The Gounti Yéna basin is located in the first 3 municipal districts of the 5 that make up the city of Niamey. The name of Gounti Yéna, which means “cool bottom” in Djerma, was given to this valley by the first inhabitants of the district in reference to the wooded valley with a cool microclimate. The Gounti Yéna watershed has an area of 61 km². It has an elongated shape (compactness index of Gravilius KG = 2.13) in a North-South direction where fruit gardens, market gardens (vegetable garden) and flower gardens have gradually developed [12]. The watershed has an altitude of 258 meters in its upstream part and an altitude of 179 meters in its downstream part at the level of its outlet which is located between the congress center and the Gaweye hotel, with a hydraulic path of about 25. The slope of the Gounti Yéna basin is generally low except in the main thalweg, downstream of Dar Es Salam where there are slopes of 10 m to 30 m/km, with a tendency to erosion and sinking of the bed which lasted until the relatively recent developments of the bed.

The Niamey region is subject to a semi-arid, Sahelian-type climate and is characterized by two very distinct seasons [13]: a short rainy season (June to September) and a dry season, from long duration (October to May) with average annual precipitation ranging from 350 to 750 mm and average annual potential evapotranspiration of 200 mm [14]. However, [15] states that in recent years, rainfall (annual total) rarely exceeds 600 mm/year.

Niamey is located in the valley of the Niger River, a river which constitutes the primordial element in environmental, economic and sociological terms and also in terms of risks for the surrounding populations. However, the Niger River is not the only flow vector and numerous Koris (generally temporary watercourses) mark the hydrography of the sector. Indeed, the Niger River with a maximum flow of 2340 m³/s, crosses the city of Niamey over a length of more than 15 km. However, it experiences a disturbance of its regime due to the phenomena of silting, sedimentation and water erosion.

On the hydrological level (Figure 1), the city of Niamey [16]:

- is bordered to the north and east by the Kori Ouallam and the Kori de Goudel Gourou;

- is crossed by the Kori Gounti Yena, which is the most important Kori crossing the city;

- is also at the crossing by the kori of Tondibia-Goudel, which has its source on a rocky outcrop to the north and which sands up the district of Goudel, and the kori of Yantala which is born on a rocky outcrop not far from the hospital national [17];

- present on the left bank, on the edge of the terraces overlooking the river, many very localized Koris, in particular towards the industrial area and the Saga district;

- present on the right bank of the Koris which descend from the plateau and some of which lead directly to the urbanized area.

Groundwater is the preferred water resource for drinking water, as it is safer from pollutants than surface water [18]. They are groundwater, layers of permeable land saturated with water.

The first water table encountered under the ground is the water table located less than 50 meters deep and generally separated from the surface by a few layers of permeable soil in certain places. To capture this water, wells, boreholes and springs are used according to the depth of the water table and rarely according to the needs.

The geology in the Gounti Yéna basin is made up of three (3) geological entities (Figure 2): at depth, the Meta-Liptako basement of lower Proterozoic age consists of plutonic (granites, granulites) and metamorphic (gneiss, quartzites, greenschists) in different states of alteration; this basement is covered by the Continental Terminal of Middle Eocene to Pliocene age and made up of alternating more or less clayey sandstone and versicolored clays with intercalations of levels of ferruginous oolites, these are the outcrops in the region at the west of Niamey belong to a vast quasi-horizontal sandy-loamy spreading reinforced by sandstone or lateritic levels and crowned by an armored plateau [19]. However, Niamey being located on the extreme southwestern edge of the Iullemmeden basin, it has only one layer known as CT3 [20]. Finally, near the Niger River and at the level of the koris are the Quaternary alluvia which are composed of coarse sands that are not very compacted.

The three geological formations give rise to different aquifers: the bedrock aquifer captured by deep boreholes, the Continental Terminal 3 (CT3) aquifers and the alluvial aquifer captured by surface wells and sumps. The piezometric levels measured in 2020 and 2021 confirm an underground flow which takes place mainly from the NNW towards the South [21] towards the river and drainage towards the Gounti Yéna which is superimposed on this main axis of flow [22]. The hydrogeological functioning of the Continental Terminal water table has been studied and detailed by [23].

Two types of measurement were carried out within the framework of this study with regard to laboratory measurements of water quality: these are physicochemical and bacteriological (E. coli) parameters.

For the physicochemical analyses, major ions and some heavy metals were measured, these are: Calcium, Magnesium, Sodium, Potassium, Bicarbonates, Chloride, Sulphates, Nitrates, Lead, Ammonium, Cadmium, Copper, Nickel, Chromium, Zinc, Phosphorus and Iron. These elements were measured and monitored in the different samples mentioned above at monthly measurement intervals for one year during the period from November 2020 to October 2021.

