Study of flow and transport in fractured granitic rock

[摘要] The hydrogeology of the Tono basin, Japan, is strongly influenced by the hydraulicproperties of faults, especially the main Tsukiyoshi fault, which extendsthrough the centre of the assessment area and has an E-W strike. According to theresults of borehole investigations, the fault has N800W strike, 700 dip, 10 - 30mwidth and approximately 30m vertical off-set. Hydraulic head discontinuities overthe main fault in the basin are about 40 m as a result of the low permeability of thefault acting as a barrier to flow perpendicular to it. The fracture data from the Tonobasin was analysed in order to establish a correlation between geologic/geometricattributes of a fracture and associated permeability of the interval that contains thefracture, if any. Pressure response transients to excavation of two shafts that aremonitored at various boreholes within the study site show that proximity to a faultis a key attribute that determines the ability of the fracture to conduct water. Theresponses in boreholes that are close to the fault are vertically invariant, indicatinga large vertical permeability. This is not the case in boreholes that are furtherfrom 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 otherwiseunconnected aquifers. The Tsukiyoshi fault therefore acts as barrier to flowperpendicular to it but also acts as conduit to vertical flow and flow parallel to thefault. A three-dimensional model that simulates groundwater flow in the Tono basinis constructed in order to study the dynamic fluid flow before and after it was disturbedby 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 faultis near the land surface. This condition indicates high vertical permeability in thoseboreholes.This thesis introduces a new approach of using pressure response data to doan inversion calculation for the effective porosity of the granite. Pressure responsetransients have been analysed using a numerical inversion procedure to estimatethe specific storage of the granite. The specific storage was calculated using thepressure 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 calculationin order to study the impact of the main fault on the transportation of hypotheticalcontaminants in the basin. Particle tracking was used to investigate and demonstratethe effects of the fault on path lines. The fault was found to have a strong influenceon the transportation of contaminants. The general trend of the transportation ofthe contaminants follows groundwater flow from the northern high elevations towardthe southern low elevation. This shows that the contaminants are transportedmainly by advection. However, this trend is interrupted by the Tsukiyoshi faultthat blocks horizontal flow and sends water toward the surface. An interesting featuredemonstrated by the model is that, within the fault core, no contaminants were found. The contaminants rise through the high-permeability damage zone and crossover the fault through the weathered granite. However, at depths where the waterchanges direction slowly because of the fault barrier to horizontal flow, the contaminantsseem to be able to cross the fault. The explanation is that diffusion becomesthe dominant mode of transport at the point where the water moves at slow velocities.

[发布机构] University of the Free State