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

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Fig. 6. — The Swakop and Khan Rivers watershed.
cases. The fresh water thus seeping in the sand forms a fresh
water lens over saline water, and is a major source of water.
These water lenses all constitute a major local source of water
in many desert and generally underdeveloped areas. Their contamination
by intruding salt water constitutes a pollution problem
of major economic and social importance.
4b. Exploitation of fresh water lenses floating over saline water.
It appears that all the theoretical problems outlined in sections
2 and 3 are present in this problem. First of all, a chemical
stratification is present. Absorptive and exchange reactions
between the moving fluid and the porous medium will be
greatly favoured by the large surface areas of the medium.
It should be pointed out that the equilibrium time of these
reactions is usually very much smaller than the characteristic
time scales of the flow. Thus, in computing the flow of the
moving fluid, the chemical reactions are usually assumed to
occur instantaneously (25, 23).
In the exploitation of fresh water floating lenses, it is not
possible to use the classical methods of fighting salt-water
intrusion. One of these classical methods is used in the case
of fresh water coastal aquifers in the United States, where