Historical Trends and Current Inputs of Hydrophobic Organic Compounds in an Urban Estuary: The Sedimentary Record

Abstract

Sediments collected and examined for total aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) were used to assess current inputs, as well as historical trends in Narragansett Bay. The historical trends in contaminants were dominated by three distributions: (a) a subsurface maximum with decreased contaminant levels both to the surface and with depth in the cores; (b) maximum contaminant concentration at the surface, decreasing with depth; and (c) relatively homogeneous contaminant concentrations with depth. Sedimentation rates ranged from 0.11 to 5.5 cm yr-1 in the Providence River area. From this, three different depositional zones were postulated that approximately correlate with water depth and degree of human impact. The Providence River sediments accumulated organic toxicants at a much greater rate than the remainder of the estuary, comprising nearly 90% of the accumulation, but only about 9% of the surface area. By using a box model, it was established that the average sediment flux of PCBs (∼100 kg yr-1) and PAHs (∼2 mt yr-1) was greater than input source averages. In the case of aliphatic petroleum hydrocarbons (average sediment flux = 300 mton yr-1), the sediments were not accumulating at a rate sufficient to account for estimated inputs.