Hydrogéochimie du Fluor dans les Eaux Souterraines Algériennes du Sahara Septentrional Oriental. Cas des Nappes du Complexe Terminal

Abstract In the Northern Algerian Sahara region, inhabitants of the Eastern one consume groundwater from the terminal complex whose fluoride concentration exceeds the standard 1,5 mg/l. Geology, Hydrogeology, and the Hydrodynamics of the water in these aquifers have been the subject of many studies. Our research concerns notably the fluoride Hydrogéochemistry in the Eastern groundwater of Northern Sahara having in mind the aim at mapping the fluoride’s groundwater as well as understanding the origin and transportation conditions of fluoride in water. The study region belongs to the sector Northern Sahara under eastern one, including the regions of Ouargla, Ghardaïa, Touggourt, El Oued, Biskra and Hassi Messaoud. The climate of the region is hyper-arid. The terminal complex groundwater belongs to the “post-Paleozoic” in under basin oriental sedimentary “Mya”, of the Melrghir basin. Water converges towards the area of “Chotts” where the higher ages (> 40000 years). This confirms that the water of the chotts area is the most concentrated also by fluoride. Less deep water in the region of Touggourt and Ouargla is more concentrated in fluoride which implies an influence of climate on the content of salt in groundwater in this region. Evaporation is another important factor leading to the emergence of high fluoride in groundwater levels. The fluoride levels are higher in deep water, in the area to sodium facies (upstream of the sense of flow) including the Senonien, the Pontien and the Eocene. Water with the calcium facies is less concentrated in fluoride when it is under saturated in Fluorite (Miopliocène in the regions of Ouargla and Biskra). Thermodynamic testing showed that the fluoride concentrations depended on the State of fluorite saturation, the development activities of items in the balance, depends directly on the balance of water with the calcite, gypsum and anhydrite, as well as the formation of calcium complexes. The presence of the gypsum in aquifers of the study region hampers fluoride between 2.3 and 3 mg/l levels increased according to the area and considered aquifer. The conveyed Fluoride in the form of complex MgF+ exists in significant quantities in groundwater in the study region