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good. with a total dissolved solids or salts (TDSI* content of up to about 500 mg r 1 ,
which is much the same as the surface water in the river channel. For their continued
satisfactory performance, well points generally need to be back-washed every 6 - 12
months. The installation of shallow well points in river beds was frequently resorted to
as an emergency source of water, when many of the rivers in NatallKwaZulu virtually dried
up during the severe drought years of 1982/83 and again in 1992/93. Shallow well point
systems currently serve as a source of domestic and industrial water on the Buffalo and
Nondweni rivers in the interior of the province, and on the Matigulu, Nonoti, Mdloti, Lovu,
Mpambanyoni, Mtwalume and Imbizana rivers in the coastal zone.
• Deep screen wells
Deep screen wells have been installed in several of the upper estuarine reaches of rivers
in NatallKwaZulu, where alluvial and estuarine sediment depth is considerable. The depth
of screen wells, from which water is obtained by means of a pump situated in a sump
beneath the screen, varies in the approximate range of 15 - 30 m. Potential screen well
sites must be subjected to a preliminary test boring investigation, in order to ensure the
accurate location of the screen in the sandy aquifer - given that impermeable silt and clay
horizons are frequently also present - and to optimize screen opening size in terms of the
particle size distribution of the sand. Yields from (20 - 25 m deep), 200 - 300 mm
diameter screen wells vary in the range of 60 - 180 m 3 h- 1 . Groundwater from screen
wells generally has a higher iron content (due to the chemically reducing conditions
prevailing atthe abstraction depth in the aquifer), than is the case for groundwater derived
from shallower depths down to about 5 - 6 m. An iron content of up to about 40 mg r 1
is frequently encountered in the deeper groundwater. The iron is readily removed by
aeration and storage. If located too close to the shoreline of the "estuary" (within a
distance of about 2 km therefroml, the chloride content of the deeper groundwater is often
somewhat marked. If screen wells are sited within a distance of about 1 km from the
shoreline, sea water intrusion becomes a possibility with continuous long term extraction.
Generally, the quality of the groundwater obtained from deep screen wells in alluvial and
estuarine sediments is good to moderate, with TDS values usually in the range of
400 - 1 200 mg r'. Deep screen wells have been installed as a source of urban
*
The terms "total dissolved solids" and "total dissolved salts" are both used in the scientific literature.