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

The natural radionuclides Th-231 and Be-7 have known sources in estuaries a nd strongly and rapidly associate with particles. They are thus good tracer s of the transport and sources of particles in the suspended sediment reser voir, and, by implication, of particle-associated contaminants such as heav y metals. In the Hudson River estuary, Th-234 is produced from decay of dis solved U-238, which varies linearly with salinity, while Be-7 is added dire ctly to the estuary from the atmosphere. The Th-234/ Be-7 activity ratio el iminates variations in radionuclide specific activities that result from ch anges in sediment grain size or composition. Sampling in the Hudson under c onditions of low and high river flow shows that Th-234/Be-7 in suspended se diment increases from low salinity to higher salinity waters at all samplin g times. The Th/Be activity ratio may thus be used as a tracer of where par ticles are labeled with these radionuclides in the estuary and of the relat ive 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 Th-234/ Be-7 by normalizing the metals to Fe. Iron a nd 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 sour ce(s) and chemical behavior affect the metal/Fe ratios. The strongest posit ive correlation with Th-234/ Be-7 is seen for Ag/Fe, reflecting the dominan ce of lower estuary sources of this trace metal to the Hudson. Relationship s between Pb/Fe, Cu/Fe and Th-234/ B-7 are less strong, reflecting multiple sources of these elements. No correlation is seen for Cd/Fe with (234)Tn/B e-7, indicating either a source up-river or release of Cd as its K-d decrea ses with increasing salinity. (C) 1999 Elsevier Science B.V. All rights res erved.