Field permeability and hydraulic potential measurements in crystalline rock and solute transport through finely fractured media

Drill stem tests were performed to depths of 500 metres during borehole advancement in crystalline rock using a single packer system. Apparent equilibrium shut-in pressures were obtained to provide an estimate of the vertical hydraulic potentials. A downhole pneumatic piston was used to create an instantaneous pressure pulse in the isolated test section and the transient pressure decay was monitored. Equivalent single-fracture apertures and equivalent rock mass hydraulic conductivities, Kerm, were calculated from the transient pressure decay data. Equivalent single-fracture apertures are presented which range between 1 and 125 microns for tests performed in a 150 m deep borehole into a granitic rock mass. It can be shown from modeling studies that diffusion from a fracture into the adjacent matrix, fracture aperture and grounwater velocity through the fractures are the controlling transport parameters for non reactive solute transport through fractured media. Finite difference model results for a constant porosity system illustrating solute breakthrough curves and profiles along the length of a fracture indicate the importance of matrix diffusion in particular. If the average water velocity in a 300 μm fracture in a fractured clay or shale is 3m/d, the average velocity of a conservative solute under the simulated conditions is approximately 08m/d