Contribution to the Hydrochemical Study of Groundwater from the Continental Terminal in Saloum

This paper focuses on the study of the aquifer of the Continental Terminal in the south of Saloum river. This study aims to participate in the knowledge of the physicochemical quality and to help understand the origins and processes of the mineralization of the water of the Continental Terminal (CT). Physicochemical treatments show an average pH of 6.61 indicating a slightly acidic water overall. The electrical conductivity varies between 37.4 and 12,320 µS·cm −1 with an average of 729.3 µS·cm −1 . High conductivities are recorded around the ocean and the Saloum River, indicating higher mineralized waters in these areas. The geochemical study and multivariate statistical analysis indicate three groups of the sampled waters. Group 1 is mainly formed of the Ca-HCO 3 , Na-HCO 3 , Ca-Cl and Na-Cl facies. This group is the most common one and is found throughout the southern Saloum area. Group 2, mainly made up of the Ca-HCO 3 and Na-HCO 3 facies, is located in the center, east, west and north of the zone. The mineralization of these two groups is believed to be of carbonate, evaporitic and/or anthropogenic origin. Group 3 is formed from the Na-Cl facies. This group is located in the north and west of the area (near the ocean and the Saloum river). This group 3 suggests pollution of marine and anthropogenic origin. The calculated base indices suggest cationic exchanges between the waters and the formations of the water table of the terminal continental.


Introduction
Between the end of the 1960s and of the 1990s, West Africa was affected by transformations from a significant decrease in rainfall [1]. This decrease in rainfall is a result of climate change. In Senegal, new climate variations are perceived through precipitation and temperatures [2]. The decrease in rains and high temperatures in the southern area of Saloum lead to a decrease in surface runoff and freshwater inflows. The salinity of surface waters in this area is increasing. High salinities are observed (greater than 60 g/l) and may be (greater than 140 g/l) during the dry season, in the upstream part of the Saloum river estuary throughout the year [1]. Work carried out in the area [1] [3] [4], indicated significant values of salinity in the Saloum estuary, which may even exceed the salinity of seawater. As a result, surface waters are dominated by a high salt content except for temporary runoff in the rainy season. Groundwater is therefore the main source of water for the population. Thus, with the presence of brackish water in places, found in the Maastrichtian water table and high fluorine contents that may exceed WHO standards [1], the Continental Terminal (CT) water table is the major source of water for the inhabitants of the area. In this area of Saloum, the population's water supply is mainly based on the exploitation of groundwater from the Continental Terminal aquifer [5]. Due to the decrease in rainfall, high temperatures and population growth in the area, groundwater recharge decreases, evaporation and pumping increases. Consequently, the Continental Terminal (CT) water table in its coastal area is exposed to saline intrusion. According to Fadili [6], high pumping in coastal areas decreases the potential of the existing aquifer, by penetrating marine waters, changing its quality, following an imbalance between fresh water and water from the sea. Today, the local population has given up operating wells because of their high chloride content (3195 mg/l) [7]. In many arid and semi-arid regions, the intrusion of salt water into coastal and continental aquifers is a widespread phenomenon that can sometimes irreversibly degrade the fate of these waters [3]. Several studies have been carried out in this southern zone of Saloum [8] [9] [10] [11] [12]. These studies focus on geology, hydrogeology, major and minor ion hydrogeochemistry, isotopes and hydrogeological modeling. They made it possible to identify the sources of salinity, the mechanisms and degree of salinization, the surface water-groundwater relationships in this southern area of Saloum [13] [14] [15] [16] [17]. These authors indicated that in addition to the appearance of salt water on the coastal part, the increase in the salinity of the waters of the Saloum River also contributes to the contamination of groundwater in this southern area of Saloum [8]. These studies aim to understand the Saloum system to allow its good management. The detection and monitoring of salinity are essential in the mechanisms of its intrusion into coastal aquifers, within the framework of sustainable management of water resources [3]. Our objective is to participate in the knowledge of the physicochemical quality of the water of the Continental Terminal (CT) groundwater and to contribute to understanding the origins and Journal of Water Resource and Protection processes of the mineralization of these water sources.

