Infiltration History and Spatial Variability Derived from Chloride Mass Balance

Chloride mass balance was applied to drill cuttings collected from the unsaturated zone surrounding the Yucca Mountain Project. Samples correspond to four Nye County Early Warning Drilling Program boreholes where air was used as the drilling fluid to preserve sample integrity. Infiltration dates before present and pore velocities were calculated using a range of annual chloride deposition rates obtained from the literature. The lower chloride loading corresponds to contemporary values, and the upper loading corresponds to an attempt to correct for either past greater chloride deposition or a past higher precipitation with chloride concentration remaining constant. In each borehole, pore velocities present two distinct slopes corresponding to different infiltration regimes. The first one, near the surface, presents the slowest infiltration rate. The second pore velocity corresponds to a past wetter period (late Pleistocene to early Holocene) with much faster pore velocities. Results indicate that pore velocities among the boreholes differ at most by a factor of approximately 3.5. Boreholes located in areas of little or gradual slope present faster infiltration rates than those in areas of greater slope. Borehole NC-EWDP-22S, near Fortymile Wash east of Yucca Mountain, exhibits the most rapid pore velocities where as boreholes further from the wash demonstrate lower velocities. These results denote the effects climate change, and runoff and run-on at the surface have over infiltration rates in arid regions.