FAVORABLE GEOLOGIC CONDITIONS AT THE PLANNED CRESCENT JUNCTION, UTAH, SITE FOR DISPOSAL OF URANIUM MILL TAILINGS FROM NEAR MOAB, UTAH

The Uranium Mill Tailings Radiation Control Act requires the DOE to stabilize 16 million tons of uranium mill tailings at the former Atlas Minerals Corporation millsite near Moab, Utah, in a disposal cell that will maintain its integrity for up to 1,000 years and be protective of human health and the environment. After the disposal site selection process was detailed in a 2005 EIS, the site chosen in the Record of Decision as the best alternative for disposal of the tailings is about 30 miles north of Moab and 1 mile northeast of Crescent Junction. The site is just south of the Book Cliffs on Crescent Flat, which slopes gently south at 2-3 percent grade. Up to 25 feet of Quaternary deposits, consisting mainly of alluvial and colluvial mud and silt deposited by successive sheet wash episodes, cover Upper Cretaceous Mancos Shale bedrock. About 2,400 feet of Mancos Shale, composed mostly of silty claystone deposited in an open-marine setting of the Western Interior Seaway, overlies and isolates the site from the Dakota aquifer – the first ground water in the Lower Cretaceous Dakota Sandstone and Cedar Mountain Formation.

Characterization of the site that will contain the 250-acre disposal cell was done to determine the geologic and hydrologic suitability for permanent tailings disposal. Investigations included field geomorphic reconnaissance, aerial photography analysis, surface geologic mapping, drilling of ten 300 feet coreholes and 100 geotechnical boreholes, and excavation of 5 test pits. Coring found some fracturing in the weathered Mancos Shale with no natural fractures deeper than 100 feet into bedrock. No saturation was noted in the Quaternary material or in cored bedrock, although small amounts of highly saline connate water have seeped into several open coreholes. No faults are in the disposal cell area or within 2 miles of it; nearest faults to the east and west are not seismogenic and are related to incipient collapse caused by dissolution of thick, deep salt deposits in the Pennsylvanian Paradox Formation. Rockfalls and landslides will not affect the disposal site because these processes are more than 1,000 feet to the north along the base of the Book Cliffs. Aggradation rather than erosion is the principal on-going geologic process on the gently south-sloping surface of the site.