The statistics of the physical parameters analyzed are presented in Table 2.

The concentration of H+ protons in the water is measured by the pH of the water, which recapitulates the stability of the equilibrium between the forms of carbonic acid and is linked to the buffer system developed by carbonates and bicarbonates [22] . pH values range from 3.71 to 7.72 with an average of 5.89 in rainy seasons and from 4.01 to 7.47 with an average of 5.68 in dry seasons. It was noted that 71% and 94% of samples in the rainy season and dry season respectively had pH values outside the [26] and Benin standard norms, which range from 6.5 to 8.5. These recorded pH values indicate that the waters are generally acidic to slightly neutral. This means they could have a corrosive tendency on metals and distribution channels [23] [24] . In addition, acidic waters can cause certain adverse health effects, such as gastrointestinal disorders [25] and mucosal deterioration in humans. The acidity of water can be attributed to humic acid from organic or plant decomposition and possibly other geochemical processes [24] .

The temperature of the water sampled ranges from 26˚C to 31.8˚C, with an average of 29.08˚C, and from 26˚C to 33.9˚C, with an average of 29.23˚C for high and low-water periods respectively. It is close to the average atmospheric temperature. This indicates a thermal equilibrium between well water and the

atmosphere. It is possible that hot water influences the dissolution rate of minerals and promotes the proliferation of potentially harmful microbes, which may, in turn, influence the taste, color, odor, and corrosive tendency of water [26] . In addition, studies have shown that warmer waters generally contain less oxygen [24] [27] , indicating that they are anaerobic [28] . Such a situation could lead to 1) the reduction of nitrates to nitrites by reducing microbes, 2) an increase in the concentration of iron (Fe) in the water, and 3) subsequent discoloration of water transported through pipes when aerated [24] [27] .

The electrical conductivity (EC) of water is generally an indicator of the presence of ions and dissolved solids [24] . Conductivity ranges from 24 µS/cm to 920 µS/cm, with an average of 333.25 µS/cm in rainy seasons, and from 27 µS/cm to 1205 µS/cm, with an average of 320.46 in dry seasons. A median of around 266 (dry season) and 295 µS/cm (rainy season) indicates moderately mineralized water (Table 2).

We note that almost all the samples analyzed comply with the WHO standard of 1000 mg/L for dry residue (TDS). The waters analyzed are therefore low in mineral content and, on the whole, soft (TDS < 1000 mg/L).

The average TDS/CE ratio is 0.5. This suggests a high mineralization of the sampled waters [29] . Generally, this ratio should be close to 0.7 for waters with low mineralization. However, these ratios do not agree with the conductivity values obtained, which indicate weakly to moderately mineralized waters. This discrepancy between TDS/EC ratios and conductivity classes is thought to be due to the existence of unmeasured minor ions [29] [30] .

1) Major ions

The statistical description of major ions in groundwater is presented in Table 3. Overall, average concentrations of major ions in 2021 groundwater samples were higher than in 2020, except for SO 4 2 − , PO 4 3 − , and Mg²⁺.

The average concentration of cations in groundwater samples is in the order of Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ for both sampling periods. Ca²⁺ is the dominant cation present in the water samples collected. Ca²⁺ concentrations ranged from 0.71 to 107.26 mg/L in the rainy season and from 2.01 to 197.8 mg/L in the dry season. Most of the water analyzed had Ca²⁺ concentrations within the WHO acceptable limit of 75 mg/L and the Benin limit of 100 mg/L, except at five wells (Abattoir (P13), Agongbomè (P15), Tohouikamè (P8), Ouinda (P25) and Topkota (P26)) in the dry season and four wells (Agongbomè (P15), Abattoir (P13), Topkota (P26) and Gbedjromede (P14)) in rainy These high calcium levels could cause intestinal diseases, kidney or bladder stones, and urinary irritation [31] .

Mg²⁺ ranged from 0.44 to 29.79 mg/L and from 0.49 to 35. 21 mg/L in high and dry seasons respectively. Mean Ca²⁺ and Mg²⁺ concentrations are 25.21 and 31.54 mg/L, and 7.16 mg/L and 6.26 mg/L at high and low water respectively. Carbonate dissolution is an important origin of Ca²⁺ and Mg²⁺ in water, but as the aquifer system has no carbonate minerals, the contribution of Mg²⁺ ions would probably be due to the dissolution of silicate minerals and cation exchange processes [9] . Mg²⁺ concentrations were observed to be within the [26] and Benin standard of 50 mg/L.

