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pumping. As the system reaches close to steady condition, S will approach
a uniform value.
Figure 7. Variation of estimated S -values in relation to the distance
between the observation and pumping borehole as obtained in the
fractured-rock aquifer on the University of the Orange Free State (UOFS)
campus (after Lloyd, 1999).
FACTORS AFFECTING WELL YIELD
Drilling of a successful well for groundwater in hard rocks is to a great
extent a matter of chance. Groundwater exploration in hard rock formations
should be carried out with due considerations to lithological, structural and
geomorphological setting of hard rock formations. Plotting of structural data
viz. poles of joints and foliation planes on Schmidt's equal area net is useful
in the design and orientation of wells in such rock formations. Shape of cone
of depression and base flow from streams are of help in determining the
relative role of different fracture planes as groundwater conduits.
As fractures tend to close with depth, an overall decrease in well yield
with depth is reported from various crystalline terrains. The optimum depth
of wells is regarded between 50 and 100 m (Table 3). Although generally
lithologies do not have any significant influence on well yield but coarse-
grained rocks viz. pegmatites are more permeable. Fine grained and micaceous
rocks viz. phyllites and schists have poor yield. Weathered granites usually
have lower permeabilities as compared with fractured rocks due to the
development of secondary clay minerals. Lineament zones are more productive
for the construction of bore-wells.
In areas underlain by hard crystalline and meta-sedimentaries viz., granite,
gneiss, schist, phyllite, quartzite, etc., occurrence of groundwater in the fracture
system has been identified down to a depth of 60 m and even up to 200 m
locally. In most of the granite-gneiss country, the weathered residuum serves
as an effective groundwater repository. It has been found that the deeper
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