J.F. Brennan Co. Starts Dredging at Ashland Superfund Site Following Successful Pilot Project
VEIT Optimizes Dredging Operations Using EIVA NaviSuite 3D Software with Trimble GCS900™ Grade Control System
By Anna Townshend
J.F. Brennan started work in mid-May on the dredging portion of an approximately $40 million Superfund project at the Ashland/Northern States Power (NSP) Lakefront Site in Wisconsin. Upland work for the project began in 2012, and the phase two waterway cleanup began in 2017, after a successful pilot project demonstrated that dredging could meet the project requirements.
The overall site has contaminated land and sediment along the shore of Lake Superior in Ashland, Wisconsin. The site contains property owned by Northern States Power Company, a Wisconsin corporation (d.b.a. Xcel Energy, a subsidiary of Xcel Energy Inc. (NSPW)); a portion of Kreher Park, a city-owned property fronting on the bay that includes a former lumber mill and a former municipal waste water treatment plant (WWTP) and waste fill site; an inlet of Chequamegon Bay containing contaminated sediment directly offshore from the former WWTP; and a railroad right-of-way, a church and school, as well as private residences.
Historically, Chequamegon Bay was an important transportation route for shipping materials to and from Ashland, including iron ore, lumber, pulp and coal. During the late 19th and early 20th centuries, Ashland was one of the busiest ports in the Great Lakes. In recent times, the shipping volume through the bay has declined because of the decrease in the mining and lumber industries in the region, while recreational activities have increased. The City of Ashland has a waterfront development plan for the future that includes the impacted portions of Kreher Park and the Chequamegon Bay.
In 1989, during exploratory work to expand the WWTP, the City of Ashland encountered contaminated soil and groundwater. The city notified the Wisconsin Department of Natural Resources (WDNR), closed the WWTP, and built a new facility a few miles away. In 1994, WDNR initiated an investigation and evaluation of the area to characterize the extent of contamination on the property. In 1998, EPA was petitioned to evaluate the site for inclusion on the National Priorities List (NPL) and cleanup under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), also known as Superfund. The site was nominated for inclusion on the NPL in 2000, and was formally added to the NPL in 2002.
In 2010, the Environmental Protection Agency (EPA) issued its Record of Decision (ROD), documenting the selected remedy plan, chosen in accordance with CERCLA and the Superfund Amendments and Reauthorization Act (SARA). When the ROD was issued, EPA was concerned that wet dredging the nearshore sediments would not successfully achieve the cleanup goals and performance standards due to significant amounts of wood waste and debris commingled with tar and oil in the form of non-aqueous phase liquid (NAPL) in the nearshore sediments. At that time EPA selected a combination remedy of dry excavation for the nearshore sediments and wet dredging of offshore sediments (Alternative SED-6), but allowed for performance of a pre-design pilot test to determine if wet dredging could achieve the cleanup goals and performance standards in the nearshore area.
According to the ROD, the site contains soil, sediment and groundwater contaminated by polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs). The most abundant constituents in each of these compounds include benzene, a VOC, and naphthalene, a PAH. Additionally, free phase hydrocarbons (free product) were present as NAPL, which does not readily mix with water. NAPL has also been found in subsurface sediments in the near shore area. The report said sediment contamination tends to be higher with depth below the sediment/water interface and is highest in the near shore area, decreasing with distance from the shoreline. Much of the contaminated sediment was also co-mingled with wood waste.
Remediation of the contaminated sediments commenced in 2016. A pilot test was successfully performed in 2016 and EPA, in consultation with the Wisconsin Department of Natural Resources, agreed to allow wet dredging as the alternative remedy for the nearshore area, and issued an Explanation of Significant Differences (ESD) to the ROD. The pilot test demonstrated that wet dredging of nearshore sediments can achieve the cleanup goals and performance standards in a manner protective of human health and the environment.
