MERCURY DEPOSITION RECORDS FROM LAKE AND ESTUARINE DEPOSITS AROUND LONG ISLAND SOUND: ATMOSPHERIC DEPOSITION VERSUS LOCAL SOURCES

Long Island Sound (LIS) sediment carries up to 800 ppb Hg, whereas marshes and cove deposits along the main incoming rivers (Connecticut River, Housatonic River) have several ppm Hg. The Hg distribution in the Sound is influenced both by sediment transport processes and the location of sources. We present here Hg data from lakes on Block Island, which is a remote glacial relic island with many ponds near the Rhode Island coast as well as Hg data from marsh and sediment cores in and around LIS. The Hg records from Block Island reflect largely atmospheric deposition over the last few 100 years, with some sediment focusing. A gentle onset of Hg contamination starts in the mid 1800s, with a much stronger rate of contamination starting around World War II. The Hg accumulation rates have decreased strongly since the mid 1970s. Marsh cores near Jarvis Creek, central LIS, show modest concentrations (up to 350 ppb Hg) but relatively large Hg accumulation rates, and an earlier onset of strong Hg contamination (early to mid 1800s). The ratio of Hg and excess ²¹⁰Pb core inventories clearly shows that part of the LIS Hg stems from local point sources and is not the result of focused atmospheric Hg deposition. Mercury point sources along LIS are the historic hat-making industry around Danbury (CT) and Norwalk (CT), which left the Still River / Housatonic River sediments heavily polluted with Hg (up to 100 ppm Hg!!). The many dams in the Housatonic river retain much of the Hg-polluted sediment. Coves along the Connecticut River have up to 4 ppm Hg which may derive from urban sources around Hartford (CT) as well as Hg spills from a power plant (1928-1961) that used Hg as the working fluid instead of steam. The many sewage treatment plants that discharge into LIS also provide a steady flux of Hg into the Sound, which can be traced in sediment through correlation with Clostridium perfringens spore abundances, a sewage provenance indicator. A large part of the Hg burden in LIS therefore stems from local sources, and it is only in remote locations as the Block Island lakes where the pure atmospheric deposition regional signal can be found.