The History of Metals Pollution in Narragansett Bay as Recorded by Salt Marsh Sediments

Sediment cores from 5 salt marshes from the head to the mouth of Narragansett Bay and an additional core from a lagoon on Block Island Sound were analyzed for 210Pb and for Fe, Mn, Cu, Pb, Cr, Zn, Ag, and Ni in order to examine the long-term variation of metal inputs to Narragansett Bay. The 210Pb results were used to determine accretion rates for each core. The distributions of Fe and Mn were used as indicators of the chemical conditions in the sediment cores and the Cu, Pb, Cr, Zn, Ag, and Ni distributions over time were compared with known or estimated source inputs to examine the long-term variation of pollutant metal inputs to Narragansett Bay. At one location, duplicate cores were sampled to look at the variability within a marsh. At another location both a high marsh core, rece1vmg predominantly atmospheric inputs, and low marsh core, rece1vmg waterborne and atmospheric inputs, were sampled so that atmospheric and tidal contributions to the low marsh could be determined. A comparison was also made of the distributions of metals in bay cores and in the lagoon core. The 210Pb determined accretion rates showed that all of the marshes accrete at rates equal to or greater than local sea level rise (0.26 ± 0.02 cm/yr). The concentrations of most pollutant metals began to increase in the mid to late 1800s, based on the 210Pb determined dates. Early increases appear to correspond to atmospheric inputs from coal burning, while steeper increases, starting in the early 1900s, apparently reflect intensified industrial activity and/or the relocation of sewage discharges from Providence, and thus waterborne inputs. Maximum concentrations were reached in the 1950s for some metals and for some cores and 1970s for other metals and other cores, and concentrations have declined almost continuously to the present. The observed reductions have been attributed to implementation of and improvements to sewage treatment and to controls on atmospheric emissions. Despite a large variability between cores sampled at the same site, all cores along the bay transect showed features which occurred at the same time, within the errors on the ages of core depths. This suggested that there has not been a movement of pollution down the bay with time, but that variations were observed at all locations at about the same time, although with an intensity that decreased with distance from the head of the bay. The decreasing gradient in metals with distance from the head of the estuary implies that the source of metals is in the upper bay. The gradient in concentrations, m addition to the observation that all cores were affected simultaneously, strongly suggests that reductions of metal discharges at the head of the estuary will result in bay-wide reductions in metal concentrations in the sediment. Comparison of the high and low marsh cores suggested that the predominant source of metals to the marshes has been tidal. The distribution of metals in the lagoon core showed peak concentrations in the 1970s and it is speculated that declines in concentration at this site are primarily the result of legislation restricting atmospheric emissions or changes in highway drainage