pH are neutral to basic, with median values of 7.32 in surface water and 7.89 and 7.35, respectively, in boreholes and wells ( Table 1 ). Acidic pH values have been measured in some wells tapping the alluvial aquifer. According to Deckers et al. (1996) , this potential acidity is linked to the high pyrite concentration at shallow depths in the subsoil of many soils in the delta and, by extension, the lower valley. The oxidation of pyrite by rising groundwater causes the formation of sulfuric acid, which could reduce the pH of the water. On the other hand, on the marine terraces and in the dunes, the water samples show slightly basic values (pH = 8) due to the presence of shell clusters (Anadaris senilis) in the soils, whose dissolution can enrich the water with hydrogen carbonate ions (HCO₃) according to the reaction:

${\text{CaCO}}_3+{\text{CO}}_2+{\text{H}}_2\text{O}\to {\text{Ca}}^{2+}+2{\text{HCO}}_3$ (1),

neutralizing soil acidity and forming gypsum

${\text{Ca}}^{2+}+{\text{SO}}_4^{2-}\to {\text{CaSO}}_4+2{\text{H}}_2\text{O}$ (2)

which raises the pH

In surface waters, mineralization ranges from 80 to 294 µS/cm. The lowest mineralized waters (80 - 100 µS/cm) are found in samples taken from the main course of the river. Water samples taken from the river’s tributaries, such as Lake Guiers, show higher conductivities ranging from 100 to 294 µS/cm. It should be noted that, at present, the saline charge in these tributaries is exclusively dependent on related human activities around these waterways (irrigation, domestic activities, livestock farming, etc.). In the wells, the electrical conductivity values of the groundwater sampled are fairly heterogeneous, ranging from 210 to 9300 µS/cm ( Table 2 ). In general, the lowest mineralized waters are found in the Ogolian dune sediments and in the alluvial plain close to the hydraulic axes, or in the eastern part where the alluvial water table is semi-free. In the extension of the marine terraces and in the inter-dunes, well water often has higher concentrations. These saline waters, most of which are captured by piezometers, are contained in Nouakchottian or Inchirian sediments. They are unsuitable for human consumption and irrigation, and can considerably increase the salinization of under-irrigated land. Furthermore, the electrical conductivity values of water taken from boreholes are moderately to highly mineralized. They range from 692 to 3500 µS/cm. However, the lowest mineralized waters are found in boreholes tapping the Saloum Formation (ex-Continental Terminal) and in Eocene limestone. On the other hand, in boreholes tapping the Maastrichtian, the water is more highly mineralized. This high mineralization may be linked to the lithological nature of the tapped sediments and their position in relation to the saline central band, which contains ancient sedimentary brines (Travi, 1993) .

Water chemistry in the study area is mainly characterized by Na-Cl, Ca-Mg-HCO₃ or mixed facies ( Figure 4 ). The cations, classified as follows: Na > Ca > Mg > K > F > Fe, are largely dominated by Na ions, most of which exceed even the WHO standard norms (200 mg/L) for drinking water in groundwater (WHO, 2014) . The order of abundance of anions is Cl > HCO₃ > NO₃ > SO₄ > Br. Chloride ions in groundwater also exceed WHO drinking water standards (250 mg/l) in over 50% of samples. In waters with low ion concentrations, the dominant ions are Ca²⁺, Mg²⁺ and ${\text{HCO}}_3^{-}$. On the other hand, in samples with high total dissolved solutes, Na⁺ and Cl⁻ are the dominant ions ( Figure 5 ).

The Piper’s diagram (Piper, 1953) shows different types of chemical facies ( Figure 4 ). Surface waters are dominated by Ca-Mg-HCO₃ facies. This type of chemical facies (around 20% of samples) has also been observed in the alluvial plain close to the hydraulic axis, in well water located at the foot of the limited-extension Ogolian dunes and in the high, fractured zone of the Guiers dome. It is also found in deep reservoirs that tap the water table of the Eocene limestone and the sandy-clay formations of the Continental Terminal. However,

groundwater collected from the dunes (P9; P10) as well as from the alluvial aquifer collecting Nouakchottian and Inchirien sediments, generally with a high mineral content, essentially shows sodium chloride chemical facies dominated by Na-Cl. This type of chemical facies Na-Cl dominates in boreholes tapping the deep Maastrichtian aquifer. In its central band, in some areas, these two main facies can migrate to mixed Ca-Cl or HCO₃-Na facies, reflecting the mixed character with surface waters and are mainly found on floodplains and dunes. These waters are associated with shallow wells with dilute chemistry and intermediate mineralization (TDS < 235 mg/l). These facies are probably the result of mixing processes in the floodplains and dune formations aquifer. Base exchange or reverse base exchange processes in freshwater systems tend to extract divalent ions such as Ca²⁺ from solution and replace them with monovalent ions such as Na⁺ (Hem, 1992) .

