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УДК 613.471:577.4
DOI 10.17021/2018.13.2.110.120
© A. Adjagodo, T.C. Guidi, K. V. Chegnimonhan, M. Agassounon Djikpo Tchibozo, N.C. Kelome, D. Mama, E. Agbossou, Y. Ameyapoh, 2018
ECOLOGICAL MONITORING OF THE QUALITY OF NATURAL RESERVOIRS
OF WATER IN SOUTHERN BENIN
Adjagodo Antoinette, Laboratory of Standards and Quality Control in Microbiology, Nutrition and Pharmacology (LNCQMNP), Faculty of Science and Technology (FAST); University of Abomey (UAC), 01BP526 01 / 01BP1636RP Cotonou, Benin; Thermal and Energy Laboratory of Nantes (LTEN), CNRS, Polytech Nantes, Chantrerie, Christian Pauc Street,44300 Nantes, France; Laboratory of Microbiology of Food Quality Control, UL-Lome-Togo, BP 1515 Lome, Togo.
Guidi Tognon Clotilde, Laboratory of Processes and Technological Innovations (LaPIT), University Institute of Technology of Lokossa (IUT-Lokossa), BP 133, Lokossa, National University of Sciences, Technologies, Engineering and Mathematics of Benin.
Chegnimonhan K. Victorin, Laboratory of Processes and Technological Innovations (LaPIT), University Institute of Technology of Lokossa (IUT-Lokossa), BP 133, Lokossa, National University of Sciences, Technologies, Engineering and Mathematics of Benin; Thermal and Energy Laboratory of Nantes (LTEN), CNRS, Polytech Nantes, Chantrerie, Christian Pauc Street, 44300 Nantes, France.
Agassounon Djikpo Tchibozo Micheline, Laboratory of Standards and Quality Control in Microbiology, Nutrition and Pharmacology (LNCQMNP), Faculty of Science and Technology (FAST); University of Abomey (UAC), 01BP526 01 / 01BP1636RP Cotonou, Benin; Thermal and Energy Laboratory of Nantes (LTEN), CNRS, Polytech Nantes, Chantrerie, Christian Pauc Street, 44300 Nantes, France.
Kelome Nelly Carine, Laboratory of Standards and Quality Control in Microbiology, Nutrition and Pharmacology (LNCQMNP), Faculty of Science and Technology (FAST); University of Abomey (UAC), 01BP526 01 / 01BP1636RP Cotonou, Benin; Laboratory of Geology, Mines and Environment, Faculty of Science and Technology, University of Abomey-Calavi, 01 BP 526, Benin.
Mama D., National Institute of Water, University of Abomey-Calavi, 01 BP 526, Benin.
Agbossou Euloge, National Institute of Water, University of Abomey-Calavi, 01 BP 526, Benin.
Ameyapoh Yaovi, Laboratory of Microbiology of Food Quality Control, UL-Lome-Togo, BP 1515 Lome, Togo.
Context: The Oueme River is the largest one in Benin and also the most exploited by the population. This water is used by the riparian population for their daily overall water needs involving consumption. Firstly, the impact of anthropogenic activities on the water of the Oueme River, physicochemical analyses have been carried out during the low water period in April 2016, then prognosis has been made relating to human health.
Materials and methods: Eleven (11) sampling sites were selected and at each site four samples were taken, meaning a total of 44 water samples. Several physical parameters (temperature, pH, suspended solids, turbidity, dissolved oxygen and conductivity) were measured in situ. Some chemical parameters (nitrates, nitrites, ammonium phosphates and sulphates) and chemical oxygen and biochemical oxygen demand were respectively determined by spectro-photometry and BOD-meter.
Results: The results obtained revealed that the average contents of nitrates, nitrites, phosphates and sulphates were respectively equal to 4.95 mg/l; 0.17 mg/l; 0.85 mg/l and 23 mg/l. The average values obtained for the temperature, the pH, the conductivity, the dissolved oxygen, the turbidity, the suspended solids and the Solid Total Dissolved are respectively 31.24°C; 7.18; 447.95 ^S/cm; 6.72 mg/l; 47.18 NTU; 31.09 mg/l and 198.63 mg/l. These values meet national and WHO standards, except temperature, nitrites and phosphates. BOD5 values vary from 1 mg/l to 30 mg/l with an average of 9.73 mg/l. The Chemical Oxygen Demand (COD) varies from 3.68 to 81.63 mg/l with an average of 27.47 mg/l. The results of the Principal Component Analysis (PCA) revealed two gradients: the first one represents the degree of pollution while the second describes the mineralization of water.
