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11.6.4 Aquifer characteristics of geological formations and faults
(a) Proterozoic and older granitic and related rock formations
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In these rocks, moderate to good yields of groundwater of the order of 5 - 10m 3 h- 1 are
often obtained in lower rainfall areas, where such rocks occur in the Dry Interior (River)
Basins of the province. Water quality is usually good with TDS values mostly less than
500 mg r 1 for boreholes with good yields (in areas of moderate and higher rainfall). Due
to their generally linearly well-foliated and jointed nature (and to a lesser extent the faulting
therein), the most favourable sites for boreholes in these formations are on valley lines and
intersections. In areas of more massive granite and gneiss, basins of weathering of the
rock (which are good groundwater reservoirs), are frequently present on joint-eontrolled
valley intersections. Many successful boreholes have been drilled on rocks of these
formations in the south and centre of the province.
(b) Natal Group formations
The mainly sandstone rocks of the formations comprising this group are the best and most
consistent secondary aquifers in NatallKwaZulu. Groundwater in these formations is
contained on bedding planes, and on joints and fractures therein. Yields from 150 mm
diameter boreholes (80 - 100 m deep), are usually. in the range 2 - 10m 3 h- 1 , although
higher yields of 15 - 20 m 3 h- 1 are frequently encountered. Dry boreholes are rare. Water
quality in boreholes drilled in formations of the Natal Group is generally very good, with
TDS values often less than 500 mg r 1 (much of this being temporary bicarbonate
hardness). For the most part, sandstones ofthe Natal Group form topographically elevated
areas which have a high rainfall. The best locations for boreholes are on joint or fault­
related valley intersections or valley lines.
(c) Dwyka Tillite Formation
This formation is the poorest secondary aquifer in NatallKwaZulu. However, reasonable
yields are sometimes obtained in boreholes drilled in favourable locations. In general, it is
only worthwhile drilling on faults and major joints (which are reasonably frequently
present) in the formation. Within the formation itself, yields when obtained at all, are less
than about 3 m 3 h- 1 , the lowest yields being associated with the drier areas such as the