Introduction Water supplies of many cities around the world depend on groundwater resources. But growing and uncontrolled urbanization, particularly in poor countries, has a negative impact on the quantity and the quality of this precious resource [1]. In Cotonou, economic capital of Benin, low incomes and remoteness of drinking water drains from some dwelling, especially the poorest, push them to use dubious quality waters, particularly those of wells [2]. Well water is used in households for several domestic purposes [3]. The water table that feeds the well is shallow and therefore vulnerable to pollution. Various forms of pollution affect water resources [4] one of the most common and widespread is chemical pollution. The chemical composition of water from the natural environment can be altered when toxic or undesirable substances come into contact with water table [5]. Studies about the quality of well water in urban environment of south-Benin showed that it is unsuitable for human consumption [2, 6, 7]. The polluted water consumption is the main source of waterborne diseases [8] covering 70% to 80% of diseases treated in hospitals [9]. These diseases affect the most, the poor area in developing countries [10, 11]. The present work focuses on the study of the chemical pollution of well waters used by the watersides populations of the sixth district of Cotonou. It aims to determine the nitrogenous pollutants (nitrate, nitrite) and the toxic micropollutants metals such as the lead and mercury identified like priority dangerous substances in the decision 2455/2001/CE of the Council of Europe [12]. Materials and Methods Study Area. The sixth district of Cotonou is located at between 6 22'0'' & 6 23'30'' North latitude and between 2°24'30'' & 2°26'0'' East longitude. It is situated in the peripheral area of Cotonou in the South of Benin. It is limited in the north by Lake Nokoué, in the south by the fifth district, in the east by the lagoon of Cotonou, and in the west by the seventh, eighth and ninth districts of Cotonou (Fig. 1). Fig. 1. Location of the study area It is the second district, the most peopled of the thirteen in the municipality of Cotonou. Nine of them (Vossa, Ahouansori-Agata, Ahouansori-towéta (I and II), Ladji, Hindé (I and II) and Djidjè (I and II)) were selected for this study because of the precarious situation of households and the massive recourse to well water. These are areas with inadequate access to safe water and sanitation [13]. The climate is equatorial with an alternation of two rainy seasons and two dry seasons. During the long rainy season, the district is threatened by severe flooding. The relief is flat with water table level which fluctuates between 0 and 5 m. Soil permeability is high and accelerates the infiltration of rainwater. The soil infiltration speed is higher than 8.3·10-5 m/s [14]. The relief and hydrogeology of Cotonou facilitate groundwater recharge and then flooding. Sample Collection. Samples were collected during the long rainy season of 2015 by Rodier method [15].Total number of samples analyzed comes from thirty wells distributed on the study areas (Fig. 2). Selection criteria of these wells are the distance separating them from a potential source of pollution, accessibility, owner's assent and level of frequentation. They were collected in plastic bottles of 1 liter. Each sample is identified by these references (date, place, number, time of sampling and measurements in situ) and remarks (potential sources of pollution, coverage state of wells and others). Determination of physical and chemical parameters. Parameters such as temperature, hydrogen potential (pH), electrical conductivity (EC), turbidity and dissolved oxygen were measured in situ by respective use of pH/Oxymeter with a probe WTW 340i, HACH-DR890 colorimeter and multiparamètre HACH-DR. 890. The chemicals compounds were dosed in the samples by using the spectrophotometer HACH DR1900. Nitrates, nitrites, lead and mercury were respectively evaluated by methods of cadmium reduction, diazotization, fast extraction of LeadTrak ™column and preconcentration at the cold vapor. The results were compared with the criteria of Benin [16] and EU /OMS [17]. Statistical analysis. A descriptive statistical analysis has been made by averages, minima and maxima determination. Principal Components Analysis was carried out in order to bring out the relation between studied parameters. These analyses were made by using software STATISTICA version 6. Fig. 2. Wells waters sampling sites Results and Discussion Physico-chemical quality of well waters. Results obtained (Table 1) show that water temperatures are between 26.7°C and 29.6°C with an average of 