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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

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Relative Impacts of Slag and Liquid Effluent Discharge on Trace-Element Concentrations in Bed Sediment

Multiple lines of evidence indicate that the influence of slag on trace-element concentrations in the sediments of the middle and lower reaches of Lake Roosevelt are likely secondary to the influences of liquid effluent discharges. First, the low percentages of cadmium and lead, and to a lesser degree zinc and copper, present in the residual of the leached sediment samples indicates that these trace elements are predominantly associated with the sediment grain surface rather than the interior matrix of the sediment grains. If slag were the primary source of these trace elements, most of the mass of the element would be in the interior matrix of the slag particles and not susceptible to leaching with hydroxylamine hydrochloride reagent.

Second, the widespread and approximately concurrent decreases in trace-element concentrations observed in the trace-element profiles above the 1964 horizon in the Columbia River cores are more consistent with trace-element discharges in the liquid effluent than discharges in slag material. Trace elements sorbed to very fine-grained sediment surfaces typically originate as dissolved trace elements or trace elements bound to or associated with colloidal material such as that discharged in the liquid effluent of the Trail smelter. Dissolved and colloidally associated trace elements are more easily transported throughout the length of the reservoir than slag that is discharged as predominantly sand-size particles and would require more time for even the smaller size fractions to be transported to the lower reaches of the reservoir. Transport of most of the slag particle-size fractions within the middle and lower reaches of the reservoir is limited to times when flow is great enough to transport larger grain sizes. As a result, the effects of slag discharge on trace-element concentrations in the accumulated bed sediments would take longer to be observed in the lower reaches of the reservoir and would likely be less pronounced.

Although slag was observed below lacustrine bed sediments at RM 680 and in core CCR-668, the presence of large slag deposits like that shown in figure 7 in or below the lacustrine sediments of the reservoir is most likely an uncommon occurrence. Direct observation of slag grains in reservoir sediments generally was confined to the upper reaches of the reservoir above RM 700. Using a point-count microscopic examination, Bortleson and others (1994) estimated that slag made up about one-half of the sediment sample from RM 745 near the International Boundary, about 28 percent of the sediment at RM 738, and only about 5 percent of the sediment at RM 730. Likewise, Bart Canon (Northwest Microprobe, written commun., 2002) estimated that while slag composed a substantial fraction of sediments from near RM 744 and RM 748, slag was a negligible fraction of sediments from near RM 723.

Third, the timing of most of the major decreases in trace-element concentrations occurring within the trace-element profiles occur throughout the period following the CS-137 peak associated with the mid-1960s. The more gradual reduction observed in the trace-element profiles from the cores also is more consistent with the trend in the reduction of trace-element loading in liquid effluent (fig. 7) than compared to trend in historical reductions of slag discharge. This is most clearly observed for mercury and cadmium and to a lesser degree lead, which are very minor components of slag (Cominco, 1991), but substantial components of the liquid effluent discharge (British Columbia Ministry of Environment, 1976). Zinc is a substantial component of both liquid effluent and slag material discharged by the Trail smelter.

The concentration of copper in bed sediments may be a useful indicator of the presence of slag in sediments from Lake Roosevelt. Copper is distinctive among the six trace elements of concern because it is discharged by the Trail smelter primarily as slag. Using estimates of slag percentage in sediments from the upper reach of the reservoir (Bortleson and others, 1994) and assuming that all copper reported in the measured concentration for the corresponding samples is due to the presence of slag, the calculated concentration of copper in slag at those locations would be expected to range from 0.7 to 1.5 percent. This is consistent with the measured and reported range of copper concentrations in slag from the Trail smelter of about 0.6 to 1 percent. If the presence of elevated copper concentration in sediments is largely the result of slag particles (a conservative assumption based on the sediment leaching data), sediment samples with less than 125 mg/kg of copper would be expected to contain less than 1 percent slag in addition to a background concentration of about 25 mg/kg of copper. Applying this reasoning to the longitudinal gradient of copper in Lake Roosevelt sediments observed by Majewski and others (2003), and Bortleson and others (1994), the major influence of slag in sediments is limited to reaches upstream of about RM 687. Thus, the trace-element concentrations of zinc, lead, cadmium, mercury and to a lesser extent arsenic in sediments in the lower and middle reaches of the reservoir are largely due to the input from liquid effluent discharged to the Columbia River, which can easily be transported the length of the reservoir as dissolved or colloidal material.

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