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111.BIII!I!111
The yields of boreholes drilled on or into dolerite intrusions are variable, although they can
be as much as 10 m 3 h-' or more in some instances. The quality of groundwater obtained
from boreholes drilled into intrusions of Karoo dolerite is generally good. Quality however,
can be markedly influenced by the characteristics of the groundwater present in the
enclosing rock formation, particularly where borehole yields are low. The quality of
groundwater in dolerite often reflects the prevailing rainfall conditions, with waters with
a low TDS content of 300 - 750 mg r' found in high rainfall areas. A higher TDS content
in the range 750 - 1 500 mg r' is evident in areas of relatively low rainfall.
(h) Faults
The numerous faults of Jurassic age which characterize the hard rock geological
formations of the coast and coastal hinterland, are often very good sources of
groundwater. Boreholes must usually intersectthin inclined zones of fault breccia at some
depth below ground level for good yields to be obtained, although in the case of broad
fault zones, boreholes drilled wholly within the fault zone breccia also frequently provide
very good yields of groundwater (contained within the secondary cemented fragmented
fault breccia, which may be either or both silicified and kaolinised). In the case of the
normally very poor yielding Dwyka TiIlite Formation, virtually the only locations on which
boreholes can be drilled with a reasonable chance of success, are on fault zones within or
bounding the formation.
While occasionally on thin fault zones, dry holes may be encountered on drilling, more
frequently, yields of up to 40 - 50 m 3 h- 1 can be obtained in boreholes drilled on or into
fault zones. Given the large yields usually associated with fault zones, the quality of the
groundwater is normally good, with a TDS value of the order of 700 mg r' or less. Thin
fault zones in areas of low rainfall however, in which relatively low yields are obtained, can
provide poor quality water - reflecting the nature of the groundwater in the enclosing rock.
IiI Makatini. Msinene and 5t Lucia formations (Cretaceous)
The aquifer characteristics of these soft rock sediments (which comprise a basal
conglomerate of the Makatini Formation and overlying siltstones of the formation plus the
other overlying formations), are very poor. These formations are effectively limited to the
Zululand Coastal Plain in the north east, where for the most part, they occur under a