Sediment mixing and accumulation rate effects on radionuclide depth profiles in Hudson estuary sediments

Abstract

Measured anthropogenic radionuclide profiles in sediment cores from the Hudson River estuary were compared with profiles computed by using known input histories of radionuclides to the estuary and mixing coefficients which decreased exponentially with depth in the sediment. Observed 134Cs sediment depth profiles were used in the mixing rate computation because reactor releases were the only significant source for this nuclide, whereas the inputs of 137Cs and 239,240Pu to the estuary were complicated by runoff or erosion in upstream areas, in addition to direct fallout from precipitation. Our estimates for the rates of surface sediment mixing in the low salinity reach of the estuary range from 0.25 to 1 cm2/yr, or less. In some areas of the harbor adjacent to New York City, where fine‐particle accumulation rates are generally >3 cm/yr, and often as high as 10 to 20 cm/yr, sediment mixing rates as high as 10 cm2/yr would have little effect on radionuclide peak distributions. Consequently, anthropogenic radionuclide maximum activities in subsurface sediments of the Hudson appear to be useful as time‐stratigraphic reference levels, which can be correlated with periods of maximum radionuclide inputs for estimating rates and patterns of sediment accumulation.