The microbiological characterization was carried out on the contents of faecal contamination indicators described by [25] [26] [27]. The contents of indicators of faecal contamination were focused on the determination of the rate of E. coli. These levels were determined and their evolution followed in the same samples. This time, for three periods: the rainy season, the cold dry season and the hot dry season.

The entomological survey took place during the period from October 2020 to June 2021 in order to cover all seasons of the year: rainy season; cold dry season and hot dry season. Each of the three study sites was subject to three visits due to one visit per season, considering 24 concessions, including 20 for indoor spraying and 4 for light traps.

As part of this study, campaigns were carried out in order to take samples which are then analyzed at the Quali-Control-Lab laboratory, at the Faculty of Agronomy of the University of Niamey and at the IRD representation in Niger. These samples are intended for physicochemical and bacteriological quality control.

The water is taken from two different, sterilized 500 ml bottles. The vial intended for physicochemical analyzes is filled to the top to avoid the formation of air bubbles. For the determination of the bacteriological parameters, the bottle is not completely filled in order to allow the bacteria to develop well. Both vials are kept in a cooler and transported to the laboratories.

For the determination of the physicochemical parameters, the materials used are: the HACH digital titrimetry, the HACH DR/3800 spectrophotometer and the JENWAY PFP 7 brand flame photometer.

For microbiology, we needed several tools such as 2 pipettes: 1 ml and 9 ml, 1 pipette with 8 tips, 4 trays, 4 sterile 15 ml bottles, sterile tips, lactate ringers, 1 marker, gloves, a support for flasks and a waste bin.

The rainfall data used come from FAZA and Dar_Es_Salam schools rainfall survey station.

For the entomological survey, the tools consist of white sheets, aerosol insecticide, light traps and petrie dishes.

Survey forms were developed and administered directly to the managers of the various health centers.

Ö Physicochemistry

The different analytical techniques for chemical parameters are as follows:

● Volumetry: The chemical parameters determined are: Calcium, Magnesium (EDTA test), Bicarbonates (sulfuric acid test), Chloride (silver nitrate test).

● Spectrophotometry: The parameters determined by this technique are Nitrites, Nitrates, Iron, Sulphates, Zinc, Phosphorus, Copper, Cadmium, Ammonium, Nickel and Chromium.

● Flame photometry: For this technique, the apparatus used is the flame photometer allowing the determination of Sodium and Potassium.

Ö Microbiology

Drinking water must be free of pathogenic microorganisms and must be free of any bacteria indicative of faecal contamination. Bacteria belonging to the coliform group as indicators of faecal contamination define the reference bacteria with Escherichia coli (indicators of recent pollution) as the main representative of this group of bacteria.

For the enumeration of Escherichia coli, the microplate technique using MUG has been recognized as being more specific, more precise, more accurate and faster than the methods used previously. It represents an important evolution compared to the usual techniques for Escherichia coli which, among the microorganisms indicating faecal contamination, is of particular interest.

The aim here is to carry out a qualitative and quantitative characterization of the micro-organisms (Escherichia coli: indicator of faecal contamination) of the various water samples along the Gounti Yéna basin.

The plates have 96 wells including 12 rows of 8 wells. The manipulation consists in developing the micro-organisms of the water sample in an incubator, after it has gradually undergone several successive dilutions (4 in our case), brought into contact with ringers lactate which will nutrient function for microorganisms then put in the 96 wells of our plate and closed with plastic to prevent evaporation and any other contamination. The incubator should ideally be at a temperature of 44˚C, because Escherichia are thermo-tolerant and this will already allow a kind of filtration to be carried out by eliminating other types of micro-organisms that cannot tolerate such heat. Incubation can be done at 44.0 ± 0.5˚C for a minimum of 36 hours and up to a maximum of 72 hours, in the context of this study, it is carried out at 44˚C and for 48 hours. After incubation, the wells showing blue fluorescence under UV at 366 nm are considered positive and later the Most Probable Number (MPN) is calculated. The most probable number represents the statistical estimate of the concentration of microorganisms sought in the sample analyzed. This estimate follows Poisson’s law. The lower and upper limits correspond to a 95% confidence interval. The detection limit is 75 MPN/100 ml.

The different results for the physicochemical and bacteriological parameters will be presented in correlation with rainfall.

