Evaluation of the Physico-Chemical Quality and Potability of Groundwater Consumption in Department of Collines at Benin

The purpose of this study is to assess the physical and chemical quality of borehole water intended for consumption in the collines department in Benin. At the end of a sampling campaign, twenty-one (21) drinking water points were sampled. Different physico-chemical parameters were measured using standard analysis methods. The results of the analysis of the samples, showed that the groundwater of the department of the hills is characterized by a neutral pH and an average electrical conductivity in accordance with the WHO and Benin standard relating to the potability of groundwater. With this pH neutrality which would be linked to the nature of geological formations made up of crystalline rocks, the water in the hills is moderately hard with hardness values of 208 mg/L on average for magnesium ions of 22.54 mg/L on average and calcium ions of the order of 46.03 mg/L on average at the scale of the various localities and an alkalinity is of 43.81 mg/L on average. The values between


Introduction
Benin has a significant renewable water resource potential estimated on average at 13 billion cubic meters per year for surface water and an average annual recharge of about 2 billion cubic meters for groundwater (Azonsi et al., 2009). In terms of uses, around 90 million cubic meters of water are currently withdrawn from available groundwater resources, it's mean less than 5% of the potential, while surface water reservoirs can only store one around thirty million cubic meters, or less than 0.3% of surface water resources drained by the country's hydrographic network (Directorate General for Water, DG Eau 2006). In basement regions, groundwater resources are the most in demand for the DWS because of their technically favorable and financially less costly operating conditions. Suddenly, there is strong pressure on this component that can affect its availability in quantity and quality in the medium and long term (Tossou, 2016).
In the Collines department in Benin, a region known to be hydrogeologically difficult where the drinking water supply sector is dependent on groundwater, the excessive use of chemical fertilizers and pesticides to increase agricultural yields constitutes a danger for the quality of groundwater. It should be remembered that the quantitative and qualitative composition of groundwater in dissolved and suspended matter, of mineral or organic nature, determines its quality (Jain et al., 2005) and this quality can be altered when external substances enter in contact with the aquifer. This is the case with undesirable or even toxic substances which make groundwater and surface water unsuitable and toxic for various uses, in particular for the use of drinking water (Méhounou et al., 2016). The poor quality of drinking water is a public health problem (WHO, 1981;World Health Organization, 2006). Thus, since the different water user and consumer sectors can induce more or less negative effects on the quality of groundwater resources, it is important that this water be subject to systematic and regular analyzes of its quality. This reflects the objective of this study, which is to assess the physical and chemical quality of borehole water intended for consumption in the hills Department in Benin.

Hydrogeological Framework of the Study
The Hills Department, which covers an area of 13,931 km 2 , is located between 7˚27'' and 8˚46'' North latitude and between 1˚39'' and 2˚44'' East longitude.
Rainfall and temperature are two factors that influence the availability of water in hill towns. The rainfall regime follows a bimodal distribution in the South and  -Ganta, 1987;Boko, 1988;Afouda, 1990;Houssou, 1998;Ogouwalé, 2006). And despite the amount of water flowing and seeping in, drinking water supply remains problematic due to the geological nature of the subsoil. Geologically, the department of hills is located on the scale of the structural unit of the plain of Benin and presents a great complexity of geological formations affected by several phases of deformation, metamorphism and magmatism (Boukari, 1982). Figure 1 shows outcropping lithological formations. In the southern part, migmatitic and gneissic formations are exposed, intruded by granitic plutons in circumscribed masses and a volcano-sedimentary series (Breda, 1989;Adissin, 2012). These outcrops have a general north-south direction (Dubroeucq, 1967). In the Northeast, we observe in places, plutons of porphyroid granite with biotite appearing in the form of plissote massifs placed in the host gneissic to migmatitic, the North-West being dominated by migmatites and gneiss of the Pira group.
Studies (Kamagaté, 2006;Kamagaté et al., 2007;El-Fahem, 2008;Kamagaté et al., 2008) on the hydrogeology of the basement aquifers of Benin, particularly in the upper valley of the Ouémé and on the characteristics of the aquifers in the study area, particularly in its southern part in Dassa-Zoumé and its surroundings (Boukari, 1982;2007) have shown the existence of two main reservoirs,

