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Mechanism of salt contamination of karstic springs related to the Messinian deep stage. The speleological model of Port Miou (France)

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Mechanism of salt contamination of karstic springs related to the Messinian deep stage.
The speleological model of Port Miou (France).
by Eric Gilli1 & Thomas Cavalera2
1University Paris 8 and UMR Espace 6012 Nice.
2University of Provence
Submarine karst springs are common on the Mediterranean shore but most of them are brackish, which limits
their usefulness. Various experiments to limit freshwater/seawater mixing were attempted in the past but have failed.
This study examined Port Miou (Cassis, France) showing that salinity is present at 2300 m from the entrance and at a
depth of 179 m bsl. The geometry of this setting is inherited from a complex palaeogeography. The lowering of the
Mediterranean during the Messinian salinity crisis has made possible the existence of caves several hundred meters
below present sea level. The presence of titanium in the sediment of the cave seems to be a residual product of an
alumina factory that is discharged at a depth of 300 m, 6 km south of the cave. This supports the hypothesis of a deep
aspiration of seawater by a Messinian gallery. A similar example exists in Kefalonia Island, where a marine intrusion is
observed in coastal sinkholes.
1. Introduction
Submarine karst springs are common on the Mediterranean shore but most of them are brackish, which limits
their usefulness. For the southeast coastline of France, these aquifers are estimated at 10 m3/s. For the entire
Mediterranean basin, the potential is approximately 1000 m3/s which theoretically supports the domestic needs of 350
million persons. Numerous attempts to catch these springs (dams, flexible or rigid artificial insulations, pumping, etc.)
ended in failure (Gilli, 2003). The principle was generally to artificially augment the hydraulic gradient in order to
lower the contact between fresh water and sea water. Recent explorations of deep submarine caves explain why these
attempts failed.
Cave divers or ROV have reached important depths in several places : 308 m (224 m b.s.l.) in the Fontaine de
Vaucluse (France), 179 m (179 m b.s.l.) in the Port Miou spring (France). Similar examples exist also in Greece, Italy,
Spain and Turkey. These depths are more important than the lowering of the water table related to the Quaternary
glacio-eustatism (120 m b.s.l.) and several authors imagine a possible settlement of the karstic systems during the
Messinian stage [-5.95 to –5.35 Ma] when a large drop in sea-level, down to 1500 m, occurred in the Mediterranean
Sea (Ryan et al, 1973 ; Gautier et al, 1994 ; Krijgsman et al, 1999). It caused the sedimentation of important evaporitic
levels on the bottom of the sea, and the presence of deep submarine canyons on the margins of the basin. Deep canyons
were also formed inland that are now filled with alluviums. This drastic drop in base level affected the local karst
systems and caused the deepening of the water circulations and the possible reopening of palaeokarsts. Several authors
pointed out the influence of the Messinian event on the French karst (Julian and Nicod, 1984 ; Audra et al, 2004) and it
has been widely recognized in Italy (Bini, 1994), in Greece (Arfib, 2001) and in Spain (Fleury, 2005). Recent works on
karstic springs suggest the possibility of a deep salt contamination of the main Mediterranean brackish springs, due to
the presence of deep karstic galleries related to that Messinian model (Gilli, 2001). The current study of the shoreline
aquifers of southeastern France has enabled us to propose an operating model.
2 . The Port Miou system
In Cassis ( South-eastern France) the springs of Port-Miou and Bestouan are the outlets of an important system
of submarine karstic galleries explored since the 50's (table 1). The average discharge is between 2 to 5 m3s-1 but the
water is brackish and cannot be used for water supply. The Port Miou cave is a 2.4 kilometers long gallery that extends
in the limestone series of Calanques. The total length of Bestouan is 3.7 kilometers. Dye tests and monitoring have
proved the relation between the two caves but no connecting passage has been discovered. At Port Miou a dam was
built in the 70's inside the cave, to prevent the marine intrusion, but in spite of a noticeable decrease of the salinity
down to 3 gL-1, it had not been possible to obtain drinkable fresh water upstream of the dam (Potié, 1974) (fig. 1). The
use of helium and later rebreathers by cave divers made possible the exploration of a vertical pit, down to 179 m below
the sea level, at the end of the cave. At that depth, the water is still brackish.
