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 $\sp{210}$Pb 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 $\sp{210}$Pb results were used to determine accretion rates for each core. Distributions of Fe and Mn were used as indicators of chemical conditions of sediment cores and Cu, Pb, Cr, Zn, Ag, and Ni distributions with 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 variability within a marsh. At another location, a high marsh, receiving predominantly atmospheric inputs and a low marsh, receiving waterborne and atmospheric inputs, were sampled so that atmospheric and tidal contributions could be determined. A comparison was made of the distributions of metals in bay cores and in the lagoon core. All the Rhode Island marshes accrete at rates equal to or greater (avg. 0.40 cm/yr) than the local rise in sea level (0.26 cm/yr). Based on the $\sp{210}$Pb chronologies, pollutant metals began to increase in the mid to late 1800s, corresponding to coal burning emissions to the atmosphere. Steeper increases in the 1900s reflect industrial and sewage discharges. Maximum concentrations were reached in the 1950s and have declined almost continuously since then. Observed reductions were attributable to implementation of and improvements to sewage treatment, and controls on atmospheric emissions. All cores in the bay showed variations which occurred at about the same time, but concentrations in upper bay cores were greater than in lower bay cores. The observed gradient and the simultaneous changes in concentrations in all cores suggests that discharge limitations at the head of the estuary will result in bay-wide reductions. Comparison of high and low marsh cores suggested a predominately waterborne source of metals. Concentrations in the lagoon core peaked in the 1970s and appear to follow atmospheric emission restrictions and/or changes in local highway drainage patterns