234Th and 7Be as tracers for transport and sources of particle-associated contaminants in the Hudson River Estuary

The natural radionuclides 234Th and 7Be have known sources in estuaries and strongly and rapidly associate with particles. They are thus good tracers of the transport and sources of particles in the suspended sediment reservoir, and, by implication, of particle-associated contaminants such as heavy metals. In the Hudson River estuary, 234Th is produced from decay of dissolved 238U, which varies linearly with salinity, while 7Be is added directly to the estuary from the atmosphere. The 234Thr 7Be activity ratio eliminates variations in radionuclide specific activities that result from changes in sediment grain size or composition. Sampling in the Hudson under conditions of low and high river flow shows that 234Thr 7Be in suspended sediment increases from low salinity to higher salinity waters at all sampling times. The ThrBe activity ratio may thus be used as a tracer of where particles are labeled with these radionuclides in the estuary and of the relative importance of resuspension and particle transport through the estuary. The distribution of heavy metals in the suspended sediment reservoir can be compared with that of 234Thr 7Be by normalizing the metals to Fe. Iron and Al are well correlated in suspended sediments of the Hudson, indicating that either element can be used to normalize grain size effects on specific concentrations of metals. Ratios of Ag and Pb to Fe in suspended particles generally increase down-estuary at all sampling times, whereas Cu, Zn and Cd-to-Fe ratios show more scatter and less consistency in trends. Both source s. and chemical behavior affect the metalrFe ratios. The strongest positive correlation with 234Thr 7Be is seen for AgrFe, reflecting the dominance of lower estuary sources of this trace metal to the Hudson. Relationships between PbrFe, CurFe and 234Thr 7Be are less strong, reflecting multiple sources of these elements. No correlation is seen for CdrFe with 234Thr 7Be, indicating either a source up-river or release of Cd as its Kd decreases with increasing salinity.