Hydrological properties of Topopah Spring tuff under a thermal gradient—Laboratory results

Abstract

Abstract Hydrological properties of a naturally fractured Topopah Spring welded tuff sample from Yucca Mountain at the Nevada Test Site, Nevada, have been studied. Confining pressure, sample temperature and pore pressure were held at values that simulated the in situ near-field conditions expected shortly after emplacement of a high-level waste container. Water permeability of the sample was determined as functions of temperature and time. Electrical resistivity and electrical impedance tomographs were used to infer the distribution of moisture in the sample during dehydration and rehydration processes. During the almost 5 month duration of the experiment, water permeability decreased about three orders of magnitude, from 1.3 × 10 −14 m 2 to about 10 −17 m 2 . Scanning electron microscope images and chemical composition of the water that flowed through the sample suggest that both dissolution and deposition of silica occurred on the fracture surface. These processes may have smoothed the fracture surface, decreasing overall effective aperature. This possibility is consistent with our findings at isothermal conditions. The distribution of electrical resistivity, and electrical impedance tomographs taken at the midplane of the sample indicate that, as under isothermal conditions, during the dehydration water first left the matrix adjacent to the fracture, then escaped through the fracture aperture. Rehydration was not the reverse of the dehydration process. In this case, fracture did not dominate the rehydration. Under the thermal gradient, dehydration of the sample was very nonuniform.