28.28°C. Table 1 Physico-chemical quality of well water analyzed Parameters Acceptable limit Averages Minima Maxima Na* Non-compliance rate, % Temperature, °C 25 28.28 ± 0.73 26.70 29.60 30 100 pH 6.5 to 8.5 6.95 ± 0.33 6.14 7.46 2 6.67 Conductivity, µS/cm 2 000 1162.07± 404.26 295.00 1 975.00 0 0 Turbidity, NTU 5 5.53 ± 4.92 0.00 20.00 12 40 Dissolved oxygen, mg/l 5 2.79 ±1.64 1.08 8.28 27 90 *A.samples numbers (Non-compliant). Temperature values of all studied wells exceed 25°C recommended by the WHO. The same observations were made by Degbey et al. [6] during their study about wells waters in Abomey-Calavi. According to these authors the high values can be explained by the influence of ambient heat on taken water and also by the geothermic gradient of the zone. pH values are between 6.14 and 7.46 with 93.33% of samples are out of the required values (6.5 to 8.5); 53.33% of samples are lightly acidic. The concentration in dissolved oxygen is between 1.08 mg/l and 8.28 mg/l. As for the conductivity’s values, they vary from 295 µS/cm to 1975 µS/cm. The turbidities varying between 0 and 20 NTU. These values, on the average (5.53 NTU) can be explained by the dilution of wells waters by the rains during the rainy season [18] (Table 1). Analyzed Nitrogenous pollutants in well waters: Case of nitrates and nitrites. Studied well waters present the concentrations of nitrates varying from16.38 mg/l to 349.85 mg/l with an average of 82.56 ± 94.37 mg/l. The non-compliant samples (PV2, PV3, PA3, PA4, PA5, PT2, PT4, PT5) exceed twice the limit value. The affected wells are located in Vossa Ahouansori-agata and Toweta areas. Nitrites concentrations are between 0.016 mg/l and 2.63 mg/l with a mean of 0.16 ± 0.47 mg/l. Compared to Benin criteria of drinking water, all studied wells waters are lower than the limit value for nitrite (3.20 mg/l), While 26.67% are non-compliant to the WHO criteria which is 0.1 mg/l. the results show that well waters’ concentrations in nitrates are higher than those of nitrites (Table 2). Table 2 Pollution’s level of Wells water by nitrogenous pollutants Settings Acceptable limit Averages Minima Maxima bN* Non-compliance rate, % Nitrates, mg/l 45 82.56 ± 94.37 16.38 349.85 8 26.67 Nitrites, mg/l 0.1 0.16 ± 0.47 0.02 2.63 8 26.67 *B.samples numbers (Non-compliant). Indeed nitrites are found rarely in significant concentrations in natural waters [16, 19] meanwhile nitrates are the dominant nitrogenous form in rivers and groundwater aquifers [20]. Nitrates and nitrites are ions naturally present in the environment [3]. In the absence of contamination, the nitrates content of groundwater varies from 0.1 to 1 mg/l, whereas it exceeds 50 mg/l often nowadays, the standard retained for drinking water by WHO [21]. The nitrate in water consumption is attributable to human activities and urban insalubrity [22]. Higher concentrations of nitrates in the studied wells water may be due to domestic solid waste, domestic waste of water, human or animal faeces [18]. This is the example of wells (PV2, PV3) located at Vossa, very near respectively of the wild discharges and the latrines abandoned. These results are in accordance with those of Mpakam et al. [23] who showed that there is a highly significant relationship between the position of the source of pollution (latrine, septic tanks) and the wells. In general, the pollution of all sampled wells with nitrates may be related to inadequate sanitation facilities and garbage collection. Indeed, 68% of the population in the study area evacuates household wastes in nature; 52% throw out domestic wastes water and 26% defecate in nature [3]. Infiltration of various wastes in nature may be favored by rains. 95% of the populations use well water for various domestic purposes whereas nitrates and nitrites can cause various health complications such as irritation, allergies, abortions, cancer and chemical poisoning [20]. An example of the danger is that nitrites cause iron’s oxidation of hemoglobin in red blood cells. The result of methemoglobin which in high levels jeopardizes the ability of blood to carry oxygen in the body cells [24]. Analyzed metals micropollutants of wells waters: Case of lead and mercury. Results ‘analysis of these toxic substances (Table 3) in the suited well waters show the presence of lead and mercury in all samples. Table 3 Pollution’s level of wells waters by the toxic metallic micropollutants Settings Acceptable limit Averages Minima Maxima cN* Non-compliance rate, % Lead, ug/l 10 2.20 ± 1.19 1.00 5.00 0 0 Mercury, ug/l 1 0.51 ± 0.38 0.10 1.50 4 13.33 *С. samples numbers (Non-compliant). Indeed, lead concentration varies of 1 mg/l to 5 mg/l with a mean of 2.20 ± 1.19 mg/l. These