Selected Projects

Spokane Digester Failure
Exponent was retained by the City of Spokane to investigate the nature and cause of the structural failure of a digester at the City's water reclamation plant. The sudden failure of the digester dome left one city employee dead and two severely injured. Exponent's initial investigation focused on the structural failure and digester operations and process conditions in the 10-day period prior to the event. Engineers investigating the structures, process, and operations quickly and independently reached the conclusion that the digester had been overfilled. However, key questions remained to be answered, including why overflows were closed, why a transfer was unsuccessful, and why the sludge level in the digester was not detected by the sensor. When testing of the level sensor at the plant and in our laboratory found no problem with the sensor, Exponent began a complete investigation of the digester operations and process conditions. We reviewed more than 300,000 data points from the site control and data acquisition (SCADA) records and the interviews and re-interviews of 30 people before we were able to develop a complete understanding of the nature and the operational factors that contributed to the event. The Exponent team included engineers from the Environmental Sciences, Civil, Mechanics and Materials, and Electrical practices. Exponent is still working with the City of Spokane to implement recommended changes and assess the reliability of the SCADA system components.

Natural Resource Damage Assessment Support, Lower Duwamish River, Seattle, Washington
Federal (NOAA), state (Washington), and tribal trustees have alleged that chemical releases (primarily polychlorinated biphenyls, or PCBs) from Boeing Company facilities and from other parties have injured natural resources within the Lower Duwamish River in Seattle, Washington. The Boeing Company initially contracted with Exponent to investigate the nature and extent of sediment contamination and to evaluate the potential for natural resource injuries associated with releases from Boeing Company facilities. Exponent has participated in numerous negotiations with the trustees and has helped Boeing develop potential habitat restoration options.

More recently, the Lower Duwamish River was designated as a Superfund site and Exponent has been participating in a cooperative effort with a group of potentially responsible parties (PRPs) to conduct a remedial investigation. The first phase of the remedial investigation has been completed, including comprehensive ecological and human health risk assessments based on previously existing data. Key issues are the potential effects of sediment contamination on outmigrating juvenile chinook salmon, a listed species under the federal Endangered Species Act, and risks to humans who might consume seafood from the estuary. Planning for the second phase of the remedial investigation, including extensive field studies, is currently under way.

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