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decomposition in the hard rock Ito a depth of 80 - 100 m} generally yield 1 - 5 m 3 h- 1 .
Towards the western base of the lebombo Range, numerous north-south trending dykes
are present in the basalt. Boreholes drilled on these dykes give somewhat better yields.
In this area of relatively low rainfall, groundwater quality in the basalt is usually poor with
a TDS value of 750 - 2 500 mg ["1. Boreholes drilled in the rhyolite and dacite of the
lebombo Range itself often have a low yield of the order of 1 - 3 m 3 h- 1 . Such rocks also
contain several north-south trending dolerite dykes, the drilling of boreholes on which
lespecially where they cross valleys), can give rise to yields of 5 m 3 h- 1 or more in places.
Groundwater quality in the more elevated (higher rainfall) western part of the lebombo
Range is relatively good with a TDS content of 600 - 1 200 mg ["1. However, on the
drier, lower eastern slopes of the Range, the water quality deteriorates and saline waters
with a TDS content of 750 - 2 000 mg ["1 predominate.
(g) Intrusive Karoo dolerite
Intrusions of Karoo dolerite, in the form of sheets or sills and dykes, occur in all the hard
rock geological formations in the province (as described above). Supplies of groundwater
are normally associated with the contacts of these features with the country rock into
which they have been intruded. In the case of sheets or sills, higher volumes of
groundwater are usually associated with the basal contact of the intrusion. rather than
with its upper contact - the generally well-jointed nature of the dolerite facilitating
groundwater movement therein. For this reason, springs and seepage lines often occur
where the lower contacts of dolerite sheets (particularly the thicker ones), have been
exposed on the valley sides by erosion. Due to disturbance and fracturing of the enclosing
rock. enhanced groundwater movement frequently takes place in the country rock
immediately adjoining the intrusion. This situation also obtains in the case of linear,
vertical or steeplyinclined dykes of Karoo dolerite. groundwater movement being enhanced
by the general close-jointing of the dolerite itself, and by the frequently fractured nature
of the immediately adjoining enclosing rock. Accordingly, in the case of dykes.
groundwater can be found both within the dyke itself as well as on its margins. In general,
the latter situation results in greater flows of groundwater in boreholes, than is the case
in the dyke itself. Particularly good yields of groundwater are obtained from boreholes
drilled on or in dolerite dykes, where these features cross, or are aligned along valley lines.