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Benthic Habitat Mapping
Capping
When testing of dredged material indicates chemical contamination, the material requires special disposal. One alternative for open-water disposal is capping. Capping refers to the placement of dredged material over contaminated disposal sediments. This layer of clean sediment isolates the contaminants from the water column and from benthic organisms that may colonize the disposal site. What does a sediment cap look like? Disposal of contaminated dredged material at open-water sites has been managed by capping since the late 1970s. Since then, a large number of capping projects have been completed. Capping at the Mud Dump Site in the New York Bight provides an instructive example of how monitoring at capped sites is conducted. In both 1993 and 1997, dioxin-contaminated sediments were dredged from container ports in Newark Bay, New Jersey. These sediments were deposited in 24 meters (78.7 feet) of water on the seafloor in two separate mounds at the southern end of the Mud Dump Site. Bathymetry surveys and sediment profile imaging (SPI) were used for monitoring construction of both the 1993 and the 1997 capped mounds. Comparison of bathymetric surveys conducted before and after disposal of dredged material detected thicker layers of contaminated dredged sediment on the mound apex. Meanwhile, transects of SPI stations were used to detect thin layers of contaminated sediment on the mound aprons. Since the capping procedure developed by the U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency required that the entire area (or "footprint") of contaminated dredged material be capped with at least 1 meter of clean sand, both surveys were essential to properly manage the site. A post-cap bathymetric survey confirmed that 1 meter of cap material was present.
Since completion of the capping operations at Mud Dump Site, a variety of techniques have been used for long-term monitoring of cap stability and effectiveness. Periodic bathymetric surveys have indicated no significant changes in disposal mound topography. Sediment cores and subbottom profiling surveys confirm the presence of more than 1 meter of sand cap material overlying the contaminated sediments. Additionally, chemical analyses of cap sediment and tissue from benthic organisms living on and within the cap have shown negligible levels of dioxin. SPI results have also demonstrated the consistent presence of clean cap sand over the surface of the mounds.
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