Paintbrush Tuff

The location in the geologic section from which the B and T tunnel-complex water samples were obtained, comparison of their chemical composition with data obtained from laboratory experiments on both vitric- and crystalline-tuff, and hydrologic data from the Rainier Mesa system, combine to imply that the vitric material (Rainier Mesa Member and Paintbrush Tuff) of Rainier Mesa is responsible for the water quality observed above the zeolitized zone. 

If it is assumed that precipitation of montmorillonite of this composition is occurring contemporaneously with dissolution of vitric material in the Paintbrush Tuff, the ratios of maior cations (in solution) consumed is approximately Mg Ca Na K = 1 0.40 0.14 0.14. 

The assumption that montmorillonite of reported composition is being produced and is the only authigenic phase is probably only an approximation. The composition may be in error or may vary areally within Rainier Mesa as a result of areal variations in water quality. Although no zeolites or other clay minerals were reported in the bulk of the Paintbrush Tuff, very small amounts may remain undetected by either x-ray diffraction analysis or thin-sectlon petrography and may affect the aqueous composition by precipitation or by ion exchange. 

Thordarson (1) and Benson (LBL, Berkeley, written commun., 1978) both describe the Paintbrush Tuff as partially saturated. Core samples obtained from the lower part of the Paintbrush in the tunnel complex were reported by Diment and others (2 ) to have saturation levels of 55-91 percent, with an average of 77 percent. Benson (LBL, Berkeley, written commun., 1978) reported an average value for water saturation in the Paintbrush of 90 percent. It would therefore be expected that the total aquifer-pore surface would not be involved in transport of water. As a matter of fact, assuming that all of the saturated-pore space is effective in transmitting water, one might predict that, on the average, somewhere between 77 and 90 percent of the aquifer surface is in contact with percolating water. 

Figure 1.2. The proposed repository is in the thick vadoze zone of volcanic tuff. TCw, PTn, TSw, and CHn stand for tuff units of Tiva Canyon welded, Paintbrush non-welded, Topopah Springs welded, and Calico Hills non-welded. Qal denotes Quaternary alluvium. Courtesy of Jim Winterle at the Center for Nuclear Waste Regulatory Analysis, Southwest Research Institute.

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