The Exploitation of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 189

in Mar Piccolo (Tarente, Italy) in the 1970s (Stefanon, 1972). A continuous
measurement setup monitoring the position of the interface enabled the
adjustment of diversions. This system was nevertheless abandoned, as it did
not solve contamination problems further upstream. A similar setup was
developed for the Mortola spring (Menton, France & Italy), where fi rst a
fl exible tarp, then a steel bell capped the spring, but there too the remaining
salinity of 1.4 g·L -1 is too high to make exploitation worthwhile.
Some springs, however, have zero salinity, when the aquifer is well-
protected from the marine environment by impermeable formations,
or when the pressure is high enough in the passageways to prevent the
intrusion of salt water. The outlet must then be isolated from the sea by a
circular dam (Aurisiana, Italy).
Exploitation by increasing the hydraulic charge has been attempted,
in order to lower the fresh water/salt water interface. A subterranean
dam was thus built in the submerged gallery of the Port Miou spring, but
it did not yield the hoped-for results (see chap. A9-2.5). Several attempts
to raise the level of the Almyros of Heraklion pool also failed to lower the
salinity to a satisfactory level. An attempt to increase piezometric level by
totally blocking the conduit was unsuccessfully attempted on the Cabbé
spring (France). A circular dam around the Anavalos spring (Greece) was,
however, successful. The idea was to create a column of fresh water above
the emergence, so that the resulting pressure would counteract that of the
sea water and prevent it from contaminating the spring. Water is drawn off
so as to not cause a drop in the level. If the level of the dam is too high, the
discharge of the spring decreases; if its is too low or the freshwater level is
lowered through pumping, saltwater contamination occurs.
b) Diversion in the aquifer
Given the emplacement mechanisms of coastal karst systems, which enable
deep contamination, it is generally preferable to draw on the resource
through wells upstream of the emergences, but for aquifers with large karst
conduits, drawdown provokes the migration of the saltwater intrusion, in
all three spatial dimensions, with a very irregular shape. Salt water can
reach and contaminate the catchment in an unpredictable manner. If the
aquifer is large, compartments isolated from the marine environment can
be sought out.
One solution consists of drawing only upon the upper layer of the
aquifer, using several shallow wells, in order to induce only minimal
drawdown.
Subhorizontal drainage galleries can also be used, at the top of the
aquifer. Increases in discharge must then be obtained by lengthening the