TRACE ELEMENTS IN SLAG BEARING SEDIMENTS FROM LAKE ROOSEVELT, WA: A SEQUENTIAL EXTRACTION AND SEM STUDY

Previous studies have demonstrated the occurrence of elevated concentrations of trace elements in the biota, and the bed sediments in Lake Roosevelt, the 217 km reservoir in the Columbia River behind the Grand Coulee Dam. However, limited information is available about where these elements are hosted in (or on) the sediment particles and their potential to be transferred to the biota and water. The present study seeks to better understand the chemical forms, modes of binding, and associations of these trace elements within the sediment by combining sequential extraction (an operationally defined approach) with SEM secondary electron imaging and energy dispersive X-ray analysis (direct observation).

Sediment samples were collected using a box core at three different points in Lake Roosevelt. These samples were analyzed for Fe, Ni, Cr, As, Zn, Mn, Cd, Pb, and Cu following sequential extraction by the European Commission's BCR procedure. (Quevauviller 2002) Within the sediment, this procedure distinguishes three labile fractions: (1) the acid soluble fraction (interpreted as carbonates), (2) the reducible fraction (interpreted as hydrous Fe- and Mn-oxides), and (3) the oxidizable fraction (interpreted as sulfidic and organic material). In addition it distinguishes a "residual" fraction.

A shortcoming of sequential extraction methods is that the fractions are operationally defined. Therefore, to further our understanding of the elements' associations we use the SEM to characterize changes in grain surface chemistry (e.g., disappearance of Fe- and Mn-oxide grain coatings) and grain surface morphology (e.g., etching/dissolution textures) following each step of the process.