Conclusion: The values recorded for the nitrogen and phosphate parameters contribute to the eutrophication of the river.Due to the presence of nitrogen pollutants (nitrate, nitrite and ammonium) in the Oueme River, consuming its water constitutes a potential risk to diseases and may cause detrimental effects in the short or long term on human health.
Key words: anthropogenic activities, the Oueme River, eutrophication, pollution.
Introduction. Fluvial water is an indispensable resource for health, agriculture, industry, tourism, recreation, navigation, etc. Water resources come in three forms namely: atmospheric water, surface water and groundwater. The needs in water resources are increasing, due to human activities, including urbanization, population growth, rising of living standards, increasing competition and pollution [8]. Surface waters are fragile and complex systems which functioning can be altered by human activities. Thus, the quality of river water is often contaminated by various types of pollution due to many causes, especially human activities [16, 20]. Water is often affected by excessive use of agrochemicals, uncontrolled releases from industries, and solid and liquid wastes from households [2, 25]. At the lower Oueme valley in Benin, the water of the river is under anthropic pressure and consequently, its quality is likely to be at threat. The water of this river is used for various purposes by neighbor populations. It serves as a dwelling place for lake populations; it is also a way of transportation of persons and goods. Water is also useful for domestic purposes [3]. However, recent agriculture practice using chemical fertilizers within 30 meters of the surface water induces pollution of soils, water tables and rivers [27] by nitrates, nitrites, ammoniums and phosphates. Similarly, the use of sodium tripolyphosphates as additives in laundry detergents and dishwashing products is also a source of surface water contamination by phosphates. Among the major anthropogenic sources of phosphorus and nitrogen, domestic wastewater discharges account for a large portion of water pollution by phosphates [9]. The lower Oueme valley is not on the margins of these chemical pollution activities [3]. Thus, this study was undertaken to shed light on water pollution, furthermore to evaluate the physicochemical quality of the water of the lower valley of the Oueme.
Materials and methods.
Figure 1. Location map of the study area
The lower valley of the Oueme (Figure 1) is located between 6 °25 'and 6 °57' north latitude and 2 °21 'and 2 °38' east longitude to the South of Benin in the Department of Oueme. The valley is supplied by the biggest river of Benin that is the Oueme River. This river crosses four Communes of the lower valley namely Dangbo, Aguegues, Adjohoun and Bonou. The climate is subequatorial there with four seasons of unequal distributions, two rainy and two dry seasons. The hydrological regime of the Oueme is characterized by a minimum flow in March and a maximum one during the high-water period in September. But the period of low water extends from January to May inclusive. The flood arrives in June and the flow increases until September; it stays near the maximum during the month of October [19].
Sampling. The sampling water points (Figure 2) were chosen vertically at the level of the Oueme River, taking into account the distance (50 cm to 1 km) of polluting anthropogenic activities. Eleven (11) sampling sites were selected and at each site four samples were taken, meaning a total of 44 water samples. Water was sampled during the recession period (April 2016) for physico-chemical analyzes. The samples were taken at 50 cm from the surface in 1 litre flasks previously rinsed with the sampling water. Each sample was identified by means of a sampling sheet (date, place, time of collection and number). The so taken water samples were then stored at 4°C during transportation. The coordinates of the sampled points were taken with the GARMIN type GPS.
Figure 2. Location map of sampling points
Determination of physicochemical parameters. Physicochemical parameters such as pH, electrical conductivity, Total Dissolved Solids, turbidity and dissolved oxygen were measured in situ by Oxi 340i pH-meter, HACH-DR890 colorimeter and HACH-DR890 multiparameter. The Chemical Oxygen Demand was determined by the AFNOR method (T90-101). The Biochemical Oxygen Demand (BOD5) was found using an OxiTop BOD meter. The chemical parameters were identified using a DR 1900 spectrophotometer. Nitrates, nitrites, ammoniums, phosphates and sulphates were measured respectively by reduction to the cadmium, diazotisation, Nessler, PhosVer 3 and SulfaVer 4 methods. The WHO standard criteria [22] have been used to interpret the results as well as those of the coastal water quality assessment system [26]. This is the surface water evaluation grid with five classes:
• Class I: Water of very good quality, if it is represented in blue.
