TRACE METAL CHRONOLOGIES IN SEDIMENT CORES FROM NYC DRINKING WATER RESERVOIRS

Dated sediment cores from two New York City drinking water reservoirs were used to develop trace metal chronologies extending back to the early twentieth century. Cores were collected in the West Basin of the Ashokan Reservoir located in rural Ulster County, NY and the Muscoot Reservoir in suburban Westchester County. Radionuclide analyses of core sections were carried out in our lab using gamma spectroscopy with an ORTEC intrinsic germanium well detector. Dating information was based on the known atmospheric input history of fallout, Cs-137, and the decay atmospheric (excess) Pb-210. Depth profiles of radionuclide activities were modeled with respect to drainage basin holdup times, sedimentation rates, mixed layers, and radionuclide inventories and focusing factors following Chaky (2003). Levels of lead, copper, zinc, and chromium were measured in the core sections using our Xepos III X-ray fluorescence spectrometer. In the Ashokan cores, levels of Pb peaked around 1970, consistent with the history of domestic consumption of leaded gasoline. Levels of zinc were also maximum at about the same time suggesting additional significant sources, perhaps for both metals. In Muscoot Reservoir sediments, maximum levels of zinc were somewhat higher, ~250 ppm compared to ~170 ppm in the Ashokan, but occurred much later, increasing through the 1980s and into the 1990s. This may reflect the influence of municipal solid waste incineration and a major facility located about ten miles upwind of the Muscoot.