Table 1 : History of explorations and studies in Port Miou and Bestouan (Cassis, France)
Figure 1 : Cross section of the Port Miou gallery.
Discharge, temperature and salinity of the spring have been collected for several months in the 70's, the
discharge varied from 2 m3s-1 to 100 m3s-1 and the salinity was from 20 gL-1 during low water to 0 gL-1during the
floods. New data is recorded since December 2004 with an average discharge of 3 m3s-1 and salinity between 14 and
3.4 gL-1. The average values indicates a medium flow of seawater close to 1 m3s-1 but the mechanism of the
contamination is still unknown. Two hypotheses are possible that offer two different possibilities to reduce the salinity
- a classical hypothesis where the fresh water circulates in a deep karstic gallery connected to a matricial
fissured zone polluted by sea water. The diminution of the salinity is possible by augmenting the pressure of fresh
water in the conduit (Arfib, 2001). In Port Miou the study of the discharge/salinity graphs did not make it possible to
see such a mechanism (Arfib et al, 2006 ; Cavalera, 2007).
- a speleological hypothesis where the fresh water circulates in a deep karstic gallery connected to the sea by
another gallery (Gilli et al, 2004). This suggests that the permeability of the limestone is low.
In the area of Port Miou, this second hypothesis is supported by the existence of karst features below current sea
level. The bathymetric map of Lion gulf (Berne et al, 2002) and a morpho-bathymetric study (Collina-Girard, 1996)
reveal the existence of a limestone plateau that extends a few kilometers south to the Calanques, with dolines at a depth
1953 1s
true exploration in the Port Miou cave by the EOLE team (J. Blanc, J. Picard, et M. Galerne)
Exploration and study of submarine galleries by O.F.R.S. (Cdt Cousteau) : the team reach 280 m in
Port Miou and 40 m in Bestouan. Topographic, thermographic and faunistic data is collected
(CORROY et al, 1958)
1960 A US scientific diver (C. Limbaugh) get lost and die during a photography session in Port Miou
1964-1966 Divers of G.E.P.S. (Claude Touloumdjian) reach 400 m in Port Miou et 450 in the Bestouan.
1968 Discovery in Port Miou of an aerial zone at 530 m from the entrance et exploration of the gallery up to
870 m.
1968-1973 Study and construction of an underground dam by the SRPM. (POTIÉ,1974)
1978 C. Touloumdjian reaches 1365 m in Port Miou and Francis Leguen 1400 m in Bestouan
1981 Discovery in Port Miou of a vertical shaft at 2230 m from the entrance by B.Léger. The terminal depth
is -82 m bsl
1982 Two Italian and Swiss divers get lost and die in Port Miou.
1980-1983 The divers C. Touloumdjian and F. Leguen reach 2290 m in Bestouan
1990-1993 Series of explorations by C.R.P.S. (Comité Régional de Plongée Souterraine) during which the divers
reach 3000 m in Bestouan. In Port Miou M.Douchet goes down to -147 m bsl (DOUCHET,1993).
2001 Starting of new geological studies (GILLI, 2001)
2005 J Meynié, using rebreather, reaches -172 m bsl in Port Miou.
2008 X. Meniscu, using rebreather, reaches -179 bsl in Port Miou
of 150 m below present sea level. A deep submarine valley, the canyon of Cassidaigne, whose bottom is at a depth of
1000 m, cuts this plateau. This canyon looks like a karst pocket valley and is not connected to a continental valley.
Several dives with small submarines have revealed on the walls the existence of caves with speleothems (oral
communication from COMEX). We assume that, during the Messinian deep stage, the underground river of Port-Miou
was flowing 200 or 300 hundred meters below its current position and has excavated the canyon of Cassidaigne. At the
end of the Messinian deep stage, the system was breached by the sea, causing the fresh water to flow through an upper
gallery. Now the presence of a deep paleo-drain, filled by seawater, could provoke a saltwater intrusion into the karst
3. Three pipes model
A three pipes model can be used to understand the mechanism in static conditions (Drogue,1993) (fig.2, left
panel). At equilibrium :
(H+H1)ρ1 = (H+H2)ρ2 = (H+H3)ρ3
With H depth of mixing zone, H1 karst hydraulic gradient, ρ1 density of fresh water,
H2 altitude of brackish spring, ρ2 density of brackish water,
H3 variation of sea level, ρ3 density of sea water.
