Exponent served as RI/FS contractor for the Ketchikan Pulp Company (KPC), a timber company closing a dissolving sulfite pulp mill in southeast Alaska. Exponent conducted a focused investigation of both upland areas and offshore marine sediments, and provided technical support for the Ward Cove Sediment Remediation Project.
Exponent scientists first reviewed the historical database for Ward Cove and proposed a phased approach to delineating potential areas of concern. A key objective of the work was to discriminate effects due to toxic chemicals from effects due to conventional variables such as ammonia, sulfide, and oxygen-demanding organic material. Site-specific sediment chemistry, bathymetry, circulation, and toxicity data were collected to support site assessment. We then applied a suite of four sediment toxicity tests representing a variety of exposure regimes, aeration designs, test species, and endpoints to allow identification of toxicity designations based on a preponderance of evidence (rather than the results of a single test). This approach also differentiated between potential effects of toxic substances and conventional variables on sediment toxicity in the cove.
Human health and ecological food web evaluations applied existing tissue data and sediment data to realistic estimates of exposure and determined that risks associated with seafood consumption were within acceptable levels. Additional work was conducted to assess the severity and causative agents associated with sediment toxicity to benthic organisms. In a second phase, more detailed evaluations were conducted in the areas of concern identified in Phase 1. These studies identified the vertical extent of substances of concern within a confined area and better characterized the substances that are likely responsible for any observed toxicity. The detailed evaluations included evaluations of pore water, Toxicity Identification Evaluations (TIEs), and various forms of sediment manipulation. Problem chemicals were determined to be non-persistent chemicals (e.g., ammonia, sulfide, 4-methylphenol) that are natural degradation products of organic matter and woody debris. Modeling and a case study analysis were conducted to assess the rate of natural recovery.
The feasibility study focused on development of a practical remedy for a large volume of low-toxicity sediment, as identified using site-specific effects thresholds in a weight-of-evidence approach. The proposed remedy will consist of a combination of natural recovery, limited maintenance dredging, and capping. As required under Section 7(c) of the Endangered Species Act, Exponent is currently conducting a Biological Assessment of the impacts of the proposed remediation on three endangered species: humpback whales, peregrine falcons, and Steller sea lions.