Integrated Hydrochemical assessment and irrigation suitability of the groundwater of the Continental Terminal aquifer in the coastal sedimentary basin of Togo

1 Introduction

Water is essential to the functioning of ecological systems and the sustenance of life (Baran, 2008). Groundwater is a strategic resource, as it provides a significant proportion of the water supply, helps regulate the water cycle and ensures the stability of ecosystems (Currell et al., 2012; Wang et al., 2019). However, their availability and quality are declining due to increasing pressure from irrigation, industrial use and domestic water supply (Chidambaram et al., 2009; Shah, 2005). In many contexts, this growing gap between supply and demand makes it difficult to manage water resources sustainably. At the global scale, coastal aquifers constitute strategic groundwater resources that are highly vulnerable to salinization, overexploitation, and anthropogenic contamination. Seawater intrusion, driven by increasing groundwater abstraction and reduced freshwater recharge, represents a major threat to the quality of coastal groundwater resources (Hussain et al., 2019). In this context, the investigation of coastal aquifers in West Africa, particularly the Continental Terminal aquifer of the Coastal Sedimentary Basin of Togo, contributes to a better understanding of the interactions among natural processes, anthropogenic pressures, and the hydrochemical evolution of coastal aquifer systems.

Like other developing countries, Togo relies heavily on groundwater to meet its socio-economic needs. This reliance leads to increased pressure on aquifers, which are vulnerable to overexploitation and various sources of contamination. The Togo Coastal Sedimentary Basin (BSCT), which accounts for approximately 6% of the national territory, constitutes a hydrogeological unit of major interest. The Continental Terminal (CT) aquifer is, in this respect, an easily accessible reservoir and the most heavily used in the aquifer system. Its hydrogeochemical dynamics are strongly influenced by anthropogenic inputs (Bleza, 2023).

Previous studies have highlighted significant exceedances of the WHO guideline values for several physicochemical parameters, particularly nitrates (Djongon et al., 2025; Bleza, 2023). These high concentrations indicate contamination primarily linked to structural deficiencies in domestic sanitation (septic tanks, effluent seepage), failures in the wastewater drainage networks, and industrial discharges. These clear alterations in the surface water table are likely to affect deeper aquifers where hydraulic connections exist, and represent a challenge to the sustainability of the resource and to regional water security. The processes responsible for groundwater degradation exhibit distinct hydrochemical signatures. Seawater intrusion is generally characterized by an enrichment in chloride and sodium, with specific ionic ratios that allow the marine influence to be distinguished from other salinity sources (Custodio, 2005; Werner et al., 2013). Natural mineralization mainly results from water-rock interactions, particularly the dissolution of carbonate minerals and ion exchange processes within the aquifer system (Appelo and Postma, 2005). Agricultural and domestic activities are frequently associated with increased nitrate concentrations due to fertilizers, wastewater discharge, and individual sanitation systems (Foster et al., n.d.). Industrial activities may also modify groundwater chemical composition through the input of specific elements depending on the nature of the released contaminants. In this study, the combination of hydrochemical approaches, multivariate statistical analyses, and spatial mapping allows the identification and differentiation of the various mechanisms controlling groundwater quality in the Continental Terminal aquifer.

Despite previous investigations conducted in the Coastal Sedimentary Basin of Togo, the hydrochemical dynamics of the Continental Terminal aquifer remain insufficiently documented at the basin scale. The last comprehensive study characterizing groundwater quality across the entire aquifer was carried out within the framework of the PNUD program in 1975. Since then, most investigations have focused on specific areas or particular issues, without providing an updated overview of groundwater quality evolution at the basin scale. However, the hydrological and anthropogenic context has undergone significant changes due to increasing urbanization, agricultural intensification, and rising groundwater abstraction. These changes are likely to influence mineralization processes, increase nitrate contamination risks, and locally promote saline intrusion in coastal areas. Therefore, an integrated and updated assessment is required to characterize groundwater chemical composition, identify the processes controlling its evolution, and locate areas with increased vulnerability.

In this context, international frameworks for sustainable management emphasize the need to identify vulnerable areas promptly to minimize the risk of pollution and protect groundwater resources in the long term. Delineating these areas is a crucial step in anticipating changes in water quality and guiding management actions.

This study falls within this framework and aims to provide an integrated assessment of the groundwater quality of the Continental Terminal aquifer in the Coastal Sedimentary Basin of Togo (BSCT) by combining Groundwater Quality Indices (GWQI), Hierarchical Cluster Analysis (HCA), Sodium Adsorption Ratio (SAR), and Geographic Information Systems (GIS) as complementary tools for diagnosis, classification, and spatial analysis. More specifically, the objectives are to (i) characterize groundwater hydrochemical composition and identify the main processes controlling mineralization, (ii) assess overall groundwater quality through GWQI and distinguish the respective contributions of natural processes (marine salinization) and anthropogenic activities (agricultural and urban contamination) to quality degradation, and (iii) map priority areas for sustainable groundwater resource management.

2 Materials and methods

2.1 Location

The coastal sedimentary basin of Togo is located in the southern part of the country, along the Atlantic coastline. It covers an area of approximately 3,450 km², representing nearly 6% of the national territory. It stretches from west to east, from the Ghanaian border to the Mono River, which marks the border with Benin. To the north, it reaches the crystalline basement, whilst to the south it is bounded by the coastline of the Gulf of Guinea (Figure 1).

Administratively, the catchment area encompasses several prefectures in the Maritime Region, notably those of Golfe, Agoè-Nyivé, Lacs, Vo and Bas-Mono, as well as parts of the prefectures of Yoto, Zio and Avé. Lomé, Aného, Tsévié, Tabligbo, and Vogan are the main urban areas within this area. These urban centers account for nearly 41% of the national population (RGPH Récencement Général de La Population et de l’Habitat, 2022) and are home to the majority of the country’s economic activity. This population density is driving growing demand for water and increasing the pressure on the basin’s groundwater resources.

2.2 Hydrology and climate

Togo’s coastal sedimentary basin is situated in a marginal-littoral environment where freshwater from rivers mixes with brackish lagoon water. The Zio, Haho, and Mono rivers interact locally with the groundwater table, particularly in the alluvial plains. The low elevation and proximity to the sea make several areas susceptible to saline intrusion. The coastal lagoon system, covering approximately 64 km², comprises Lake Zowla and the Lomé lagoons, connected to the sea via the Aného channel and the mouth of the Mono. This system is severely degraded by domestic and industrial effluents. Salinity levels vary between 1.8 and 2.5 g/L, with high concentrations of trace metals (Ayah et al., 2015).