Sampling and Assay Methods
The assessment of the physico-chemical quality of groundwater in the hills was possible using water sampling taken during a campaign in October 2019. Thus, the physico-chemical parameters measured in situ and at laboratory, were used to assess the quality of this drinking water.  Sampling Sampling was done at 21 water points which are boreholes intended for the drinking water supply (DWS). Figure 2 shows the spatial distribution of the sampling points. The size of the sample covers the south of the department widely. To do this, 1.5 liter plastic bottles previously washed and rinsed in the laboratory were completely filled with water. These are then hermetically sealed to prevent any gas leakage. At the same time, a 250 ml glass beaker rinsed with  The pH, the temperature (T˚C), the total dissolved salt (TDS) and the Electrical Conductivity (CE) were measured by a pH/Oxi meter WTW 340i whose probe rinsed and immersed in a beaker containing the sample displays on the screen and after stabilization, the relative values of the three parameters in situ.
Chemical parameters such as calcium (Ca 2+ ), magnesium (Mg 2+ ) and chloride (Cl − ) are measured by the volumetric method. Other parameters including color, sulfate ( iodide (I − ), Ammonium ( 4 NH + ) and iron (Fe 2+ ) are made by the method of spectrometry. The assay protocols for these parameters were made by standard analysis methods as described by Rodier et al., (2009) or according to the catalogs of the equipment used.

Normalized Principal Component Analysis (ACPN)
The statistical study carried out by ACPN gives numerous results which are presented in Table 1 and Table 2. In Table 1, are recorded the eigenvalues, the variances expressed for each factor and their accumulations. The factor F1, with an expressed variance of 47.58% is the most important of all, then the factors F2, F3, F4 and F5 with respectively 13.06%; 10.33%; 7.36%; and 6.07% of the expressed variance. These five factors reflect most of the information sought and make it possible to represent the cloud of points in a significant way because the sum of the variance expressed by these factors is 84.40%. The contribution of the different variables in the definition of the main factors is given in Table 2. Each factor is defined by a certain number of essential variables in the demonstration of the mechanism of water mineralization. This table shows that the factor F1, the most important is defined by the electrical conductivity, the hardness, the alkalinity and the ions Cl − , Mg 2+ , Ca 2+ ,  Significant links existing between the different parameters are given by the correlation matrix. These links reflect the different correlations that exist between the parameters analyzed. Based on the critical correlation coefficient r = 0.64 (Mangin, 1970), electrical conductivity is strongly correlated with the majority of ions (r > 0.5). Note that electrical conductivity (  PO − and F − ions. Nitrogen compounds come from the degradation of organic matter by microorganisms in the surface layers of the soil, with the production of CO 2 which is then carried away in depth with the seepage water (Ahoussi et al., 2008). Better still, the naturally occurring 3 NO − ions which are part of the nitrogen cycle represent the most soluble form of nitrogen. Mainly used as inorganic fertilizers for plant growth and the synthesis of organic nitrogen compounds, excess nitrates can be found quickly in groundwater. Waste containing organic nitrogen also represents a source of nitrates obtained from various biochemical processes (ammonification and nitrification) (Aghzar et al., 2001;Amadou and al., 2014). This plan highlights the superficial exchanges that take place between the site's water and runoff from precipitation and soil drainage. The graphic representation in space of the statistical units of the factorial plane F1 -F2 and especially F1 -F3 highlights three main groupings of water points (Figure 4). Class 1 gathers waters of medium to high electrical conductivity, the ionic acquisition of which is under the control of mineralization

Variation of pH and Temperature (T ˚C)
The pH of water provides information on its acidity and alkalinity. The pH of natural waters is generally between 6.6 to 7.8 (Nisbet et al., 1970) and it varies from 7.2 to 7.6 (Rodier, 1984). Usually, pH values are between 6 and 8.5 in natural waters (Chapman & Kimstach, 1996). The nature of the land crossed by the water is the natural cause, causing significant variations in pH. The analysis of these waters revealed that the pH is close to neutral, at the level of all the water points (Figure 6), the average pH values at the level of the study area were within the drinking water standards of groundwater (6.5 -8.5 according to WHO standards). Water temperature is an important factor in the aquatic environment as it governs almost all physical, chemical and biological reactions (Chapman & Kimstach, 1996). In the study area, the measured temperature shows almost no variation ( Figure 6). The observed value is around 29.5˚. These values remain acceptable for drinking water standards.