We have developed a laboratory model of the system, with 3 pipes respectively filled with colored sea water,
fresh water and brackish water (fig. 2-right). It perfectly simulates the mechanism. When fresh water is injected in the
fresh water pipe (H1 augments), a current moves towards the brackish outlet where the density is lower than in the sea
pipe, ρ2 decreases by dilution and this creates a dilution and an aspiration (negative H3) in the seawater pipe.
Figure 2 : Three pipes model.
In Port Miou, the presence of an anthropic tracer supports this affirmation. In the area of Gardanne a factory of
alumina generates important quantities of residual products locally called "red mud". The red mud contains an
important quantity of heavy metals notably titanium and chromium. This mud is mixed with water and is transported
since the 70's by a submarine pipe towards the canyon of Cassidaigne where it is discharged into the sea, at a depth of
300 m (fig. 3).
Figure 3 : The speleological model of Port Miou (Cassis, France).
Several samples of sediments (surface and core samples) were collected in the Port Miou gallery upstream and
downstream of the dam. Most of samples upstream the dam present a thin level of red mud laying on a thick level of
gray to brown marine silt. The upper layer contains a much more important quantity of titanium than samples collected
in this area (table.2). When analyzing the cave sediment a few centimeters below its surface the concentration of
titanium is much lower. Downstream of the dam the concentration is also lower. This shows that red particles, rich in
titanium, have been recently transported by a current of brackish water, from the bottom of the cave to the entrance.
This supports an aspiration of sea water with particles of red mud by a deep gallery connected to the canyon of
Cassidaigne. Other possible explanations have been unsuccessfully studied. The factory, where the alumina is
processed, is on an impervious substratum. The inland wastes have low concentration of titanium. There are no leaks
on the pipeline. Environmental studies (Arnoux and Stora, 2003) show that the red mud, discharged in the canyon,
remains at an important depth and is never present close to the spring in the continental platform. One explanation
could be an underground sea current in a karstic conduit that transports the particles of red mud towards the north. New
analysis is underway to confirm it.
Sample Cr Cu Ti V Fe Mn Ni Pb Zn
"Red muds" (in factory) 1815 23,4 32 415 717 184 500 351 n.d. 94,5 38,3
Marine sediment with red muds in
Cassidaigne canyon 64 31.5 168 37,4 26 003 1113 n.d. 25,4 81
PM100705 surface sediment of Port Miou gallery 36.6 12,9 1992 71,3 21 101 888,65 36,3 22,6 197,2
PM100705_1 surface sediment of Port Miou gallery 1600
PM 191105_1 surface sediment of Port Miou gallery 400
PM 191105_2 surface sediment of Port Miou gallery 640
PM160405 sediment of Port Miou : 10 cm depth 0.48
PM160405 sediment of Port Miou : 40 cm depth 0.26
PM160405 sediment of Port Miou : 80 cm depth 0.22
BE140606 sediment of Bestouan gallery : surface 40 n.d. 680 0,07 27 1500 40 20 160
BE140606 sediment of Bestouan : 5 cm depth 360
BE140606 sediment of Bestouan : 15 cm depth 260
BE140606 sediment of Bestouan : 30 cm depth 220
BE140606 sediment of Bestouan : 38 cm depth 430
Continental karstic sediment (terra rosa) 610
Roucas Blanc (sediment of a brackish karstic spring) <10 <10 20 10 1100 -. -. <10 -.
Continental waste of La Barasse (ground) 490 20 17 0,6 275 000 -. - 370 -
Continental waste of La Barasse (leachate) 60 30 1,1 0,13 38 000 - - 60 -
Table 2 : Concentration of heavy metals in Port Miou and the area of Cassis (France).