Southern Togo has a sub-equatorial climate with two wet seasons and two dry seasons linked to the movement of the Intertropical Convergence Zone. The main wet season runs from April to July, followed by a short dry season. A second wet period occurs in September–October. The main dry season runs from November to March. Annual rainfall varies from 864 mm in Lomé to 1,445 mm in Tabligbo (Gnazou, 2008). Rainfall decreases from the north-east towards the south-west, a phenomenon known as the ‘climatic anomaly’ of southern Togo (Rodier and Rodier, 1976). Groundwater recharge occurs mainly between May and July.

Average temperatures range from 25.6 °C to 29.7 °C, with small daily variations. Relative humidity remains high (76–81%) due to the influence of the maritime trade winds. The proximity of the sea moderate’s temperatures and increases humidity, particularly in Lomé.

2.3 Geology and hydrogeology

The geological setting corresponds to the Togolese part of the Gulf of Guinea basin, which was formed during the opening of the Atlantic Ocean. The sedimentary sequence spans from the Maastrichtian to the Quaternary (Slansky et al., 1959; Monciardini et al., 1986; Johnson, 1987; Da Costa et al., 2013). It lies in fundamental discordance with the Pan-African basement, which consists of migmatite gneisses.

The Mesozoic-Cenozoic sequence dips gently towards the south, where the thickness of the deposits increases (Figure 2). The formations are generally younger towards the coast, except for the Quaternary alluvial and fluvio-lagoonal deposits. The whole sequence forms a monoclinal series overlain by the Continental Terminal formations.

The stratigraphy begins with Campanian detrital deposits interbedded with clay or carbonate layers. These are followed by the Maastrichtian, which features clay facies rich in organic matter in the north, becoming marly and calcareous in the south. The Palaeocene consists of sands and limestones at the base, followed by predominantly clayey phosphate-bearing layers. The Lower and Middle Eocene is characterised by clay-limestone deposits containing phosphatic layers. The Miocene–Pliocene corresponds to the sands and clays of the Continental Terminal, locally overlain by alluvial deposits. The Quaternary comprises fluvial-lacustrine, lagune, and coastal deposits. The sedimentary sequence is affected by a network of faults responsible for differential subsidence (Padaro et al., 2021). The lineaments correspond to features inherited from the bedrock. The dips generally vary between 1° and 2° towards the south-east.

The sedimentary formations of the Togo coastal basin form a multi-layered aquifer system comprising three main units (Castany, 1964; Gnazou et al., 2017). The stratigraphic section (Figure 2) shows, from the base to the top: the Maastrichtian aquifer, consisting of sands interbedded with limestone; the Palaeocene aquifer, formed of alternating layers of limestone, marl and sand; and the unconfined aquifer of the Continental Terminal, composed of sandy to gravelly deposits of Miocene–Pliocene age.

These units exhibit limited lateral continuity due to stratigraphic dipping towards the north and marked lateral variations in facies. These constraints govern the heterogeneity of the hydrodynamic properties, in particular transmissivity, permeability and recharge patterns. The hydrogeological characteristics of the aquifers are summarized in Table 1.

Table 1. Characteristics of the aquifers in the coastal sedimentary basin (prepared by the authors).

In addition to this formation is the Quaternary aquifer, consisting of sands and sandy clays of marine and fluvial-lacustrine origin. Although this surface unit is locally thin, it plays an important role in surface water exchange, particularly in coastal areas subject to variations in salinity and seawater intrusion.

2.4 Data collection

This study is based on a single sampling campaign conducted between June and July 2025. A total of 246 samples were collected from groundwater exploitation facilities of the Continental Terminal aquifer, covering the entire study area. Each sample was subjected to physicochemical analyses. The sampling strategy was designed to represent the hydrogeological variability of the Continental Terminal aquifer and the main factors influencing groundwater quality. The 246 sampling points were selected according to a spatial distribution approach integrating the extent of the Continental Terminal aquifer, land use patterns, and the degree of anthropogenic pressure. The sampling network covered the main hydrogeological contexts of the basin, including recharge areas located in the northern part, highly urbanized areas of Greater Lomé, agricultural zones, and coastal areas potentially affected by marine salinization. Non-functional wells were first purged using an electric pump for at least 15 min to ensure the renewal of the water column before sampling. Samples were collected in two polypropylene bottles previously rinsed with distilled water and conditioned with the water to be analyzed. A 125 mL acidified sample was used for the determination of major cations, while a 250 mL non-acidified sample was used for the analysis of anions. Functional wells were sampled after pumping.

The in-situ parameters (pH, electrical conductivity, and temperature) were measured directly in the field using a HANNA multiparameter device to ensure the stability of values sensitive to environmental conditions. Laboratory analyses focused on the main major ions (Table 2). The concentrations of HCO₃⁻, Cl⁻, SO₄²⁻, NO₃⁻, Ca²⁺, Mg²⁺, Na⁺, K⁺, F⁻, and Fe²⁺ were determined at the Laboratory of Applied Hydrology and Environment (LHAE) according to AFNOR standard protocols (AFNOR, 1979; Rodier, 2005), ensuring analytical reproducibility and data comparability.

Table 2. Equipment and techniques used to measure physicochemical parameters.

Total dissolved solids (TDS) were calculated by ionic summation using the Diagram software, a method commonly applied to assess the overall degree of groundwater mineralization.

2.5 Data processing

2.5.1 Ion balance

The collected data were organized and analyzed in order to characterize the physicochemical variability of the groundwater at the Continental Terminal. A preliminary check of analytical quality was carried out by calculating the ionic balance using the formula below, based on a comparison between the sum of cations and the sum of anions expressed in meq/L. Samples with an error of between −5 and +5% were considered analytically consistent.

CBE = ∑ Cations + ∑ Anions ∑ Cations + ∑ Anions ∗ 100 (1)

The CBE (Equation 1) values obtained for the 246 groundwater samples ranged between −5 and +5%, indicating acceptable analytical accuracy and good internal consistency of the chemical analyses. Therefore, all samples met the commonly accepted hydrochemical quality criterion (|CBE| ≤ 5%) and were retained for subsequent analyses and interpretations.

2.5.2 Statistics

A descriptive statistical analysis was applied to all the measured parameters. This analysis was used to determine the minimum, maximum and mean values, as well as the standard deviations, in order to identify general trends and spatial contrasts within the study area.

