U.S. Army Corps of Engineers – Seattle District
- Conduct a CERCLA Remedial Investigation
- Perform chemical data management, evaluation, review, and validation
- Conduct Baseline Human Health and Ecological Risk As
Big Cliff Reservoir is situated between the Big Cliff and Detroit Dams on the North Santiam River in central Oregon. During the construction of Detroit and Big Cliff Dams, from May 1947 through December 1953, the Site was the location of a construction camp used to support construction operations. Materials used and stored at the Site could have been spilled and/or disposed of during the construction period. The Big Cliff Reservoir Construction Site is approximately 7.3 acres and is typically submerged with the depth of water varying from more than 20 ft to as little as 3 ft throughout the year. The sediment surface is predominantly compacted gravel and sand with a transient silt layer that is seasonally deposited. Because releases occurred when the site was terrestrial and the sediments are not easily transported, the sediment and potential contaminants are not subject to typical sediment transport mechanisms.
This work was performed for the U.S. Army Corps of Engineers (USACE) Seattle and Portland Districts. Avatar prepared all required Plans including a UFP Quality Assurance Project Plan and Field Sampling Plan and conducted a large-scale field investigation in October 2017. The planning and sampling was timed to meet a critical reservoir drawdown period. Surface and subsurface sediment samples were collected from various locations within the former Construction Area during the reservoir drawdown week.
Random Grid Sampling:
Using ArcGIS®, a grid consisting of 33 individual grids at 125 ft x 125 ft was overlaid on the Site area. A surface sample was collected from each grid at a randomly assigned location.
Target samples were identified for biased sampling at specifically targeted locations. A total of 29 surface and eight subsurface target samples were collected based on visual or suspected contamination areas. Step out surface samples were collected from a subset of target stations to evaluate conditions immediately upstream and downstream of a targeted sample location.
Sediment samples were collected from the Big Cliff Reservoir at upstream and downstream locations as well as adjacent to the former Construction Area.
Sediment was collected from eight locations within the Construction Area in the event that comparisons of sediment chemistry concentrations with sediment-based ecological benchmarks showed the potential need for cleanup based on toxicity to benthic organisms.
Historical data and data collected in October 2017 were used in the Remedial Investigation Report which included a Human Health and Ecological Risk Assessment. The RI used a combination of targeted samples and randomly selected station locations to support the baseline risk assessments. The risk assessments used a variable baseline approach to characterize the primary risk drivers for the site. This was performed by selectively removing targeted sample results in the exposure point calculations. Five different iterations were performed. The combined randomized and targeted sampling grids for the RI combined with variable baseline approach for the risk assessments proved to be an effective method for identifying locations that were driving risk at the former construction area. A subset of well-bounded locations appears to be driving the majority of both human health and ecological risk at the Site. The results of the risk assessments are being used to determine if a Feasibility Study is needed.
Soil sampling was conducted upstream of the Site at an industrial area with limited access (Maintenance Yard). The principal reason for the effort was to add additional soil sampling locations to supplement a data gap. Historical surface soil samples reflected elevated concentrations of arsenic, copper, lead, and manganese that exceeded selected risk-based criteria. In 2018, fifteen surface soil samples and 5 subsurface samples were collected and analyzed for metals and pH. Sample results were summarized in a technical memorandum that included results compared with screening levels from USEPA and Oregon DEQ. The potential human health receptor was the industrial site worker and potential ecological receptors were terrestrial plants, soil invertebrates, birds, and mammals.
In December 2016, the USACE conducted an investigation to assess if contaminated sediment located within the submerged former construction site is affecting surface water at concentrations above Oregon DEQ and USEPA water quality criteria for human health and ecological receptors. The investigation focused on surface water sample collection above known contaminated sediment locations, as well as locations upriver of the Site to serve as a point of reference, and downstream of the Site to assess potential downstream impacts. The December 2016 samples were analyzed for DRO, RRO, GRO, total and dissolved TAL metals (plus vanadium), hardness, PAHs, phthalates, and polychlorinated biphenyl (PCB) congeners. The USACE prepared a conceptual site model (CSM) including a human health and ecological risk screening which showed some USEPA human health criteria exceedances but no ecological exceedances.
In 2018, surface water sampling was conducted at the Construction area via boat under lower flow conditions to supplement data collected in 2016. Six surface water samples were collected: four samples in the construction area, one sample upstream provided background information, and one sample downstream provided information regarding impact to surface water from the Site. Surface water samples were analyzed for PCB congeners, total suspended solids, total metals, SVOCs, PAHs, and alkalinity. Sample results were summarized with a comparison to USEPA and Oregon DEQ screening levels for humans and ecological receptors.
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