At Yucca Mountain, Nevada, a potential nuclear waste repository may be constructed 200 to 300 m above the water table in the Topopah Spring Tuff (Tpt). The composition of dissolved solids in water percolating downward from the surface changes by interaction with the rock. The chemical composition of the pore water at the potential repository level must be known to assess possible effects on corrosion of waste canisters. The water percolates through about 100 m of the densely welded part of the Tiva Canyon Tuff (TCw), about 50 m of nonwelded and highly porous tuffs of the Paintbrush Group (PTn), and about 300 m of the densely welded part of the Tpt (TSw). Mean volumetric water contents and standard deviations for core from boreholes USW SD-7, USW SD-9, and USW SD-12 are: 5.5±2.1 percent (TCw); 16.3±5.0 percent (PTn); and 8.1±2.5 percent (TSw). Water contents combined with thicknesses of the hydrogeologic units indicate that 70 to 90 percent of the total water contained in tuffs above the potential repository is in the welded TCw and TSw rather than in the much more porous nonwelded PTn. Chemical analyses reported for pore water in 1996 and 1998 by A. Yang were only from nonwelded tuff, because pore water could not be extracted from the welded tuffs, which have greater compressive strength. During the past year and a half, the USGS has been extracting pore water from welded TSw by ultracentrifugation of core obtained along a 2000-m reach of a drift across the potential repository block. Mean concentrations (mg/L) and standard deviations of dissolved ions in 25 samples are: Ca⁺⁺, 116±55; Mg⁺⁺, 15.8±8.2; Na⁺, 83.5±26.4; K⁺, 10.2±2.9; Cl^-, 34.5±16.7; SO₄^--, 55.5±35.5; HCO₃^-, 357±123; F^-, 3.3±2.4; and SiO₂, 51.0±6.4. The chemical variability of the pore water extracted from the TSw high-silica rhyolite contrasts with the chemical homogeneity of these rocks shown by Z. Peterman and P. Cloke in 2002. The variability in dissolved-ion concentrations of pore water samples at the potential repository level, therefore, is imparted by water-rock interaction during infiltration and percolation through the overlying stratigraphic units, and it is not being smoothed out either by advective flow or by diffusion in TSw.