A thorough understanding of hydrochemical processes in the unsaturated zone is important in characterizing the potential and time for water to infiltrate from the surface to the water table at Yucca Mountain. Several milliliters of water contained in the pores of the volcanic rocks (pore water) at Yucca Mountain were extracted from volcanic samples at Yucca Mountain and analyzed:

• Chemical composition reflects the extent of water-rock interaction between the water that infiltrates from the surface and the rock matrix. Unsaturated-zone pore water has substantially larger concentrations of major ions and dissolved solids than saturated-zone water.
• Tritium and carbon-14 contents of the pore water provide insight as to the nature of flow mechanisms, residence times, percolation flux, and transport velocity.
• Stable-isotopic compositions of hydrogen and oxygen in fracture and matrix water can help to identify fracture/matrix interactions and sources of water.

Methods of extracting pore water from rock samples

Ultracentrifuge Method: Approximately 150 to 200 grams of core rubble is loaded into three titanium cups which are attached to a rotor and spun for 6 hours at 15,000 revolutions per minute. Depending on the moisture content of the rock, 0.5 to 3 millilters of pore water were extracted.

Uniaxial Compression Method: Approximately 400 to 900 grams of core or rubble are loaded into the compression cell. Pressure is applied with progressive loading of the samples to a stress level of 827 megapascals to extract the pore water.

Methods for pore-water extraction for chemical-composition analysis were developed by the USGS for the Yucca Mountain studies. These methods are documented in the following reports:

Yang, I.C., Turner, A.K., Sayre, T.M, and Montazer, Parvis, 1988, Triaxial-compression extraction of pore water from unsaturated tuff, Yucca Mountain, Nevada: U.S. Geological Survey Water-Resources Investigations Report 88-4189, 68 p.

Mower, T.E., Higgins, J.D., Yang, I.C., and Peters, C.A., 1994, Pore-water extraction from unsaturated tuff by triaxial and one-dimensional compression methods, Nevada Test Site: U.S. Geological Survey Water-Resources Investigations Report 93-4144,
79 p.

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