U-SERIES ISOTOPES, WATER/ROCK INTERACTION, AND EVIDENCE OF UNSATURATED-ZONE FLOW AT YUCCA MOUNTAIN, NEVADA

²³⁴U/²³⁸U and -²³⁰Th/²³⁸U ratios determined by TIMS in both water and volcanic rocks vary throughout the >500-m-thick unsaturated zone (UZ) at Yucca Mountain. Infiltration, with ²³⁴U/²³⁸U activity ratios (AR) of about 1.5 to 1.8, is modified as it percolates downward by addition of ²³⁴U via a-recoil and preferential leaching from pore walls and fracture surfaces. Water in shallower parts of the UZ commonly has ²³⁴U/²³⁸U AR of 2 to 4, whereas values of 7 to 10 are common at greater depths. Water perched in welded tuffs near the base of the UZ also has large ²³⁴U/²³⁸U AR (7 to 8), as does ground water in the upper parts of the saturated zone. In contrast to water, ²³⁴U is depleted in whole-rock samples throughout the UZ. ²³⁴U/²³⁸U AR typically ranges from 0.94 to 1.0, although values reach as low as 0.76. Most whole-rock analyses have ²³⁰Th/²³⁴U AR from 0.92 to 1.02 and plot close to the equiline and infinite-age lines on ²³⁴U/²³⁸U vs. ²³⁰Th/²³⁸U AR plots. Systematic differences in the degree of disequilibrium are not observed between subsamples collected from fracture surfaces or fragment interiors. However, samples from the footwall of the Bow Ridge fault, a zone of focused percolation, plot well to the right of the equiline with larger ²³⁰Th/²³⁴U AR (1.07 to 1.22) than samples away from the fault.

Results imply that U-series isotopes respond differently to water/rock interactions under different UZ hydrologic conditions. Progressive increases in ²³⁴U/²³⁸U AR with depth in UZ pore and fracture water are consistent with preferential leaching involving small water/rock ratios. Simple steady-state U-removal models can simulate some, but not all, of the observed whole-rock compositions. Long-term leaching leads to ²³⁴U-²³⁰Th-²³⁸U compositions that reflect differences in small amounts of ²³⁸U loss and significant ²³⁴U/²³⁸U fractionation. Compositions plotting to the left of the equiline require Th loss or U gain. In contrast, large ²³⁰Th/²³⁴U ratios observed at the Bow Ridge fault indicate a more aggressive hydrologic environment with greater ²³⁸U leaching and less fractionation of ²³⁴U/²³⁸U. These studies may help define zones of increased ground-water percolation, constrain water-flux models using mass balances, and quantify the extent of Quaternary water-table fluctuations.