Well 12A Superfund Site
Under a task order issued by the USACE Seattle District under our USACE Kansas City PRAC, KEMRON, as the prime contractor, completed the Well 12A Shallow Excavation Remedial Action located within the larger Commencement Bay – South Tacoma Channel Superfund Site. The site is a former oil recycling facility where off-site migration of chlorinated solvent-contaminated groundwater impacted the City of Tacoma’s drinking water well (Well 12A). Other site soil and groundwater contaminants include polychlorinated biphenyls (PCBs), volatile organic compounds (VOCs), metals and dense non aqueous phase liquid (DNAPL). Following a decrease in the effectiveness of the previous pump and treat remediation strategy, a Record of Decision (ROD) Amendment specified a combination of soil excavation, in situ thermal remediation, and enhanced anaerobic bioremediation to reduce groundwater contaminant mass leaving the site by 90%.
KEMRON prepared a Remedial Action Work Plan (RAWP) which described the intended remedial action to include abandonment of existing monitoring wells; removal of the 12-inch concrete pad and associated rebar; a geophysical survey using EM61 and GPR; removal of any USTs and other metal debris; and removal of the highly contaminated soil and filtercake. Metal debris was targeted for removal to prevent interference during future implementation of thermal remediation. Use of green construction technologies was also prioritized as part of the excavation strategy. Excavation was planned for two areas approximately 50 ft by 50 ft predicted to contain greater than 1,000 ug/kg VOCs in shallow soil (0-5 ft bgs) based on modeling of historical soil concentrations measured in nearby borings. In addition, the RAWP included a Green Remediation Plan, a Waste Minimization Plan and an Accident Prevention Plan (APP).
Unforseen field conditions encountered increased excavation complexity and impacted project schedule. . First, following removal of the concrete pad, elevated soil contamination levels encountered led to multiple rounds of soil sampling and analysis to better delineate the excavation footprint and determine disposal requirements and options. Second, the initially unknown dimensions of the one known UST were identified as being on the upper end of anticipated sizes (8-foot diameter and 38 feet long). Furthermore, the UST, thought to be nearly empty, contained a concrete “plug” approximately 3’ by 4’ that blocked the former access point, nearly 7,000 gallons of liquid waste, and 35 tons of pea gravel that had to be removed. The pea gravel was believed to be non hazardous based on previous data but was sampled and shown to exceeded Land Disposal Restriction (LDR) treatment standards for trichloroethylene (TCE) and lead. Additionally, oily filtercake and soil near the northwestern corner of the UST was sampled and determined to contain much higher contaminant concentrations. This material was excavated and fount to extend vertically to approximately 15 ft bgs. The horizontal extent of the filtercake abutted the former Time Oil building footings requiring excavation using trench boxes, piano key removal and use of controlled density fill. On-site chemical oxidation treatment using persulfate (and hydrated lime as an activator) was proposed and implemented as an innovative solution to the extremely high concentrations of VOCs which otherwise would have required disposal via incineration. A total of 2,131 tons of filter cake and contaminated soil were treated on site, excavated, then loaded and trucked offsite for disposal at a Subtitle C landfill. The excavation area was backfilled with clean backfill, new storm drains and storm piping were installed and then asphalt was installed to complete the restoration.
KEMRON implemented an on-site in-situ chemical oxidation treatment of the material containing high concentrations of VOCs, metals and PCBs to reduce VOC concentrations to below LDR thresholds. This allowed for excavation and disposal to a landfill approximately 270 miles away as material that will meet LDRs. This is an environmentally sound and cost effective alternative to simply digging and hauling to an off-site incinerator (located approximately 900 miles away).