values are all below the allowable limit for drinking water which is 10 mg/l [22] and 50 mg/l [16]. Mercury concentrations are between 0.1 mg/l and 1.5 mg/l with a mean of 0.51 ± 0.38 mg/l. 13.33% of wells have concentrations out of the mercury standard. Lead and mercury, toxic metals, have already been detected in wells waters by several authors [6, 25, 26]. Lead and mercury are for a large majority of anthropic origin [27, 28]. Their presence in sampled well water can be related to toxic waste of the wild discharges installed along the shore of Lake Nokoué and Cotonou’s lagoon [29]. Mercury comes in either from everyday objects, such as fluorescent lights, toggle switches, products of telecommunication and information technologies, domestic thermometers and thermostats and dental amalgam [30]. Lead and mercury belong to the metals identified like priority dangerous substances, and are subjected to an objective of zero discharge in the groundwater, according to the Decision 2455/2001/CE of the European council [12]. At high levels of exposure, lead causes cognitive and neurobehavioral disorders [31]. In pregnant women, it crosses the placental membrane and can harm the development of central nervous system of foetus. In young children, the permeability of the blood brain barrier increases sensitivity to the toxic effects of lead. Lead poisoning is also one of the related disorders from excess of lead and results in clinical disorders, biological anomalies and varied histopathologic changes [6]. The mercury detection in these samples confirms the dangerous level of pollution of wells because it is the most toxic heavy metals existing in the environment [27]. It is considered by WHO as one of the ten chemicals or groups of extremely alarming chemicals compound for the public health [32]. In addition to this feature, the mercury can also travel long distances in the air. These characteristics make it transborder contaminants that could be deposited in a country other than their origin country. Its effects will be felt by widely separated populations of contamination sources and it is very persistent, several generations will be affected. Studies have also shown that methyl mercury is the most toxic form [30]; it may increase the risk of cardiovascular diseases [33] and infertility [34]. Classification of pollution’s state of the studied wells waters. This analysis was realized on thirty (30) wells and variables including four chemical pollution parameters (nitrates, nitrites, lead and mercury) and five physicochemical parameters (temperature, pH, turbidity, dissolved oxygen (O2dis) and electrical conductivity (CE)). The F1-F2 factorial design can explain 56.81% (F1: 34.89% and F2: 21.92%) of the total variability. The projection of the variables on the factorial plane F1-F2 (Fig. 3) shows that nitrates, lead, and mercury are positively correlated with F1. Fig. 3. Determined parameters’ projection on the factorial design Fact./Fact2 This axis, by its positive pole, informs about the majority of the parameters which determine the degree of the nitrogenous and toxic pollution of water. The conductivity is positively correlated with F2 which expresses the water mineralization. The analysis of the projection of the individuals on the factorial design F1-F2 (Fig. 4) defines a distribution of the wells in four groups divided respectively into these proportions. Fig. 4. Determined wells’ projection on the factorial design Fact./Fact2 Group I (20%): it includes the wells PV3, PV5, PA4, PT4, PT2 and PA3; these waters have a high degree of nitrogen pollution (NO3-) and high toxic metal (Pb Hg). Group II (30%): it includes PV1, PA1, PA2, PH3, PH5, PD4, PL3, PD1 and PD5; these waters are characterized by a low degree of nitrogen and toxic metal (NO3- Pb, Hg). Group III (20%): it contains the wells PV2, PT1, PT3, PL1, PL2 and PL4 which are high mineralization and low nitrite. Group IV (6.67%): it includes the two wells PA3, PD5; they are high degree of nitrite with low mineralization. Conclusion This study highlighted the state of chemical pollution of 30 traditional wells waters in the sixth district of Cotonou by nitrogenous pollutants and toxic metallic micropollutants. Studied well waters present a high nitrate and mercury pollution and are weakly polluted by nitrite and lead. Water pollution can be attributed to toxic waste, dumps garbage installed in study areas. Inadequate infrastructures of sanitation can be also incriminated. The infiltration of various wastes released into the environment can be facilitated by the period of the study which is the rainy season. The health risks associated with consumption of these waters without any treatment are important.