not impact on-site personnel. Upon completing dredging activities, Sevenson performed confirmatory multi-beam surveys to verify and document that targeted volumes of contaminated sediment were successfully removed.

Sevenson transported dredged debris and sediment to the loading/offloading pad via scows, which then transported the contaminated material to the SPA. Sevenson placed dredged sediment onto the SPA to facilitate gravity dewatering and so that field crews could separate large debris for disposal. The SPA also served as the site where crews amended with Portland cement. The offloaded and dewatered/amended material was stockpiled and placed into bins while awaiting transport by truck to the disposal facility. The staging of sediment was managed within the SPA, allowing Sevenson to sequester and treat all water within the stockpiling area. Water released from stockpiled material was pumped directly to the on-site water treatment system. Sediment dredged from the NAPL Area was TSCA level material. Under TSCA regulations, PCB-contaminated material (including sediment) containing or exceeding 50 ppm must be disposed of at a hazardous waste landfill permitted under the Resource Conservation and Recovery Act (RCRA) or at a TCSA-approved chemical waste landfill.

Dredging, backfilling, and capping typically provoke resuspension of sediment and chemical constituents, thereby increasing turbidity issues and chemical concentrations in the water column. Sevenson was also cognizant that the presence of NAPL in the materials being dredged would produce a sheen in the dredge area. To address these short-term water quality issues, Sevenson implemented the necessary measures to meet the applicable water quality criteria. Sevenson utilized an innovative method of containing all NAPL during dredging activities. A moonpool was used during all dredging activities. The moonpool system was constructed of a square group of sectional barges with an interior and exterior turbidity curtain installed around the interior and exterior perimeter of the barge sections. The interior curtain was non-permeable and the exterior curtain was permeable. A system of telescoping spuds were used to lower and raise the curtains as needed according to the water depth.

Sevenson also performed real time turbidity monitoring 24/7.  Upstream and downstream turbidity monitors were used to measure in river turbidity levels during all dredging activities.

Sevenson’s scope of work also included installation of an engineered cap over 1.15 acres of post-dredge surfaces. The engineered cap consisted of a sand residual cover layer, a 4% sand/organoclay mixture with a minimum thickness of 12”, a filter stone layer with a minimum thickness of 12”, and an armor stone layer with a minimum thickness of 36”. All capping layers were verified with a multibeam hydrographic survey.  Three long-term monitoring ports were also installed within the cap footprint to allow for future testing access. Sevenson utilized a system of feed hoppers, a variable speed conveyor, a quad screw mixer, and a stacking conveyor upland to achieve a reactive layer mixture of 4% Organoclay and 96% sand.   The organoclay/sand mixture was mechanically loaded into hopper scows and transported to the cap area, where a PC 800 placed the material with a clamshell bucket. The filter stone and armor stones were loaded onto deck barges and transported to the capping area for subsequent placement. Filter stone was placed with a clamshell bucket and the larger armor stone was placed with an orange peel attachment.

Navigational Waterways Coordination

The Port of Monroe is an active waterway that provides access routes for two to four commercial vessels each week. Additionally, recreational boaters frequent this section of the River Raisin on a daily basis. Both commercial and recreational boat traffic passed by the work area along the southern portion of the navigation channel outside the NAPL Area. Sevenson worked with Port of Monroe, USACE, Lake Carriers Association, and commercial freight authorities to coordinate dredging operations so as to ensure work activities did not disrupt marine traffic. Sevenson regularly apprised authorities of project schedule objectives and updates regarding ongoing work.

Site Restoration

After confirming that remedial objectives were achieved, Sevenson commenced decontamination, site restoration, and demobilization activities. Field crews removed resuspension controls from the river; dismantled the loading/offloading pad and drip apron; decontaminated all equipment, tools, and the SPA; and pumped residual water to the on-site treatment system.

Project Description

Site Preparation and Mobilization

Sevenson began work on pre-mobilization submittals, such as all required work plans, and obtained a sidewalk closure permit. Crews removed portions of the existing chain link fence; erected temporary fencing; performed clearing and grubbing operations; installed the necessary soil erosion and sediment controls; constructed the temporary staging pad(s) and access roads; and established temporary offices. Sevenson’s temporary facilities consisted of an office and break trailers, temporary access roads, a temporary rerouting of White Pine Trail, as well as parking lots for site personnel. Sevenson contacted the local “Call 811” service to locate existing utilities within the work zone, such as overhead electric lines, subsurface gas piping, and storm sewers. Utility infrastructure was marked out. Several overhead electric lines exist north of Rum Creek within the excavation area. These lines were supported and protected in order to perform excavation and backfilling operations. Sevenson contacted the local utility company and the utility company secured the pole while the field-crews performed work activities.

Upland Soil Excavation

Sevenson excavated 6,844 CY of contaminated soil from various areas of this site. The northern excavation area consisted of three areas (N1, N2A and N2B) that required different depths of excavation, ranging from 3 ft. to 5 ft. In order to complete excavation operations along the shoreline, Sevenson installed a temporary cofferdam consisting of a combination of sheetpile and water-filled bladderdam. The cofferdam ends were installed north and south of the excavation areas and connected to the shoreline to provide an adequate seal and allow for a dry excavation along the shoreline for excavation areas N1 and N2.

