The St. George Basin Environment and Possible Consequences of Planned Offshore Oil and Gas Development
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Proceedings of a Synthesis Meeting: The St. George Basin Environment and Possible Consequences of Planned Offshore Oil and Gas Development Anchorage, Alaska - April 28-30, 1981 M. J. Hameedi (Editor) Outer Continental Shelf Environmental Assessment Program Juneau, Alaska March 1982 United States United States Department of Commerce Department of the Interior Malcolm Baldridge, Secretary James E. Watt, Secretary National Oceanic and Bureau of Land Management Atmospheric Administration Robert F. Burford, Director John V. Byrne, Administrator Office of Marine rdlution hwssment R. Lawrence Swanson, Director NOTICES This report has been reviewed by the U.S. ~k~artmentof commerce, National Oceanic and Atmospheric Administration's Outer Continental Shelf Environmental Assessment Program Office, and approved for publication. The interpretation of data and opinions expressed in this document are those of the authors and synthesis meeting participants. Approval does not necessarily signify that the contents reflect the views and policies of the Department of Com- merce or those of the Department of the Interior. The National Oceanic and Atmospheric Administration (NOAA) does not approve, recom- mend, or endorse any proprietary product or proprietary material mentioned in this publication. No reference shall be made to NOAA or to this publication in any advertising or sales promotion which would indicate or imply that NOAA approves, recommends, or endorses any proprietary product or proprietary material mentioned herein, or which has as its purpose an intent to cause directly or indirectly theadvertised product to be used or purchased becauseof this publication. PREFACE The Outer Continental Shelf Environmental Assessment Program (OCSEAP) was established by a basic agreement between the National Oceanic and Atmospheric Administration (NOAA) and the Bureau of Land Management (BLM) to conduct environmental research on Alaskan continental shelf areas identified by BLM for potential oil and gas development. OCSEAP's activities include review of existing data, planning and management of original research on all aspects of marine science, and transfer of information to BLM and other users. Scientific information is gathered and disseminated in journal articles, monographs, and books; digital data products; progress reports and final reports from individual research projects; and proceed- ings of disciplinary program reviews and synthesis meetings. OCSEAP synthesis meetings are interdisciplinary and address environmental issues and resource use conflicts which have arisen in a proposed oil and gas lease area. OCSEAP investigators, other scientists, OCSEAP management, BLM personnel, and representatives from the State of Alaska, petroleum industry, local residents, and other interest groups attend these meetings. Synthesis reports are based on the proceedings of the meeting and include interpretation of data by scientists and others knowledgeable about the lease area or the environmental problems of offshore oil and gas development. Within OCSEAP, synthesis reports are the most direct avenue from scientists to decisionmakers. Interpreting a variety of scientific data so as to predict the effects of oil and gas development is complex. It requires a comprehensive scientific and objective understandingof the environment that may be moredifficult and time-consuming than the tasks some scientists have undertaken in the past. Nonetheless, active participation of scientists in this process is crucial and within OCSEAP it will continue. This report evaluates available environmental data in light of proposed oil and gas de- velopment in the St. George Basin. Many issues and topics have been addressed and resolved; a number of questions remain unanswered due either to a paucity of data or to lack of specificity in the questions or problems to be addressed. Future synthesis meetings will be held as necessary and as more refined and analytical interpretation of environmental data are available. Esther C. Wunnicke Manager, Alaska Outer Continental Shelf Office Bureau of Land Management Herbert E. Bruce Director, Alaska Office, Office of Marine Pollution Assessment National Oceanic and Atmospheric Administration ACKNOWLEDGMENTS This report was prepared with contributions and assistance from scientists within and outside the Outer Continental Shelf Environmental Assessment Program (OCSEAP), representa- tives of several federal agencies, the State of Alaska, the petroleum industry, interest groups, and the public at large. Special thanks are due the Workshop Chairpersons and meeting participants who submitted text and graphic material for the report. This