• Class II: Good quality water, if it is represented in green.
• Class III: Medium quality water, if in yellow.
• Class IV: Water of poor quality, if orange.
• Class V: Water of very poor quality, if it is represented in red.
Data processing. The values of the determined physicochemical parameters were submitted to a descriptive statistical analysis (average, minimum and maximum). The correlation matrix and the Principal Component Analysis (PCA) were also performed. The PCA makes it possible to highlight the relations between the physicochemical parameters of the waters of the Oueme River and their distribution in the various sites sampled. The MINITAB version 14 software was used for the realization of the correlation matrix and to achieve the Principal Component Analysis (PCA).
Results. Physical quality of the river water on the vertical during the low water period.
The physical water quality analysis results (Table 1) of the river show that the water temperature oscillates between 30.2 ± 0.05°C and 33.1 ± 0.05°C, respectively at Gbodje and Bembe 1 and Avrankanme with
an average of 31.24 ± 0.98°C for all the sampled waters. The measured temperatures are all higher than the WHO standard of 25°C. The pH values ranged from 7.1 ± 0.01 (at Dannou and Bembe 1) to 7.27 ± 0.009 (Gbodje) with an average of 7.18 ± 0.06. These values are within the range set by WHO which is 6.5 <pH <8.5. The highest electrical conductivity values (1967 ± 0.05 ^S/Cm) and Total Dissolved Solids (879 ± 0.5 mg/l) are recorded at Nokoue Lake junction and the lowest conductivity values (55.4 ± 0.09 ^s/Cm) and Total Dissolved Solids (24.4 ± 0.09 mg/l) are recorded at Avrankanme. Suspended solid water content ranges from 8 ± 0.5 mg/l (Bembe 1) to 118 ± 0.57 mg/l (Gboa) with an average of 29.45 ± 34.33 mg/l. Dissolved oxygen is from 2.49 ± 0.008 mgO2/l (Nokoue Lake junction) to 12.3 ± 0.05 mgO2/l (the Porto-Novo lagoon junction) with an average of 6.72 ± 3.39 mgO2/l. The turbidity values are from 14 ± 0.5 to 158 ± 0.5 NTU with an average of 44.82 ± 44.84 NTU. The most turbid water is obtained in the region of Gboa and the least turbid one appeared in Bembe 1.
Table 1
Physical quality of the water sampled during the recession period
Points of sample pH T °C Turbidity (Turb) (NTU) Electrical Conductivity (EC) (^S/cm) Suspended Solid (SS) (mg/l) O2dis (mg/l) Total Dissolved Solid (TDS) (mg/l)
Avrankanme 7.19 ± 0.005 33.1 ± 0.05 95 ± 0.5 55.4 ± 0.09 67 ± 0.5 9.27 ± 0.005 27.7 ± 0.09
Gboa 7.23 ± 0.008 30.5 ± 0.08 158 ± 0.5 58 ± 0.08 118 ± 0.5 6.44 ± 0.005 24.4 ± 0.09
Sotinkpekon 7.21 ± 0.005 30.6 ± 0.08 77 ± 0.5 66.5 ± 0.09 49 ± 0.5 3.97 ± 0.05 28 ± 0.5
