U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5090
Vertical Distribution of Trace-Element Concentrations and Occurrence of Metallurgical Slag Particles in Accumulated Bed Sediments of Lake Roosevelt, Washington, September 2002
Summary and Conclusions
In 2002, the U.S. Geological Survey conducted a study of Lake Roosevelt to collect information on the vertical distribution of trace elements within the accumulated bed sediments and pore water of Lake Roosevelt, the 217-km-long impoundment of the Columbia River upstream of Grand Coulee Dam, Washington. Vertical distributions of trace-element concentrations were determined from six sediment cores collected in September 2002. The analysis of cesium-137 data indicate that two sediment cores contained sedimentary records extending to the initial reservoir filling in 1941 and the remaining four cores contained at least 60 percent of the sedimentary record. Pore-water samples were collected and analyzed from the top 10 cm at the three upstream core sites.
Concentrations of the trace elements of concern, arsenic, cadmium, copper, lead, mercury, and zinc, varied greatly in the sediment core profiles, often covering a range of 5 to 10 fold. Maximum concentrations in sediment samples were as high as 2,200 milligrams per kilogram (mg/kg) for zinc, 920 mg/kg for lead, and 250 mg/kg for copper, with median concentrations of 970, 260, and 53 mg/kg, respectively. Concentrations of arsenic, cadmium, and mercury were lower. Maximum concentrations were as high as 34 mg/kg for arsenic, 23 mg/kg for cadmium, and 2.8 mg/kg for mercury, with median concentrations of 13, 7.5, and 0.58 mg/kg, respectively. Concentrations of cadmium and mercury exceeded the upper range of reference sediments by 10 times or more in 77 and 47 percent of the samples analyzed, respectively.
Concentrations of lead and zinc exceeded reference sediments by 10 times in less than 25 percent of the samples analyzed. Prominent temporary decreases in the concentrations of cesium-137 and trace elements indicate an influx of sediments from landslides. Of these trace elements of concern, concentrations of arsenic and copper were least elevated above concentrations in reference sediments. All samples exceeded cleanup guidelines adopted by the Colville Confederated Tribes for cadmium, lead, and zinc and more than 70 percent of samples exceeded cleanup guidelines for mercury, arsenic, and copper. Although 100 percent of samples exceeded sediment guidelines for cadmium, lead, and zinc, surficial concentrations of arsenic, copper, lead, and mercury in some cores were less than the sediment-quality guidelines.
Trace-element concentrations were highest beneath surficial sediments, typically in the lower one-half of each profile. With the exception of copper, the trace-element profiles of the five cores collected along the pre-reservoir Columbia River channel typically showed trends of decreasing concentrations from the 1964 sediment horizon to the sediment-water surface. Along the longitudinal profile of the reservoir, surficial concentrations of zinc, copper, lead, cadmium, and mercury showed trends of decreasing concentration, while concentrations of arsenic tended to increase slightly downstream. Trace-element concentrations and profiles in sediments from the Spokane Arm of the reservoir showed distinct differences compared to the similarities observed among the cores from along the pre-reservoir Columbia River. These differences likely are due to the greater influence of sediment inflow from the Coeur d'Alene basin to that portion of the reservoir.
Overall, the depth concentration profiles typically show maximum concentrations of arsenic, cadmium, lead, mercury, and zinc often occurred in the lower to mid sections (closer to the bottom) of the trace-element profile and the concentrations of copper often were highest (closer to the surface) in the trace-element profiles. The concentration profiles for arsenic, cadmium, lead, zinc, and to a lesser extent mercury, also clearly show decreasing concentrations in more recently deposited sediments throughout the reservoir as reported by Majewski and others (2003). However, the concentration profiles for copper generally show trends of increasing concentrations in more recent sediments with the exception of core CCR-692. Within individual cores, the concentration profiles of arsenic, cadmium, copper, lead, mercury and zinc showed similar patterns of relative concentrations. Variation in the concentration of zinc, lead, cadmium, and mercury were consistent to the extent that concentration profiles were often near parallel. In some core intervals, the influx of sediments from bank material accentuated the near parallel concentration profiles.
Concentrations of dissolved arsenic, cadmium, copper, lead and zinc in pore waters were often higher than ambient dissolved concentrations measured in Columbia River water and thus may be a source of trace elements to the overlying surface water. The limited pore-water concentration data was inconclusive with respect to the extent of remobilization of trace metals from buried sediments to the overlying water. Additional information is needed on possible seasonal variation in pore concentrations that may be affected by the changing pool elevation in the reservoir. The leaching experiment indicated that most of the mass of trace elements associated with the contaminated sediments is associated with the surface of the sediment particle where it is more accessible to physical, chemical, and biological processes that could remobilize the trace element into the overlying water column.
Selected samples were analyzed for the presence of metallurgical slag, a trace element-rich byproduct of metals smelting processes, which was discharged into the Columbia River by a smelter in Trail, British Columbia, upstream of the reservoir prior to 1995. Particles that had physical and chemical characteristics of slag discharged to the Columbia River were present in the sediments of Lake Roosevelt. Particles of slag were isolated from three cores based on their distinctive physical characteristics, appearance (smooth, glassy-lustrous, black in color, and being both angular and rounded in shape), and chemical composition measured using a scanning electron microscope equipped with an energy dispersive spectrometer (SEM/EDS). The chemical composition of the interior matrix of slag collected from a black-sandy beach near the International Border closely approximated the reported elemental concentrations of fresh smelter slag. Slag particles isolated from the core sediments appeared more weathered due to the lack of the glassy-lustrous appearance; however, their chemical composition closely matched the composition of unweathered particles except that the concentration of calcium was roughly 20 percent smaller. SEM/EDS examination of the morphology of the surface of many of the slag particles from the core sediments showed the development of exfoliation flakes suggesting that the glassy slag material was undergoing hydration and chemical weathering.
Slag, while common in the uppermost reaches of the reservoir above river mile 720, may be present only in minor amounts in sediments of the lower and mid reaches of the reservoir. Trace elements in sediments in the lower and mid reaches of the reservoir are thus largely due to the liquid effluent discharged to the Columbia River, which can easily be transported the length of the reservoir. Relative concentrations of trace elements in the cores from those reaches of the reservoir resemble relative loading of trace elements from the liquid effluent, further supporting that conclusion.
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