To compare the water, we represented the chemical analyzes in the form of triangular (Piper) and semi-Logarithmic (Schoeller-Berkaloff) diagrams.

Ö Piper diagram

The Piper diagram [28] can be used to demonstrate the hydrochemical facies of the water samples analyzed. This diagram provides a convenient method for displaying, classifying, and comparing water types based on the ionic composition of different water samples [29]. The report of the annual averages of the results of the chemical analyzes of the water samples on the Piper diagram (Figure 3) shows: 1/3 of the samples are located at the sodium and potassium pole, 1/3 at the calcium pole and and 1/3 in the mixed enclosure at the level of the cation triangle. All the samples (100%) are found at the bicarbonate pole against at the level of the anion triangle.

The projection of these samples on the diamond diagram gives two types of facies (Figure 3); this is the sodium and potassium bicarbonate facies for two samples (W_FAZA pond and Deyzeibon resurgence point) and another type of calcium and magnesium bicarbonate facies for the Oriba station pond.

The concentration of the anions follows the following decreasing order: HCO 3 − -Cl⁻- SO 4 2 − - NO 3 − and that of the cations decreases following this order: Na⁺-Ca²⁺-Mg²⁺-K⁺.

Nitrates make up most of the anions when mineralization is high, while bicarbonates are more significant when mineralization is low [30], so when faced with relatively low mineralization, bicarbonates dominate over nitrates.

Ö Schoeller Berkaloff Diagram

The Schoeller Berkaloff diagram is a semi-logarithmic diagram which allows to graphically represent the results of chemical analyses. Thus, the annual average of the results of the chemical analyzes of the three (3) water samples from the study area reported on the Schoeller Berkaloff diagram (Figure 4) show that the peak obtained at the bicarbonate pole and at the level of the sodium pole and potassium in Shoeller Berkaloff’s Digram, confirm the name of the facies, it is of

the sodium bicarbonate and potassium type. The parallelism of the curves representative of the samples implies that they are of the same origin (Figure 4).

A total of 612 measurements were taken, i.e. 204 measurements per sample on the three sampling points chosen.

➢ Major elements

1) Calcium

Calcium is an alkaline earth metal extremely widespread in nature and in particular in limestone rocks in the form of carbonates. It is the major component of water hardness. The arrival of water therefore corresponds to a drop in the calcium content. We notice through Figure 5 that as soon as there is a rainfall the Calcium contents drop. These contents vary from 18 to 64 mg/l (Figure 5).All these values are below the standards set by the WHO.

2) Magnesium

Magnesium is the second significant element of water hardness after calcium. A maximum content of 59 mg/l is obtained(Figure 6).

3) Sodium

Sodium salts (eg sodium chloride) are present in virtually all foods (the main source of exposure) and in drinking water. Although sodium concentrations in drinking water are generally below 20 mg/l, they can be much higher in some countries. Levels of sodium salts in the air are normally low compared to levels measured in food or water. In surface water, it is difficult to establish a relationship with rainfall, the variation in these levels remains heterogeneous at the sampling points (Figure 7).

The sodium absorption rate (SAR) of the different samples and their conductivities (Figure 8) show that the latter have a low alkalizing power through the Riverside diagram, which implies that these water can then be used on n’ any type of soil in the context of irrigated crops without danger of alkalinization of the soils used.

4) Nitrite

Nitrites come either from an incomplete oxidation of ammonia, or from a reduction of nitrates under the influence of a denitrifying action. Water that contains nitrites is to be considered suspect. The nitrite ion ( NO 2 − ) is usually not detected in significant concentrations except in reducing environments, because nitrate represents the most stable oxidation state. Nitrite can be formed by microbial reduction of nitrate and, in vivo, by reduction of ingested nitrate.

In general, the largest source of human exposure to nitrates and nitrites comes from vegetables (nitrites and nitrates) and meat foods (nitrites are used as preservatives in many deli meats). Since the WHO standard is 3 mg/L, all the values measured remain below this standard(Figure 9).

5) Nitrates

The nitrate ion ( NO 3 − ) occurs naturally in the environment and is an important nutrient for plants. It is present in varying concentrations in all plants and is one of the links in the nitrogen cycle. The absorption of nitrates by ingesting vegetables, meat or water is rapid and greater than 90%; they are eventually excreted in the urine. In humans, nearly 25% of ingested nitrates end up in the saliva, 20% of which are converted into nitrites by the action of bacteria in the mouth. All forms of nitrogen are likely to be the source of nitrates by a process of biological oxidation. The contents are higher in the dry period than in the rainy one. This temporal evolution of NO₃ was also observed in the work of [31].