Study of the Spatial Distribution of Chemical Parameters
The

Assessment of the Quality of Drinking Water
The quality of groundwater intended for consumption in the Hills was assessed following the study of the pollution parameters and the interpretation of a simplified grid (Table 3)   Very bad >to 3000 >1000 >100 Figure 13. Spatial distribution of the rates of nitrites and nitrates.  (Nouayti et al., 2015). Analysis of the samples has shown that the water quality is generally good or even excellent (Table 4). However, a cross-reading shows very poor-quality waters (zongo, toui and Aizon) of poor quality (karré, Missi, Logbo, Lozin, Ouédemè and Gbedjè) given the nitrate contents.

Discussion
The quality of water depends above all on its physicochemical parameters. The groundwater of the Collines Department is characterized by a neutral pH that meets WHO and Beninese standards for groundwater potability. These values are contrary to the values obtained in the municipality of Pobè (Lagnika et al., 2014) and in the district of Dêkin in Dangbo (Adéké, 2017), where they obtain acidic pH despite the significant presence of limestone in the area southern Benin. However, similar results have characterized the groundwater of Ulmès in Morocco (Dadi et al., 1997). The decomposition of organic matter in the surface layer of the soil produces the CO 2 responsible for the acidity of groundwater in humid tropical environments (Ahoussi et al., 2010). The neutrality of the pH values obtained could therefore be linked to the nature of the geological formations made up of crystalline rocks, where certain rocks by dissociation in water release carbonate ions 2 3 CO − .
The pH of water depends on the temperature. The temperatures obtained are almost invariant and higher than the value allowed by the WHO standard. These values are slightly lower than those found by Lagnika et al. (2014), Adéké (2017) and Tossou (2016). The latter give this rise to the influence of the ambient temperature and by the geothermal gradient mentioned by Degbey et al., 2010in Maoudombaye, 2015. The slight decreases in temperature observed in our study could be explained by the period and the time of sampling.   Kamagaté et al., 2007;El-Fahem, 2008;Kamagaté et al., 2008). This nitrate value is higher than that found by Adéké (2017)  nitrates. The latter reaches the groundwater by filtration and may be the main factor in the deterioration of the quality of these waters (Aghzar et al., 2002).
This pollution of groundwater in the department of hills by nitrates comes from human activities of agricultural origin. Likewise, the fairly high levels of total nitrogen in organic matter could also have an impact on the vulnerability of the water table to this pollution.
Iron is a trace element beneficial to the body at low concentrations. Its content in groundwater in the hills is 0.28 mg/L on average, below the norm. But it appears more at the level of Zoumè with contents higher than the standard. This value obtained is lower than the value recorded in the Tiassalé region in Côte d'Ivoire (1.38 -8.75 mg/L) by Oga et al. (2009), in the district of Dêkin commune of Dangbo in Benin (5.49 mg/L) by Adéké (2017). The latter as well as the results of Ahoussi et al. (2013) attach the high iron content of drilling water to the ferralitic nature of the more or less leached soil. The soil of the region of the hills of Benin not being ferralitic, the presence of iron in the groundwater at this polluted point could be of anthropogenic origin because it is fed by rainfall recharge from the surface. Indeed, according to Oga, iron gives water an unpleasant metallic taste, and a reddish brown color. The coloring of the water obtained in the area is then due to the presence of iron. This statement is confirmed by the analysis of variables which shows a correlation between colored water and Fe 2+ ions.

Conclusion
This study made it possible to assess the quality and potability of underground water resources in the hills. The analyzes carried out on twenty-one (21) drinking water points are carried out on physical (temperature, pH and electrical meters are higher than those of the WHO unlike those of the electrical conductivity which conform to it. The water sampled is highly mineralized in Mg 2+ ions. These ions are suspected of natural origin. The strong presence of Cl − and especially 3 NO − ions (a very good indicator of the vulnerability of aquifers) makes it possible to suspect the presence of derivatives of phytosanitary products and pesticides in drinking water. These products cause toxicity in the consumer.
Likewise, the relatively high total nitrogen content of organic matter could also have an impact on the vulnerability of the water table to this pollution. It is urgent to conduct a study of the biological quality of its sampling sites to target localities exposed to a health risk. This study thus reveals the non-drinkability of water resources in the hills.