CHEMICAL COMPOSITION OF PORE WATER FROM BOREHOLES USW SD-6 AND USW WT-24, YUCCA MOUNTAIN, NEVADA

Analyses of pore water extracted from drill cores show significant vertical and lateral variation in dissolved major-ion concentrations. Chemical compositions of pore water extracted by compression and by ultracentrifugation from adjacent core samples are generally in agreement, within the sampling and analytical error of 10 to 15 percent except for silica. The larger concentrations of silica obtained by compression probably are due to localized pressure solution of silicate minerals during compression. The agreement by the two extraction methods indicates that compression does not significantly alter the compositions of pore water. The shallower water from core in borehole USW SD-6 was extracted from nonwelded units collectively referred to as the Paintbrush Tuff nonwelded (PTn). Pore water about 300 m deeper was from core in both boreholes USW SD-6 and USW WT-24 in the nonwelded units referred to as the Calico Hills nonwelded (CHn).

Mean Ca⁺⁺, Mg⁺⁺, SO₄^--, and NO₃^- concentrations were significantly smaller in the CHn than in the PTn, and HCO₃^- concentration and (Na⁺+K⁺)/(Ca⁺⁺+Mg⁺⁺) ratio were significantly greater in the CHn than in the PTn. The decrease in NO₃^- and the increase in HCO₃^- could be the result of denitrification through the oxidation of organic matter. The decrease in Ca⁺⁺ and associated increase in (Na⁺+K⁺)/(Ca⁺⁺+Mg⁺⁺) is the result of ion exchange with zeolites in the CHn in borehole WT-24. This effect is not as pronounced in borehole SD-6, probably reflecting a smaller amount of zeolitization of the CHn in SD-6. The existence of the compositional variability in pore water within individual units in both boreholes SD-6 and WT-24 at the decimeter to meter scale implies that neither advective nor diffusional mixing is effective in homogenizing the concentrations of dissolved ions in pore water. Possibly, this variability reflects very local interaction of pore water and fracture water at a rate greater than diffusion or advection can smooth the compositional differences.

Geochemical calculations using the PHREEQC code indicate that the pore water from both boreholes SD-6 and WT-24 is uniformly undersaturated in anhydrite, gypsum, and amorphous silica, but supersaturated in quartz and chalcedony. The saturation of calcite, aragonite, sepiolite, and dolomite is more variable from sample to sample.