Evolution de la qualité géochimique des eaux souterraines utilisées dans l’irrigation et leur impact sur la dégradation des sols de la plaine du Bas-Chéliff

Abstract

In the plain of Lower-Cheliff, a northwest Algerian region with a semi-arid climate and high rates of evapotranspiration, the recourse to irrigation for most of the crops is indispensible. For the last twenty years, the water shortage allocated to agriculture has constrained the farmers to privilege the use of the groundwater resource which is of poor quality. This recourse has been accentuated these years witnessing a deterioration of the irrigation supply network. The mapping of groundwater salinity in the plain of Lower-Cheliff has shown the advanced state of this resource vulnerability to salinity where more than 60% of the underground water resource has EC > 3dS/m. The use of indicator Kriging method has improved the mapping of groundwater salinity in the region with a better evaluation of the obtained surfaces by optimizing the estimates of the extreme values often male estimated by the ordinary Kriging. The assessment of the water groundwater quality of the Lower-Cheliff plain, compared to the hydro geochemical process which controls its ionic load and its aptitude of being used for agricultural needs showed that the most diluted water is likely to precipitate salts containing divalent elements (Ca and Mg) in the form of calcite, gypsum and dolomite. That will give more mobility to the sodium which is likely to be fixed on the exchange complex, causing thus, an impoverishment of the irrigated soil. The concept of generalized residual alkalinity, simplified, has confirmed these results. The water with residual alkalinity calcite, positive sepiolite, and gypsum are those provoking the degradation of the irrigated soil. The experimental scheme carried out through this study showed that the concentration of a water of RSC1>0 causes an increase in the SAR, at the same time in soils with balanced or clayey texture, whereas they do not show a risk of sodisation according to the traditional methods of classification such as that of Richards (1954). The concept of residual alkalinity constitutes an indicator of the groundwater quality used for irrigation, whereas the SAR cannot be considered in certain particular cases, i.e. when the sign of AR is positive. Finally, the various experimental and theoretical results showed high risks related to the use of groundwater for soils irrigation in the plain of Lower-Cheliff. In addition, the agronomic aptitude of these soils is threatened, which can be a determining factor in the subsistence and the viability of agriculture in the area.