THE OCCURRENCE OF TRACE ELEMENTS AND METALLURGICAL SLAG IN SEDIMENT CORES FROM LAKE ROOSEVELT, WASHINGTON STATE

Bed-sediment samples from 6 gravity cores were collected in September 2002 from Lake Roosevelt, the 217-km-long impoundment of the Columbia River formed upstream of Grand Coulee Dam. Cores were obtained from locations throughout the reservoir in deeper sections near the pre-reservoir river channel. Core lengths varied from 45 to 164 cm and were sectioned into 12 to 19 intervals that were analyzed for 39 trace elements and Cesium-137. Selected samples containing larger concentrations of Zn, Ag, and Sb were analyzed for the presence of slag, a glassy trace element-rich byproduct of ore smelting.

In past studies of surficial bed sediments of Lake Roosevelt, elevated concentrations of As, Cd, Cu, Pb, Hg, and Zn have been widely observed ranging up to 200 times background levels. Although natural sources of these trace elements are present in this region, the elevated concentrations within lake sediments have been attributed primarily to mining activities and ore-processing wastes transported to the reservoir by the Columbia River.

Cesium-137 data indicate that the cores contained a nearly complete record of reservoir sedimentation. Trace-element concentrations observed in core profiles were similar to concentrations in surficial sediments observed in previous studies; however, the concentrations at depth were often 2 to 5 times larger than those observed near the upper surface of the core. Maximum concentrations of Zn, Pb, and Cu were as high as 2,200, 920, and 250 mg/kg, with median concentrations of 930, 230 and 52 mg/kg, respectively. Concentrations of As, Cd, and Hg were smaller, with maximum concentrations as high as 34, 21, and 2.3 mg/kg, and median concentrations of 13, 7.5, and 0.44 mg/kg, respectively.

Slag grains transported to Lake Roosevelt by the Columbia River were opaque when examined by petrographic microscope. Analysis of the groundmass of slag particles using energy-dispersive X-rays indicated the median element-weight percentages of Fe, Si, Ca, O, and Zn were about 37, 13, 10, 10 and 2.6 respectively. The element-weight percentages were used to identify slag grains in the core sediment samples.