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NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by the National Science Foundation under Grant No. EAR-0625247, the U.S. Geological Survey under Award No. 06HQGR0197, and the Chevron Corporation. The opinions, findings, and conclusions or recommendations contained in this document are those of the authors and do not necessarily reflect the views of the National Science Foundation, the U.S. Geological Survey, or the Chevron Corporation.

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