Port-Miou is the submarine spring of a vast karstic hydrosystem in the lower limestone Provence, characteristic of the peri-Mediterranean karsts.
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Coordination : Hervé Jourde / Naomi Mazzilli
Data Base : Juliette Fabre
The submarines springs of Port Miou and Bestouan form a continuous flow of water from the continent that discharges into the sea. This underground water flows in karstic conduits of metric to decametric dimensions that were formed in the subsoil of the calanques from Marseille to Cassis, in lower limestone Provence. This hydrosystem of Port-Miou, represented by the emblematic spring of Port-Miou, goes far beyond the coastal zone and extends over a large part of the geological units of Beausset-Calanques and Sainte Baume (catchment area >400km²), up to more than 1000 meters of altitude. At the outlet, the groundwater has a variable salinity over time depending on the flow, acquired at great depth below sea level, at more than 235 meters deep, several kilometers inland. Upstream, it is a strategic water resource for the region, close to the areas of need of the metropolises, and sheltered from the natural hills of the Sainte-Baume Natural Regional Park and the Calanques National Park.
This hydrosystem is also a real open-air laboratory for the study of the water cycle in the Mediterranean region, in the immediate vicinity of the University of Aix-Marseille. The coastal karst springs are particular objects of study, characterized by a brackish water with variable salinity acting as a natural marker which allows to study the hydrodynamic functioning of the karst and the mechanism of saline intrusion. The long-term evolution of groundwater flow and salinity is directly related to global changes (climate, eustatic variations). Coastal springs are the result of flow and transport processes on the mainland, in the spring’s recharge watershed, and interactions with the sea, which enters the land at depth and receives this fresh or brackish water input in the nearshore zone. The Port-Miou hydrosystem thus contributes to the following objectives:
- monitoring of a network of saline intrusion in the regional aquifer and local eustatic variations ;
- study of the saline intrusion mechanism in heterogeneous coastal aquifers and consequences on water resources ;
- quantitative and qualitative assessment of groundwater inputs to the sea: flows, biodiversity ;
- methodological development for the hydrodynamic study of heterogeneous aquifers, modeling and geochemistry ;
- study of karstification in carbonate reservoirs and its impact on groundwater flow dynamics and storage ;
- study of the relationship between rainfall, land use and floods.
To learn more : www.karsteau.fr
A privileged experimental site
The Port Miou observation site is unique for the study of peri-Mediterranean karsts. It is spread over the Port-Miou watershed, overlapping the Huveaune watershed, and the surrounding watersheds (Le Las, La Reppe, Le Gapeau, Le Cauron, L’Issole). The Sainte-Baume mountain range forms a central culminating point, which receives a large part of the rainwater. Two isotope monitoring stations are deployed in the north and south of the massif. Inland, the physico-chemical evolution of various springs is monitored, as well as the main river Huveaune. At the outlet, the underwater spring of Port-Miou was developed in the 1970s, and is now an experimental laboratory. It is an artificial tunnel that allows access to the underground karstic gallery drowned 500 meters from the sea inland. Measurements are carried out on water (CTD, fluorescence, water isotopes, major ions) and on reservoir rocks (morphological evolution of the karst, 3D structures, geochemistry) at the scale of the regional hydrosystem.
The underground dam of the underground river and the artificial gallery form an exceptional site, with many assets :
- easy access for the installation of high performance measurement equipment in hydrogeology, hydrochemistry, hydrobiology ;
- a monitoring point 500 meters upstream from the source at sea to limit the disturbances linked to the direct intrusion of sea water through the conduit;
- the only easily accessible point known to observe the groundwater without being a diver;
- a unique site of experimentation and observation in the world with this underground dam (there are other sites in the world in coastal areas but they do not have this underground dam).
The results obtained are exportable to continental karsts and to karsts around the Mediterranean.
Long term observations
The objective of an observatory site is to acquire data, repeated over a long period of time. The observation is spread over three study compartments of the critical zone: the atmosphere through measurements on precipitation water, rivers and groundwater in the basement.
In the watershed:
- The isotopic signal of the rain is monitored by the CEREGE, coupled with the SNO RENOIR (National Network for the Observation of Isotopes in Rainfall).
- State services monitor the hydrometric stations of the Huveaune and the Saint-Pons spring.
- Water quality monitoring is also carried out with the help of the Syndicat du Bassin Versant de l’Huveaune.
And at the outlet, on the Port-Miou underground site:
- Groundwater in the coastal zone is monitored by water level, salinity, temperature, atmospheric pressure, fluorescence and water geochemistry measurements, monitored by the CEREGE laboratory (OSU PYTHEAS).