Hierarchical Ascending Classification (HAC) was also applied to the hydrochemical data in order to identify groups of water bodies exhibiting significant chemical similarities. This multivariate method, widely used in hydrogeochemistry, is based on the calculation of a distance matrix between samples, followed by an iterative clustering process that progressively groups individuals into homogeneous classes (Benzécri, 1985; Das, 2026; Ayenew et al., 2009; Boudjema et al., 2024; Guettaia et al., 2025). The resulting dendrogram provides a concise hierarchical representation of the relationships between samples, facilitating the identification of the internal structures of the dataset and the chemical similarities between different types of water (Liu et al., 2021; Bu et al., 2020).

2.5.3 Determination of the quality index

Water quality management relies on the collection and analysis of large volumes of data, the spatial and temporal variability of which makes interpretation particularly complex (Uddin et al., 2021). To simplify this assessment, a number of methodological tools have been developed (Debels et al., 2005; Jha et al., 2015; Kannel et al., 2007; Sun et al., 2016; Hurley et al., 2012), of which the Water Quality Index (WQI) is one of the most widely used. This model is based on an aggregation function that transforms large datasets into a single value that represents the overall condition of the resource (Cherif et al., 2020; Srinivas et al., 2011). Thanks to its ability to simplify matters, the IQE is widely used by organisations responsible for water management and supply, which find it to be an operational indicator that is easy to interpret and well-suited to environmental monitoring needs.

In this study, the IQE was applied to assess the quality of groundwater in the Continental Terminal aquifer. International standards, notably those of the World Health Organisation (WHO), were incorporated into the calculation of the sub-indices. Thirteen (13) physicochemical parameters were selected: pH, Electrical Conductivity (EC), TDS, HCO³⁻, Cl⁻, SO₄²⁻, NO₃⁻, Ca²⁺, Mg^2+, Na⁺, K⁺, TH and Fe²⁺. The index obtained allows water to be classified into quality categories such as excellent, good, poor or very poor, thus facilitating the analysis of the influence of natural factors (geology, water-rock interactions, residence time) and anthropogenic factors (domestic, agricultural or industrial pollution) on the chemical composition of the waters of the Continental Terminal.

The IQE is calculated using the weighted arithmetic mean method (Brown et al., 1972; Travi and Mudry, 1997; Chatterjee and Raziuddin, 2002; Yidana and Yidana, 2010; Talhaoui et al., 2020). In this context, each parameter is assigned a relative weight Wi, reflecting its importance in the assessment of water quality. These weights, shown in Table 3, are calculated using the following formula (Equation 2):

Wi = k Si (2)

Table 3. Water quality parameters, standard, ideal, and relative values (Cherif et al., 2020; Srinivas et al., 2011).

k is a proportionality constant, which is taken to be 1.0 (Babu et al., 2006);

Si, is a standard value for assessing water quality parameter i; for the purposes of this study, these are the standards and limits set out in the WHO guidelines for water intended for human consumption.

Next, a quality index (Qi) is calculated for each parameter by dividing the concentration by the standard for that parameter and multiplying the result by 100, as shown in the following formula (Equation 3):

Qi = ( Ci − Vi Si − Vi ) ∗ 100 (3)

Qi: a scale for assessing the quality of each parameter;

Ci: the concentration of each parameter in mg/l.

Vi: The ideal value for the i-^th water quality parameter, obtained from standard tables; Vi for pH = 7 and for the other parameters (Srinivas et al., 2011), it is equal to zero. Finally, the overall quality index is calculated using the following (Equation 4):

IQE = ∑ ni = 1 Qi ∗ Wi ∑ i = 1 n Wi (4)

Five quality classes can be identified based on the values of the WQI water quality index (Table 4).

Table 4. WQI classification (Srinivas et al., 2011).

2.5.4 Sodium risk assessment: sodium adsorption ratio (SAR)

The suitability of groundwater for irrigation was assessed using the Sodium Adsorption Ratio (SAR), a chemical parameter widely used to estimate the alkalinity and sodium risk of water in relation to agricultural soils. The SAR was first introduced by Richards (1965). Since then, this index has been widely used in studies assessing the quality of irrigation water to characterize hydrochemical variability and the suitability of water resources for agricultural purposes (Nijesh et al., 2021; Tampo et al., 2022; Neissi et al., 2020; Singh, 2020).

The SAR expresses the ratio between the concentration of sodium (Na⁺) and that of calcium (Ca²⁺) and magnesium (Mg²⁺), the latter two ions playing a decisive role in maintaining the structural stability of soils. Indeed, calcium and magnesium tend to counteract the dispersing effect of sodium on clay particles. A high SAR value promotes the adsorption of sodium onto the soil’s cation exchange complex, leading to the dispersion of clays, structural degradation, reduced permeability and, ultimately, a reduction in infiltration and agricultural productivity. In this study, the SAR was calculated from ionic concentrations expressed in milliequivalents per litre (meq/L), using the following formula (Equation 5):

SAR = Na + Ca 2 + + Mg 2 + 2 (5)

This index thus makes it possible to assess the extent to which irrigation water contributes to cation exchange reactions in the soil and to classify water types according to their level of sodium risk.

Four (04) quality classes can be identified based on the sodium adsorption rate values (Table 5).

Table 5. SAR classification (Richards, 1965).

2.5.5 Spatial analysis using inverse distance weighted (IDW) interpolation

Inverse Distance Weighting (IDW) is a predictive mapping method widely used in the geosciences for the spatial estimation of continuous variables (Romain, 2007). This is a local interpolation technique, based on the assumption that values measured close to the point to be estimated have a greater influence than those further away (Huisman and de By, 2009). This approach is particularly well suited to irregular or spatially dispersed datasets, such as environmental measurements or geophysical surveys (Travi and Mudry, 1997). The value to be estimated at a point within the study area is calculated as a weighted average of the values of the surrounding points, with the weights determined by the distance between each measurement point and the target point. Thus, the method directly reflects Tobler’s first law, which states that ‘everything is related, but things that are close are more closely related than things that are far apart’ (Miller, 2004). In other words, the similarity between values is assumed to decrease with distance, in accordance with the principle of spatial decay (Figure 3).

To ensure reliable interpolation, the application of IDW requires a sufficient number of sample points, generally at least 14, with an appropriate spatial distribution at the local scale (Setianto and Triandini, 2013). The power parameter p is one of the most significant factors affecting the model’s accuracy, as it determines the relative weight given to nearby points compared to distant points (Setianto and Triandini, 2013; Burrough et al., 2015). Furthermore, the choice of neighbourhood (the size of the analysis window and the number of neighbours selected) also influences the quality of the final result.

