HISTORICAL RECORDS OF MERCURY CONTAMINATION IN SEDIMENT CORES IN CONNECTICUT AND LONG ISLAND SOUND

About 400 surface and core sediment samples from a sampling grid in Long Island Sound (LIS) and 16 cores from salt marshes and riverine wetlands were analyzed for mercury (Hg), Clostridium perfringens (C. perfringens, a sewage tracer) and many other chemical and physical parameters. The Hg concentrations in LIS surface sediments vary between 30 and 650 ppb, with the highest values in the western end of LIS. A trend of increasing Hg concentrations to the west correlates with increasing abundances of fine-grained sediment and C-org; normalization of the Hg data to mean grain size still shows a pronounced east-west trend. The linear relationship between C. perfringens and Hg in sediments from the New York Bight, where sewage disposal is a well defined source, was used to estimate the contribution of waste water treatment plants to Hg in LIS sediments. Many LIS surface samples have between 10-40 percent sewage-derived Hg, with values up to 80 percent in western-most LIS. The remaining Hg is largely imported from the surrounding watersheds with fine-grained sediments, and hence is a focused flux of the integrated regional atmospheric Hg deposition. The direct atmospheric deposition of Hg into LIS is only a small part of the overall LIS Hg sediment budget. The east-west trend in Hg concentration is thus due to fine sediment focussing in western LIS as well as additional Hg sources in that region (mainly sewage treatment facilities). Sediment cores have contamination profiles with background Hg values of 50-100 ppb, and peak values between 200-500 ppb. The Hg concentrations decrease in sediments deposited over the last 30 years. The highest Hg concentrations (1200 ppb) are found in the Housatonic River basin. The onset of Hg contamination in the cores (around 1840 A.D.) coincides with the first elevated C. perfringens levels, indicating an anthropogenic origin for the Hg contamination.