Decades of industrial, agricultural and urban use along the Columbia Slough led to elevated concentrations of contamination in the bottom of the slough. As a result, a fish consumption advisory was issued for fish that live their whole lives in the Columbia Slough, which are called resident fish.
Contaminated sediments between Moore and Wright islands and the former Pacific Carbide site are the focus of this sediment hot spot cleanup project and part of the overall Columbia Slough sediment cleanup plan.
View the Story Map: Moore and Wright Islands Natural Area Sediments Cleanup
Project details
From July to December 2025, DEQ led a sediment cleanup and habitat restoration project in the Columbia Slough – south of Moore and Wright islands. The project included:
- Capping contaminated sediment to isolate it
- Removing polluted bank soil
- Restoring shoreline habitat
Administrative controls will be prepared in 2026, including signs to alert boaters of the in-water cap and recording environmental restrictions on official documents.
In-water cleanup
A 2.8-acre protective cap was placed over contaminated sediment between Moore and Wright islands and the former Pacific Carbide site. The cap includes a reactive core mat that traps contamination, and it is covered with fish-friendly gravel armoring and a sand habitat layer. Some nearby areas received a thin layer of sand mixed with activated carbon to bind contamination in the sediment.
The in-water remedy is expected to reduce concentrations of polychlorinated biphenyls, also known as PCBs, by about 85% in sediment porewater in the project area. Sediment porewater is the water between sediment particles. It will protect bottom-dwelling organisms and reduce the amount of PCBs that build up in resident fish over time.
Habitat restoration
The bank was regraded to reduce steep slopes and provide flood mitigation, resulting in approximately 2,600 cubic yards of polluted bank soil being removed and disposed of at Wasco County Landfill. Soil amendments along with native seeding and plants were installed along the regraded bank to improve ecological habitat. As plants grow along the restored bank, they
will also help provide shade and improve habitat along the waterway. Vegetation
maintenance will be performed for three years to ensure plant survival.
Background
Pacific Carbide was developed along the Columbia Slough in the 1940s and operated as a calcium carbide manufacturing plant until 1987. Prior to and during the 1970s, significant amounts of waste material entered the Columbia Slough through discharge pipes and from one or more catastrophic failures of the slough bank adjacent to the settling ponds. Contaminated slough sediments between Moore and Wright islands and the former Pacific Carbide facility represented an in-water area that needed to be addressed as part of the Columbia Slough cleanup plan.
In 2013, Pacific Carbide entered into an agreement with DEQ called a Consent Judgment to implement an upland soil cleanup remedy and pay into the Lower Columbia Slough settlement account to address the in-water sediment contamination.
Funding
In 2023, DEQ and Department of State Lands applied for and received a $1 million EPA Brownfield Cleanup Grant. In May 2024, DSL and DEQ entered into an interagency agreement to perform the work. DSL administered the Brownfield Grant funds and DEQ managed the day-to-day grant and cleanup activities.