Interpolation is defined mathematically by the following expression (Equation 6):

Z 0 = ∑ i = 1 N Zi . d i − n ∑ i = 1 N d i − n (6)

Z₀: the estimated value of the variable z at the target point 0,

Zi: the measured value at sample point I,

d_i: the distance between the estimated target point 0 and the sample point I,

N: the total number of neighboring sample points used in the interpolation (summation limit),

n: the power parameter (exponent) that controls the distance-based weight decay.

The search radius R determines the spatial range within which sampling points are taken into account when calculating each interpolated value. A low value for the power parameter p gives greater weight to distant points, thereby producing a smoother interpolation surface. Conversely, high values of p accentuate the influence of nearby points, generating marked spatial variations. The use of a barrier polyline B allows the search for input points to be constrained by imposing geometric or hydrogeological limits, thereby improving the relevance of the interpolation in complex structural contexts.

The Water Quality Index was mapped using the IDW method to visualize its spatial distribution within the coastal sedimentary basin. This geostatistical mapping provides a detailed picture of quality gradients, enabling water points to be classified according to different index categories and supporting decision-making on resource management. The resulting maps are an essential tool for:

• determine the intended use of the water (drinking water supply, industrial use, or other applications),
• identify areas with potential for sustainable exploitation, and.
• identify potential sources of pollution or high-risk areas requiring closer monitoring.

3 Results and discussion

3.1 A statistical analysis

Descriptive statistical analysis reveals significant hydrochemical heterogeneity (Table 6), as evidenced in particular by the wide ranges observed for EC, TDS, total hardness, and the Na⁺ and Cl⁻ ions. These parameters indicate contrasting degrees of mineralization, suggesting varied mineralization conditions, ranging from weakly mineralized waters to waters heavily enriched by the processes of possible saline intrusions. pH values remain generally consistent with natural waters, despite a few slightly acidic occurrences which may reflect the influence of soil CO₂ or organic environments. The high and scattered concentrations of NO₃⁻ and Fe, meanwhile, suggest anthropogenic sources (infiltration of agricultural or domestic effluents). Finally, the major parameters Ca²⁺, Mg²⁺ and HCO₃⁻ show consistent levels. Taken together, these results provide a basis for understanding hydrogeochemical processes and justify the use of multivariate approaches such as CAH to better distinguish between water types and their controlling mechanisms.

Table 6. Statistics parameters.

Ascending Hierarchical Classification (AHC) was performed using the physicochemical and chemical parameters of groundwater to identify groups of samples with similar hydrochemical characteristics. The classification was carried out using XLSTAT software according to Ward’s method, with the Euclidean distance used as a measure of dissimilarity between samples. The optimal number of clusters was determined based on the dendrogram and the analysis of the loss of inertia during the clustering process.

The results of the Ascending Hierarchical Classification (AHC) reveal the existence of three main water groups (I, II, and III), which are themselves subdivided into six distinct sub-groups (Figure 4), reflecting distinct hydrochemical facies within the aquifer. In order to characterize these groups, the median value of the main chemical parameters was calculated for each class, a method that helps to limit the influence of extreme values. The corresponding summary results are presented in Table 7, providing a basis for the comparative interpretation of the different water types.

Table 7. Median chemical composition of hydrochemical parameters in six groups from the CAH.

Group 1, which is highly representative of the dataset (147 samples), is characterized by low values of EC (651 μS/cm), TDS (386 mg/L), Ca²⁺ (27.2 mg/L), Mg²⁺ (19.2 mg/L), and Na⁺ (62 mg/L). The moderate HCO₃⁻ concentration indicates weakly mineralized waters, characterized by limited interaction with the aquifer matrix and relatively short residence times. This hydrochemical signature corresponds to recent recharge waters, consistent with isotopic observations reported by Djongon (2022) and Bleza (2023), which indicate that groundwater in the Continental Terminal aquifer mainly originates from precipitation, with a contribution from precipitation affected by evaporation. The low dissolved element concentrations observed in Group 1 suggest rapid groundwater circulation in areas favorable to recharge. However, the median NO₃⁻ concentration (38 mg/L) indicates a moderate anthropogenic influence, likely associated with agricultural activities or diffuse domestic inputs. Group 2 is characterised by extreme values for EC (10,380 μS/cm), Cl⁻ (2,803 mg/L), Na⁺ (930 mg/L), and SO₄²⁻ (605 mg/L). This group reflects high mineralization of saline origin. The compositions indicate a significant marine influence. This hydrochemical evolution is consistent with the observations of Dun et al. (2022), who showed that seawater intrusion, combined with cation exchange processes and water-rock interactions, controls the chemical composition of coastal aquifers. Similarly, Kumar et al. (2022) reported that enrichments in Na⁺, Cl⁻, and SO₄²⁻, along with high electrical conductivity values, represent indicators of salinization in coastal aquifers. Nitrate concentrations are very low, indicating denitrification processes in an environment close to neutrality (Diouf et al., 2022). Group 3 exhibits high EC values (3,564 μS/cm) and elevated HCO₃⁻ concentrations (402.6 mg/L), associated with high levels of Ca²⁺ and Mg²⁺. These characteristics indicate groundwater influenced by carbonate mineral dissolution and subjected to cation exchange processes during its circulation within the aquifer. Similar processes have been reported in several coastal aquifers (Tziritis et al., 2024), in the Rhodope coastal aquifer in Greece, showed that groundwater hydrochemical evolution is mainly controlled by water-rock interactions, particularly carbonate dissolution and ion exchange processes (Yidana et al., 2011), in the Keta coastal basin in Ghana, reported that mineral weathering and exchange reactions between groundwater and the aquifer matrix control variations in groundwater chemical composition.

Group 4 shows high concentrations of Na⁺ (922 mg/L), Cl⁻ (1,482 mg/L) and SO₄²⁻ (320 mg/L), combined with marked mineralization (6,781 μS/cm), indicating a combination of saline processes and the dissolution of sulphate evaporites (gypsum, anhydrite). This group also exhibits a slightly acidic pH, which may locally promote the solubilization of certain minerals. In coastal aquifers, ionic ratios such as Na⁺/Cl⁻, SO₄²⁻/Cl⁻, Ca²⁺/Cl⁻, and Cl⁻/HCO₃⁻ are widely recognized indicators for distinguishing the effects of seawater intrusion, cation exchange processes, and water-rock interactions (Custodio, 2005). In the present study, the analysis of the Cl⁻/HCO₃⁻ ratio shows that 44% of the samples belong to moderately to strongly affected classes, including 18% classified as adversely polluted and 16% as highly affected, indicating a significant chloride influence in some areas of the aquifer. This distribution confirms that the most mineralized waters of Group 4 are likely subjected to more pronounced salinization processes compared with the other groups. Similar observations were reported by Edet (2017) and Aladejana et al. (2021), who showed that salinization of coastal aquifers generally results from the combined effects of saline intrusion, cation exchange, and mineral dissolution during groundwater circulation. Therefore, the chemical composition of Group 4 reflects highly evolved waters, whose mineralization is progressively acquired through water-rock interactions, mineral dissolution, and saline inputs.