report is published as part of OCSEAP, a program of marine environmental research established through Basic Agreement between the Bureau of Land Management, U.S. Department of the Interior, and the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. EXECUTIVE SUMMARY The continental shelf southeast ofthe Pribilof Islands has been proposed for oil and gas lease sale (OCS Sale 70). The proposed sale area, which includes the St. George Basin, would consist of 479 blocks covering over 1 million hectares at water depths of 98-154 m. This sale is scheduled for February 1983. The proposed lease sale area is located on a flat and relatively featureless part of the continental shelf. Two prominent submarine canyons, Pribilof Canyon and Bering Canyon, cut into the outer shelf and slope and appear to have been dominant factors in the distribution of sediment. Circulation over the shelf is bound to the west by a northwesterly boundary current with mean speeds of 5-10 cmls over the continental slope. Although this current, per se, does not flow onto the shelf, exchange of mass or momentum does occur between theoceanic and shelf regimes and cross-shelf current pulses have been observed. Shelf circulation is dominated by tidal currents and, particularly over the middle shelf, is responsive to episodic meteorologic conditions, i.e., those associated with migrating storms or outbreaks of cold and dry continental air over water. Mean currents averaged over many tidal cycles and wind events are usually 1-5 cmls. Vector mean wind speeds observed on St. Paul Island are about 1 rnls in summer and 2.5 rnls in winter. These winds would transport surface oil at about 3-8 cmls. Much stronger winds, 10-30 mls, can be expected during the frequent storms. Such winds would move oil on the sea surface at 30-90 cmls or about 35-100 kmld. Surface trajectories have been calculated for oil released from a point source located at 55"30rN and 166'W and at other locations for summer and winter conditions. In summer (June-August),spilled oil would have moved 85 km to the east and spread over about 250 km2after 10 days. Oil released from sites in the southern lease sale blocks could reach the coastline along the Alaska Peninsula within 30 days. In winter (December-May), oil spill trajectories would reflect a more energetic flow. After 10 days, oil would have moved 150 km west-southwest and spread over about 225 km2. Spilled oil is not likely to reach the shoreline in a winter spill. The calculated greater area covered by spilled oil in summer is due to temperature considerations alone. If 50,000 bbl of crude oil were spilled, dissolved oil in concentrations shown to be toxic to various fish and sheilfish larvae would probably persist for about 10 days in the upper mixed layer and subsurface water over the shelf. The oil would cover 100-300 km2. Crude oil from a 50,000 bbl spill could be delivered to the botton in concentrations on the order of g/m2 over about 100 km2; lesser concentrations could occur over a much larger area. These estimates do not take into account the effects of weathering and ad- vective processes. The physical factors which influence oil transport vary little spatially over the shelf; concentrations of dissolved oil, area of impact, and trajectories that spilled oil might follow would be similar, whatever the location of the spill. On the other hand, physical factors in summer and winter differ markedly. The low topographic gradient, age of sediment, lack of modern sediment input, sediment texture, and little reworking of sediment by bottom currents strongly suggest that sediment is probably normally consolidated and stable as a foundation material. There is substantial evidence for potential sediment instability along the continental slope from the Pribilof Canyon to Bering Canyon and for possible gas charging in deep subsurface sediment (300 m or more below the surface sediment) of the St. George Basin. The St. George Basin is a graben bounded by growth faults. Seismic events probably occur along these boundary faults. The probability that a randomly selected site within the Basin will experience ground motion in excess of 0.2 g acceleration within 40 years is 11 percent; for 0.5 g acceleration it is about 3 percent. Great earthquakes and ground motion of 1 g acceleration are likely in locations south of the Alaska Peninsula, such as the Shumagin region, between 159" and 162"W. There is also a potential for the generation of local tsunami heights of about 30m in the region. Such events could endanger human life as well as structures and facilities associated with oil and gas development. 11. The probability of volcanic activity in the Pribilof Islands is very low. On the other hand, several volcanoes on the Alaska Peninsula and the Aleutian Islands are highly likely toerupt and could damage life