SEDIMENT SEQUENTIAL RISK MITIGATION AT THEA FOSS WATERWAY SUPERFUND SITE

摘要

Sequential risk mitigation is a three-phase process designed to: (1) reduce the ecological and human health risks posed by highly contaminated sediments through confinement or capping; (2) reduce the risks associated with moderate levels of pollution on a less urgent schedule and at lower cost; and (3) address areas of limited contamination through a combination of natural attenuation and enhanced natural attenuation. Natural attenuation has as its basis the observation that ecosystem recovery appears to be largely a function of time. Sediment decomposition and the mixing of old sediments by bottom-dwelling organisms contribute to reduced contaminant concentrations. Knowledge of these processes is critical to the development of ecosystem recovery and waste management strategies. To evaluate the applicability of natural attenuation, information should be collected to determine whether surface sediment chemical concentrations will reach cleanup standards within a 10-year period through a combination of natural attenuation and adequate source control. The remedial design for the Thea Foss Waterway Superfund Site in Ta-coma, Washington, employs the sequential risk mitigation approach. The EPA has selected a remedy involving natural attenuation (21 acres [8.4 hectares]), enhanced natural attenuation (4 acres [1.6 hectares]), dredging (650,000 cubic yards [494,000 cubic meters]), and capping (30 acres [12 hectares]) of this 8,000-foot (2,438-meter)-long waterway. The dredged material will be disposed of in a near-shore confined disposal facility. Specialized design elements include sorbent capping of active oil seeps, about 400 feet (122 meters) of permanent sheet pile bulkhead, and other features related to long-term monitoring of the remedy and to controlling contaminant sources.