The Long Haul: Results of Monitored Natural Recovery of the Field Verification Program Mound after Thirty Years

摘要

Overview. Monitored natural recovery (MNR) is a sediment management technique thatcan require decades to evaluate. A comprehensive study was initiated in 1982 to evaluatethe environmental consequences of dredged material placement under aquatic, wetland,and upland conditions. This study presents the results of MNR nearly 30 years after theexperimental aquatic placement of uncapped dredged material in Long Island Sound.Background/Objectives. An experimental mound was created in Long Island Sound aspart of the joint USEPA/USACE Field Verification Program (FVP). The FVP objectivewas to field-verify existing methods for predicting the environmental consequences ofdredged material placement. The FVP unconfined open-water disposal mound wascreated from the disposal of 55,000 m3 of Black Rock Harbor (BRH) sediment in 1983.BRH sediment consisted of black, fine-grained silt and clay with high water content andelevated concentrations of metal and organic contaminants. Exposure to BRH sediment inlaboratory tests resulted in both chronic and acute toxicity in several test species, as wellas PCB and PAH bioaccumulation. The underlying assumption was that if adverse biologicaleffects were observed in the laboratory, they should also be measurable in thenatural environmental following placement of the unsuitable material in open water. Repeatedmonitoring of the mound has shown a wide range of benthic communityresponses, from an initial classic primary successional recovery following disturbance, toepisodes of retrograde succession following Hurricane Gloria and hypoxic events in LongIsland Sound. After 1988, the New England District continued to monitor the mound toobserve and document the longer term environmental responses.Approach/Activities. Benthic community conditions were surveyed with sediment profileimaging (SPI) in 1991, 1993, 1995, 1999, 2005, and 2011. Bulk sediment from grabsand cores has been collected and analyzed, most recently for PAHs, PCBs, and metals in2005 to assess distribution of contaminants within the mound. In 1997, 2000, 2005 and2011 high resolution acoustic data were collected to assess small scale (< 10 cm vertical)changes in the surface of the mound and surrounding seafloor due to sediment transportprocesses.Results/Lessons Learned. Multiple lines of evidence (SPI images, sediment chemistry,bathymetry, backscatter and side-scan sonar) demonstrated that the surface sediments atFVP support a robust, actively bioturbating benthic community with no evidence of