High-level radioactive waste contains long-lived nuclides that require complete confinement for long periods of geologic time. Rock salt long has been acclaimed as the preferred geologic medium for the ultimate disposal of these wastes, inasmuch as its unique self-healing properties make it impervious to the intraformational circulation of groundwaters. Data have been compiled on the nature and extent of all major salt deposits in the conterminous United States; however, specific site studies have been confined to the Permian basin, to central Kansas, and to a large tract of federally owned land in southeastern New Mexico. A series of coreholes has been drilled to provide the critical data for selecting appropriate disposal levels and assessing the hydraulic characteristi s of the overlying and underlying formations. Selective parts of the cores have been analyzed to determine the quantities and characteristics of various minerals in the evaporite section and, in particular, of any hydrated minerals and rocks that could dehydrate upon heating because of radioactive decay of the wastes. Measurements of the physical properties of these rocks also have been made to calculate their deformational behavior.

Because of the need for long-term waste confinement, several unique studies have been initiated. The long geologic history of relative quiescence, coupled with data on historic earthquakes, strongly suggests that the Permian basin will continue to be tectonically stable for the next few hundreds of thousands of years, or for the effective lifetime of the wastes. In addition, studies of subsurface salt dissolution show that the rate of basinward migration of the relatively shallow edges

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of the salt during Quaternary time has averaged only a few miles per million years. Also, in central Kansas present rates of denudation have been found to average less than 1 ft per 1,000 years while stream incisions in the same area during Quaternary time have not exceeded several hundred feet. Finally, investigations have revealed that the buried wastes would not be affected adversely by the advance of a new continental ice sheet.

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