Testing the Concept of Drift Shadow at Yucca Mountain, Nevada

Paces, J.B. and Neymark, L.A. and Ghezzehei, T. and Dobson, P.F.

Proceedings of the 11th International High Level Radioactive Waste Management Conference, IHLRWM, pp. 278-285 , 2006.


If proven, the concept of drift shadow, a zone of reduced water content and slower ground-water travel time beneath openings in fractured rock of the unsaturated zone, may increase performance of a proposed geologic repository for high-level radioactive waste at Yucca Mountain. To test this concept under natural-flow conditions present in the proposed repository horizon, isotopes within the uranium-series decay chain (uranium-238, uranium-234, and thorium-230, or sup 238U-sup 234U-sup 230Th) have been analyzed in samples of rock from beneath four naturally occurring lithophysal cavities. All samples show sup 234U depletion relative to parent sup 238U, indicating varying degrees of water-rock interaction over the past million years. Variations in sup 234U/sup 238U activity ratios indicate that depletion of sup 234U relative to sup 238U can be either smaller or greater in rock beneath cavity floors relative to rock near cavity margins. These results are consistent with the concept of drift shadow and with numerical simulations of meter-scale spherical cavities in fractured tuff. Differences in distribution patterns of sup 234U/sup 238U activity ratios in rock beneath the cavity floors are interpreted to reflect differences in the amount of past seepage into lithophysal cavities, as indicated by the abundance of secondary mineral depositsmore ยป