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Suspended sediment plays an important role in the transport and geochemical cycling of aquatic trace metals. The sampling and subsequent analysis of suspended sediment has been used to locate ore deposits, determine water-quality trends, and identify anthropogenic pollution sources. Many laboratories and organizations determine suspended sediment-trace metal concentrations by the method of difference. However, this procedure may lack sufficient sensitivity to accurately and precisely determine suspended sediment metal levels when either metal or suspended sediment concentrations are low. Many of the difficulties associated with the method of difference can be eliminated or minimized by a direct analysis of suspended sediment. The classical procedure for concentrating (dewatering) suspended sediment entails in-line filtration using preweighed 0.45-µm membrane filters; subsequent analysis entails the digestion of the sediment plus filter, analysis of the digestate, and the application of a blank correction for metal contributions from the filters. This procedure can be prohibitively expensive and time consuming, especially when large-scale studies with numerous sampling sites or high sampling frequencies are required. Potential alternative methods of concentration (dewatering) of suspended sediment which could be used in lieu of in-line filtration and for direct quantitation of trace metals while reducing costs were investigated. Three potential substitute procedures—batch centrifugation, settling-centrifugation, and backflush-filtration—were compared to in-line filtration, and were field and laboratory tested. Results indicate that both the batch centrifugation and the settlingcentrifugation procedures are comparable with in-line filtration, and are amenable to large-scale sampling and trace-metals analysis programs.
suspended sediment, dewatering, chemical analysis, trace metals, river water
Research chemist, U.S. Geological Survey, Doraville, GA