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Coastal and Marine Geology Program > Seafloor Studies of Mamala Bay, Honolulu, Hawaii

Seafloor Studies of Mamala Bay, Honolulu, Hawaii

USGS Fact Sheet

Mike Torresan
Mike Torresan
"Disposal of dredge spoils in the near offshore area, coupled with the rapid growth of Honolulu and other nearby municipalities, has placed increased stress on the environment of Mamala Bay. No satisfactory bathymetric map of the seafloor had existed, and little information has been compiled about the effects of these activities to determine whether modifications to the operation and management of the designated dump sites and sewage outfall locations were necessary."

- Mike Torresan, U.S. Geological Survey

Half a century of dredge spoils disposal has had an unknown effect on the Mamala Bay ecosystem.

Location map.
Location map of the Hawaiian Islands showing designated dredged spoil disposal sites for Hawaii's harbors. [larger version]
No comprehensive study of the effects of disposal of dredge spoils has been conducted to determine if the environment has suffered. The U.S. Army Corps of Engineers (USACE) has regularly dredged the shipping channels of Honolulu Harbor and Pearl Harbor for commercial and military purposes. The 5-year frequency for new dredging activity has led to the formation of extensive offshore wide deposits of relatively coarse sediments being created in a sedimentary environment that naturally collects much finer-grained materials. At the same time, the rapid growth of Honolulu and its suburban region over the past 3 decades has added nutrient-enriched sewage outfall to the artificially-heavy sediment load. The combined effects of dredge-spoil disposal and contaminant loads are not well-documented, and are poorly understood. The U.S. Geological Survey (USGS) began a comprehensive study to characterize the seafloor of Mamala Bay.

In 1993, the USGS initiated a program sponsored by the USACE and the U.S. Environmental Protection Agency that produced a detailed map of the seafloor, especially of the distribution of dredge spoils, and a preliminary analysis of the environmental effects of dredge-spoils disposal and the chemical effects of the introduction of nutrient-rich outfall. Part of the work would involve sediment sampling, biological testing, and evaluation. Of particular interest is tissue sampling and analysis of endemic shrimp to determine the rates of absorption of toxic elements, such as heavy metals, that are easily brought into the food chain.

USGS researchers have compiled a sonar image map of the Mamala Bay seafloor.

This map was compiled from sidescan sonar surveys with electronics designed to provide noise-free, shipboard images of maximum resolution. The image map clearly shows the locations of disposed dredge materials as acoustically-bright areas; areas not covered by dredge materials appear acoustically dark. Other interesting seafloor features are large enechelon sand waves or ripples, possibly the result of a disposal technique or military activity.

General interpretive map. General interpretive map showing trackline coverage (numbered lines), disposal sites, bathymetry, and surficial geology of the seafloor of Honolulu, Hawaii. [larger version]

USGS scientists have described a series of mounds on the sea-floor around dredge spoil disposal sites.

The mounds average about 500 feet in diameter and up to 4 feet high, and are thought to be associated with disposal of dredged spoils. Each mound represents about 2,500 cubic yards of sediment, the amount that is contained in a dredge barge. These mounds appear as bright spots on sonar images. USGS researchers explain that the dredging process artificially coarsens the deposits as the suspended fine-grained muds are lost with draining seawater. Those muds included with dredged materials create a plume that lasts over an hour when dumped. USGS studies indicate that such plumes may have significant effects on the health of biological species. USGS researchers will analyze the geologic and biologic distribution of contaminants.

Sidescan-sonar mosaic. Sidescan sonar mosaic of the seafloor off Honolulu, Hawaii. Dredged spoils leave high backscatter, circular to subcircular footprints or mounds that coalesce to form the two large deposits in the central portion of the mosaic. [larger version]

Sub-bottom profile.
Representative 3.5 KHz seismic reflection profile showing mounds formed by dredged spoil disposal. Vertical exaggeration is 22:1. [larger version]

Previous studies provide few definitive results as to the extent of biological uptake of contaminants in the food chain. As a part of the study, USGS scientists, in cooperation with colleagues at the University of Hawaii, will trap bottom-dwelling shrimp to analyze their tissues for toxic element uptake. Fauna in Mamala Bay are directly exposed to sewage outfall from the Honolulu and Pearl Harbor areas, as well as to other non-point sources of contaminants such as from the leaching of soils containing agricultural nutrients and lead paint from shipping activities. In addition, USGS scientists are concerned about the distribution and ultimate fate of sediments to which contaminants are attached. The potential adverse effects from resuspension of contaminants on sediments resulting from continued disposal of dredge spoils are not well known.

Contact Information
Mike Torresan
U.S. Geological Survey
Bldg 15, McKelvey Building
Menlo Park, CA 94025
Phone: (650) 329-5493

Related Research Projects:

Fate of Dredge Disposal Material and Polluted Sediment, Offshore Honolulu, Hawaii
USGS Coastal & Marine Geology Program

Related Publications:

Sea-Floor Geology of a Part of Mamala Bay, Hawaii
Originally published in Pacific Science

Acoustic Mapping of the Regional Seafloor Geology in and Around Hawaiian Ocean Dredged-Material Disposal Sites - USGS Open File Report 00-124
USGS Coastal & Marine Geology Program

Final Report: Acoustic Mapping of Dredged Material Disposal Sites and Deposits in Mamala Bay, Honolulu, Hawaii - USGS Open File Report 95-17
USGS Coastal & Marine Geology Program

K1-95-HW: Cruise Report 1995 - Preliminary results. - USGS Open File Report 95-839
USGS Coastal & Marine Geology Program

Related Links:

University of Hawaii
Manoa, Hawaii

U.S. Army Corps of Engineers
U.S. Army

U.S. Environmental Protection Agency
U.S. Federal Government

Coastal and Marine Geology Program > Seafloor Studies of Mamala Bay, Honolulu, Hawaii

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