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2.3.2 Geothermal hypothesis
The deep contamination of Florida's aquifer by marine convection currents,
induced by geothermal fl ux, was put forward by Henry and Kohout (1972),
to explain the salinity of Florida's coastal karsts, but this hypothesis is
diffi cult to extrapolate to shallow systems.
2.3.3 Inherited drain hypothesis
The presence of very deep drains, inherited from a Messinian paleogeography
or from tectonic movement, is a reality, as some of them have been partially
explored by speleologists. This allows the consideration of the behavior of
brackish springs by defi ning a simple model with 3 connected vases, fi lled;
respectively, with sea water, fresh water, and brackish water (Lismonde,
2001). At equilibrium:
(H+ΔH 1 1 = (H+ΔH 2 2 = (H+ΔH 3 3
where H is the depth of the drain relative to average sea level, ΔH 1 is the
hydraulic head of the karst, ρ 1 is the density of fresh water, ΔH 2 is the
elevation of the brackish spring, ρ 2 is the density of the brackish water,
ΔH 3 is the variation in the elevation of the sea, ρ 3 is the density of brackish
water.
This model easily explains the presence of brackish springs above sea-
level (Figure 38).
The term ΔH 3 corresponds to variations in sea level, or to eventual
depressions, in the case of the submarine estavelles of Bali (Crete), or of the
sea-mills of Argostoli (ΔH 3 is then negative). It is therefore most commonly
zero, if tides are neglected. Where ρ 1 = 1, for each system the result is:
H . ρ 3 = H+ΔH 1 = (H+ΔH 2 2
so ΔH 2 = H (ρ 3 - ρ 2 )/ρ 2 and since ρ 3 is always greater than ρ 2 , ΔH 2 is always
positive.
Taking the example of drainages at a depth of 100 m and salinity at the
spring of 5 g·L -1 (so a density of 1.004), one obtains a theoretical elevation
around 2.5 m. Springs of the same salinity located at 27 m (Slanac) can
therefore be explained by drains at a depth of approximately 1000 m,
which would fi t with the Messinian hypothesis and the thickness of peri-
Mediterranean limestones.
It is certain that if such deep circulations are being considered, it
also becomes necessary to take the geothermal gradient and water's
temperature-related density variations into account.
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