Group 5 is clearly distinguished by very high concentrations of NO₃⁻ (115.3 mg/L), well above WHO standards. The moderate levels of Na⁺ and K⁺, combined with the concentrations of HCO₃⁻, indicate natural mineralization superimposed on significant domestic pollution. These waters pose a major health risk, particularly for infants.

Group 6 shows relatively low values for EC (1,662 μS/cm) and TDS (764 mg/L), and moderate levels of Na⁺ and Cl⁻. The presence of NO₃⁻ (47.2 mg/L) reflects a moderate anthropogenic influence, whilst the concentrations of Ca²⁺ and Mg²⁺ suggest the dissolution of carbonate deposits. These waters are intermediate between natural and anthropogenic types.

The correlation matrix (Table 8) reveals significant relationships between the main physicochemical parameters. Electrical Conductivity (EC) shows a very strong positive correlation with TDS, reflecting their shared influence by the overall mineralization of the water. EC and TDS also show high correlations with total hardness (TH), Ca²⁺, Mg²⁺, Na⁺, Cl⁻, and SO₄²⁻, indicating that these major ions make a decisive contribution to the dissolved load.

Table 8. Correlation matrix.

The quasi-linear correlation observed between Na⁺ and Cl⁻ suggests a marked saline influence, probably linked to marine intrusion. Furthermore, the positive correlations between Ca²⁺, Mg²⁺ and total hardness (TH) confirm the predominant role of the dissolution of carbonate minerals and the base exchange process in determining the chemical composition of the waters.

3.2 Spatial distribution of parameters

3.2.1 Physical parameters

The physical parameters of water allow for a rapid initial assessment in hydrogeology, describing the condition of the water without the need for detailed chemical analysis. The spatial distribution of these physical parameters is shown in Figure 5.

3.2.1.1 Electrical conductivity (EC, μS/cm)

The electrical conductivity of groundwater reflects its ability to conduct electric current, which is proportional to the concentration of dissolved ions. It is widely used as a rapid indicator of mineralization and salinity levels. High EC values may indicate several processes, including strong water-rock interactions, intense evaporation, or saline intrusion in coastal areas.

In this study, electrical conductivity shows marked spatial variations across the basin (Figure 5a). The observed values, ranging from 65.85 to 11,180 μS/cm, highlight a strong spatial heterogeneity. EC increases in the western part, particularly on the Agoè plateau, along the coastal strip, and in the eastern part near the Mono River, where values locally exceed the WHO guideline value of 1,500 μS/cm (WHO, 2011). Conversely, low conductivity values recorded in the northeastern and central sectors correspond to weakly mineralized waters associated with recharge zones, where residence time and water contact with aquifer formations are limited.

The high conductivity values observed in the southern and eastern sectors result from several hydrogeochemical processes, including evaporation and cation exchange. In coastal areas, increased mineralization is likely related to seawater intrusion, as indicated by the high concentrations of Na⁺ and Cl⁻. This interpretation agrees with the findings of Yomo (2024), who demonstrated, based on hydrochemical, isotopic, and electrical resistivity tomography data, that the upper aquifers of the Coastal Sedimentary Basin, particularly the Continental Terminal aquifer, are affected by lateral seawater intrusion and saline water upconing. These processes promote the development of sodium chloride and sodium-potassium chloride hydrochemical facies. This salinization is enhanced by intensive groundwater abstraction in the southern part of the basin, which facilitates the inland migration of seawater.

This interpretation is consistent with the detrital nature of the Continental Terminal aquifer, mainly composed of sands and sandy clays, whose high permeability facilitates hydraulic exchanges with the marine environment (Bleza, 2023). In urban areas, wastewater infiltration and domestic discharges also contribute to groundwater enrichment in dissolved elements. In the eastern part of the basin, the increase in ionic content is promoted by the dissolution of salts present in superficial formations and aquifer materials. Lamouroux (1966) showed that this area is characterized by halomorphic soils developed under the influence of saline waters, containing high amounts of soluble salts dominated by sodium chlorides and sulfates. The dissolution of these salts contributes to groundwater enrichment in Na⁺, Cl⁻, and SO₄²⁻, explaining the high mineralization observed in this part of the basin.

This spatial distribution is consistent with the hydrogeological characteristics of the Continental Terminal aquifer described by Djongon et al. (2025). According to this author, the saturated zone consists of a heterogeneous assemblage of detrital facies ranging from coarse sands to sandy clays. This lithological heterogeneity is associated with significant variations in hydraulic conductivity, as shown by the mapping of Gnazou et al. (2017) which identified several sectors with contrasting hydrodynamic properties. These differences control groundwater residence time and the intensity of water-rock interactions. Areas with slower groundwater flow favor prolonged contact with aquifer materials and greater mineral dissolution, whereas recharge zones characterized by faster groundwater renewal exhibit lower mineralization levels. This hydrogeological organization contributes to explaining the spatial heterogeneity of electrical conductivity observed in the Continental Terminal aquifer.

3.2.1.2 pH

pH measures the concentration of hydrogen ions (H⁺) in water and indicates whether it is acidic or alkaline. It governs the solubility of minerals, the mobility of metals, and the carbonate–bicarbonate equilibrium. pH is influenced by water-rock interactions, the partial pressure of CO₂ in the soil, and redox processes. Natural groundwater generally has a pH between 6.5 and 8.5. In this study covering the entire basin, pH varies from 5.2 to 8.6 (Figure 5b). This indicates that the waters are slightly acidic to slightly alkaline. The highest pH values, ranging from 7 to 8.6, are found mainly in the south, particularly in the Lake Togo area. Conversely, the northern part has more acidic waters with pH values below 7. The acidic nature of the groundwater can be attributed to acid precipitation resulting from atmospheric pollution and to the siliceous nature of the aquifer matrix. This observation is consistent with previous studies (Djongon, 2022; Akpataku et al., 2020; Fambi et al., 2021; Bleza, 2023).