Gouke 7.22 ± 0.005 31.8 ± 0.05 32 ± 0.5 77.1 ± 0.09 22 ± 0.5 11.53 ± 0.02 32 ± 0.5
Dannou 7.1 ± 0.05 31.4 ± 0.05 24 ± 0.5 82.2 ± 0.17 13 ± 0.5 3.74 ± 0.008 34.6 ± 0.08
Hetin Sota 7.12 ± 0.008 31.3 ± 0.05 26 ± 0.8 132.4 ± 0.12 15 ± 0.5 9.65 ± 0.01 35 ± 0.5
Kessounou 7.18 ± 0.005 31.4 ± 0.05 26 ± 0.8 83.9 ± 0.09 11 ± 0.5 4.5 ± 0.08 55.6 ± 0.05
Bembe 1 7.1 ± 0.01 30.2 ± 0.08 14 ± 0.5 275 ± 0.5 8 ± 0.5 5.5 ± 0.09 119.4 ± 0.05
Gbodje 7.27 ± 0.009 30.2 ± 0.05 23 ± 0.5 406 ± 0.05 10 ± 0.5 4.51 ± 0.01 176.2 ± 0.05
Emb lac Nokoue 7.25 ± 0.008 32.7 ± 0.05 18 ± 0.5 1967 ± 0.05 11 ± 0.5 2.49 ± 0.008 879 ± 0.5
Emb lag. Porto-Novo 7.15 ± 0.008 30.4 ± 0.05 26 ± 0.5 1724 ± 0.09 18 ± 0.5 12.3 ± 0.05 773 ± 0.5
Mean Values 7.18 ± 0.006 31.24 ± 0.98 47.18 ± 44.84 447.95 ±701.6 31.09 ± 34.33 6.72 ± 3.39 198.63 ± 414.67
Minima 7.1 ± 0.01 30.2 ± 0.05 14 ± 0.5 55.4 ± 0.09 8 ± 0.5 2.49 ± 0.008 24.4 ± 0.09
Maxima 7.27 ± 0.009 33.1 ± 0.05 158 ± 0.5 1967 ± 0.05 118 ± 0.5 12.3 ± 0.05 879 ± 0.5
Normative criterion 6.5<pH<8.5 25° C <5NTU 2000 ^S/cm - - -
Compliance rate 100 % 0 % 0 % 100 % - 90.91 % -
Chemical quality of the river water on the vertical during the recession period. The results of the chemical analysis (Table 2) carried out on the waters sampled on the Oueme River reveal sulphate concentrations ranging from 8 ± 0.5 mg/l to 66 ± 0.57 mg/l with an average value of 23 ± 18.45 mg/l. The highest concentrations are obtained at the junctions of Lake Nokoue and the Porto-Novo Lagoon. The lowest concentrations are obtained at Dannou and Hetin-Sota. Nitrate concentrations ranged from 3.54 ± 0.005 mg/l to 9.3 ± 0.5 mg/l with an average of 4.95 ± 1.73 mg/l. Maximum values for nitrate concentrations are observed at Gboa and Sotinkpekon and the minimum values are observed at the junctions of Lake Nokoue and the Porto-Novo lagoon and at Gbodje. The maximum nitrite values measured at the sampled waters of the Oueme River are obtained in Gboa (0.41 ± 0.005 mg/l) and Sotinkpekon (0.26 ± 0.005 mg/l). The minimum values are performed at Hetin- Sota, junctions of Lake Nokoue and the Porto-Novo lagoon with a concentration of 0.09 ± 0.005 mg/l. High concentrations of ammonium are also observed in Gboa and Sotinkpekon and the lowest concentrations are obtained at Hetin-Sota and at the junctions of Lake Nokoue and the Porto-Novo lagoon. The phosphate concentrations observed in these waters are between 0.61 ± 0.008 mg/l and 1.69 ± 0.005 mg/l with an average of 0.85 ± 0.31 mg/l. The highest concentrations are acquired in Gboa and Sotinkpekon while the lowest concentrations were found in Gbodje and in the junction of Lake Nokoue.
