Introduction
Mamala Bay, off the south shore of the island of Oahu, has been used as a repository of dredged material
primarily from Pearl and Honolulu Harbors for over a century. The U.S. Geological Survey, U.S. Army
Corps of Engineers, and the U.S. Environmental Protection Agency are conducting an integrated study
on the distribution and character of dredged materials as well as the effects of dredged material on the
marine environment. A three phase study is providing information to evaluate the effects on seafloor
substrate and the benthic fauna. The studies include geophysical profiling and imaging, bottom
photography, sampling, chemical and physical analyses of sediment, and evaluations of the benthic
population, population density, and adverse impacts to the benthic fauna.
Phase 1, conducted in 1993, inventoried the seafloor via remote sensing. Sidescan sonar and subbottom
profilers characterized the seafloor in and around the disposal sites, and the resulting products reveal the
character and extent of the dredged material. These data were used to plan Phase 2 in 1994, a sampling
program that employed subbottom profilers, video and still photography, and seafloor sampling to ground
truth the sonar mosaic and identify the seafloor substrates responsible for the various acoustic signatures
on the sonar images and subbottom profiles. Box coring provided the samples necessary to distinguish
dredged material from native sediment, and for the chemical analyses used to determine contaminant
concentrations. Phase 3 studies conducted in June of 1995 consisted of box core sampling for chemical
and biological analyses. Specific studies include: infaunal taxonomy and population density,
bioassay/bioaccumulation, sediment chemistry, and post-disposal resuspension and transport.
The 1995 survey, conducted June 14 through 17, resulted in the collection of 39 box cores from 20
different stations. Multiple box cores were composited at 7 different locations occupied in 1994, to
provide the material required for the 7 bioassay and bioaccumulation analyses currently underway (Figure
1). Seventeen of the 20 stations occupied provided the biological samples for the benthic infaunal
identification and population density study conducted by Dr. Julie Brock of the University of Hawaii, and
the sediment chemistry analyses conducted (and completed) by Quanterra Environmental Laboratories
(Figure 1). Seven of the 20 stations occupied in 1995 were occupied in 1994, and provide the data for
direct comparison of sediment chemistry at the same sites from two consecutive years. The sum total of
the data collected from all three phases of the monitoring program will provide the U.S. Army Corps of
Engineers and the U.S. Environmental Protection Agency with the information required to make
informed decisions as to the management of the South Oahu disposal site in Mamala Bay.
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First posted May 27, 2003
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