3.2.1.3 TDS (total dissolved solids, mg/L)

Total dissolved solids represents the total concentration of dissolved substances (mineral salts, ions, organic matter). It corresponds to the sum of the major cations and anions. An increase in TDS indicates advanced chemical changes or saline contamination. Drinking water generally has a TDS of less than 1,000 mg/L (WHO, 2011). The distribution of TDS in this study confirms this spatial pattern (Figure 5c). The highest concentrations are found in the coastal zone, whilst they decrease gradually towards the interior of the basin.

3.2.1.4 Total hardness

Total hardness (TH) corresponds to the total concentration of calcium (Ca²⁺) and magnesium (Mg²⁺), expressed in mg/L of CaCO₃. It indicates the hardness of the water. It results mainly from the dissolution of carbonates and dolomites. A high TH promotes limescale formation, but has no significant direct impact on health. The results from the chemical analyses in this study (Figure 5d) show TH values ranging from 27.9 to 3399.9 mg/L of CaCO₃. It remains low to moderate in the northern and central parts. Conversely, it becomes high in the south-west, along the coast, and in the eastern part of the basin, where it exceeds the WHO recommended value (WHO, 2011).

3.2.2 Chemistry of major cations

The concentrations of dissolved inorganic elements in groundwater depend on the geological and geochemical characteristics of the environment through which the water flows, the duration of water-rock contact, and, increasingly, human activities (Freeze and Cherry, 1979). Analysis of the major cations provides a better understanding of the mechanisms governing groundwater mineralization in the Continental Terminal. The spatial distribution of each element is shown in Figure 6.

3.2.2.1 Calcium (Ca²⁺)

Calcium originates mainly from the dissolution of carbonate minerals [calcite CaCO₃, dolomite CaMg(CO₃)2] and calcium silicates. It is often the dominant component in the Ca-HCO₃facies, which is typical of recently recharged waters. Its concentration increases with residence time and the intensity of water-rock interactions. In the study area (Figure 6a), Ca²⁺ concentrations also show marked variation, with values ranging from 4 to 796 mg/L and an average of 72.8 mg/L. Across the entire catchment, concentrations are mostly between 50 and 100 mg/L. However, values exceeding 200 mg/L are observed in the south-east and west.

Overall, the major cations analyzed in the waters of the Continental Terminal aquifer show considerable variability. The maximum values are well above the averages, indicating the presence of areas of high mineralization. The spatial distribution thus confirms marked heterogeneity, linked to the geological context and marine influence.

3.2.2.2 Potassium (K⁺)

Potassium is found in potassium feldspars and micas. It is generally less mobile than sodium because it is bound by clays. High concentrations are rare and may indicate human influence. In our study area, K⁺ concentrations range from 0.3 to 154 mg/L, with an average of 10.1 mg/L. The low values are mainly found in the north-east of the basin. In contrast, the highest concentrations occur in the west, along the coastline, as well as in the south-east. The spatial distribution, therefore remains varied, even though overall potassium levels remain moderate (Figure 6b).

3.2.2.3 Magnesium (Mg²⁺)

Magnesium originates from the dissolution of dolomites, ferromagnesian minerals and silicates. It contributes to total hardness. A predominance of Mg may indicate a dolomitic influence or cation exchange processes. In this study, the concentration of Mg²⁺ ranges from 0.5 to 343.2 mg/L, with an average of 30.9 mg/L. Across the entire basin, concentrations remain below 50 mg/L. However, they show a marked increase in the coastal zone and in the south-east, where they exceed 150 mg/L, with locally higher peaks (Figure 6c).

3.2.2.4 Sodium (Na⁺)

Sodium is abundant in alkali feldspars and evaporites. It is released through the weathering of silicates and by cation exchange. In coastal areas, a high concentration may indicate marine intrusion. Enrichment in Na⁺ may also result from Ca-Na exchange on clays. In the present study, Na⁺ shows the greatest variations (Figure 6a). Concentrations range from 5.2 to 1,290 mg/L, with an average of 149.2 mg/L. The highest values are found in the south-west, along the coastal strip and in the eastern part of the basin. Conversely, the lowest concentrations, below 50 mg/L, are found in the north and center.

3.2.3 Chemistry of major anions

Analysis of the major anions (Figure 7) enables the identification of the factors responsible for the mineralization of the waters of the Continental Terminal. These elements result mainly from the dissolution of geological formations and water-rock interactions, but also from the marine influence in the coastal zone, which enriches the waters with Cl⁻. Added to this is the effect of human activities, particularly in urban and agricultural areas, which can locally increase concentrations.

3.2.3.1 Chloride (Cl^–)

Chloride is a conservative ion. It does not participate in major geochemical reactions and serves as an indicator of salinity or marine intrusion. It may originate from: marine aerosols; evaporites; wastewater.

Concentrations > 250 mg/L affect the taste of the water. In this study, Cl⁻concentrations ranged from 6 to 3904.3 mg/L, with an average of 299.8 mg/L. The highest concentrations are found in the south-western part, along the coastline, and in the south-eastern part, where they exceed 250 mg/L; concentrations then decrease gradually inland (Figure 7a). The trend in the chloride gradient, which increases from the coastal zone towards the north, clearly reflects the influence of the sea. These results highlight a hydrochemical gradient between fresh groundwater of continental origin and groundwater affected by saline inputs, particularly in areas influenced by seawater intrusion. The presence of sodium chloride hydrochemical facies in coastal sectors has also been reported by Bleza (2023) and Yomo (2024). The high mineralization observed in the southern part of the aquifer confirms the influence of saline intrusions, previously identified in the coastal aquifer by Odeloui et al. (2022). Similarly, the enrichment in chloride and sodium observed in some boreholes agrees with the findings of Yidana et al. (2011) and Yomo (2024), who attributed this hydrochemical evolution to seawater mixing, cation exchange processes, and groundwater residence time.

