The northern Sahara contains vast transboundary aquifer system, including the Continental Intercalaire aquifer (CI) which is the main water reserve for those hyperarid regions. It is shared between Algeria, Libya and Tunisia.
The piezometric map shows that the main groundwater flow comes from the “Occidental basin” considered as the principal recharge area of CI aquifer. However, it is the less studied part of all the aquifer.
The hydrogeochemical data are really scare in the occidental basin even that some recent researches have demonstrated, using mathematical modelling of fluxes and satellite technics, that CI aquifer should receive more important recent recharge below dunes of occidental basin, where CI is unconfined, than it was expected in older studies.
Our study has identified processes that control hydrochemistry of CI groundwater and discussed information got from stable and radioactive isotopes in the northern part of the occidental basin (Ghardaia region and neighbouring cities).
We concluded that CI groundwater, there, behaves as two distinct groups, according to geographical localisation (north: Ghardaia region, and south: El Goléa region); the dissimilarities are much contrasted. Samples situated between 31.55° and 31.57°N of latitude represent a transition within the occidental basin. This finding corroborates previous observations from regional piezometry suggesting groundwater divide situated in this zone.
In north, samples are highly mineralized, they belong to SO4-Cl-Na type; in south, they are poorly mineralized and predominantly belong to HCO3-Ca type. Actually, in north, CI aquifer is clay and gypsum-enriched, however, toward south it is almost totally sandy.
Stable isotope data (δ18O and δ2H) shows that all samples are much depleted in both δ18O and δ2H comparing to the actual rainfall isotopic signature, but in south CI groundwater are more evaporated than in north. Such pattern suggests that CI groundwater has infiltrated under climate regime cooler than the current one. Moreover, southern samples seem to indicate a mixture with recent evaporated water.
36Cl cosmogenic isotope results show that 36Cl/Cl ratio in south region is higher than 116×10-15 at/at considered in literature as “the initial ratio”. This finding gives two indications:
• First, it confirms that CI groundwater in south is younger than it was expected previously.
• Secondly, the initial 36Cl/Cl ratio supposed in literature is less than its real value.
Later, we will use 14C and noble gases to reinforce our results and calibrate an initial 36Cl/Cl ratio more accurate with real data.
As perspective of this work, we suggest to implement a monitoring network of CI in the occidental basin in the aim of more sustainable management of this valuable resource.