4. The Argostoli system
Examples of marine aspiration are already known in Mediterranean sea at Argostoli (Drogue, 1989, 1993 ;
Maurin and Zötl, 1965) and Bali (Greece) (Arfib, 2001) or Moraig-Toix (Spain) (Cortes et al, 2000). Near the small
city of Argostoli, on the western coast of Kefalonia island, a sea intrusion is observed in coastal sinkholes. The flow
rate of the seawater is sufficient to power mills (fig.4). A dye test conducted in 1963 showed a communication, after 16
days, with the brackish springs of Sami on the eastern coast of the island (Maurin and Zötl, 1965). In the sinkholes, the
level of the seawater sumps is 1 m bsl. In the Sami springs it is 1 m asl.
Figure 4 : Argostoli mill in Kefalonia (Greece).
5. Conclusion
This speleological model can probably be extended to other karstic springs in the Mediterranean sea. It opens
two main directions for the catchment of water : drilling inland, far enough from the coast, to reach the aquifer
upstream of the mixing with sea water, or obstructing the deep gallery to prevent the seawater intrusion. A comparison
with the Floridian karst, where geothermal conditions are evoked (Kohout et al, 1977) to explain the presence of deep
karstic galleries should be very interesting to do.
Acknowledgement : This study was realized with the help of the Conservatoire du littoral (Mr Estève), the ONF (Mr
Vincent), the Société des Eaux de Marseille( MM. D'Aspe, Onatsky and Lieutaud ), the Alcan-Rio Tinto society (Mrs
Raignault), the Camargo fundation (Mr Dautricourt). Monitoring and sampling was done by the divers : C.
Touloumdjian, M. Douchet, J. Meynié, F. Tessier and by our colleagues B. Arfib, D. Chevaldonné.
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Experiment Findings
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In the Calanques of Cassis, close to Marseille (southeast France) the submarine springs of Port Miou and Bestouan are major karstic features. They are explored since more than one century, and important works were realised in the years seventies to attempt to catch the water. The study of ancient waterflow measurements in the submarine spring of Port Miou tends to indicate an average discharge of 7 m3/s. This value would implicate the existence of a large impluvium that includes most of the Basse Provence area. The classical basin in the urgonian limestone of Bassin du Beausset unit and the limestones of Sainte Baume mountain is not large enough to explain this discharge. A study of small basins in this area indicates that the infiltration modulus is close to 8 l/s/km 2. Part of the infiltrated water already feeds several karstic springs but the total value is not sufficient and reveals a water deficit close to 4 to 5 l/s/km 2 that could explain the discharge of the Cassis springs.The surface of the basin should be more than 1000 km 2 , including limestone areas towards north, up to the Sainte Victoire Mountain. The Cassis springs could be the equivalent of Vaucluse spring and be inherited from the messinian period.
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It is difficult to explain the position and behaviour of the main karst springs of southern France without calling on a drop in the water table below those encountered at the lowest levels of Pleistocene glacio-eustatic fluctuations. The principal karst features around the Mediterranean are probably inherited from the Messinian period (“Salinity crisis”) when sea level dropped dramatically due to the closing of the Straight of Gibraltar and desiccation of the Mediterranean Sea. Important deep karst systems were formed because the regional ground water dropped and the main valleys were entrenched as canyons. Sea level rise during the Pliocene caused sedimentation in the Messinian canyons and water, under a low hydraulic head, entered the upper cave levels.The powerful submarine spring of Port-Miou is located south of Marseille in a drowned canyon of the Calanques massif. The main water flow comes from a vertical shaft that extends to a depth of more than 147 m bsl. The close shelf margin comprises a submarine karst plateau cut by a deep canyon whose bottom reaches 1,000 m bsl. The canyon ends upstream in a pocket valley without relation to any important continental valley. This canyon was probably excavated by the underground paleoriver of Port-Miou during the Messinian Salinity Crisis. Currently, seawater mixes with karst water at depth. The crisis also affected inland karst aquifers. The famous spring of Fontaine de Vaucluse was explored by a ROV (remote observation vehicle) to a depth of 308 m, 224 m below current sea level. Flutes observed on the wall of the shaft indicate the spring was formerly an air-filled shaft connected to a deep underground river flowing towards a deep valley. Outcroppings and seismic data confirm the presence of deep paleo-valleys filled with Pliocene sediments in the current Rhône and Durance valleys. In the Ardèche, several vauclusian springs may also be related to the Messinian Rhône canyon, located at about 200 m below present sea level. A Pliocene base level rise resulted in horizontal dry cave levels. In the hinterland of Gulf of Lion, the Cévennes karst margin was drained toward the hydrologic window opened by the Messinian erosional surface on the continental shelf.