Total hardness recorded values ranging from 5.36 mg/L to 358.20 mg/L and from 9.01 to 613.87 mg/L in wet and dry seasons respectively. The maximum total hardness value for drinking water is 600 mg/L [32] . However, hardness above 300 mg/L could lead to kidney and heart problems [32] . Concerning total hardness results, all water samples were identified as suitable for human consumption except those from P26 (613.87), P25 (485.60), and P8 (373.13) in the dry season and those from P14 (358.20), P15 (304.35) and P22 (Avlékété) (307.05) in the rainy season. Almost all samples therefore comply with the potability standard.

Mg²⁺/Ca²⁺ ratios ranged from 0.06 to 2.73 in rainy seasons and from 0.03 to 1.42 in dry seasons. They illustrate the relative proportions of these two chemical elements in a solution of water. Thus, hardness is more strongly influenced by fluctuating quantities of magnesium than by variations in calcium.

Na⁺ concentration varied from 0.07 to 96.20 mg/L in the rainy season and from 1.30 to 95.53 mg/L in the dry season, with an average of 15.04 and 19.24 mg/L, respectively. Silicate dissolution can increase Na⁺ concentration in samples. In addition, Na⁺ can also result from cation exchange [33] . Na concentrations comply with the [26] guideline value of 200 mg/L. The dominant processes in the contribution of sodium and chloride ions to the Continental Terminal aquifer would therefore be leaching, by rainwater, of soils salinized by the deposition of sea spray, dilution/concentration phenomena linked to recharge/evapo- ration processes and mixing with salt and brackish water [9] .

Average K⁺ concentrations were 4.17 and 5.60 mg/L in 2020 and 2021, respectively. Maximum K⁺ concentrations (12.38; 12.86; 13.59; 17.53; 22.98; 26.32 mg/L) were observed in Takon (P29), Agongbomè (P15), Gbèdjromèdé (P14), Sedjè-Denou (P31), Avlékété (P22) and Tchakpè-codji (P21) wells in the rainy season, and (12.86; 20.58; 22.93; 25.64; 49. 41) in wells P13, P21, P31, P26, P22 in the dry season. It was observed that 16% and 14% of samples in rainy and dry seasons respectively recorded an excessive K+ concentration above the acceptable limit set by [26] , which is 12 mg/L. It should be noted that in these areas, the water table is shallow (less than 5 m) and mostly collects the Quaternary aquifer, except at P31 and P29, where it is the border of the Continental Terminal aquifer. In addition, these wells are less than 5 m from sources of contamination, such as crop fields where fertilizers are used, and garbage dumps in densely urbanized areas. This means that these waters are subject to the effects of human activity. Most of the potassium in groundwater comes from human activities and surface salt and brackish water. A significant fraction of this potassium is involved in basic exchange phenomena (fixation by clay and organic matter) [9] . These water samples are considered to present a risk of hyperkalemia to their consumers [34] . Excess potassium can cause severe cardiac disorders, up to and including cardiac arrest and death in the absence of urgent chelator treatment.

HCO 3 − is the dominant anion in groundwater samples, with average concentrations in the order of HCO 3 − > Cl⁻ > NO 3 − > SO 4 2 − in the rainy and dry seasons. HCO 3 − ranged from 6.10 to 294. 50 mg/L in the rainy season and from 2 to 565.80 mg/L in the dry season, with an average of 66.93 and 74.03 mg/L, respectively. In addition, 5% and 10% of samples had HCO 3 − levels exceeding the [26] guide value of 250 mg/L in rainy and dry seasons respectively.

The presence of sulfates in groundwater may be due to natural or anthropogenic causes. Sulfate levels in the water varied from 0 to 93.6 in the rainy season and from 0 to 85.8 mg/L in the dry season, with an average of 10.27 and 10.02 mg/L, respectively. SO 4 2 − concentrations were within the [26] guideline value of 250 mg/L and the Benin (2001) guideline value of 500 mg/L.

Chloride levels ranged from 5.01 mg/L to 156.05 mg/L in the rainy season and from 2.30 mg/L to 152.30 mg/L in the dry season, with an average of 26.25 and 34.64 mg/L, respectively. All the groundwaters studied had chloride concentrations below the WHO and Benin permissible limit (250 mg/L), indicating low chloride contamination. Consequently, consumers of these waters are not exposed to health risks due to the ingestion of excess Cl.