Table 2
Chemical quality of the water sampled during the low water period
Points of sample NO3- (mg/l) NO2- (mg/l) NH4+ (mg/l) SO42 (mg/l) PO43- (mg/l)
Avrankanme 5.27 ± 0.005 0.26 ± 0.005 0.33 ± 0.005 19 ± 0.5 1.09 ± 0.005
Gboa 6.53 ± 0.005 0.41 ± 0.005 0.53 ± 0.005 25 ± 0.5 1.69 ± 0.005
Sotinkpekon 9.3 ± 0.05 0.26 ± 0.005 0.34 ± 0.005 20 ± 0.009 1.02 ± 0.009
Gouke 5.76 ± 0.005 0.18 ± 0.005 0.24 ± 0.008 13 ± 0.5 0.82 ± 0.008
Dannou 4.17 ± 0.005 0.15 ± 0.005 0.19 ± 0.009 8 ± 0.5 0.78 ± 0.005
Hetin_Sota 4.4 ± 0.009 0.09 ± 0.005 0.12 ± 0.005 8 ± 0.5 0.65 ± 0.005
Kessounou 3.75 ± 0.005 0.17 ± 0.005 0.32 ± 0.01 10 ± 0.8 0.75 ± 0.008
Bembe 1 4.2 ± 0.05 0.16 ± 0.005 0.21 ± 0.008 15 ± 0.5 0.69 ± 0.005
Gbodje 3.56 ± 0.005 0.14 ± 0.005 0.13 ± 0.005 19 ± 0.5 0.61 ± 0.008
Nokoue lake junction 3.54 ± 0.005 0.09 ± 0.005 0.11 ± 0.008 50 ± 0.5 0.63 ± 0.008
Porto-Novo lagoon junction 3.99 ± 0.009 0.11 ± 0.008 0.12 ± 0.005 66 ± 0.5 0.68 ± 0.005
Minima 3.54 ± 0.005 0.09 ± 0.005 0.11 ± 0.008 8 ± 0.5 0.61 ± 0.008
Maxima 9.3 ± 0.05 0.41 ± 0.005 0.53 ± 0.005 66 ± 0.5 1.69 ± 0.005
Normative criterion 45 0.1 0.5 250 0.5
Compliance rate 100 % 18.2 % 63.64 % 100 % 0 %
Mean values 4.95 ± 1.73 0.17 ± 0.09 0.22± 0.13 23 ±18.45 0.85 ± 0.31
Chemical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD5). The BOD5 of the water analysed (figure 3) varies from 1 ± 0.5 to 30 ± 0.5 mg/l with an average value of 9.73 ± 7.83 mg/l, while COD values ranged from 3.68 ± 0.005 at 81.63 ± 0.005 mg/l with an average of 27.47 ± 20.92 mg/l. The highest COD and BOD5 values are observed at the Porto-Novo lagoon junction and the lowest values in these two parameters are observed at Kessounou level. High concentrations of COD and BOD5 are also observed at Avrankanme, Gboa, Sotinkpekon and Nokoue Lake junction.
Figure 3. BOD5 and COD contents of the Oueme River during the low water period
Taking into account the surface water evaluation grid according to SEQ Littoral [25], the average values of the physicochemical parameters obtained are divided into five pollution levels ranging from the least polluted (Class I) to the more polluted (class V) ( See Table 3).
Table 3
Grid of classification of waters of the Oueme River during the low water period
Parameters Very good Good rating Average Bad Very bad
Temperature (°C) 20 21.5 25 28 (31.24)
pH Min 6.5 6 (7.18) 5.5 4.5
pH Max 8.2 9 9.5 10
SS 2 25 38 (29.45) 50
BOD5 3 6 10 (9.73) 25
COD 20 30 (27.47) 40 80
O2 8 6 4 (6.72) 3
EC Min 180 120 (447.95) 60 0
EC Max 2500 3000 3500 4000
Turbidity 1 35 70 (44.82) 100
NO3- 2 10 (4.95) 25 50
NO2- 0.03 0.3 (0.17) 0.5 1
nh4+ 0.1 0.5 (0.22) 2 5
PO43- 0.1 0.5 1 (0.85) 2
SO42- 60 (23) 120 190 250
Values in parentheses represent the mean values of the measurements.
On the basis of this classification, it appears that the analysed sample is of good quality for the pH, COD, EC, NO3-, NO2-, NH4+, but of average quality for the following parameters: SS, BOD5, O2, turbidity and PO43-. On the other hand, it is of very poor quality for the temperature. From this classification, it follows that the water of the Oueme River is of average quality.
