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Considerable public attention has focused on the environmental stress in and around the Gulf of the Farallones National Marine Sanctuary (NMS).The U.S. Geological Survey (USGS) provides geological information in support of studies related to proposed siting of offshore areas for disposal of dredge spoils and to determining locations of barrels of radioactive waste. The potential for damage to the marine environment from disposal of dredge materials and from rupture of waste containers is difficult to assess without a detailed knowledge of geology, oceanography, and the movement and ultimate fate of transported sediments. USGS studies perform a critical role in the preliminary study of potential disposal sites for dredge materials.Cooperative work since 1990 with the Environmental Protection Agency (EPA), the Army Corps of Engineers, and the U.S. Navy has reduced the number of candidate disposal sites from six to three in an approximate 1,000 square mile area west of San Francisco. Sidescan sonar surveys were conducted in all areas in cooperation with the private sector and geological interpretations were derived from these images in preparation for site-specific studies conducted by EPA. A key set of geological characteristics for a proposed dredge-materials disposal site, among other logistical attributes, includes a featureless plain with gentle slopes, no evidence of mass movement of sediments or rock, a lack of strong currents that might redisperse dredge materials, and low-level biota. The ideal site would be geologically stable with net deposition rather than erosion or sediment instability.
The role of USGS scientists is simply to interpret the geological and geophysical data for regulatory agencies. USGS does not make judgments about the suitability of an area as a disposal site. Officials responsible for enforcing environmental regulations use these geologic interpretations along with other criteria to select appropriate disposal sites. USGS scientists have mapped a small portion of the area known to contain hazardous wastes, and have made significant advances in interpreting sonar signals attributed to the containers.
Between 1946 and 1970, nearly 50,000 drums of hazardous and radioactive wastes were dumped over a 350 square nautical mile area that overlaps the Farallones NMS. The task facing environmental managers is to assess contamination of the environment around the 55-gallon drums. However, managers do not know the precise locations of these containers and therefore cannot put together an effective sediment and water sampling program that will provide clues to the extent of contamination, if any. USGS mapping, in cooperation with the National Oceanic and Atmospheric Administration (NOAA), to date covers just 15 percent of the potentially contaminated area. USGS scientists have determined that the sonar backscatter signal can be digitally enhanced to distinguish non-geologic targets such as waste containers, from geologic targets. USGS scientists understanding of the geology in the marine environment is a key to selecting appropriate mapping technology.Broad-beam sonar scanning used for reconnaissance mapping of the Exclusive Economic Zone (EEZ) yields insufficient detail. High-resolution scanning yields very fine detail but these surveys take much time and money to complete. USGS researchers use a mid-range frequency for this type of surveying in 3,000 feet of water to resolve features on the seafloor that, with correct interpretation, can lead to a generalized map showing geology as well as other targets of interest. The importance of such USGS maps is realized when the presence and location of specific targets such as hazardous waste containers is sought. USGS scientists work towards finishing the mapping of the Marine Sanctuary.Research in cooperation with NOAA and with the private sector continues to produce newer and better means for surveying the seafloor. As the remaining 85 percent of the area is mapped, techniques for resolving seafloor features and for detecting targets of interest are documented for application to other marine environments, such as those offshore from major metropolitan areas or in the vicinity of existing and proposed marine sanctuaries. A library of data combining near- and offshore surveys is available for use in applications requiring geological information.
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Related Publications: Beyond the Golden Gate—Oceanography, Geology, Biology, and Environmental Issues in the Gulf of the Farallones - USGS Circular 1198
Detection of Barrels that contain Low-level Radioactive Waste in Farallon Island Radioactive Waste Dumpsite Using Side-scan Sonar and Underwater-Optical Systems—Preliminary Interpretation of Barrel Distribution - USGS Open File Report 92-178
Current patterns over the shelf and slope adjacent to the Gulf of the Farallones Executive Summary - USGS Open File Report 92-382
Map of Seafloor Declivity and Fall Lines on the Continental Slope, Gulf of the Farallones, Central California - USGS Open File Report 93-298
Related Links: Environmental Protection Agency U.S. Army Corps of Engineers U.S. Navy National Oceanic and Atmospheric Administration Gulf of the Farallones National Marine Sanctuary
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