3.2.3.2 Bicarbonates (HCO₃^–)

Bicarbonate originates from the dissolution of CO₂ in water and the weathering of carbonates. It is the dominant component in continental recharge waters. It controls the pH buffer system and reflects the partial pressure of CO₂ in the soil. In this study, HCO₃⁻ values range from 24.4 to 1,464 mg/L, with an average of 179.6 mg/L. Concentrations above 380 mg/L are found mainly in the south-western and south-eastern sectors. Conversely, the lowest values, ranging from 24 to 76 mg/L, are found in the far north-east of the basin (Figure 7b). In the northeastern part of the basin, the occurrence of bicarbonate-rich waters in the sandy Continental Terminal aquifer likely reflects hydrogeochemical interactions with the underlying Paleocene limestone formations. Recharge waters, enriched in CO₂ within the unsaturated zone, dissolve carbonate minerals from the Paleocene formations, generating Ca²⁺ and HCO₃⁻ ions. Hydraulic exchanges between the two aquifers, facilitated by local hydraulic continuity or discontinuities within semi-permeable layers, allow the transfer of this bicarbonate signature into the Continental Terminal aquifer. When the hydrochemical facies evolves toward Na-HCO₃, this change likely reflects cation exchange processes between dissolved calcium and sodium adsorbed onto clay minerals. In the southern sectors, high HCO₃⁻ concentrations mainly result from groundwater enrichment in CO₂ derived from soils and organic matter decomposition. These elevated bicarbonate levels may also be associated with the influence of anthropogenic activities related to urbanization. The infiltration of domestic wastewater and the degradation of organic matter increase CO₂ production in the unsaturated zone, promoting bicarbonate formation in groundwater (Vengosh and Keren, 1996).

3.2.3.3 Nitrates (NO₃^–)

Nitrate is a major component of the nitrogen cycle. In natural environments, its concentration is generally low in deep aquifers. High concentrations indicate agricultural or domestic pollution. A concentration > 50 mg/L can cause methaemoglobinaemia in children. In the coastal sedimentary basin (Figure 7c), nitrate (NO₃⁻) levels range from 0 to 221.5 mg/L, with an average of 21.4 mg/L. Concentrations remain low in the majority of the waters analyzed. However, certain areas show high values, particularly in Greater Lomé, along the coastline and in the eastern part of the basin, where concentrations exceed 50 mg/L. These concentrations reflect the influence of human activities. The high nitrate concentrations observed in the Continental Terminal aquifer are mainly explained by anthropogenic inputs and nitrogen transformation processes. Several studies conducted on the transboundary aquifer system have shown that nitrate is a marker of contamination associated with domestic discharges, septic tanks, and agricultural activities (Guidah Chabi et al., 2023; Alassane Zakari et al., 2026). In the presence of dissolved oxygen, these nitrogen inputs undergo nitrification, which oxidizes ammonium (NH₄⁺) into nitrate (NO₃⁻), the predominant and highly mobile form in groundwater. These results are consistent with those of Akpataku et al. (2020), who attributed the presence of nitrogen compounds in groundwater of Lomé to anthropogenic sources, including septic tanks, latrines, landfills, leaking sanitation networks, and wastewater discharge. In agreement with their conclusions, high nitrate concentrations represent a potential health risk, particularly due to the risk of methemoglobinemia in infants and pregnant women, while elevated ammonium levels indicate both reducing conditions and confirmed contamination of the groundwater system.

3.2.3.4 Sulfate (SO₄^2–)

Sulphate originates from the dissolution of gypsum, the oxidation of sulphides (pyrite) or human activities. High concentrations may indicate oxidation processes or industrial contamination. In this study, SO₄⁻ ion concentrations remain predominantly low to moderate throughout the basin. Values range from 3.4 to 605 mg/L. Concentrations increase in the eastern and western parts, where they exceed 250 mg/L. Conversely, they decrease gradually from the center toward the north of the basin (Figure 7d). The low sulfate concentrations observed across most of the basin indicate a limited contribution of evaporitic formations to groundwater mineralization. In contrast, the elevated concentrations recorded in the eastern and western sectors are likely the result of a combination of processes. In coastal areas, marine aerosols and interactions with seawater can enrich groundwater in sulfate (Gomis-Yagües et al., 2000; Appelo and Postma, 2005). In highly urbanized areas, wastewater infiltration, landfill leachates, and industrial effluents also represent potential sources of sulfate (Kumar et al., 2022). In addition, the localized oxidation of pyrite occurring within certain sedimentary formations may contribute to sulfate enrichment (Appelo and Postma, 1993). The combined influence of these processes explains the high sulfate concentrations observed in the eastern and western parts of the basin, whereas the low concentrations in the central and northern sectors reflect a weaker influence of marine inputs and anthropogenic pressures.

3.3 Hydrochemical facies

Analysis of hydrochemical facies from the Piper programme (Figure 8a), carried out using the Diagram software (Piper, 1944), reveals a distinct spatial distribution of groundwater. The results show a clear predominance of chloride-type waters (82%), reflecting a high concentration of Cl⁻ions, generally associated with Na⁺ and K⁺ cations. This hydrochemical behavior suggests a high degree of mineralization within the studied aquifer. Calcium and magnesium bicarbonate facies account for approximately 10% of the samples and are mainly located in the northern part of the basin. These waters, which are weakly mineralized, are characteristic of recharge zones where water-rock interactions remain limited. The chemical signature (HCO₃-Ca²⁺-Mg²⁺) reflects mineralization controlled primarily by the dissolution of carbonate indices. These results indicate a short residence time and a limited influence of secondary geochemical processes.

Conversely, the high concentrations of Cl⁻, Na⁺ and, to a lesser extent, K⁺ recorded in the southern part of the basin suggest a residual marine influence on the composition of the groundwater. This chemical signature may result either from the chemical legacy of sedimentary formations of marine origin or from a contemporary saline intrusion. The Cl-Na⁺-K⁺ facies, widely represented in the south (58%), reflect a chemistry typically associated with coastal environments or evaporitic and marine formations. The salt-rich clay-sand layers, frequently observed in the Continental Terminal reservoir, gradually release chloride and sodium ions during water-rock interaction, reinforcing the progressive mineralization observed along the north–south trend (Figure 8b). The abundance of these ions may also be amplified by absorption/desorption phenomena and Ca-Mg ← →Na-type cation exchange, which is particularly active in clay layers of marine origin.

3.4 Chemical quality of the groundwater

Table 9 shows the classification of the water quality of the Continental Terminal aquifer, determined using the Water Quality Index (WQI) for the period June–July 2025. The results show that water quality is largely satisfactory in relation to the parameters studied. Out of a total of 246 samples, 172 (70%) correspond to excellent water quality (0 < WQI < 25), whilst 30 samples (12%) fall into the good category (25 < WQI < 50). In total, 82% of the samples fall within a range compatible with drinking water criteria, indicating an overall acceptable chemical quality for human consumption.

Table 9. Classification of the quality of water intended for human consumption.

