Discrimination in the Mineralization Processes of the Surface Water Table of the Senegal River Delta

Abstract

Located in the north-west of Senegal, the Senegal River Delta (DFS) is a strategic region for the development of irrigated agriculture. Thus, with agricultural intensification, we are witnessing a salinization of the alluvial groundwater that could even lead to a degradation of the cultivated soils. The main objective of this study is to carry out a hydro-geo-chemical characterization of the alluvial groundwater and to discriminate between the different processes responsible for the mineralization of these waters in order to provide a model for acquiring the chemical composition of waters. To this end, two sampling campaigns were carried out, one in June 2011 during the dry season and another in December 2012. During these campaigns, 12 piezometers and 17 micro-piezometers were sampled. The river waters as well as the drainage waters were sampled. The results allowed a geochemical characterization of the waters of the DFS surface water table. Two groups of waters are individualized. These are fresh waters (rainwater and river water) of bicarbonate calcium facies (HCO33-Ca) and salt waters (seawater and drainage water from irrigated areas) of chlorinated sodium facies (Na-Cl). The study of the origin of the mineralization showed that the dominant elements (Na and Cl) derive mainly from the old contamination of seawater. Their concentration under high evaporation explains their evolution towards brines. Marine pollution is the main factor in the acquisition of mineralization. The dissolution of minerals such as gypsum, calcite and to a lower extent dolomite contributes to the mineralization of water. These phenomena, as well as the basic exchanges, then constitute the evolutive factors of the chemical composition of the alluvial groundwater.