A 22 YEAR RECORD OF DREDGED MATERIAL MONITORING: RESULTS FROM THE FIELD VERIFICATION PROGRAM (FVP) MOUND IN LONG ISLAND SOUND

摘要

In the spring of 1983, 55,000 m~3 of contaminated sediment from Black Rock Harbor, CT was placed at the Field Verification Program (FVP) dredged material disposal mound in central Long Island Sound. Placement of the material was designed to support extensive field and laboratory experiments as well as biological, physical, and chemical testing protocols in order to develop a framework for monitoring the biological effects of dredged material disposal. The mound was left uncapped to serve as a 'negative control' to observe relative recolonization rates and long-term stability of the benthic community. While monitoring has occurred at regular intervals since the original disposal event, the most recent monitoring survey in June of 2005 was conducted to provide baseline information for future potential capping of this previously uncapped mound. A retrospective of the 22 years of data provides an unprecedented record of the active diagenetic and biological processes occurring at the surface of a dredged material deposit. Black Rock Harbor (BRH) sediment consisted of black, high water content, fine-grained sediment with elevated concentrations of metal and organic contaminants. Surface sediment samples collected immediately after disposal at the center of FVP revealed a mixture of gray sand and clay within a matrix of black organic silt. Sediment quality triad studies of BRH sediment showed extreme chronic and acute toxicity; sediments from near FVP collected in 2000, however, showed no sediment toxicity and no differences in benthic community structure relative to reference. Cores collected in June 2005 were sampled for assessment of sediment contaminants and physical properties. Concentrations of sediment contaminants in the surface sediments were similar to samples collected at the reference area, while samples collected below 10-15 cm in each core showed an increase in most measured contaminants, consistent with penetration into material mixed with BRH sediment. The highest concentrations measured were close to the lowest measured in the original BRH study. The deposit has been monitored frequently using a sediment-profile imaging (SPI) camera both to track the spread of dredged material and follow benthic recolonization of the mound. SPI surveys conducted since mound formation have consistently shown that the flanks of the mound have converged to near-reference conditions, with some variation in results in images collected from the center portion of the mound. SPI images collected in 2005 were consistent in demonstrating the return to a benthic community very similar to that of the reference images, especially on the flanks of the historical mound. The active diagenetic processes of bioturbation, diffusion, and ambient sedimentation have contributed to alteration of the surface sediments of FVP, although the lack of contaminants in the top of the cores suggests that active sedimentation is the most rapid process occurring at the sediment surface.