Prior to the pilot project, the upland excavation in phase one included soil removal and groundwater treatment at the Copper Falls aquifer, Kreher Park and Upper Bluff. The waterway cleanup began with the construction of a breakwater, completed in the fall of 2015. The breakwater was needed to operate equipment in the rough waters of Lake Superior, said Tyler Lee, project manager for J.F. Brennan. The company dug the footprint for the breakwater in 2015, prior to the pilot project.
The breakwater is approximately 840 feet long and is located 400 to 500 feet from the shoreline, creating a semi-sheltered basin. It is made of quarried stone materials (bedding, core, filter and armor). The exterior armor layer of stone ranges from 2 to 6 tons.
In March 2017, the Foth Infrastructure & Environment/Envirocon Joint Venture released its final design and report on the Phase 2 wet dredge project, following the success of the 2016 pilot project. The design report said the, “governing factor in the armor stone sizing is extreme ice conditions/loads rather than extreme wave conditions/loads. The design crest elevation is +8.0 feet low water datum (LWD), which allows for 1 foot of settlement.”
The report also said that discussions during the breakwater design process indicated that dredging operations in the basin could occur as long as the significant wave height in the area of interest was no more than 2 feet and modeled the design based on that threshold. Significant storm events during the 2016 pilot project work, demonstrated the strength of the wave height modeling and breakwater design.
During dredging operations in 2017, the east and west gaps in the breakwater have been temporarily closed. The gap closure system includes geotextile tubes with a layer of bedding stones to level the lake bottom below the tubes.
Pilot Success: Phase Two Dredging
To demonstrate the effectiveness of this dredging operation, the pilot project began in 2016, and covered around 40,000 square feet. Around 8,000 cubic yards of impacted sediments and debris were removed from the lakebed, and extensive monitoring efforts throughout the project assured that it adhered to water quality standards. In December 2016, the pilot project was deemed a success.
Full scale dredging operations began earlier this year. To perform the dredging, J.F. Brennan is using two 140,000-pound long reach excavators and one 8-inch swinging ladder hydraulic dredge. The excavators have a custom Brennan environmental bucket, equipped with a lid to minimize turbidity and assist in removing debris from the lake bed. The buckets were built by B&D Fabricators.
“The material is highly variant and includes dense sandy silt, stiff clay, wood chips, large wood debris and soft sediments. The lidded bucket is well suited to the variable material types that are being encountered,” Lee said. At the end of the project, he noted that they would have to spend time scraping and cleaning equipment of any debris residue and specifically NAPL, a sticky heavy product that remains on equipment.
In addition to the two excavators and hydraulic dredge, J.F. Brennan is using six hopper barges to move sediment to an offload point, about 1,500 feet from the furthest dredge location. Another contractor takes the material at the offload point, where it is treated for landfill, and transported there about 60 miles away.
At the offloading pad, debris and sediment is removed from the barges. Large debris is sorted or removed via vibratory screens, before additional mechanized processing takes place at the adjacent sediment processing structure. The final design report said decant water from the barges will also be pumped to geotubes located within the sediment processing tent.
To meet off-site transportation requirements, the dredged material and smaller debris get a stabilization agent and the debris piles are passively dewatered. Dredged material is consolidated and blended in a mixer where the stabilization agent is added. The processed material exits the mixer and falls onto a transfer conveyor, then to a jumper conveyor to a radial stacker, where processed material is reworked over a two to three-day period. Cured material is then relocated to the east end of the sediment processing tent and staged for transport and disposal.
In the water, approximately 3,600 linear feet of 10- to 18-foot impermeable turbidity curtains from Spilldam Environmental have been deployed to isolate the work area. Additionally, about 1,500 feet of oil booms and 80,000 feet of oil absorbent booms have been used to corral and absorb oil sheen. Lee said the booms and curtains are also helpful in mitigating odors. The project has a comprehensive air quality system installed and has met all guidelines set forth.
Most of the work is being done with the mechanical dredges. The hydraulic dredge with the Vic Vac™, a dredge suction attachment for removing residual sediment from the bottom, to limit the suspension of small particulates and organic solids, will finish the work.
In 2018, the contractor will return to install a 6-inch restorative layer of coarse sand. Lee said that layer may likely be supplemented with rock piles for additional fish habitat