Study of flow and transport in fractured granitic rock
The hydrogeology of the Tono basin, Japan, is strongly influenced by the hydraulic properties of faults, especially the main Tsukiyoshi fault, which extends through the centre of the assessment area and has an E-W strike. According to the results of borehole investigations, the fault has N800W strike, 700 dip, 10 - 30m width and approximately 30m vertical off-set. Hydraulic head discontinuities over the main fault in the basin are about 40 m as a result of the low permeability of the fault acting as a barrier to flow perpendicular to it. The fracture data from the Tono basin was analysed in order to establish a correlation between geologic/geometric attributes of a fracture and associated permeability of the interval that contains the fracture, if any. Pressure response transients to excavation of two shafts that are monitored at various boreholes within the study site show that proximity to a fault is a key attribute that determines the ability of the fracture to conduct water. The responses in boreholes that are close to the fault are vertically invariant, indicating a large vertical permeability. This is not the case in boreholes that are further from the main fault, where there is depth dependence in the pressure responses. Near the fault, the damage zone seems to be equilibrating the heads between otherwise unconnected aquifers. The Tsukiyoshi fault therefore acts as barrier to flow perpendicular to it but also acts as conduit to vertical flow and flow parallel to the fault. A three-dimensional model that simulates groundwater flow in the Tono basin is constructed in order to study the dynamic fluid flow before and after it was disturbed by production and the excavation of the shafts. In the steady-state calculation, the model predicts that the hydraulic head at depth in boreholes near the fault is near the land surface. This condition indicates high vertical permeability in those boreholes. This thesis introduces a new approach of using pressure response data to do an inversion calculation for the effective porosity of the granite. Pressure response transients have been analysed using a numerical inversion procedure to estimate the specific storage of the granite. The specific storage was calculated using the pressure response data and ranged from 4:12 x 10-7 to 8:93 x 10-6m-1. These values of the specific storage were used to do a transport calculation in order to study the impact of the main fault on the transportation of hypothetical contaminants in the basin. Particle tracking was used to investigate and demonstrate the effects of the fault on path lines. The fault was found to have a strong influence on the transportation of contaminants. The general trend of the transportation of the contaminants follows groundwater flow from the northern high elevations toward the southern low elevation. This shows that the contaminants are transported mainly by advection. However, this trend is interrupted by the Tsukiyoshi fault that blocks horizontal flow and sends water toward the surface. An interesting feature demonstrated by the model is that, within the fault core, no contaminants were found. The contaminants rise through the high-permeability damage zone and cross over the fault through the weathered granite. However, at depths where the water changes direction slowly because of the fault barrier to horizontal flow, the contaminants seem to be able to cross the fault. The explanation is that diffusion becomes the dominant mode of transport at the point where the water moves at slow velocities.