Correlation analysis between physicochemical parameters during the low water period
The correlation analysis is done through the matrix correlation (Table 4) and the Principal Component Analysis (Figures 4 and 5). The correlation matrix gives the significant links that exist between the different variables studied. They are expressed by the correlations that are greater than 0.5. The physicochemical parameters determined are strongly correlated with one another. Positive and significant correlations can be noticed between Turb-NO3 (0.68); Turb-NO2 (0.94); Turb-NH4 (0.94); Turb-PO4 (0.97) SS-NO3 (0.65); SS-NO2 (0.93); SS-NH4 (0.93); TDS-PO4 (0.91); TDS-EC (1); NO2-NO3 (0.79); NH4-NO3 (0.78); NH4-NO2 (1); PO4-NO3 (0.73); PO4-NO2 (0.96); PO4-NH4 (0.96); EC-SO4 (0.90); COD-EC (0.60); BOD5-EC (1).
The results obtained with the Principal Component Analysis of the physicochemical parameters reveal that the first two axes account for 72.32% of the variability under the influence of physicochemical parameters on the river water. The projection of the variables on the factorial plane F1-F2 brings out three groups of water. The first grouping concerns the localities of Avrankanme, Gboa, Sotinkpekon and Gouke, which have very turbid waters of suspended matter, nitrate, nitrite, ammonium and phosphate. The second grouping takes into account the junctions of Lake Nokoue and the Porto-Novo lagoon which have waters rich in BOD5, COD, SO42", EC and TDS as opposed to the third group involving the sampling of the localities of Kessounou, Dannou, Gbodje, Bembe 1 and Hetin-Sota where the contents in BOD5, COD, SO42", EC and TDS are low.
Figure 4. Projection of physicochemical variables on the factorial plane F1-F2
4 -3 -2 -1 0 1 2 3 4 5
fiftCT 1 : 28.47 °/o
Figure 5. Projection of sampling sites on the factorial plane Note: 1 - Avrankanme, 2 - Gboa, 3 - Sotinkpekon, 4 - Gouke, 5 - Dannou, 6 - Hetin-Sota, 7 - Kessounou, 8 - Bembe 1, 9 - Gbodje, 10 - the junction of Lake Nokoue, 11 - the junction of the Porto-Novo lagoon
Table 4
Correlations between physicochemical and bacteriological variables
NO3 NO2 nh4 S04 PO4 pH Oxy EC COD SS Turb TDS BOD5
NO3 1
no2 0.79 1
nh4 0.79 1 1
S04 -0.22 -0.14 -0.14 1
PO4 0.73 0.96 0.96 0.01 1
pH 0.14 0.14 0.14 0.29 0.19 1
-0.23 -0.06 -0.06 0.02 -0.00 0.15 1
Oxy -0.07 0.09 0.09 0.13 0.14 -0.19 0.04 1
EC -0.52 -0.52 -0.52 0.90 -0.39 0.19 0.12 0.00 1
COD 0.04 -0.06 -0.06 0.77 0.13 0.12 -0.05 0.42 0.60 1
SS 0.65 0.93 0.93 0.09 0.97 0.32 0.02 0.08 -0.31 0.15 1
Turb 0.68 0.94 0.94 0.05 0.97 0.32 0.01 0.06 -0.35 0.12 1 1
TDS -0.51 -0.51 -0.51 0.91 -0.38 0.18 0.12 0.02 1.00 0.60 -0.31 -0.34 1
BOD5 0.06 -0.06 -0.06 0.76 0.13 0.11 -0.07 0.41 0.60 1.00 0.14 0.12 0.59 1
Discussion. The temperatures recorded within of the Oueme River are all higher than 25°C, which is the value allowed by [22] waters. This high temperature is at the period of study that is relatively hot and at the low water period as well. According to [24] and [12], temperatures between 24 and 35°C are favourable to good growth of high value fish species. The pH values that oscillate from 7.1 to 7.27 are good for aquaculture according to [1], if the pH is between 6.5 and 9.
