Geochemical and isotopic study of groundwaters in the Mitidja coastal plain, north‑central Algeria

The deterioration of the environment and more specifically the aquatic medium has nowadays become progressively an issue of great concern worldwide. In the current context of climate change and demographic / economic growth, Algeria is subjected to an increasing stress on water. Due to its relative poorness in surface water harvesting capabilities, the relatively easier exploitation of aquifers became the preferred source for water allocation both to population and to agriculture/industry in Algeria. Mitidja plain that is located south of the capital city Algiers, is the country’s most expanded sublittoral Mediterranean plain.It occurs in the central-north of Algeria, it is oriented East-West and extends over a length of 80 km and a width of 10-20 km. It comprises aquifers that are the main source of drinking water for the whole north-central part of the country. The intensification of industrial and agricultural activities combined with demographic growth and urbanization of the plain, resulted in a clear increase in demand for water accompanied by significant problems of degradation of the groundwater quality. The major ion hydrochemistry and environmental isotope composition (¹⁸O, ²H,³H) of Mitidja alluvial groundwaters were investigated to identify the sources and the processes that affect groundwater composition, water origin, and timing of recharge.

The interpretation of the hydro-geochemical data suggests that groundwater composition is largely controlled by the water-rock interactions, particularly the dissolution of evaporate minerals (Halite,Gypsum/Anhydrite) and ion exchange processes. Elevated content of nitrates indicates that agricultural activities are probably the most significant anthropogenic sources of nitrogen contamination.

The concentrations of Fe, Mn, Pb, and Cd in most samples were found higher than the prescribed limits defined by the World Health Organization (WHO). The application of the Principal Component Analysis (PCA) for trace metals identified two sources of pollution- natural and anthropogenic sources. Information inferred from Oxygen-18 and deuterium ratios reflected the existence of recharge through non-evaporated modern rainfall mostly originating from Mediterranean air masses. Measurable tritium concentrations in groundwaters allowed qualitative identification of the present-day component. It is confirmed thus that this component of the alluvial aquifer is supplied by recent rainfall taking place during the rainy season.