FIELD-SCALE WATER TRANSPORT IN UNSATURATED CRYSTALLINE ROCK

Safe disposal of toxic wastes in geologic formations requires minimal water and gas movement in the vicinity of storage areas, Ventilation o f repository tunnels or caverns built in solid rock can desaturate the near field up to a distance of meters from the rock surface, even whe n the surrounding geological formation is saturated and under hydrosta tic pressures. A tunnel segment at the Grimsel test site located in th e Aare granite of the Bernese Alps (central Switzerland) has been subj ected to a resaturation and, subsequently, to a controlled desaturatio n, Using thermocouple psychrometers (TP) and time domain reflectometry (TDR), the water potentials psi and water contents theta were measure d within the unsaturated granodiorite matrix near the tunnel wall at d epths between 0 and 160 cm. During the resaturation the water potentia ls in the first 30 cm from the rock surface changed within weeks from values of less than -1.5 MPa to near saturation. They returned to the negative initial values during desaturation, The dynamics of this satu ration-desaturation regime could be monitored very sensitively using t he thermocouple psychrometers, The TDR measurements indicated that wat er contents changed dose to the surface, but at deeper installation de pths the observed changes were within the experimental noise. The fiel d-measured data of the desaturation cycle were used to test the predic tive capabilities of the hydraulic parameter functions that were deriv ed from the water retention characteristics psi(theta) determined in t he laboratory. A depth-invariant saturated hydraulic conductivity k(s) = 3.0 x 10(-11) m s(-1) was estimated from the psi(t) data at all mea surement depths, using the one-dimensional, unsaturated water flow and transport model HYDRUS [Vogel er al., 1996], For individual measureme nt depths, the estimated k(s) varied between 9.8 x 10(-12) and 6.1 x 1 0(-11) The fitted k(s) values fell within the range of previously esti mated k(s) for this location and led to a satisfactory description of the data, even though the model did not include transport of water vap or.