Geology Reference
In-Depth Information
Figure 6.
Variation in permeability values of crystalline rocks as a function
of scale of measurement. Bars mark the permeability range based on several
reported values, and stars represent single values (after Clauser, 1992).
In a recent study, Shapiro (2003) has shown that although the hydraulic
conductivity measured from borehole tests in individual fractures varies over
more than six orders of magnitude (10
-10
to 10
-4
ms
-1
), the magnitude of the
bulk hydraulic conductivity of the rock mass was the same from aquifer tests
over 10's of metres and kilometre-scale estimates inferred from groundwater
modelling. In contrast, the magnitude of the formation properties controlling
chemical migration viz. dispersivity and matrix diffusion increases from
laboratory size tests to field tests on kilometre scale.
Most of the earlier work on well hydraulics was based on porous
homogeneous media. However, as the hard rock aquifers have varying degree
of heterogeneity, due to the complexity of geometry of fracturing, the
conventional well flow equations developed for homogeneous porous aquifer
will not adequately describe the drawdown response in fractured aquifers.
Theis' method, although originally developed for porous and homogeneous
media, can be used for fractured rock aquifers with due care if the correct
part of the time-drawdown curve is matched with the Theis' type curve.
Experience in South Africa has shown that the estimated values of
S
in
fractured rocks decrease with increase in the distance of observation well
from pumping well (Fig. 7). Such a behaviour is explained by the rapid
release of water from larger fractures near the pumping well while the pressure
gradient between the small and larger fractures will be smaller away from
the pumping well. Further, a variation is noted only at the early time of
