Geochemistry and hydrogeochemical process of groundwater in the Souf valley of Low Septentrional Sahara, Algeria

Abstract

Ground water is the only hydrous resource exploited in the Souf valley of Algerian Sahara. Several techniques and conventional graphical plots were used to define the geochemistry of the aquifer. Based on the ionic components, hydrochemical facies, the factors controlling groundwater chemistry are assessed and defined. Ground waters are highly saline (total dissolved solids (TDS being 2361-14780 mg/L) and show high contents of major ions that extensively exceed the admitted norms of portability. The relative abundance of major ions was Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ for cations and Cl^- > SO₄ ^2- > HCO₃^- > NO₃^- for anions. Two main waters types are identified; one is sodic facies rich in magnesium, the second is mixed facies. Their spatial distribution is strongly influenced by different parameters linked to the aquifer system in the study area (lithology, structure, conditions and underground circulation directions). Dissolution of gypsum, salts, weathering of carbonate and silicate, ion exchange, mixing and nitrification of the organic matter are the main hydrogeochemical processes of waters salinity. The conjugated action of the different mineralisation processes as well as their combinations had accentuated the salinity of these waters. Their identification allowed a better interrelationship between the different present ions. Indeed, the spatial chemical facies distribution observed is closely linked to the lithologic nature and to the geometry of the aquifer. In this region deprived of treatment means and softening, when choosing captive fields and implantation of drilling of potable water supply, these results would guide decision makers in selecting the right sectors containing less ionised groundwater.

Key words: Groundwater resource, hydrogeochemistry, mineralisation processes, Souf valley, Algerian Sahara.