Predicting Water Quality during Dredging and Disposal of Contaminated Sediments from the Sitcum Waterway in Commencement Bay, Washington, USA

Abstract

The Sitcum Waterway is a heavily trafficked industrial and commercial waterway located in the Port of Tacoma off Commencement Bay, Washington. Vessel traffic within the waterway, which is a cornerstone of Port of Tacoma operations, consists of large cargo ships and associated tug, barge, and service boat traffic. The waterway is about 3,000 feet long and about 700 feet wide. Large pier structures exist on both sides of the waterway. The waterway's bottom sediments have become contaminated with metals and organic compounds (primarily polycyclic aromatic hydrocarbons). The source of these contaminants is believed to be related to historical ore unloading activity and various industrial wastewater and stormwater outfalls. The Commencement Bay Nearshore/Tideflats (CB/NT) area is a Superfund site. Within the Superfund site, the Sitcum Waterway is considered one of several problem areas. The EPA has identified various options for sediment remedial action in their overall Record of Decision for the entire CB/NT Superfund site. The available options include dredging of contaminated sediment and subsequent confinement in either a capped aquatic, nearshore, or upland disposal facility. Ultimately, hydraulic dredging and confined nearshore disposal were selected as the preferred remedial option. As part of the technical support studies required for formal selection of a remedial option, we performed tests to predict water quality during dredging and disposal operations associated with nearshore filling of the contaminated sediment. Modified elutriate tests were performed on two composite sediment samples. Each sediment composite was used to perform triplicate elutriate tests. The test procedure employed was based on the modified elutriate test as described by the U.S. Army Corps of Engineers. Four metals (antimony, arsenic, lead, and zinc) were detected. Ammonia and cyanide also were detected. Receiving water composite results had similar concentrations as the seawater composite samples except for lower concentrations of cyanide, lead, and zinc. These results were used in the water quality computer models EFQUAL and PLUMES to aid in remedial design activities. EFQUAL computed dilution requirements using the total effluent discharge concentrations for two discharge locations; within the Sitcum Waterway and at the mouth of the Sitcum Waterway. The only chemical that required dilution, as determined by EFQUAL, was cyanide. Because the ambient concentration for total cyanide exceeds the marine water quality criteria, the receiving water quality standard was adjusted to 10, 20, and 30 percent above ambient concentration to evaluate the required dilution factors at each of these levels. PLUMES, developed to predict plume dilution from effluent discharge to marine and fresh water bodies, incorporated ambient water body characteristics, physical properties of the effluent, and the hydrodynamics of the outfall pipe or diffuser. PLUMES computed both volumetric dilutions and chemical concentrations as a function of distance from the diffuser port. EFQUAL was used in conjunction with PLUMES to predict the required dilution zone distances. Dilution zone distances for discharge at the mouth of Sitcum Waterway ranged from 330 to 130 feet and in the Sitcum Waterway from 27 to 4.3 feet. These results indicate dredging and nearshore confined disposal of Sitcum Waterway sediments can be accomplished without significant water quality impact.