To facilitate excavation zone dewatering, Sevenson pumped the water from within the excavation area into the Rogue River. The water was first conveyed through a bag filter housing to capture any suspended solids. The regenerative water was handled in the same manner as the initial drawdown water. Sevenson utilized a PC300 excavator to remove soil and live load tandem dump trucks. As the soil was removed, the excavation operations proceeded south. The area remained dewatered to allow for the collection of soil and sediment samples where applicable.

After all contaminated soil was removed in Area N1, Sevenson continued excavation operations in Areas N2A and N2B. Since the cofferdam had already been installed and the area had been dewatered, the excavation operation was seamless. Excavation Area N2 consisted of areas with three different excavation depths. The western shoreline area consisted of a 3 ft. excavation; the eastern adjacent upland soil required a 1 ft. excavation depth; and a small portion in the northeastern corner required a 5 ft. excavation (N2B). After field personnel completed the soil removal in Area N2, field crews proceeded to the Southern Excavation Areas, south of Rum Creek. The southern excavation areas consisted of four isolated upland excavation areas. Areas S2, S3, and S4 required the removal of 3 ft. of soil and Area S1 required the removal of 10 ft. of soil.

Once the temporary cofferdams were installed, Sevenson dewatered the area inside the cofferdams by pumping the water through bag filters into the Rogue River. Due to the excavation depth of 10 ft. for Excavation Area S1, Sevenson was required to slope a portion of the excavation to prevent slope failure during removal.

Sediment Removal

Using a PC 300 excavator, field crews also removed 8,030 CY of sediment from six different areas. These areas were located along the shoreline in the northern and southern areas of the site. Sediment Areas N1 and N2 were located to the north of Rum Creek adjacent to the shoreline of the upland excavation areas. Sediment areas S2, S6 and S7 were located south of Rum Creek at the northern portion of the southern excavation area and Sediment Areas S4 and S5 were located at the southern portion of the southern excavation area. To remove the sediment from these areas, Sevenson installed temporary sheetpile and water bladderdam cofferdams in the same manner as the excavation areas that were located in the northern area. Sevenson mobilized equipment and materials in quick fashion to install the temporary sheetpile cofferdam. Sevenson’s fleet of available equipment (sheetpile and pile installation equipment) allowed for quick installation and limited economic impact to the project.

Sediment was removed to 2 ft. to 3 ft. BGS. The sediment was loaded into off-road dump trucks and was transported to a sediment staging pad that Sevenson constructed on the upland portion of the site. After the sediment was decanted and amended with portland cement, as needed to pass the paint filter test, it was loaded into dump trucks for transportation to an offsite landfill for disposal. After Sevenson removed the required sediment, GZA (the Client’s engineer) obtained the post-excavation sediment samples for chemical analysis.

Backfill and Rip Rap Placement

Once Sevenson completed the removal of soil and sediment and received confirmation sampling approval, field crews initiated the placement of geotextile and backfill materials. An anchor trench was installed in the Rogue River parallel to the shoreline to secure the geotextile. This trench was excavated during the sediment removal operation. The rip rap was placed on the geotextile in the anchor trench first and it extended up the slope to the required elevation. Field crews installed 1,280 tons of rip rap. Each roll of geotextile was deployed with a minimum of 12 in. overlap at each seam. As backfill was placed it was graded in 12-in. loose lifts with a bulldozer and compacted with a roller. Sevenson procured the services of a local geotechnical lab to perform third-party compaction testing on placed fill. Crews then performed a minimum of 1 test per lift for every 1,000 sf.

Construction Water Management

Based on test pits conducted adjacent to S1, and its proximity to Rum Creek, the estimated ground water that would be generated during soil removal was approximately 45,000 gallons. Therefore, a frac tank was utilized during soil removal of excavation area S1 to containerize groundwater. Sevenson was required to excavate upland soils beyond groundwater elevation due to confirmation sampling and the presence of leather scraps. A minimal amount was containerized at the end of the project when equipment was decontaminated prior to removal from the site. All water was transported and disposed by GZA (the Client’s engineer).

Transportation and Disposal

The project required disposing of multiple waste streams. Upland soil and sediment were shipped to Envirosafe located in Otter Creek, Ohio. In addition to the soil and sediment, contact water from excavation area S1 and decon water was containerized, transported and disposed offsite by the Client’s Engineer.


Restoration activities required the removal of a permanent chain link fence; the restoration of asphalt on White Pine Trail; placement of backfill and topsoil; placement of an erosion control blanket; and establishment of wetland turf planting zones. Crews also constructed new concrete sidewalks and restored concrete aprons.

Equipment Decontamination and Demobilization

Upon removal of the permanent chain link fence and placement of the final cover, Sevenson decontaminated of all equipment by pressure washing. Following decontamination, all equipment was demobilized from the site.

Rooted in Remedial Construction
Expanded into Environmental Dredging

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