The remaining 18% are of poor quality (IQE > 50). The ‘poor’ (6%) and ‘very poor’ (4%) categories suggest a gradual deterioration generally linked to increases in total dissolved solids (TDS), sulphates, chlorides and, in some cases, nitrates. The 20 samples unfit for consumption (8%), characterized by an IQE > 100, reflect severely degraded hydrogeochemical conditions, associated with increased mineralization or localized anthropogenic inputs.

Figure 9 shows the spatial distribution of the CT aquifer’s WQI in the Togolese coastal basin. The best water quality is found in the northern and central areas, where the ‘excellent’ and ‘good’ categories predominate. This distribution suggests a limited influence of human activities and relatively low mineralization.

In the western part, to the west of the River Zio, where the main urban centers and industrial infrastructure are concentrated, EQI values remain generally acceptable, but show increased vulnerability due to the density of human activity. Further north, water quality remains at a level ranging from good to excellent, consistent with lower levels of human impact.

Along the course of the River Zio, water quality is predominantly poor to very poor, which is consistent with the intensive use of agricultural inputs in the riverside market gardening areas. This area in the Zio Valley acts as a discharge point contributing to the contamination of the groundwater.

Further south, water quality deteriorates even more, likely due to the combined effects of urbanization, sewage infiltration and interactions with lagoon waters, in accordance with (Bleza, 2023; Akpataku et al., 2020). A gradual deterioration is also observed towards the coast.

In the eastern part of the basin (east of Haho, north-east of Tabligbo and the Moyen Mono area), water quality indicators show greater variation, ranging from excellent to very poor. The low proportion of heavily polluted water (8%) nonetheless reflects unfavorable local conditions. The localities of Agomé-séva, Batonou and Avévé have the highest concentration of water classified as unfit for use, requiring treatment prior to any use.

Finally, in the central area between the Lili and Zio rivers, water quality deteriorates progressively from the Tsévié Plateau toward Lake Togo, reflecting the combined effect of natural mineralization, underground flows heading towards the coast, and increasing human pressure.

Table 10 shows that groundwater quality degradation, as reflected by the WQI, is controlled by two distinct yet complementary mechanisms. The strong positive correlations (r > 0.90) among Cl⁻, EC, TDS, Ca²⁺, Mg²⁺, and Na⁺ indicate that these parameters are governed by the same hydrogeochemical processes related to groundwater mineralization. In contrast, the correlations involving K⁺ and NO₃⁻ reflect the influence of anthropogenic activities. Although these two mechanisms are geochemically distinct, their combined effects contribute to increasing WQI values, indicating a deterioration of groundwater quality.

Table 10. Correlation matrix showing the contribution of the analyzed parameters.

3.5 Suitability of water for irrigation

The assessment of the sodium adsorption ratio (SAR) is a key indicator for determining the potential impact of groundwater on soil structure when used for irrigation. The values obtained for water from the Continental Terminal of Togo’s coastal sedimentary basin indicate generally favorable quality (Table 11). Indeed, 99% of the samples have a SAR below 10, indicating water of excellent quality for irrigation. This low proportion of exchangeable sodium, well balanced by calcium and magnesium, limits the risks of clay dispersion and structural degradation of the soil (Saidi et al., 2004).

Table 11. Classification of irrigation water quality (Richards, 1965).

Only 1% of the samples fell into the ‘good’ category (10–18), whilst no sample had an SAR exceeding 18, suggesting that there is no significant sodium risk in the area studied.

The assessment of the suitability of water from the Continental Terminal for irrigation, based on Richards’ diagram (Richards, 1954) (Figure 10) shows that the majority of samples fall into class S1, indicating a low sodium risk and good compatibility with the soil. However, the distribution of data points across classes C2 to C4 reveals that salinity is the main constraint on irrigation. Groups 2, 3, 4 and 5 exhibit high electrical conductivity values, reflecting significant salinization, whilst groups 1 and 6 comprise the waters most suitable for irrigation. Thus, whilst the risk of alkalization remains moderate, the use of these waters in many cases requires measures to mitigate salinity and appropriate crop selection.

Analysis of Table 12 shows that only 3.66% of the 246 samples fall into class C1S1, indicating excellent quality. Good-quality water (C2S1) accounts for 38.6% of the total. The majority (46.8%) falls into classes C3S1- C3S2, characterized by high salinity requiring restricted use for salt-tolerant crops and on well-drained soils. Highly mineralized waters (C4S1-C4S2) constitute 7.7% of the samples and are usable only under strict conditions and for crops that are highly salt-tolerant.

Table 12. Classification of irrigation water according to EC and SAR.

4 Conclusion

This study provides new insights into the hydrochemical functioning of the Continental Terminal aquifer by highlighting the combined influence of natural mineralization processes, saline intrusion, and anthropogenic pressures associated with land use on groundwater quality. The differentiation of hydrochemical facies reveals groundwater evolution controlled by water-rock interactions, saline inputs, and anthropogenic sources of contamination. The integrated approach combining hydrochemical analyses, statistical tools, and Water Quality Index (WQI) assessment enabled the identification of the main factors responsible for the spatial variability of groundwater quality and the delineation of areas with increased vulnerability.

The findings are based on a single sampling campaign conducted between June and July 2025, corresponding to the end of the rainy season in southern Togo. This temporal constraint represents a limitation of the study because the hydrochemical characteristics of an unconfined coastal aquifer may vary with seasonal conditions. Periods of intense recharge can promote the dilution of dissolved constituents, whereas the dry season, characterized by reduced recharge and increased evapotranspiration, may enhance the concentration of dissolved salts and locally intensify saline influence. Likewise, nitrate dynamics may vary according to periods of nutrient input and soil leaching. Therefore, the results presented primarily describe the hydrochemical status of the aquifer during the sampling period and should be considered as an initial spatial reference.

The coastal and highly urbanized areas identified as vulnerable zones should be prioritized for groundwater monitoring and protection. Seasonal hydrochemical monitoring covering both wet and dry periods would provide a better understanding of the temporal evolution of salinization, contamination, and recharge processes. Such complementary data would support the development of more effective groundwater management strategies for the long-term protection of water resources in the Continental Terminal Basin.

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Footnotes

Edited by: Rosa María Fuentes Rivas, Instituto Potosino de Investigación Científica y Tecnológica (IPICYT), Mexico

Reviewed by: Noureddine Er-Ramy, Abdelmalek Essaâdi University, Morocco

Sacit Mutlu, Van Yuzuncu Yil University, Türkiye

Mohamed Ait Haddou, Universite Ibn Zohr Faculte des Sciences Agadir, Morocco