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Torresan, M.E., Hampton, M.A., Gowen, M.H., Barber, Jr., J.H., Zink, L.L., Chase, T.E., Wong, F.L., Gann, J.T., and Dartnell, P., 1995, Final report: acoustic mapping of dredged material disposal sites and deposits in Mamala Bay, Honolulu, Hawaii: U.S. Geological Survey Open-file Report 95-17. |
Introduction 1,
2 K1-93 Survey Results Conclusions Figures
Apx 1: Statistics 1
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CONCLUSIONS Sidescan sonar shows that dredged material disposal leaves a remarkable imprint on the seafloor. Disposal has affected an area of about 100 km2, forming two major deposits. These deposits are characterized by high-backscatter, circular to subcircular footprints that coalesce to form a high-backscatter blanket that mantles the seafloor near the center of each disposal site. The sonar images clearly document that the dredged material deposits extend well beyond the disposal site boundaries (figures 5, 6 and plate 1), and subsequent sampling shows that dredged material below the resolution limit of the sidescan sonar exists beyond the high backscatter area shown in the images (Torresan and others, 1994a and b). The high resolution acoustic profiles collected over the sites are characterized in places by gentle seafloor undulations, but are primarily characterized by lateral discontinuities in seafloor reflectivity and hardness. Typically, the substrate within the sites and below the discontinuous seafloor reflector is featureless, and devoid of internal structure and subbottom reflectors. Subbottom stratigraphy and structure may well exist but is not visible in the 3.5-kHz profiles, possibly owing to the acoustic signal being scattered and reflected by the coarse component in the dredged material. Alternatively, the acoustic signal may be absorbed by the dredged material. In contrast, profiles collected away from the sites, especially in deeper water to the south, do not show the strong discontinuities in seafloor reflectivity, and can have subbottom reflectors rather than the typically structureless substrate seen mantling each disposal site. Diffraction patterns on most 3.5-kHz profiles in the region west of the former Pearl Harbor and the active South Oahu sites are always associated with bedforms visible on the sonar mosaic and bottom photographs collected during the 1977-1978 site designation studies (Chave and Miller 1977a, 1977b, 1978a; Neighbor Island Consultants, 1977; Tertra Tech, 1977; Goeggel; EPA, 1980) and the USGS May 1994 survey (Torresan and others, 1994a and b). East of the old Honolulu Harbor site relatively large, mound-like bedforms exist, and these bedforms extend west into the old Honolulu Harbor disposal site. With the completion of the acoustical surveys conducted by the USGS in 1993, a good acoustic data base now exists for Mamala Bay. More detailed studies are required to define the nature of the various substrates, the benthic processes, and the relative roles of the substrate and the benthic and neritic communities in the transfer and storage of contaminants associated with dredged material disposal. Analysis of sediment cores and photographs collected in May 1994 (Torresan and others, 1994b) will aid in "groundtruthing" the sonar mosaic and acoustic profiles, will evaluate contaminant concentrations in the sediment, and will help define the variety of anthropogenic stresses affecting Mamala Bay. Analyses employing sedimentological, geochemical, and biological techniques are required to better assess the impact to the seafloor, the benthic and neritic communities, the material flux, and the fate of contaminants associated with dredged material disposal. Specific studies should include evaluations of the harbor and the disposal site sediment and biological tissues for contaminants such as metals, pesticides, organics, organotins, PAHs, phenols, and phthalates, and the evaluation of the effect of bottom currents and bioturbation in dredged material redistribution and contaminant transfer. Future studies will benefit from additional field programs specifically designed to collect benthic fauna. |
URL: https://pubs.usgs.gov/of/1995/of95-017/10concl.html Maintained by: Michael Diggles Author: Florence L. Wong Last modified: May 27, 2005 (mfd)
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