Radioelement distribution in drill-hole USW-G1, Yucca Mountain, Nye County, Nevada

Drill-hole USW-G1 is one of series of test holes drilled in the vicinity of Yucca Mountain to acquire geologic, geophysical, hydrologic, and geochemical data to determine the feasibility of using the area for long-term storage of nuclear waste. The borehole is at lat 36052'00'' N., long 116027'29'' W.; the collar elevation is 4348.6 ft above mean sea level. The hole was drilled to 6000 ft. The radioelement contents (radium equivalent uranium (RaeU), thorium, and potassium) of samples collected from drill hole USW-G1 were measured to characterize the geologic units penetrated by the hole, to determine the homogeneity of the units, and to ascertain where redistribution of the radioelements may have occurred. Evidence of radioelement migration is important in potential waste-disposal sites because it suggests previous fluid movement through permeable sections of the geologic units that, in turn, reflects the relative permeability and physical competence of the units. Thorium is insoluble in high-temperature fluids or groundwater, and variations in its content probably indicate variations in original magma composition or variations in the physical mechanisms of eruptive processes. In contrast, RaeU and potassium are more soluble, and variations in the concentrations of these radioelements that are independent of thorium variations probably indicate postemplacement alteration. 7 refs., 2 figs., 2 tabs