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 U.S. Nuclear Regulatory Commission, award number NRC-04-96-078, and the U.S. Department of Energy, DE-FG08-97NV12056. The opinions, findings, conclusions, and recommendations expressed herein are those of the authors and do not necessarily reflect the view of the Nuclear Regulatory Commission or the U.S. Department of Energy.

Cover: Illustrations courtesy of David A. Feary (photographs from Yucca Mountain and Busted Butte), and Peters, R. R., and E. A. Klavetter, 1988, a continuum model for water movement in an unsaturated fractured rock mass, Water Resources Research 24(3): 416-430, copyright by American Geophysical Union (for schematic figure showing fluid in fractures).