MAGNETIC PROPERTIES AS PROXIES FOR METAL CONTAMINATION IN COASTAL SEDIMENTS OF MASSACHUSETTS

Geochemical, petrographic, and physical property (including magnetic susceptibility and isothermal remanent magnetization) studies of sediment cores collected in 1992 from Massachusetts and Cape Cod Bays (northeastern USA) were undertaken to (1) identify areas of sewage contamination, (2) estimate rates of sediment mixing and accumulation, and (3) provide a geochemical baseline for contaminant loadings against which future loadings could be compared following the $4 billion improvements to greater Boston’s sewage treatment system. The sediment-depth profiles of 210Pb and 239+240Pu profiles indicate rapid sediment mixing by organisms to depths of 25-35 cm in most cores, precluding accurate sediment dating or sediment-accumulation-rate estimates. Nevertheless, sediment initially deposited after the onset of regional industrialization and then redistributed can be recognized by elevated concentrations of metals and radionuclides, as well as by the presence of fly ash produced from coal combustion. In these post-industrial sediments, rock-derived, Ti-bearing magnetite and associated hematite, quantified by magnetic properties, correlate with Ag, Hg, Cr, Ni, Pb, and Clostridium perfringens, a mammalian intestinal bacterium spore found in sewage. High levels of these iron oxide minerals and metals are found in silty and sandy (25-66% sand) sediments southeast of Boston, a primary regional source of contaminants. This distribution is consistent with previously measured and modeled storm-driven currents that transport sediment in a southerly path along the western side of the bays, and it is consistent with the average drift of plumes from Boston’s previous sewage outfalls. Magnetic properties that correlate with sewage tracers apparently reflect sediment transport and deposition of urban sewage along with magnetic particles derived from disturbed on-shore and near-shore environments. This investigation demonstrates that the simple, inexpensive, and non-destructive determination of magnetic properties may provide a proxy for contaminant metals and thus aid in environmental studies of post-industrial coastal sediments.