Study of flow and transport in fractured granitic rock
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Date
2012-05
Authors
Ndiweni, Cliford
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
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.
Description
Keywords
Granite -- Fracture, Hydrogeology, Thesis (Ph.D. (Institute for Groundwater Studies))--University of the Free State, 2012