La théorie du Messinien prévoit que durant la fin du Miocène la mer Méditerranée s'est asséchée presque complètement, provoquant la formation d'épais dépôts d'évaporites (sel, gypse). L'abaissement général du niveau de base a provoqué une accentuation de l'érosion. Ainsi tous les fleuves tributaires de la Méditerranée ont formé de longs et profonds canyons (Nil, Rhône, Var, etc.). Un tel abaissement a également influencé la karstification comme nous le montrons pour le karst lombard autour des grands lacs sudalpins. Avant la théorie du Messinien, on pensait que le creusement des vallées des lacs était essentiellement lié aux glaciers quaternaires. Cette explication suppose que la distribution altitudinale des cavités dépend de l'abaissement du fond de la vallée après chaque glaciation. Or la sismique faite sur les lacs et les vallées a prouvé qu'il s'agissait de profonds canyons fluviatiles remblayés. En outre l'étude des grottes a montré que celles-ci sont plus anciennes que le modelé des versants et les glaciations. Les âges U/Th indiquent qu'une grande partie des concrétions sont plus anciennes que 350 000 ans (parfois > 1,5 millions d'années). Les premiers réseaux karstiques se forment dès l'émersion durant I'Oligo-Miocène. Durant le Messinien, l'abaissement du niveau de base provoque une réorganisation des réseaux karstiques qui s'approfondissent fortement. La transgression marine pliocène provoque une nouvelle révolution dans les réseaux car une partie des cavités se trouve sous le niveau de la mer. Ces phases ont été contrôlées par des climats chauds et humides. Les grandes galeries du karst lombard se placent dans ce contexte paléoclimatique.
This is a study of the particular problems of building of water storage facilities in karst terrain. The issues involved are watertightness, exploitation of the karst groundwaters, protection of them from pollution, protection from seawater intrusion in coastal areas, protection of mines from sudden inrushes of karst groundwaters and protection of building foundations. Many case studies are given of karst water storage projects.
SUMMARY In order to re-examine the validity of that knowledge of the mechanism of subter- ranean karst drainage that was gathered in Alpine territory, investigations were extended to the Mediterranean region. Thus a hydrogeological survey of the island of Kephallinia was made in 1959 and 1961. Kephallinia is known above all for its phenomenon of the sea-katavothres of Argostolion. The reappearance of the salt water that disappears into the rocks there had not been explained in spite of repeated experiments. In principle it was conceivable that the salt water entering the land there emerged again in submarine springs as well as in the brackish water springs of the coast. After careful preparation 160 kg of uranin were dissolved in the water of one of the katavothres. This colouring substance could be ascertained in the brackish water springs of the eastern coast, which is 15 km distant. The hydrogeological explanation of this movement of water is this. The polje that existed in the present Gulf of Livadi during the eustatic reduction of the sea level in the Pleistocene drained subterraneously towards the lower leve! of the eastern coast. The fresh water that even today drains off from the main massive to the east, and the old, lower-levelled karst holes that have been inundated by the sea, are the most important factors for the hydraulic processes, which resemble those that can be observed where an ejector is installed.
There is massive, continuous inflow of seawater at a place on the coast of the karstic island of Cephalonia in the Mediterranean. This current reappears at the coast on the other side of the island. this is extremely astonishing when it is considered that for water sea level represents minimum potential in the earth's gravitational field. The nature of the energy which causes the phenomenon has been the subject of numerous as yet unconfirmed hypotheses on the part of numerous authors. This study proposes a new explanation connected with the existence of a marine current which creates a hydraulic gradient between the two sides of the island but with inflow being maintained by density flow. This is made possible by a deep karstic conduit created during the Cenozoic period and which runs in the same direction as the marine current.