Study Site
The study area ( Figure 1) is located in central western Senegal, between latitudes 13˚35' and 14˚10'N, and between longitudes 15˚40' and 16˚30'W. This southern area of Saloum is limited to the north by the Saloum River, south by the border of Gambia, west by the Atlantic Ocean and east by the Kaffrine region. The climate in the area is Sudano-Sahelian, with the alternation of a wet season and a dry season. Altitudes in the area vary from 0 (western and northern part) to around 40 m (central, southern and eastern). The hydrographic network in the area is mainly made up of the Saloum river with its two branches (Diomboss and Bandiala), the Néma, the bolons, the Djikoye and the Baobolon.
Geologically, the study context is that of the Senegalese-Mauritanian basin.
This basin is from Meso-Cenozoic to Quaternary with at its base a Precambrian to Paleozoic bedrock which rises to the surface on its eastern and south-eastern side [18].
For Lienou, two-thirds of the Senegalese-Mauritanian basin is made up of Figure 1. Study area and location of measured and sampled points. Journal of Water Resource and Protection clayey, armored sands, generally variegated [19]. In the alternating phase of hot and humid climates, at the end and start of the Pliocene and Pleistocene respectively, a lateritic modification involves a deposit of armor that surpasses the Continental Terminal [3]. The Continental Terminal (CT) is made up of detrital terrain, characterized by significant ferralitic modifications accompanied by ferruginous deposits and armor-cladding, high variations of silica, neoformation of kaolinite [7]. Faye recognizes from stratigraphic correlations of borehole logs capturing the CT ( Figure 2): a diversity of detrital formations, coarse sand, fine sand, clayey sand and sandy clay, irregular and alternated by variations of clay [3]. The Continental Terminal has a heterogeneous lithology, the deposits are detrital, clayey to gravels, and this is often encountered in continental dumping grounds, that is to say, continental sediments redistributed by marine sedimentation [8]. The sedimentation of the Continental Terminal is heterogeneous and consists mainly of fine and medium sand, clay, clayey sand, sandy clay, laterite, lateritic sand, silt, limestone, marl limestone and limestone-marly. The Continental aquifer Terminal occupies most of the territory of Senegal, in the southern zone (Sine-Saloum-Gambia), it is formed of intercalation of sands and clays, while in Ferlo it is sands, sandstones and clays [3]. This aquifer is contained in the sandy, sandy-clay and clay deposits of the Continental Terminal and the Quaternary. According to Diluca, its thickness increases from 20 m in Kaolack, to more than 100 m to the south [9]. To the west and north near the

Sampling and Physicochemical Analysis
Temperature, pH and electrical conductivity (EC) were measured in situ at 46 water points (wells and boreholes) in February 2019 using a multiparameter probe. Ten (10)  The points were located with their geographic coordinates using GPS (Global Positioning System).
These coordinates were used to represent the spatial distribution of the water points sampled with ArcGIS software (Figure 1).

Data Processing
The methods applied to process the analysis results are based on Ion balance (BI) analysis, Piper diagram, Base Exchange indices (i.e.b.), Saturation Indices (SI), binary diagrams and multivariate statistical analysis.

Ion Balance Analysis
The study of ionic balance (BI) can help to verify the accuracy of test results. The results of analyses can be considered good if this ionic balance is less than 5%, for a value of the balance between 5% and 10%, the results can be maintained and when the balance is greater than 10%, the analyzes are to be rejected [20].
The ionic balance is given by the following Equation

Piper's Diagram
The Piper diagram allows a classification of water. This classification is based on a representation of the chemical facies of water on a triangle of cations (left), anions (right) and an associated diamond giving a synthesis of the overall facies.
The approximate location of an analytical result on each of these two triangles gives the first clarification on the anionic and cationic dominance [21]. This Piper hydrochemical diagram is mainly used in the literature to display the chemical facies of a set of water samples [22] [23].
The Piper diagram is used with the help of Diagramme software for the processing of hydrochemical data. ( )

Base Exchange
where: [ -if i.e.b. = 0, we have equilibrium between the chemical constitution of the water and the constitution of the surrounding land.

