Island and Coastal Aquifers - Hydrogeology: Objectives, Methods, Applications - page 67

2 COASTAL KARST AQUIFERS
On all the limestone coasts of the planet, springs emerge near the shore of
under the sea. They are common around the borders of the Mediterranean,
and have been the object of numerous studies and exploitation attempts,
particularly along the Dalmatian coast, where thirty or so high-discharge
springs (a few m 3 ·s -1 ) are known between Trieste and Kotor (Mijatovic,
1986; Breznik, 1998). In Greece, the Almyros of Iraklion (Crete) has been
the subject of catchment attempts for half a century; in Italy the Tarente and
Mortola springs have been studied since the sixties; in France, the Port Miou
spring was the subject of in-depth studies during the same time period.
The most recent studies (Bini, 1994; Gilli, 2001; Arfi b, 2001) show that the
emplacement of these systems and their functioning cannot be explained
without the existence of a paleogeographic context very different from
the present day, which would explain their behavior and the cause of the
failures to tap them.
Their use directly at the exit is generally impossible due to their salinity,
but, these springs being the exit point for continental aquifers, their study
brings precious information, which can help efforts to tap them on land. Due
to the increasing population along the coasts, and the need for alternative
resources in case of an accident, there is today a rekindling of interest in
these resources.
2.1 Particularities of coastal and submarine karst springs
Most coastal karst springs have exsurgence conditions comparable to those
for continental springs. However, unless they are separated from the marine
environment by an impermeable layer, or unless the hydraulic gradient is
very high, coastal karst springs are generally brackish. The salinity generally
varies as an inverse function of the discharge, and is therefore highest
during the dry season; yet this is generally the period during which the
demand for these aquifers is the highest. The contamination mechanisms
are not yet entirely resolved, but they can be approached based on the
Ghyben-Herzberg law.
For submarine springs, their brackish water, being less dense than sea
water, migrates vertically from the exit point towards the surface, where it
forms a clearly visible ring during calm weather. The water then spreads
out across the surface of the sea.
Near the springs, the mixing of waters, due to common ion effects,
makes the sea water aggressive and enables it to dissolve limestone. A
wave-cut notch or mushroom-shaped rocks are usually visible near such
outlets.