GEOCHEMISTRY OF PORE WATER FROM THE UNSATURATED ZONE AT YUCCA MOUNTAIN, NEVADA

Assessing the performance of a proposed high-level radioactive waste repository at Yucca Mountain, Nevada, requires an understanding of the chemistry of water moving through the host rock. Pore water was obtained from core samples from boreholes USW SD-9, SD-12, and SD-7, which are located from north to south along the Exploratory Studies Facility. Previous analyses of pore water from these boreholes by the U.S. Geological Survey were from water extracted by compression from nonwelded tuffs. Because pore water could not be extracted from densely welded tuff using compression methods, there were no pore water chemistry data from the Topopah Spring welded hydrogeologic unit (TSw), the repository horizon. Ultra-centrifugation methods allow pore water extraction from welded tuff. Pore water data from the TSw in USW SD-9 have partly filled the gap between the overlying units of the Paintbrush Tuff nonwelded hydrogeologic unit (PTn) and the deeper Calico Hills nonwelded hydrogeologic unit (CHn), increasing the understanding of percolation through the repository host rock. Pore water from the densely welded TSw shows vertical variability in dissolved-ion concentrations, but overall the chemistry evolves with increasing depth from the calcium-sulfate-chloride type in the overlying PTn, to the sodium-bicarbonate type in the underlying CHn. The concentrations of Ca²⁺, Mg²⁺, SO₄^2-, and Cl^- decrease from the PTn to the TSw to the CHn. The concentrations of HCO3^- and (Na⁺ + K⁺)/(Ca²⁺ + Mg²⁺) ratios increase from the PTn to the TSw to the CHn. The decrease in NO3^- from the middle of the TSw through the CHn coupled with the increase in HCO3^- could be the result of denitrification through the oxidation of organic matter. Similarly, the decrease in SO₄^2- could be caused by the reduction of SO₄^2- by facultative anaerobes resulting in the production of additional HCO3^-. The decrease in Ca²⁺ and Mg²⁺ and associated increase in Na⁺ and K⁺ are the result of ion exchange with zeolites. In borehole USW SD-9, this ion exchange effect in the pore water is observed at the TSw/CHn contact, whereas in boreholes USW SD-7 and SD-12 to the south, the change in pore water chemistry isn’t prominent until the deeper parts of the CHn (Prow Pass and Bullfrog Tuffs), reflecting the distribution of zeolitized horizons.