How wet are shallow granites? An experimental investigation

The water content and porosity of a granite sample have been measured to test the assumption that the porosity in crystalline rocks hosting a waste repository will be water-saturated. Undersaturation would reduce the potential role for Rock Matrix Diffusion and impact on the retention of contaminants. A core was drilled from the Grimsel Test Site, Switzerland, with minimal cooling water, and sealed with a resin coating to preserve its water content. The initial water content, porosity, water saturation and gas permeability were determined by multiple laboratory methods, including measurements under stress to recreate in-situ conditions. The initial water content was c. 0.72 vol. % while total porosity was distinctly higher (c. 1.3 vol. %). Sub-samples separated from the core by chipping had anomalously high porosity. Gas porosity reduces with stress and was used to determine a loading factor. Correcting for 450 m of overlying granite gives a total in-situ porosity of 1.03%, indicating the in-situ porosity was less than 75% saturated. Our results show that the water content and porosity of low-porosity crystalline rocks can be measured separately under in-situ conditions, although the saturation level of the cored region might have changed since construction of the Test Site.