On the other hand, in some locations such as ancient lagoon deposits, the presence of sulfate in the water is very frequent due to the dissolution of gypsum present as evaporites in the area. Dissolution processes of this gypsum lead to the release of sulfate ions in waters, explaining dominance of Na-Cl-SO₄ facies (Diaw, 2019) . However, its origin could also be anthropogenic, as traces of gypsum used during soil desalting and, above all, polluting doses of nitrates from uncontrolled fertilizer spreading have been found in the water (Da Boit, 1993) . High concentrations of nitrates and sulfates in water are a threat not only to human health but also to the ecological balance. Researches (Ward et al., 2005; Blaisdell et al., 2019; Elwood and van der Werf, 2022) have shown that many groundwater contaminants, such as nitrates ( ${\text{NO}}_3^{-}$), are suspected of affecting human health by causing, for example, cancers of the colon, rectum, ovary, bladder, gastrointestinal tract and mouth, as well as methemoglobinemia, while increasing the risk of birth anomalies. Previous studies have shown that the absorption of excessive amounts of sulfate by the human body causes a number of illnesses, including diarrhea, dehydration and gastrointestinal disorders (Man et al., 2014) . Sulfate in the aquatic environment can be transformed into toxic substances under certain conditions, leading to the loss of essential metallic elements in aquatic plants and changes in the original eco-hydrological function (Geurts et al., 2009) . Soucek et al. have shown that high concentrations of sulfate cause the death of freshwater invertebrates (Soucek and Kennedy, 2005) .

Water-rock interaction, precipitation and dissolution are represented in the graphs TDS vs Na/(Na + Ca) and TDS vs Cl/(Cl + HCO₃). Samples located in the middle of the graph are mainly those affected by water-rock interaction. The Gibbs diagram ( Figure 6 ) of aquifer data shows that groundwater mineralization is controlled by two processes: water-rock interaction and evaporation/crystallization. Water-rock interaction indicates that water chemistry is relatively linked to the aquifer’s petrographic nature, but also to the degree of rock alteration that facilitates element solubility (Gibbs, 1970) .

The position of the water in the evaporation range explains the high mineralization of this water, which even approaches the composition of seawater. Indeed, the superficial nature of the alluvial aquifers means that they are exposed to evaporation, especially in the dry season, which contributes to the concentration effect that tends to increase the mineralization of the water, especially in the late dry season. Paleosalinity linked to ancient deposits of marine sediments and terrasses (Lei et al., 2023) , and the presence of ancient water conned in the Maastrichtian, are essentially responsible for the major contribution of Na and Cl to global mineralization. This is confirmed by the Br/Cl ratios ( Figure 8(b) ), which show a good correlation, suggesting a common marine origin for these two ions. Furthermore, these two phenomena explain the highly mineralized nature of the water in the lower valley, both in the alluvial aquifer and in the deep aquifers tapped by the boreholes. No samples were located in the precipitation field.

Despite the low bromide ion concentration, binary diagrams and correlation matrices show a high correlation between EC and Na, Cl and Br ions (~0.90 in both wells and boreholes) and a fairly moderate correlation with HCO₃ ions. Cl and Br ions dissolved in natural water are close to ideal conservative tracers, due to their hydrophilic nature and small ionic size. The presence of bromide ions can be used to interpret the origin of salinity in a water table (Alcalá and Custodio, 2008; Fontes and Matray, 1993; Herczeg et al., 2001) . It can be used to interpret the phenomena responsible for the salinity of the water table: current water level, sedimentary paleosalinity, evaporites, recycling by irrigation (Davis et al., 1998, 2004) .

In our study area, bromide, chloride and sodium ions show strong correlations ( Figure 7 , Figure 8(a) , Figure 8(b) ), indicating they are of the same origin and that the Na-Cl facies present in most groundwater is the consequence of paleosalinity established during the marine transgressions and regressions of the Nouakchottian and Inchirian periods, especially in the marine terraces where evaporation, crystallization or halite dissolution are predominant (Moussa et al., 2019; Tweed et al., 2011) . The presence of sodium chloride facies (Na-Cl) in boreholes located along the axis of Lake Guiers is due to the presence saline water of Maastrichtian aquifer in its central band (Travi, 1993) .