2) Nitrogen and phosphorus

Anthropogenic inputs, such as industrial and agricultural activities, are a major source of NO 3 − in groundwater [35] . NO 3 − concentration in the study area ranged from 0 to 85.35 mg/L in 2020 and from 0 to 140.88 mg/L in 2021. Overall, in both rainy and dry seasons, 21% of samples had the highest NO 3 − concentration exceeding the guide values set by [26] and the Republic of Benin (2001), which are 50 mg/L and 45 mg/L respectively. These nitrate-rich waters are likely to cause diseases such as colon and rectal cancers, methemoglobinemia in infants, and non-Hodgkin’s lymphoma [36] .

NH 4 + varies between 0 and 0.42 with an average of 0.06 and between 0 and 5.85 with an average of 0.51 in rainy and dry seasons respectively. Approximately 12% of the samples (P29, P26, P14, and P13) analyzed had ammonium levels exceeding the Benin standard for drinking water in dry seasons. According to [37] , depending on the duration of exposure and the dose ingested, ammonium salt can cause human health problems such as pulmonary edema, dysfunction of the nervous and renal systems, and increased blood pressure.

Nitrite NO 2 − or nitrous nitrogen is a less oxygenated and less stable form, representing the transition between nitrate and ammonium. It is a toxic form and its origin is linked to agriculture or urban and industrial discharges [30] . Maximum concentrations are observed at P13 (6.17 mg/L), P14 (31.08 mg/L), and P15 (9.51 mg/L) in the dry season and P14 (8.76 mg/L) in the rainy season. These values exceed the guide value (3 mg/L) set by [26] and Benin (2001) for water intended for human consumption. This situation could have adverse effects on human health. According to [30] , excessive nitrite levels in drinking water can cause methemoglobinemia, leading to asphyxia in bottle-fed newborns, and cancer in humans. These high levels are probably due to the progressive accumulation of organic pollutants contained in urban wastewater. They are also due to the effect of oxidation of the ammonium form.

Orthophosphate concentrations recorded at the sampling point ranged from 0 to 2.12 mg/L and 0 to 0.07 mg/L, with an average of 0.19 and 0.02 mg/L in rainy and dry seasons respectively (Table 4). They are well below the permissible orthophosphate limit of 10 mg/L. Therefore, this parameter does not represent a major pollution risk for the waters studied and poses no danger for consumers.

The correlation matrices for the current study are shown in Figure 2 and Figure 3 for the rainy and dry seasons respectively. The following inter-parameter associations observed were: pH and Ca, pH and Mg, pH and HCO₃; EC and Ca, Mg, Na, K, HCO₃, Cl, SO4, and NO₃ in the rainy season. Whereas in the dry season, the inter-parameter associations observed are: CE and Mg, Na, K, Cl, SO₄. Based on the information provided, it can be deduced that alkaline earth metals contribute significantly to the pH and EC of the samples. In addition, the discrete correlation between Cl-NO₃ (r = 0.49), Na-NO₃ (r = 0.55), and K-NO₃ (r = 0.56) in the rainy season confirms that their origins are also linked to anthropogenic activities. Furthermore, the correlation between Mg-Ca (r = 0.70), Mg-Cl (r = 0.77), Mg-SO₄ (r = 0.75), Mg-HCO₃ (r = 0.74) Mg-K (r = 0.68) and Mg-Na (r = 0.67) in the rainy season and between Mg-Ca (r = 0.66), Mg-Cl (r = 0.74), Mg-SO₄ (r = 0.69), Mg-K (r = 0.73) and Mg-Na (r = 0.50) in dry season indicates that their presence in groundwater is influenced by the same source ( [31] ). The same observation applies to the high correlation between SO4-Na (r = 0.81) and SO₄-K (r = 0.81), SO₄-Cl (r = 0.84), K-Cl (r = 0.83), Na-Cl (r = 0.87), Mg-Cl (r = 0.77). The correlation matrices also reveal strong correlations between EC-Na (r = 74) and EC-Cl (r = 0.85) in the rainy season and between EC-Na (r = 80) and EC-Cl (r = 0.90) in the dry season, testifying to the influence of brackish and salt

water. Lastly, the discrete relationship between EC and NO3 (r = 60) and (r = 0.52) in rainy and dry seasons respectively, confirms that anthropogenic inputs play an important role in defining the hydrogeochemistry of groundwater in the area [28] .

For both sampling periods, factor groups with an eigenvalue greater than or equal to 1 were selected for this analysis.

