K1-95-HW: Cruise Report 1995 - Preliminary results.

Phase III: Sediment Chemistry and Biological Sampling Survey

M.E. Torresan, M.A. Hampton, J.H. Barber, Jr., and F.L. Wong

U.S. Geological Survey Open-file Report 95-839

1995

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.

Summary
Introduction
Study Area
Previous Studies
  Designation
  Monitoring 1, 2
Methods
  Vessel
  Navigation
  Sampling
  Subsampling 1, 2
Results
Acknowledgments References 1, 2

Figures
1 Location
2 Box corer

Tables
1 Stations
2 Samples
3 Analyses
4 Bioassay

Appendices
1 Box Cores
2 Custody: Quanterra
3 Custody: Batelle

SUMMARY

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.

Results acquired to date show the effectiveness of integrated assessment. Sidescan sonar shows that the dredged material is characterized by isolated, high-backscatter, circular to subcircular footprints interpreted as individual disposal events. The footprints are concentrated over the sites and form a high-backscatter blanket that covers the low-backscatter native sediment mantling the seafloor. These data imply that much of the disposed material reaches the seafloor, and does not completely disperse in the water column.

Box cores and x-radiographs differentiate the dredged material from the native sediment. Dredged material comprises a heterogeneous mixture of cohesive, olive-gray mud that is mixed with sand to cobble-size rubble, which is responsible for its high-backscatter character. Low backscatter native sediment is a beige, bioturbated, muddy carbonate sand, generally devoid of coarse clasts. Bioturbation is ubiquitous in native sediment, and in some instances is present in the overlying dredged material. This implies that at least some organisms are capable of surviving in the dredged material. Video and still photographs and some 3.5-kHz profiles show that much of the seafloor is covered by a variety of wavy bedforms, ranging from symmetrical to asymmetrical ripples. The bedforms show that current activity, possibly internal waves, may be a mechanism for secondary transport. Photography also shows that the bottom is littered with a variety of debris types including wire, barrels, military ordnance, refuse, and carbonate reef debris.

The chemical analyses conducted on seafloor sediment and dredged materials do not yield definitive results, but generally, analyte concentrations are low to nondetectable. In some instances specific contaminants exhibit higher concentrations in native sediment, relative to dredged material and vice versa. In other instances certain analytes show both higher and lower concentrations in both native sediment and dredged material. Conclusions regarding the biological analyses are pending completion of those studies.

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