NATIONAL TRENDS IN TRACE ELEMENTS (METALS) IN RESERVOIR SEDIMENT CORES FROM 1970 TO 2004 AND THEIR RELATION TO INFLUENT STREAM QUALITY

Three decades of legislation, regulation, and changing demographics and industrial practices in the United States likely have had an effect on the amount of metals delivered to the environment. To determine whether metals concentrations in streams have increased or decreased since the 1970s, the U.S. Geological Survey collected sediment cores from 42 reservoirs and lakes across the country, covering a land-use gradient from undeveloped (reference condition) to fully urbanized. Trends in seven metals (Cd, Cr, Cu, Hg, Ni, Pb, and Zn) from 1970 to present are discussed. Statistically significant downward trends (decreasing concentrations) outnumbered upward trends for all metals except Zn. The metals with the greatest proportion of downward relative to upward trends were Pb and Cr (about 10 times more), followed by Ni (five times more). Only in the case of Zn did upward trends outnumber downward trends, by 1.5 to 1 (3.5 to 1 in lakes in dense urban settings). To assess whether the trends in the sediment cores reflect the water-quality history of the influent streams, concentrations of metals at the tops of cores were compared to those associated with influent-stream suspended sediment for five watersheds. The results, combined with evaluation of relations between suspended sediment and cores with regard to land-use and regulation histories, indicate that influent concentrations and historical trends of metals are preserved in cores. The results suggest that the removal of Pb from gasoline coupled with control of point-source releases of metals to air and water have been effective in reducing metals contamination of waterways, but that in some watersheds unregulated nonpoint sources are offsetting, or in the case of Zn, overwhelming these reductions. Despite the overall decrease in contamination, concentrations of metals in lakes with dense urban watersheds remain greater than sediment quality guidelines indicating a potential environmental threat.