Saturation Index (IS)
The influence of chemical elements in geochemical mineralization processes can be given by the state of saturation, determined by the saturation index. This saturation index provides information on the level of chemical equilibrium of water with the mineral in the aquifer matrix, in water-rock relationships, as a measure of the mechanism of dissolution and/or precipitation [26]. The saturation index is defined by the logarithm of the ionic activity ratio (PAI) to the equilibrium constant (K) (Equation (3)) [27]: where: PAI: ionic product of the solution; K: equilibrium constant. Journal of Water Resource and Protection Three cases are possible: IS < 0, the solution is undersaturated with respect to the mineral considered; IS = 0, indicates saturation; IS > 0, the solution is supersaturated.
Diagrams and PHREEQC Interactive software are used to calculate saturation indices with respect to minerals (calcite, aragonite, dolomite, anhydrite, gypsum and halite). noted, in addition to specifying the types of water [10]. The chloride ion indicates the origin of the salt in the water, it is a mixing tracer absent in exchanges between water and rock and is not lost [29] [30] [31].

Multivariate Statistical Analysis
Multivariate statistical analysis applied in the processing of hydrochemical data is based on the techniques of Ascending Hierarchical Classification (AHC) and Principal Component Analysis (PCA). These analysis techniques reduce and classify the information resulting from hydrochemical data with the aim of explaining the process of water governance [22] [32] [33] [34]. They show similarities or dissimilarities by grouping chemical elements (variables) or individuals (water point) according to their chemical compositions.

Ascending Hierarchical Classification
The Ascending Hierarchical Classification (AHC) is an analysis assessing the similarity or difference between samples to give an indication of class association [35]. This analysis allows multiple data to be grouped and ordered into distinct classes on a graph to facilitate their interpretations. On this graph, the individuals are distributed in succession of nesting forming a classification tree, called a dendrogram. According to Ahmed, it is a method of hierarchical classification of parameters, widely used in earth sciences and in the processing of hydrochemical results [22]. The Ascending Hierarchical Classification (AHC) method is powerful for processing hydrochemical data and for establishing geochemical models [32] [33].  [39]; this analysis determines the essential elements making it possible to interpret the phenomena involved from correlation with the variables [40]. According to Ahmed, PCA is used to study the mechanisms governing the physico-chemical constitution of water and/or their origins, and it is a new descriptive method recommended by Farnham, Johannesson et al. [22] [41].

Principal Component Analysis
The Ascending Hierarchical Classification (CHA) and Principal Component Analysis (PCA) methods are processed using the XLSTAT software.

Physicochemical Characteristics of Ground Water
The categories of water in the CT aquifer: fresh water (EC < 900 µs/cm) and those which are salty (EC > 900 µs/cm) [1].
The results of the analysis were subjected to an electric charge balance (ionic balance) between anions and cations to check their validity (Table 1)

Hydrochemical Classification of Groundwater
An analysis by ascending hierarchical classification is carried out from the results of the analysis. This analysis showing the dendrograms (Figure 3(a) and    [1]. Despite its diversity, this group consists mainly of sites in the area with fresh water. Generally, the water of the groundwater is soft, but near the sea and the river, it is salty [10]. However, the high levels of nitrates in some works suggest contamination due to anthropogenic origin. Nitrates show that groundwater is contaminated by recently infiltrated water [8]. The calcium chloride facies (Na-Cl) present in the center of the zone which is characterized by fresh water is thought to be due to brackish water. Pockets of brackish water are increasing in the aquifer, although they are currently not well demarcated [15].        These negative indices indicate that the surrounding terrain releases Mg 2+ and Ca 2+ to fix the Na + and K + ions. The dominance of kaolinite-type clay is remarkable, over this cation exchange mechanism through clay minerals [11].

