USGS Washington Water Science Center
|U.S. Department of the Interior
U.S. Geological Survey
Release: March 7, 2005
[Note: The report can be viewed on the Web at http://pubs.water.usgs.gov/sir20045090/ .]
Decades of liquid effluent from the Teck-Cominco smelter in British Columbia contributed most of the zinc, lead, cadmium, and other trace elements detected in a recent sediment-coring study of Lake Roosevelt, according to a report published by the U.S. Geological Survey (USGS), in cooperation with the Confederated Tribes of the Colville Reservation.
In the 2002 study, USGS scientists took cores of fine-grained sediment from six sites in the middle and lower reaches of Lake Roosevelt, a 135-mile-long reservoir in northwestern Washington formed behind Grand Coulee Dam in 1941. The lower reach extends from the dam to the confluence of the Spokane river, and the middle reach extends from the confluence to Marcus Island. Over time, fine-grained sediment has accumulated at the bottom of the lake, forming a record of deposited layers. Scientists collected the cores to examine the sediment record to see how trace element concentrations varied through time. Microscopic and chemical analyses show that slag particles found in some sediments showed signs of weathering and breaking down, demonstrating that slag is not inert.
To determine how concentrations vary vertically in each core, samples were taken from sections of each sediment core and analyzed for 54 elements. Elevated concentrations of arsenic, cadmium, copper, lead, mercury, and zinc were found throughout much of the lake sediment. Concentrations were typically largest in the deeper sections of the cores, decreasing upward toward the surface. All samples exceeded the cleanup sediment-quality standards of the Confederated Tribes of the Colville Reservation for cadmium, lead, and zinc; more than 70 percent of the samples exceeded the cleanup standards for mercury, arsenic, and copper.
Using a centrifuge, scientists extracted samples of pore water (the water between sediment particles) and analyzed them for trace elements. Concentrations of dissolved arsenic, cadmium, copper, lead, and zinc in the pore water were often higher than ambient dissolved concentrations seen in water samples from the lake.
Scientists designed a lab experiment with the sediment samples to determine if slag particles, which are commonly found in sediment from the upper reach of the reservoir, were a substantial part of the trace elements found in these sediment samples. Analyses of the residual sediment after being washed with liquid chemicals showed small concentrations of trace elements, compared with the much larger total concentrations found in the unwashed sediment. "We looked at results from the lab experiment and the general, gradual decrease in concentrations of trace elements in the more recent deposits throughout the reservoir," said USGS hydrologist Stephen Cox, lead author of the report. "These and other results from our study indicate that the liquid effluent from the Teck-Cominco smelter is the primary contributor of the large concentrations found in sediment samples from the middle and lower reaches of Lake Roosevelt."
The report, "Vertical Distribution of Trace-Element Concentrations and Occurrence of Metallurgical Slag Particles in Accumulated Bed Sediments of Lake Roosevelt, Washington, September 2002," by Stephen E. Cox, Peter B. Bell, J. Stewart Lowther, and Peter C. VanMetre, is published as U.S. Geological Survey Science Investigations Report 2004-5090. The report can be viewed on the Web at http://pubs.water.usgs.gov/sir20045090/. Copies can be purchased from the U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225-0286, telephone 303-202-4200.
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