Background

The Ashland/Northern States Power Lakefront Superfund site (Site) sits on the shore of Chequamegon Bay, Lake Superior, in the city of Ashland, WI. The Site is comprised of several properties including those owned by Northern States Power (NSP) company (dba Xcel Energy). The Site was historically industrialized and encompasses several upland properties, including a former manufactured gas plant owned by NSP, former lumber operations, a former wastewater treatment plant and several acres of offshore impacted sediment. The Site has known contamination of upland soils, groundwater and bay sediments and has been divided into two remedial areas: the Phase 1 upland area and the Phase 2 sediment area.

The ROD for the Site identified the need for upland Phase 1 remedial action (source control) to be completed prior to initiation of the Phase 2 sediment remediation. Primary contaminants of concern (COCs) for the Site are a select set of volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs) including polynuclear aromatic hydrocarbons (PAHs), non-aqueous phase liquid (NAPL) and metals. The most abundant COCs include benzene, naphthalene and NAPL. Much of the impacted sediment is covered with a layer of wood waste.

Approach

Foth was engaged by NSP in 2009 to assist with remedial planning and design services prior to issuance of the EPA Proposed Remedial Action Plan (PRAP). It was during this pre-Record of Decision (ROD) timeframe that strategic planning and then negotiation with the Agencies took place with Foth and the NSP team. After nearly 1.5 years of negotiation with the Agencies, a ROD was finalized in 2010. Foth commenced final design of the Phase 1 remedy shortly thereafter and then formed a design/build Joint Venture with contractor Envirocon (FE JV) to remediate the upland Phase 1 portion of the Site, beginning in 2012. Due to the high level of volatile organic compounds in the Phase 1 soils FE JV selected a medium temperature thermal desorption technology to treat sediments to allow them to either be beneficially used as backfill on the Site or transported offsite for disposal. Impacted soils were excavated, processed through the thermal desorption unit and stockpiled for loading and transport to an approved landfill. Groundwater and process related or contact water were treated through a temporary on-site water treatment system consisting of an oil/water separator, dissolved air floatation system, granular activated carbon and sand filters. Additionally, an intricate groundwater collection and containment system, as well as a long-term water treatment system were designed and constructed by FE JV.

Phase 2 entailed completion of the in-water work. During Phase 2, FE JV and partner subcontractors, designed and constructed a rock-rubble breakwater in 2015 to create a sheltered area for dredging. Co-incident with the breakwater construction, FE JV prepared a design for a Pilot dredging project to prove the concept of conventional wet dredging at the Site could meet the stringent project Performance Standards. (The ROD preferred remedy included excavation in a dry area isolated from Lake Superior by a sheetpile wall.)

The Pilot design was approved in April 2016 and the mechanical dredging concept was shown to be successful. EPA issued an Explanation of Significant Difference (ESD) in December 2016 allowing wet dredging to be used for the Phase 2 full-scale project. FE JV then began design of a full-scale dredging project to complete remedial action for Phase 2. The design was completed in February 2017 and full-scale mechanical dredging was completed in November, with 118,000 cy of sediment dredged, dewatered (19MM gallons of water treated), amended and landfilled. Remaining Phase 2 work will be completed in 2018.

Results

• Diligence during ROD negotiations resulted in achievable Performance Standards and opportunities for cost and schedule savings with Pilot Study concept.
• Implementing an effective Phase 1 project with care taken to address local community impacts was crucial to implementing Phase 2.
• Phase 1 included excavation of approximately 53,000 cy of PAH-impacted soil, on-site medium temperature thermal desorption of 71,000 tons of that soil with on-site beneficial re-use and off-site disposal of the treated soil.
• Air emissions control, air monitoring and on-site water treatment (over 30MM gallons treated to date) were other key Phase 1 activities performed successfully by our FE JV team that were a springboard for the Phase 2 work.
• Permitting, designing and construction a bulkhead wall and slurry wall system, along with a long-term water treatment plant for treating impacted groundwater, allowing an inward gradient to be maintained, was critical to allow containment to be achieved and maintained during Phase 1.
• Permitting, designing and construction a 900 foot long robust breakwater, in a single construction season, was critical to allowing implementation of the Pilot Study and ultimately the full-scale wet dredge remedy.
• The successful wet dredge Pilot Study resulted in securing an ESD for implementing a full scale Phase 2 wet dredge project. Approximately 138,000 cubic yards of sediment and heavy debris were removed.
• Implementing the Phase 2 full-scale wet dredge project resulted in a safer project, saved the client tens of millions of dollars in construction cost and shortened the project duration by years compared to the ROD preferred remedy.