Origin of Groundwater Salinization in the Osis of Béni Abbès (Saoura Valley, South-West Algeria)
Touhami Merzougui1, Cherif Rezzoug1*, Mekkaoui Abderahmane1
1Faculty of Technology, University of Bechar, Hay Elbadr, Bechar, Algeria.
Abstract
The aquifer of the alluvial terraces of the Béni Abbès palm grove is subject to a hyperarid climate. The groundwater salinity problem is caused by several human and natural factors, causing serious irrigation problems. Hydrochemical analyses over a 40-year interval show an alarming Spatio-temporal evolution of salinity: the drying up of the Saoura valley due, on the one hand, to the impact of the construction of the Djorf Torba dam, on the other hand to the strong evaporation which reigns, have seriously affected the quality of the waters of the infero-flow. In addition to the diffusion of salinity through the natural drain, the overexploitation of water from the terraces of the Saoura valley and the anarchic proliferation of wells in search of fresh water, intended for the irrigation of the palm grove, lead to a spectacular acceleration of this destructive phenomenon of the entire hydrogeological system of this region. In this work, old and new analysis results are used to make a comparison on the evolution of the aquifer. Such a comparative analysis has the advantage of making it possible to assess certain influences subject to severe natural and anthropogenic constraints on the acquisition of the chemical characteristics of water.
Keywords: Salinity, Hydrochemistry, Saoura, Inferno-flow, Sahara, Arid
Introduction
Groundwater aquifers in arid or hyperarid zones have classic hydrogeological and hydrogeochemical characteristics, their recharge and exploitation conditions induce particular problems that must be taken into account during their characterization. The Béni Abbès palm grove extends on the left bank of the Saoura valley, 6 km long and 5 km wide on average covering an area of 40 hectares. It draws water from the large spring, the foggaras and the wells dug in the alluvial and lower-flow terraces of the Saoura wadi. However, we note that the phenomenon of groundwater salinity is becoming more and more threatening. Indeed the problem caused by several human and natural factors generates serious problems with irrigation. This palm grove has lost 60% of its area and about three tiaras of its palm trees. The “Bayoudh” disease, socio-economic transformations, and sometimes mediocre decisions have contributed to this deterioration of the palm grove. This region has known for about thirty years an important economic boom with great development, especially in agriculture associated with a strong demographic expansion. Consequently, the groundwater in the region is increasingly sought by the inhabitants of the Oases where there is a deterioration in the quality of the water in the irrigated perimeters with the presence of high salinity which poses the problem of its use as drinking water and irrigation.
Our objective is to contribute to understanding the process and origin of groundwater salinization in these areas based on the spatiotemporal evolution of major and minor chemical elements.
Geographic location
The oasis of Béni Abbès appears as the capital of the Oasis of the Saoura. It is located about 240 km south-west of Bechar and 880 km south-west of Algiers (Figure 1). The Saoura valley resulting from the junction of the Guir and Zouzfana wadis, represents the main watercourse of the North Western Sahara of Algeria. It is bordered to the east by the Grand Erg Occidental, and to the west by the Hamada du Guir and the Petite Hamada. At the level of the lower Saoura, it touches the mountains of Ougarta (Conrad & Roche, 1965).
The alluvial aquifer of the palm grove extends over an area of about 40 ha. The arid climate of the region is characterized by an average annual rainfall of between 30 and 40 mm (Scholler, 1945). The summer is hot and dry, and the winter cold and dry. Annual evaporation is high (around 2012 mm/year).
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Figure 1. carte de localisation de Béni Abbès and the situation of the Béni Abbès palm grove (Roche, 1973, Modified). |
Hydrogeological characterization of the palm grove aquifer
Condition of deposit and supply
The aquifer system features two lithological units: the limestones and sandy marls of the Mio-Pliocene and the Quaternary formations. The reservoir is made up of Quaternary deposits made up of three layers (sand, clay, sand, conglomerate) with an average thickness of fifteen meters, the whole resting on a folded Upper Devonian schistose bedrock (Figure 2). The average thickness of the alluvial aquifer varies between 10 to 30 m (Tezangi, 2016).
The recharge of the alluvial aquifer of the palm grove is ensured by the infiltration of water from the large spring (flow 28l/s); by the lateral supply of water from the erg aquifer of a natural drain (Roche, 1973) and occasionally by floods from the Saoura wadi.
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