Apport des outils hydrogéochimiques et isotopiques à la gestion des nappes du Haut et du Moyen Chéliff.

Abstract

The Upper and Middle Cheliff region, which is part of the large Cheliff basin, is located in the north-west of Algeria. In recent years, there has been an economic boom, particularly in agriculture, which has led, on the one hand, to an increase in water needs and, on the other hand, to the deterioration in the groundwater quality. The plains of the Upper and Middle Cheliff cover the area of 1070 km , situtated in a synclinal basin with a Mio-Plio-Quaternary filling, where the Mio-Plio-Quaternary aquifer is the main source of drinking water supply and agricultural irrigation. Groundwater geochemistry and stable isotopes (2H) were used to explain chemical heterogeneity, recharge regimes and hydraulic connections between alluvial and surface water in the Upper and Middle Cheliff plains. We have been able to study the groundwater geochemistry based on analysis of chemical elements (cations, anions and dry residue) in high and low water that have been carried out in the laboratory of ANRH and CRNA, during 2012 and 2017 periods. A total of 39 well and piezometric water samples were collected during two campaigns, low water 2017 and high water 2018, for the purpose of analyzing stable isotopes (182O, H). The processing and representation of the data made use of modern tools such as: Cartography, Piper and Schoëller-Berkaloff diagrams, Stabler classification, binary diagrams, base exchange indices, saturation indices, statistical analysis (PCA) and geostatistics. The interpretation of these data revealed the following: - Groundwater is characterised by the presence of two important hydrochemical facies (calcium chloride and sodium chloride) according to the diagrams of Piper, Schoëller-Berkaloff and Stabler. It is explained by the presence of the the Mio-Plio-Quaternay alluvial formations and the gypsum marls. The chemical quality of water is fair to poor, with the presence of certain contaminants resulting from nitrates used in agricultural activities and the effect of urban discharges of anthropogenic origin. Very poor near the Temoulga massif, resulting from high salinity influenced by the lithology of the water table and its flow through the Jurassic limestone formations and deep gypso-saliferous layers (the release of the fault in 1980 caused these waters to overflow) and climatic factors (evaporation and rainfall). The Br/Clratio gives indications on the origin of the salinity that the high salinity of the waters, linked essentially to chlorides, is due to the leaching of chlorinated fertilisers, salts not used by plants (particularly chlorides) and by the dissolution of evaporitic deposits. - The chemical quality of the water is from fair to poor with the presence of certain contaminants resulting from nitrates used in agricultural and anthropogenic activities originating from urban discharge, and it is very bad near the Temoulga massif, resulting from a high salinity influenced by the lithology of the aquifer and its flow through the Jurassic limestone formations and deep gypso-salt layers (the fault throw in 1980 caused these waters to overflow) and climatic factors (evaporation and precipitation). The Br/Cl ratio gives indications on the origin of the salinity that the high water salinity which is mainly linked to chlorides is due to the leaching of chlorinated fertilizers, salts not used by plants (especially chlorides) and by the dissolution of evaporite deposits. - In the diagram δ182O, O) the analytical data distributed along a line of slope equal to 4.73 and 4.38 during the two periods, low water 2017 and high water 2018, respectively. The use of the isotopic tool has identified that the origin of the recharge of the Upper and Middle Cheliff aquifers are the Atlantic and Mediterranean oceanic meteoric rain water at high altitudes.