MERCURY DEPOSITION AT A REMOTE LOCATION IN NEW ENGLAND: A 350 YEAR RECORD FROM BLOCK ISLAND, RI

Mercury contamination in New England is determined by far field sources and local point sources. The Hg point sources may dominate the local contamination patterns, and a reference “remote locality” atmospheric deposition record of Hg contamination would be desirable. We took sediment cores in a wetland (West Beach fresh water marsh-WBFM) and in Fresh Pond (FP) on Block Island, RI. This small island has no known local Hg sources. Fresh Pond and WBFM are small isolated fresh water ponds with small watersheds. The cores were analyzed for mercury with a DMA-80, dated with ²¹⁰Pb and ¹⁴C, and from ages and bulk dry sediment densities, Hg accumulation rates were calculated. The two FP cores show almost identical depth-Hg records, with Hg concentrations up to 215 ppb Hg. The WBFM sediment has higher Hg concentrations (up to 320 ppb Hg); both cores have natural background concentrations of ~ 30-40 ppb Hg.

Mean sedimentation rates were ~3 mm/year in Fresh Pond and ~1.5 mm/yr in WBFM, while sediment densities were higher in WBFM. Mercury mass accumulation rates started to increase gently around 1870 and then increased more strongly in the late 1930s. The WBFM record shows a peak in Hg concentrations and mass accumulation rates ~1965, to decrease to concentrations of 230 ppb Hg in the 21th century. The top of the FP cores was homogenized during coring so trends in the Hg record over the last two decades could not be determined. The total Hg inventories on Block Island were at the low end of the range determined from many Long Island Sound sediment cores (Varekamp et al., 2000). The Hg and excess ²¹⁰Pb inventories were larger than could be explained through in situ atmospheric deposition alone, suggesting some modest sediment focusing by wind and rain into the pond and marsh. Most important, the shape of the Hg contamination profiles is very different from those obtained from Long Island Sound sediment cores: the latter show a strong increase in the mid 1800s and then a decline since the 1970s. The much more gradual increase in Hg concentrations in Block island over the late 1800s and early 1900s suggests that the Long Island Sound Hg burdens are strongly impacted by Hg contamination from point sources, such as the hatting industry from Danbury, CT. The Block Island Hg records may provide a good estimate of atmospheric Hg deposition over time for southern New England.