USING 210Pb AND 137Cs TO IDENTIFY THE BANK VS. SOIL CONTRIBUTIONS TO EXCESS FINE-GRAINED SEDIMENTS IN URBAN AND RURAL NEW JERSEY RIVER CHANNELS

Fine-grained sediment is currently a major pollutant in New Jersey rivers as well as other states and can affect biologic systems, river aesthetics, recreational use, and water supply. Controlling sediment supply is often complicated because determining the source of this sediment is difficult. It is reasonable to simplify possible sources to a two-source model: shallow soil erosion vs. river channel bank. The sediment may originate either from widespread but shallow surficial erosion from overland flow occurring in the watershed or from the lateral erosion of vertical river channel bank material. The goal of this study was to distinguish between these two sources using their different radionuclide signatures of ²¹⁰Pb, and ¹³⁷Cs. Older ( ~ >100 years) sediment will have little ²¹⁰Pb, and ¹³⁷Cs activity due to the short half-lives of the radionuclides, while sediment will have progressively more activity with less age. Sediment generated from surficial erosion should have higher activity levels of these atmospherically-deposited radionuclides than the sediment produced from vertical channel banks. We sampled channel bank material, watershed soils, and in-stream fine sediment, and analyzed them for their radionuclide signature to identify the relative contributions of sediments from the watershed and channel banks. The sampling was done for two small watersheds in New Jersey, Cold Brook (agricultural) and East Branch of the Rahway (urban), each of which has distinct land uses. The channel bank sediment and soils had similar activity levels between the two watersheds, indicating consistent radionuclide signatures between the sources. However, the in-stream sediment from the two streams differed significantly. The urban East Branch had much higher activity levels than the rural Cold Brook, suggesting that the sediment in East Branch is not coming from channel banks but from surficial erosion. The Cold Brook appears to have more of its sediment coming from channel banks due to the low activity levels of its in-stream sediment. This knowledge will allow for improved stream and watershed management and the possible initiation of sediment-reduction programs.