(1973) n°3 - Royal Academy for Overseas Sciences

— 595 —
injection wells are often used to prevent the inward movement
of the salt-water wedge. This is sketched in figure 7.
Injection well used to prevent the inward movement of the salt-water wedge.
Another method consists in the construction of lateral barriers.
Finally, another method is used in the water supply schemes
for the towns around Walvis bay (figure 6), where floods
replenish the fresh groundwater floating lenses in one out
of three years on the average. Shallow drains across the Swa-
kop river were connected by pressure pipes to deep wells further
downstream to inject excess fresh water during floods, thereby
replacing some of the salt around the well and making it
possible to recover fresh water from the well for a limited
period ( W ip p l in g e r , personal communication).
However, none of these methods are applicable in most
arid areas because there is no other source of fresh water to
replenish the lens during the period when water is abundant.
Also, the construction of barriers to prevent lateral intrusion
of saline water is impossible because the intrusion of saline
water proceeds from below.
The recovery of a fresh water lens was thought to be possible
by drilling a well screened only in the fresh water. An approximate
theory of a partially penetrating well used for skimming