Estimating suspended sediment and trace element fluxes in large river basins: methodological considerations as applied to the NASQAN programme

In 1994, the NASQAN (National Stream Quality Accounting Network) programme was redesigned as a flux-based water-quality monitoring network for the Mis sissippi, Columbia, Colorado, and Rio Grande Basins. As the new programme r epresented a departure from the original, new sampling, processing, analyti cal, and data handling procedures had to be selected/developed to provide d ata on discharge, suspended sediment concentration, and the concentrations of suspended sediment and dissolved trace elements. Annual suspended sediment fluxes were estimated by summing daily instantane ous fluxes based on predicted suspended sediment concentrations derived fro m discharge-based log-log regression (rating-curve) models. The models were developed using both historical and current site-specific discharge and su spended sediment concentrations. Errors using this approach typically are l ess than +/- 10% for the 3-year reporting period however, the magnitude of the errors increases substantially for temporal spans shorter than 1 year. Total, rather than total-recoverable, suspended sediment-associated trace e lement concentrations were determined by direct analysis of material dewate red from large-volume whole-water samples. Site-specific intra- and inter-a nnual suspended sediment-associated chemical variations were less (typicall y by no more than a factor of two) than those for either discharge or suspe nded sediment concentrations (usually more than 10-fold). The concentration s, hence the annual fluxes, for suspended sediment-associated phosphorus an d organic carbon, determined by direct analyses, were higher than those det ermined using a more traditional paired, whole-water/filtered-water approac h (by factors ranging from 15- to 10-fold). This may be important for such issues as eutrophication and coastal productivity. Filtered water-associated (dissolved) trace element concentrations were mar kedly lower than those determined during the historical NASQAN programme; m any were below their respective detection limits. This resulted from the us e of clean sampling, processing, and analytical protocols. Hence, the fluxe s for filtered water-associated (dissolved) Ag, Pb, Co, V, Be, Sb, and Se, as well as the total (filtered water plus suspended sediment-associated) fl uxes for these constituents, could not be estimated.