Design, Construction and Monitoring of a Contaminated Sediment Cap in the Columbia River

摘要

The Dalles Tie Treating Plant is situated on a 34-hectare parcel along the Columbia River. The plant was owned by Union Pacific Railroad from 1923 to the present. In 1937, as part of the construction of the Bonneville Dam, the site was diked and the U.S. Army Corps of Engineers constructed a storm water conveyance pipeline and pump station to convey water from behind the diked area to the Columbia River. The pipeline was abandoned in 1971. rnDuring the remedial investigation of the tie plant, wood treating constituents were detected in Columbia River sediments in the area of the outfall discharge. The wood treating constituents included polycyclic aromatic hydrocarbons (PAHs) (primarily creosote compounds), Pentachlorophenol, and arsenic compounds. Historic storm water discharge from the site were the suspected cause of the presence of wood treating constituents in sediments. rnThe area of affected sediments was on a relatively steep slope within a shallow water area adjacent to Riverfront Park in The Dalles, Oregon. The affected area covered approximately 0.6 hectares, to a maximum sediment depth of approximately 6 meters (m) . The site is located within the pool of the Bonneville Dam just downstream of The Dalles Dam and water level varies at the deepest portion of the cap between approximately 3 and 6 m, depending on the operation of the dams. The highest portion of the cap is typically above water and at times submerged under as much as 1.5 m of water. rnThe primary purpose of the cap was to prevent erosion of affected sediments and to prevent direct contact between contaminated sediments and benthic organisms. The wood treating constituents are primarily bound to the sediments and have limited mobility. rnUPRR proposed to the Oregon Department of Environmental Quality (DEQ) to conduct an interim remedial investigation and feasibility study for the area of affected sediments as a separate Operable Unit from the remainder of the tie treating plant. The remedial investigation defined concentration levels for the wood treating constituents below which adverse effect on benthic organisms would not occur. The feasibility study evaluated alternatives to address the area where adverse risk to benthic organisms could occur. Several options were evaluated including dredging, no-action, and capping. A multi-layer cap was selected as the most feasible solution based on cost, effectiveness and implementability. The feasibility study was approved by DEQ as an interim action in 1994.