In the rainy season (October 2020), the first two (02) dimensions of the analysis express 62.82% of the total dataset inertia (Figure 4(a)). This means that 62.82% of the total variability of the cloud of individuals (or variables) is explained by the plane. This is a high percentage, and the first plane therefore represents the variability contained in a very large part of the active dataset. The first factor explained 50.17% of the total variance and is correlated with all parameters (EC, Mg et Ca, Na, K, HCO₃, Cl, SO₄ and NO₃, pH), except T, PO₄, NO_2, and NH₄. Clustering the latest with the other elements suggests an influence of total mineralization by anthropogenic activities. This means that component 1 alone explains the natural and anthropogenic origin of the various elements. Component 2 is positively correlated with pH, highlighting the acidity of the water. In addition, the classification performed on the individuals shows 3 classes (Figure 4(b)). Class 1 is made up of P3, P4, P6, P25, P30 and P31. This group is characterized by low values of the variables HCO₃, Ca, Mg, EC, NH₄,

SO₄, pH, Cl, K, and PO₄. Class 2 is made up of F3, P13, P27, P28 and F8. This group is characterized by high values for NH₄, HCO₃, pH, Ca, PO_4, and Mg. Class 3 is made up of P14, P15, P21 and P22. This group is characterized by high values of the variables K, Cl, SO₄, Na, EC, Ca, Mg, HCO₃, NO_3, and NO₂ and is influenced by anthropogenic pollution.

In the dry season (March 2021), the first 3 dimensions of the analysis express 66.52% of the total dataset inertia. This means that 66.52% of the total variability of the cloud of individuals (or variables) is explained by the Dim1 Dim2 and Dim2 Dim3 planes. The first factor explained 42.4% of the total variance and is also highly correlated with all parameters (EC, Mg and Ca, Na, K, HCO₃, Cl, SO_4, NH₄). This also means that dimension 1 alone largely explains the natural origin of the various elements, but its correlation with NH₄ indicates a probable influence of anthropogenic activities. The second dimension, which accounts for 13.36% of the total variance, opposes pH and NO₂, which are positively correlated with T (˚C) but negatively correlated (Figure 5(a)). Component 3 accounts for 10.76% of the total variance, and opposes PO₄, which contributes positively to this axis, and NO₃, which correlates negatively (Figure 5(b)). This shows that components 2 and 3 explain the anthropogenic origin of the variables.

Moreover, for this period, the classification carried out on the individuals reveals 3 classes (Figure 6). Class 1 is made up of P3, P4, P6, F2, P17, F6 and P30. This group is characterized by low values of Ca, HCO₃, Mg, EC, SO₄, Cl, K, Na, and pH. Class 2 is made up of P8, P13, P15 and P25. This group is characterized by high values of HCO₃, Ca, pH, and NO₃. Class 3 is made up of P14, P22 and P26. This group is characterized by high values of Mg, SO₄, EC, K, Cl, Na, Ca and NO₃. The high nitrate values in these waters show their anthropogenic influence on groundwater quality and their contribution to mineralization.

SAR values calculated from analytical results range from 0.005 to 4.52 meq/L and from 0.06 to 2.94 meq/L in rainy (Figure 7(a)) and dry seasons (Figure 7(b)) respectively. These values correspond to those of water with a low risk of soil salinization. The SAR and conductivity values at 25˚C show the classes to which the samples belong (Figure 7). Analysis of this figure reveals four classes (4): C0S1, CAS1, C2S1 and C3S1.

- Classes C0S1 and C1S1 are characteristic of water of excellent quality for irrigation and present no risk to crops or soil. They represent around 60% of samples;

- Class C2S1, which represents good quality water for irrigation, covers around 35% of samples;

- Class C3S1, representing water of acceptable quality for irrigation, covering only 5% of samples.

The values expressed as a percentage (%) of sodium content and electrical conductivity (μS/cm) plotted on the Wilcox diagram (Figure 8(a) et Figure 8(b)), for both rainy and dry seasons, show that the majority of groundwater in the delta belongs to the category of excellent quality water for irrigation, except P26, P4, and P22 in the dry season and P21, P22, P2, P20 in the rainy season, which belongs respectively to the categories of good and admissible quality water for irrigation.

The RSC calculated in this work is between −2.93 and 0.31 and −3.99 and 0.44 meq/L respectively in rainy and dry seasons. These values are all below 1.25 and indicate good water quality for irrigation. Such low levels of RSC in water have no negative effect on soil fertility and plant growth [38] [20] .