Water Saturation Indices
Box whisker ( Figure 6) are represented from the calculation of the saturation indices. These boxes show that the waters are undersaturated with respect to all the minerals (calcite, aragonite, dolomite, anhydrite, gypsum, halite). Carbonate minerals (calcite, aragonite, dolomite) tend to approach saturation, while evaporitic minerals (anhydrite, gypsum, halite) are far from the saturation line.
The dissolution of these minerals would therefore participate in the mineralization of the water. Figure 7 shows the cross diagrams of the concentrations of major elements (Na + , can be adopted to determine other phenomena related to the mixing mechanism [31].

Binary Diagrams
The representation of these major elements (Na + , Ca 2+ and Mg 2+ ) with (Cl − ) indicates strong values of correlations. The correlation coefficients (r) are greater than 0.9 (Figures 7(a)-(c)). These correlation values would indicate a common source of these ions. This source could be marine and/or a dissolution of carbonate, evaporitic minerals. The high correlation between Na + and Cl − would be due to the same origin of a dissolution of halite and/or the presence of salt water [8]. The halite saturation index, undersaturated with respect to the water samples ( Figure 6) could corroborate this. The representation of Na + as a function  when the water is contaminated by salt water, on the sea water dilution line [42].
This representation shows a weak growth in sodium ion (Na + ). Enrichment/ depletion of Na + in fresh water would result from exchange of bases ensuring the progression of facies from Ca-Cl or Na-Cl or Ca-HCO 3 to Na-Cl or Na-HCO 3 [10] [11]. The relationship between Ca 2+ and Mg 2+ (Figure 7(d)) shows a good  (Figure 7(e) and Figure 7(f)) indicate correlation coefficients respectively r = 0.55 and r = 0.65.
For these representations, calcium and magnesium are better correlated with chlorides (Figure 7(b) and Figure 7(c)) (r = 0.92 and r = 0.95) than with bicarbonates. These results seem to confirm a common, marine origin of Ca 2+ and Mg 2+ , especially in the northern and western parts of the area. However, calcium and magnesium would come from a dissolution of dolomite and/or calcite, since the waters are undersaturated with respect to these minerals ( Figure 6). The degradation of minerals would lead, from a dissolution of calcite and/or feldspar to the presence of Ca 2+ [14]. The enrichment in calcium relative to bicarbonate (Figure 7(e)) would be due to a dissolution of calcite in the presence of CO 2 . In the group of fresh waters that are not contaminated by salty waters, the waters are often of calcium bicarbonate facies (Ca-HCO 3 ) [8]. This type of Ca-HCO 3 facies occurs largely in the center, south and east of the southern Saloum area and is a characterization of freshwater. The relationship between Ca 2+ and 2 4 SO − (Figure 7(g)) (r = 0.71) and the saturation index ( Figure 6) of undersaturated water compared to gypsum indicate that calcium and sulfate ions would be bound to the dissolution of gypsum formations.

Principal Component Analysis (PCA)
Principal Component Analysis (PCA) is applied to major elements ( 3 HCO − , Cl − , 2 4 SO − , 3 NO − , Ca 2+ , Mg 2+ , Na + , K + ), conductivity (EC) and pH. This analysis gives the results of Table 2, Table 3 and Figure 8. The correlation matrix (Table 2)   The correlation coefficient varies between 0.714 and 0.988. These correlations corroborate the common origin of these major elements which could be marine and/or by a dissolution of carbonate, evaporitic minerals.
From the eigenvalues (Table 3)

Conclusion
The pH varies between 5.