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Scope/Content: Abstract: This report presents the results of an evaluation of mercury bioaccumulation from sediment collected at the Underwood In Lieu Fishing Access Site (ILFAS). The evaluation of this supplemental sample collected in May 2008 was performed to provide bioaccumulation information from a reference sample more representative of the fine sediment in Northwestern Lake than was obtained from background sampling conducted in June 2007. This information will enhance the understanding of the potential impacts of the open water release of sediments from Northwestern Lake following the proposed removal of the dam. In December 2006, Kleinfelder, under contract to PacifiCorp Energy, collected sediment characterization samples from Northwestern Lake and reported the results in a Sediment Sampling and Analysis Report (March 2007). This investigation concluded that the average concentration of mercury in the fine silts impounded behind Condit dam that comprise the Lower Basin dredged material management unit (DMMU) is approximately 0.72 milligrams per kilogram (mg/kg). During this work, the maximum detected concentration of mercury in the sediments was 2.03 mg/kg in the surface sediment at a location (boring B-1) near the dam. Laboratory bio-assays conducted on the sediment collected during this field work indicated that the sediment had no adverse effect on the life cycles of test species. The applicable current regulatory screening levels for mercury in fresh water sediments are found in the Northwest Regional Sediment Evaluation Framework (NWRSEF) (September, 2006) and are 0.28 mg/kg (lower screening level, SL1) and 0.75 mg/kg (upper screening level, SL2). The NWRSEF states, "The lower screening level (SL1) corresponds to a concentration below which adverse effects to benthic organisms would not be expected, and the upper screening level (SL2) corresponds to a concentration at which minor adverse effects may be observed in the more sensitive groups of benthic organisms." Because mercury concentrations were found in excess of the upper screening level (SL2), the Regional Sediment Evaluation Team (RSET), composed of representatives of various state and federal regulatory agencies, requested that PacifiCorp Energy collect supplemental information to confirm the elevated mercury concentrations found in sediment near the dam at boring location B-1 in Northwestern Lake, establish the bio-availability of mercury found in the sediments through bioaccumulation studies of fish, worms, and clams, and ascertain background sediment mercury concentrations in the local area. The supplemental sampling and analysis was completed in July of 2007. However, sediments collected as a reference sample from the Little White Salmon River and used as part of the bioaccumulation study were determined to not be representatively similar to the White Salmon River sediments. Specifically, the Little White Salmon River sediments were composed of less than 70 percent fines, whereas the White Salmon River (and Northwestern Lake) sediments were typically 95 percent (or greater) fines. Based on this, RSET requested an additional bioaccumulation study be performed with sediment from the Underwood In Lieu Fishing Access Site, which is on the White Salmon River and was shown to include sediments that are similar to those collected from Northwestern Lake. The data presented in this report are intended to further test the hypothesis presented in the March 2007 Sediment Report and the November 2007 Supplemental Evaluation of Mercury in Sediments Report, which suggested that the elevated concentrations of mercury encountered in the fine-grained sediments impounded in Northwestern Lake behind Condit dam are due to naturally occurring background concentrations of the native element. This hypothesis was developed based on the understanding that hydrothermal alteration associated with recent volcanic activity, such as that which has occurred with Mount St. Helens, is a known source of mercury. Additionally, there are no known anthropogenic sources of mercury Scope/Content: Dam type: gravity Scope/Content: Date constructed: 1913. Date removed: 2011.
Pre- and post-project monitoring Safety and liability Sediment and channel dynamics Dams Dam retirement