The nitrate inputs could be linked to the leaching of agricultural soils upstream of the catchment area and the discharge of wastewater from the city into the plans of water.

The levels show pollution of organic origin linked to contact with water from the cesspools (Figure 10).

The distribution of Nitrates remains very variable in space and time, despite factors favorable to the enrichment of water in nitrates, it appears that most water points have a concentration below the drinkability limit set by WHO [32] throughout the monitoring period except in September for the Oriba station pool and October and November for the W_FAZA pool.

6) Potassium

It is a mineral salt that plays an essential role for the proper functioning of the body. On the other hand, a very high potassium level in the blood can be dangerous and have serious consequences (for example serious heart problems). Potassium is widely distributed in the environment, especially in all natural water.

The levels of this element are higher at the start of the rainy season (Figure 11). The fact that these levels are higher at the start of the season means that the external inputs (atmospheric and anthropogenic) are greater during this period. Concentrations exceeding WHO standards are noted, especially during the rainy period.

7) Bicarbonates

There is no WHO standard for this element, but a high concentration of bicarbonates gives a salty flavor to the water. These levels are probably explained

by the low dissolution of carbon dioxide at ground level. They are lower during the hot dry period, particularly in February, March, April and May (Figure 12).

8) Sulphates

The sulfate anion is one of the least toxic anions. A laxative effect has been observed in people who consumed drinking water containing more than 600

mg/L of sulphates. Sulfates occur naturally in many minerals and are marketed primarily for the chemical industry. Their presence in water comes from industrial waste and atmospheric deposits. In general, the average daily ingestion of sulfates from drinking water, air, and food is about 500 mg, with food being the major source. In accordance with the work of [33], there are few sulphates. The levels recorded throughout the study period do not exceed the limited standard [34] (Figure 13).

9) Chlorides

They are often used as a pollution index [35]. Chloride in water comes from natural sources, sewage and industrial effluents, urban discharges containing road salt, and saline water intrusion. No guideline value based on health arguments is proposed for chloride in drinking water.

The chloride ion is never absorbed by geological formations, it is a special element. High concentrations of chlorides are linked to water pollution. The concentrations of this element are homogeneous with a good correlation between rainfall and chloride content (Figure 14).

➢ Heavy metals

Heavy metals likely to be found such as lead, cadmium, cyanides, mercury, etc...are dangerous, even in trace amounts.

1) Lead

The presence of lead in tap water is rarely due to its dissolution from natural sources. The standard set by the WHO is 0.01 mg/L and no trace of lead was detected in the various analyses.

2) Ammonium

The term ammonia includes non-ionized (NH₃) and ionized ( NH 4 + ) species. Ammonia in the environment is generated by metabolic, agricultural and industrial processes; it can also come from disinfection by chloramine. Intensive farming of farm animals can cause much higher levels in surface water. Ammonia contamination can also come from the interior lining of cement mortar pipes. Ammonia in water is an indicator of possible bacterial contamination or pollution from sewage or animal waste. According to WHO, concentrations should not exceed 0.003 mg/l.

Figure 15 shows contents exceeding this limit for all the sampling points and throughout its period with an increase in these contents especially in the rainy season. We note an increase in these levels in proportion to the rainfall in surface water. This increase in this period would be related to the bacterial decomposition of nitrogenous organic matter due to the slight decreases in dissolved O₂ levels recorded during these periods. These results agree with the work of [35] [36]. Ammonium is preferentially absorbed when algae simultaneously have NH 4 + and NO 3 − [37], which explains the low levels measured in the dry season, without neglecting the part of nitrification because the environment is well oxygenated.

According to [38], the dosage of ammonia gave very high results in surface water (Gounti-Yéna and pond). This ammonia pollution has also already been highlighted by [39] on a well in a garden in Gounti Yéna.

3) Cadmium

Cadmium enters the environment through wastewater; diffuse pollution is due to contamination by fertilizers and local air pollution.

The absorption of cadmium compounds depends on their solubility. Cadmium accumulates mainly in the kidneys and has a long half-life in humans (10 to 35 years). Cadmium has been shown to be carcinogenic when inhaled and the IARC has classified cadmium and its compounds as Group 2A (probably carcinogenic to humans). However, there is no evidence for the carcinogenicity of

oral cadmium and no clear evidence of its genotoxicity. The kidney is the main target organ for cadmium toxicity. All measurement values are above WHO standards except for two measurement points. These are the W_FAZA pond and the oriba station pond, respectively recording levels of 0.008 and 0.004 mg/l in April (Figure 16).

