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

— 596 —
off fresh water overlying salt water was presented initially
by W a n g . ( 3 4 ) . W ang assumed that the interface remains
sharp and non-diffusive and he was able to compute the upco-
ning of the interface beneath each well, as is sketched in figure
8. Yet as was found out in the Indus plain, in the Kalahari desert,
and in the Middle East (6, 29), for situations such as the one
sketched in figure 8, the mouvement of the interface towards the
skimming wells leads to a considerable dispersion along the flow
lines. The zone of mixing is thus widened by the effects
of longitudinal dispersion. Contamination of the well discharge
can begin when the moving interface has traversed less
than one-half the original distance to the well. It was reported
by K u n i n ( 1 5 ) that, in similar situations in the Russian deserts,
the clay interfaces merely retard the beginning of the intrusion.
Fresh
wa ter
withdrawa I
well 1
l°yer
Fig. 8. — Upconing of the interface beneath a skimming well.
int erf o o e
Another method of exploitation of fresh water lenses floating
over stagnant salt-water is now proposed.
If two withdrawal wells are located on opposite sides of the
interface (figure 9), on a common normal, the movement
of the interface can be inhibited to a considerable extent. By
using Darcy’s law, it is easy to prove that the position of the
interface will remain undisturbed provided
1. That the sinks are at the same distance from the interface;
2. That the product of the steady volumetric rate of pumping
and the dynamic viscosity are the same for both sinks.