Portland, Oregon
In 2000, the USEPA added the Portland Harbor Superfund Site to the National Priorities List. The Gasco site, is a former gasification plant operated by NW Natural’s predecessor from the 1913–1956 adjacent to the Willamette River. As a result of these activities, volatile and semi-volatile organic chemicals, cyanide, and heavy metals were deposited in the soil. High levels of benzene, toluene, xylenes and naphthalenes were also present, as well as hazardous F002-listed solvents associated by a release by the current owner of a portion of the site. These chemicals were migrating into the Willamette River via the groundwater which crosses the site.
Sevenson Environmental Services, Inc, have been working on the site since 2004. Beginning in 2004, Sevenson performed an early action that consisted of dredging and placement of a pilot cap. In 2004, Sevenson conducted initial studies and testing of a 50-gpm pilot treatment plant, supplemented by additional work in Sevenson’s treatability lab. With the results from the pilot plant and treatability studies, Sevenson designed a complex 800-gpm groundwater treatment plant (GWTP) with two pre-treatment plants: one for the F002 waste from areas of the site, and one with no F002 waste. The pre-treatment plants recover DNAPL and LNAPL oil and remove VOCs via air stripping. Stripped organics are captured in vapor-phase carbon units.
From the pre-treatment plants, water is pumped to the main treatment plant. The main treatment plant processes consist of chemical addition, settling, cyanide destruction, filtration, and GAC adsorption before final discharge. Solids handling includes gravity thickeners, polymer addition and filter press dewatering. All operations are backed with standby generators in case of power loss.
After design completion, Sevenson constructed the entire facility including the pre-treatment plants. Sevenson performed all phases: excavation, concrete foundations, installation of process units, all pumps and piping, chemical storage units, all electrical and instrumentation, and the erection of the treatment building.
The process was equipped with a SCADA monitoring and control system. Sevenson performs all operations and maintenance at the plant and staffs the plant 7 days per week, 24 hours per day. The plant began operation in September 2013 and has successfully treated over 1.3 billion gallons of water to date with no safety instances while working over 4,600 hundred man-days.
Sevenson is currently performing additional services on the site and evaluating upland remedies for remediating soil on the site as well as adjacent river sediment. In 2024, Sevenson and the plant designer, ADA, Inc. won the Grand Prize in Industrial Waste Practice for the project from the American Academy of Environmental Engineers & Scientists.