4) Copper

Copper is both an essential nutrient and a drinking water contaminant. Levels in running water or after complete flushing tend to be low whereas in standing water samples (Figure 17) concentrations are more variable and can be significantly higher (frequently above 1 mg/l). Drinking standing water can significantly increase total daily copper exposure.

5) Nickel

The contribution of water to nickel exposure can be significant when, for example, the level of pollution is high. The most commonly observed effect of nickel in the general population is allergic contact dermatitis.The WHO standard is 0.07 mg/l and the levels measured for surface water exceed the WHO standards for the two ponds in the hot dry season, particularly in February for the Oriba station pond and in April for the W_FAZA pond (Figure 18). These levels drop during the rainy season.

6) Chromium

Chromium is widely distributed in the earth’s crust. It has a range of valences from +2 to +6. Chromium (III) is an essential nutrient. The maximum value accepted by the WHO is 0.05 mg/l.All the measurements are heterogeneous and for a very large part exceed the WHO standards(Figure 19).

7) Zinc

Zinc is an essential trace element found in virtually all foods and drinking water in the form of salts or organic compounds. Zinc levels in surface water usually do not exceed 0.01 and 0.05 mg/l. The standard set by the WHO is 3 mg/L, this standard is exceeded for the Oriba station pond during the rainy season(Figure 20).

8) Phosphorus

Regarding phosphorus materials, the main indicator is the concentration of total phosphorus, which must not exceed 0.2 mg/L. Phosphorus flows have three origins: collective domestic sanitation, discharges from livestock buildings and phosphorus departures by soil erosion. This last parameter is very difficult to assess and depends not only on the type of soil, its richness in phosphorus, but also on land use and spatial planning. In surface water, phosphorus is essential for plants, but, in excessive quantities, it mainly contributes to modifying the biological balance of aquatic environments by causing eutrophication phenomena.

For all the analyses, high levels are observed in March (Figure 21). This increase in the concentration of phosphorus in the water of the Gounti Yéna basin during this period is due to the increase in organic compounds in the water, which promotes very advanced mineralization of organic matter.

9) Iron

It is soluble in the Fe²⁺ ion state (ferrous ion) but insoluble in the Fe³⁺ state (ferric ion). It is present in natural fresh water at levels between 0.5 and 50 mg/l. The presence of iron in water can promote the proliferation of certain strains of bacteria that precipitate iron. The WHO standard is 0.3 mg/L. The ponds of the oriba and W_FAZA station all have concentrations greater than or equal to the WHO standard throughout the monitoring period(Figure 22).

Observation: Depending on the time of year and the dosed parameter, the different measurement points have levels exceeding the standards set by the WHO, the return of this water to the drinking water distribution pipes of the SEEN remains a worrying fact as to the health of the population for whom this water is intended (Table 1).

The bacterial load is a little high in the hot dry season for the W_FAZA pond and the Deyzeibon resurgence point (Figure 23); this load decreases during all

the other two periods of the year for these sampling points. It is much higher in the rainy season at the Oriba station pond, then in the hot dry season and finally in the cold season (Figure 23). This can be explained by the fact that the Oriba station pond receives water from everywhere, therefore very heavy, in addition to that of the aquifer, while the W_FAZA pond receives almost only rainwater combined with rising water, of the aquifer as well as the Deyzeibon resurgence point whose nature characterizes it.

All sampling points are contaminated with agents of faecal origin. The quantity of micro-organisms presents in water increases with rainfall, although no study has so far been able to establish a simple law formally modeling these two parameters. Microorganisms are very common in surface water (Figure 23).

They are the group of pollutants that cause the most disease. It should be noted that the survival time of pathogens (bacteria) is shorter in acid soils (pH 3 - 5) than in basic soils.

The origin of water pollution can be natural or anthropogenic [40]. The main factors that control the physico-chemical and bacteriological quality of water are human activities, the hydrogeological context and the climate [41]. [42], 1996 and Kenfaoui, 2008 clarify that the main sources of anthropogenic pollution are agriculture, which applies diffusely to the territory; industries which are the source of very diversified and often localized discharges and domestic activities via waste water discharges or landfills. [43] distinguished four sources of pollution in the city of Niamey, including domestic sources, industrial sources, hospital sources and agricultural sources.

The presence of nitrites and nitrates in the water comes from deficient septic systems [44]; fertilizers, wastewater, landfills, runoff water, soil leaching by precipitation and nitrogen oxidation, oxidation of nitrites by bacteria following exchanges with wastewater.