The local authority responsible for managing the Crau aquifer resource (SYMCRAU), asked BRGM to investigate the phenomena that determine the spatial and temporal evolution of the salt-water wedge in this aquifer, namely the origin of salinity in the lower Crau aquifer, the current state of salinisation of the aquifer, and determining factors. The objective of the SIMBA project was to identify the most vulnerable areas and to improve resource management by optimising the monitoring of salinity as it evolves, through methodological developments adapted to the hydrogeological and hydraulic context.
A first monitoring network has been set up as part of the project. The data acquired within this observatory were coupled with historical data to define a methodology based on the cross-referencing of spatial and temporal changes in salinity with those of piezometric measurements in the aquifer, on different time scales. The method is based on the concept of hydrostatic equilibrium between two fluids of different densities, fresh water and salt water, and also draws on the concept of dispersive vertical transport to describe the interface by a transition zone, the thickness of which varies with its displacement. The thickness of the interface is calculated from the characteristics of the salinity profiles recorded by the piezometers for different hydrological conditions. Geochemical and isotope analyses were also used to determine where the salt water observed in the aquifer came from, with the aim of identifying possible rapid salt transfers through the darses (docking basins) and the canal from Arles to Fos-sur-Mer and interactions with surface hydrosystems.
The main results obtained have provided several keys for understanding the salt-water wedge phenomenon:
- the salinity observed at the bottom of the various piezometer boreholes studied is caused by mixing with seawater that is at least several decades old at the points studied;
- the geometry of the substratum explains the spatial variability of the salt-water interface that was observed: the slighter the slope of the bedrock towards the sea, the greater the salt intrusion;
- the movement of the salt-water front is much more sensitive to a change in the freshwater charge in the aquifer than to a change in the salt water charge.
Using the results
The SIMBA project has mapped the main results relating to the seasonal evolution of the salt-water wedge, highlighting the locations most vulnerable to salt intrusion. The map shows the isosalinity 20 contour line for different piezometric campaigns, which can be used to assess the influence of hydrological conditions on the displacement of the salt-water wedge according to geometric characteristics (slope of the substratum). The proposed method, which is simple to implement, can be easily updated by the managers of the aquifer if necessary on the basis of piezometric information. The implementation of operational monitoring (water level, conductivity, temperature) over the long term was also proposed to the Crau aquifer manager, for the most vulnerable locations in the aquifer.
- Contracting authority: SYMCRAU
- Financial partners: Rhône-Mediterranean-Corsica Water Agency, (southern PACA region)
This project has increased our understanding of the vulnerability of the aquifer to salt-water intrusion that threatens the quality of freshwater abstracted, particularly for drinking water needs. A salinity monitoring heritage network has been commissioned and the operational tools that have been developed link up with the other major projects conducted by SYMCRAU. The results obtained by BRGM are indispensable decision-making aids for the elected representatives of the region.