Furthermore, the strong correlation between Na and Cl ions and conductivity explains why these ions make such a significant contribution to groundwater mineralization. The higher their concentration in water, the higher the conductivity ( Figure 8(c) , Figure 8(d) ) which is the opposite of the case for bicarbonate ions. This is the consequence that conductivity is weakly controlled by HCO₃ ( Figure 8(e) ).

R statistical software was used for principal component analysis (PCA) and hierarchical cluster analysis (HCA). This analysis considers major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, ${\text{HCO}}_3^{-}$, Cl⁻, ${\text{NO}}_3^{-}$, ${\text{SO}}_4^{2-}$, Fe²⁺). The PCA results presented in Figure 9 show the PCA pie chart and the contribution of each variable to the three principal components. The principal component (PC) Dim1, which represents 60.6% of the samples, shows that Na⁺, K⁺, Cl⁻ and ${\text{HCO}}_3^{-}$ ions, with a low contribution, are highly correlated, suggesting a common origin for Na, Cl and K ions, which would come from halite dissolution phenomena or water/rock interactions.

The PC Dim2, representing 16.8% of the total variance, shows a positive correlation between Ca and Mg ions, nitrates and sulfates. Calcium and magnesium ions could originate from the dissolution of calcite (CaCO₃) or dolomite (CaMg(CO₃)) (Palmer & Cherry, 1984; Appelo and Postma, 2005) but also from mixtures between recharge waters (rainwater, waterlogging water or streams) as demonstrated by Diaw (2008, 2019) .

The significant presence of sulphates may be of natural origin, due to the dissolution of gypsum, or anthropogenic, due to agricultural inputs or wastewater drainage. Nitrate, on the other hand, is anthropogenic, especially at wells (cattle troughs in the neighbourhood or near septic tanks: P28, P23, P32, P25, P9). Finally, Dim3, which contributes only 8% of the total variance, shows a negative correlation between Fe and the other ions, suggesting that this element has little or no role in water mineralization in the lower Senegal River valley.

Thus, the PCA could be subdivided into 3 water groups, namely:

• Na-Cl facies water encountered in wells located on marine terraces and in boreholes tapping the highly saline Maastrichtian in this zone,

• moderately mineralized Ca-Mg-Cl and Na-HCO₃ facies waters found in wells tapping dune formations or the alluvial plain.

• and the group of low-mineralized waters with HCO₃-Ca-Mg facies, which characterize surface waters and alluvial groundwater close to hydraulic axes or low-lying Ogolian dunes. This signature of groundwater close to hydraulic axes could suggest recharge of this alluvial aquifer by surface water.

The dendrogram ( Figure 10 ) resulting from the HCA has highlighted 4 main groups (clusters) according to their dominant chemical composition, corresponding to the different facies in the Piper diagram. The first cluster (C1) is made up of surface waters with low mineral concentration and Ca-Mg-HCO₃ facies, with TDS values of less than 250 mg·L⁻¹. The second cluster (C2) represents Na-Cl, HCO₃-Ca and mixed (Na-HCO₃ or Cl-Ca) facies waters with TDS below 1000 mg·L⁻¹. This is the case for certain surface waters and groundwaters with moderate mineralization, with a codominance between ${\text{HCO}}_3^{-}$ and Cl⁻ and Ca²⁺ and Na⁺ ions. This cluster includes wells tapping water from the alluvial aquifer close to the hydraulic axes or from the deep aquifers of the Saloum Formation or Eocene. Their facies are governed by exchange phenomena between surface water and groundwater in the alluvial aquifer, or by water/matrix interaction (Bouderbala & Gharbi, 2017; Zhang et al., 2021) . Clusters 3 and 4 concern waters with sodium chloride Na-Cl facies, whose TDS even exceeds 2000 mg·L⁻¹. These samples represent water with high concentrations of Na and Cl ions, even exceeding WHO standard norms, and correspond to structures that tap the marine terrace aquifer and boreholes capturing the highly saline Maastrichtian aquifer in its central band (boreholes P28, P29, P30). These different aquifers owe their salinity to paleosalinity and the evaporation, crystallization and dissolution of halite in the marine terraces.