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Hydrogeochemical analyses were conducted on groundwater sampled from the Saloum aquifer, in southern Senegal. The objective was to identify the chemical processes that control hydrochemistry and to assess the quality of groundwater for determining its suitability for drinking and agricultural purposes. Water samples were collected from 79 wells during the dry season in May 2012, and were subjected to analysis for chemical characteristics (major ions), pH, electrical conductivity (EC) and total dissolved solid (TDS). The dominant hydrochemical facies observed for the groundwater samples are NaCl and CaHCO3. Gibbs plot depicts predominance of rock water interaction and evaporation processes controlling the water chemistry. Percentage of Na+, Residual Sodium Carbonate (RSC), Total Hardness (TH) and Sodium Adsorption Ratio (SAR) values were calculated. The results were compared with the standard guideline values recommended by the World Health Organization and agricultural water standards. The TDS in groundwater is less than 1200 mg/L and SAR values are less than 10. RSC values overall are less than 1.25 meq/L. Results show that the groundwater in the area has generally a low hardness and is fresh (95%) to brackish. The majority of groundwater samples are appropriate for domestic uses. The indexes for water irrigation compared with standard limits revealed that most of the Saloum groundwater samples fall in the suitable range for irrigation.

La salinisation des sols constitue un risque majeur du développement de l’irrigation dans le monde. Dans le Delta du Fleuve Sénégal, l’existence d’une nappe phréatique salée et proche de la surface du sol accentue ce risque. Une modélisation des transferts hydriques a été réalisée pour mieux évaluer ce risque. Elle a été faite à partir du code Hydrus sur l’ensemble du cycle cultural et a pris en compte deux types de sol et deux modes de culture de cette région : la simple et la double culture. Elle a permis d’estimer les flux hydriques descendants pendant la phase d’irrigation par submersion, et les flux hydriques et salins ascendants pendant la phase d’irrigation et de non-irrigation. Cette modélisation indique que : i) le toit de la nappe salée remonte d’environ 50 cm pendant la phase d’irrigation; ii) une zone non saturée de faible épaisseur subsiste entre la surface inondée et la nappe; iii) l’infiltration cumulée est deux fois plus forte en double qu’en simple culture; et iv) la forte évaporation durant la phase de non-irrigation peut faire remonter entre 5,5 à 6 t de sels par ha et par an, selon les sols et les conditions de culture. Cette modélisation, qui s’appuie sur des mesures in situ des paramètres hydrodynamiques, devra être confrontée à des observations in situ de flux. Elle devra intégrer des processus non pris à ce stade tel que le piégeage d’air. Les relevés piézométriques depuis dix ans montrent, par ailleurs, un léger relèvement du niveau de la nappe salée. L’ensemble de ces résultats est suffisamment préoccupant pour préconiser, dans le cas d’une irrigation par submersion, la mise en place d’un drainage superficiel pour éviter le relèvement de la nappe et assurer l’évacuation des sels hors de la nappe et, par là, la pérennisation de l’agriculture dans cette région du Delta du fleuve Sénégal, Soil salinisation is a major hazard of the extension of irrigation in the world. In the Delta of the Senegal River Valley, a highly saline groundwater close to the soil surface increases this hazard. A water and salt modeling by means of the Hydrus code has been performed to evaluate this hazard. Two soil types and two crop systems, a single and a double one, have been considered. The calculations have been performed on the complete agricultural cycle to determine water flows going down during the irrigation phase, and water and salt flows ascending to the soil surface during the irrigation and non irrigation phases. The modeling shows that: 1) the recharge due to the irrigation period induces an elevation of the water table of about 50 cm; 2) an unsaturated zone persists between the flooded surface and the water table; 3) the cumulative infiltration is two times higher in the double crop rice system than in the single crop system; 4) the high evaporation during the dry phase causes an ascension of from 5.5 to 6 t of salts per ha and per year according to the soil type and the crop system. This modeling, which was based on field measurements of hydrodynamic properties, has to be validated by field observations of the fluxes. Some processes such as air trapping should be tested. Piezometer monitoring over the past 10 years indicates a slight rise in the saline water table. All these results demonstrate the relevance of setting up a surficial drainage system for the flooded irrigation fields, to avoid an elevation of the water table and to evacuate salts from the soil surface, in order to perpetuate agriculture in the Delta of the Senegal River Valley.

Seawater has entered and concentrated in the Saloum hydrologic system up to 100 km upstream, contaminating both the surface water and the superficial 'Continental terminal' (CT) groundwater resources, and large proportions of cultivated lands. In the areas affected by saltwater contamination, chloride concentrations as high as 3,195 mg/l have been measured in the groundwater samples collected from wells located in the vicinity of the Saloum River, making the water inadequate for drinking water purposes. This paper presents the results of a study designed to characterise the current extent of the saline groundwater and the mechanism of saline surface water body/fresh groundwater mixing in relation to the groundwater flow trends. It also describes the groundwater chemical and isotopic composition and geochemical processes controlling the chemical patterns. Four major water types occur in the study area, namely Na-rich saline groundwater, Ca–Na-rich saline groundwater, Na-dominant fresh groundwater and Ca-dominant fresh groundwater. A hydrogeochemical zonation of the aquifer, based on the presence of different water families and on the groundwater flow, led to the identification of the main processes controlling the groundwater chemistry. Cation exchange reactions on the kaolinite clay mineral, calcite dissolution in the eastern zone where calcite minerals have been identified, reverse cation exchange reactions in the saline-contaminated band along the Saloum River and, to a lesser extent, a gypsum dissolution are the predominant processes. Results of δ18O and δD analysis in 15 groundwater samples compared with the local meteoric line indicate that the groundwater has been affected by evaporation, and the groundwater is isotopically lighter as the depth of water table increases. In this study, δ18O data were used in conjunction with chloride data to identify the source of high chloride. Results show a departure of the contaminated water samples from the seawater mixing line, which indicates that other processes rather than mixing between modern seawater and native groundwater alone increase the chloride concentrations.