The peaks in SS values and the turbidity observed at Gboa (118 mg/l for SS and 158 NTU for turbidity) and Avrankanme (67 mg/l for SS and 95 NTU for turbidity) and Sotinkpekon (49 mg/l for SS and 77 NTU for turbidity) reflect the influence of anthropogenic activities on the water. The average concentrations of the dissolved oxygen in the river water are all above 3 mg/l O2. But at the junctions of Lake Nokoue and the Porto-Novo lagoon, the average concentrations of dissolved oxygen in the river water are less than 3mg/l O2. The river is polluted because [10] described the water as if it had dissolved oxygen content below 3 mg/l. As to conductivity and TDS, the trend is the same, as reported by [30] for the water of the Oueme delta. Based on our results, it can be deduced that the high concentrations of COD and BOD5 obtained in Avrankanme, Gboa, Sotinkpekon and Gouke and also at the junctions of Lake Nokoue and the Porto-Novo lagoon are due to the decomposition of plants and to the releases of wastewaters into the river [3]. On the other hand, a low COD (3.68 mg/l) and BOD5 (1 mg/l) is recorded at Kessounou level. These low levels may result from the toxic elements present in the environment because each sampling point is the place of embarkation of people, food and petroleum products. According to [13], in a clearly polluted environment, obtaining low values of BOD5 can be linked to the presence of toxic inhibitory elements.
Nitrate levels obtained in waters meet the WHO standard of 45 mg/l. Average levels of nitrite (0.17 mg/l) and ammonium (0.22 mg/l) were detected in the analysed waters. These values obtained for low water period are higher than those presented by [6] during the same period surface waters in Beterou in northern Benin; such deviations can be justified by the fact that surface water pollution is more pronounced downstream than upstream. Water containing nitrites is considered suspicious or toxic, even for fish at low doses [28] and is a constraint to the development of aquatic fauna because of toxicity [7]. The nitrate and nitrite content detected in the water of the Oueme River constitute a risk for the population consuming this water. Nitrates and nitrites can be at the origin of various health complications such as: irritations, allergies, abortion, cancers and chemical intoxications [5, 14, 15]. Nitrates are not directly dangerous for humans. However, their transformation in the body into nitrite can alter the transport of oxygen in the blood (methemoglo-binemia). Thus, consumption of water containing nitrate concentrations above 45 mg/l has been associated with methemoglobinemia in infants and pregnant women. In addition, some studies suspect that the nitrates in water can participate in the formation in the stomach of nitrosamines which is potentially carcinogenic [21]. Similarly, nitrates and nitrites can also be the cause in men and women of goiter [29], type II diabetes [17], congenital malformations and abortion [29]. Phosphate ion contents (0.85 mg/l) are also detected in the analysed waters. Phosphates are necessary for the growth of children and the proper functioning of the body, but when they are in excess, they lead to serious behavioral problems. In sensitive people, phosphate poisoning causes a disruption of metabolism by blocking the secretion of noradrenaline hormone of the adrenal glands, which controls and regulates the flow of cerebral nerve excitations [23]. Phosphates present in the water of the Oueme River may originate from rural works and also from domestic activities (laundry, dishes, bath etc.). According to [11], a phosphate concentration of more than 0.5 mg/l in water is sufficient, in the presence of nitrate and ammonium, to trigger excessive vegetation growth. This explains the phenomenon of eutrophication already reported by [18] in his study on Lake Nokoue in Benin. According to the assessment grid of SEQ Littoral, the quality of the water in the Oueme River in its lower valley has got average quality. To remedy, monitoring is essential for protection of this resource.
The correlation matrix based on the physicochemical parameters shows that there is a strong link between the physicochemical variables. The correlations of the physicochemical variables express the relationship between them. Strong correlations are obtained between the nitrate, nitrite, ammonium, suspended solids (SS) and turbidity contents. These correlations show that the concentrations of the nitrogen and phosphate parameters in the river water are influenced by SS and turbidity. Similarly, correlations obtained between the biochemical oxygen demand and the conductivity on the one hand, and between the chemical oxygen demand on the other, show that the evolution of the two parameters (BOD5 and COD) is influenced by the conductivity. The Principal Component Analysis shows that Avrankanme, Gboa, Sotinkpekon and Gouke have very high turbidity, suspended solids, nitrate, nitrite, ammonium and phosphate values in the river water. These results indicate that water is mineralized in these localities [4] and that agricultural activity occurs at locations less than 50 cm from the river water. Similarly, domestic activity (laundry, bathing, dishes, etc.)
takes place along the river [3]. All these activities carried out by the populations constitute a source of pollution of the river water by these pollutants. On the other hand, at the junctions of Lake Nokoué and the Porto-Novo lagoon, the water withdrawn has high levels of BOD5, COD, SO42-, TDS and EC. This analysis indicates a mineralization of the water and consequently an organic pollution of the water of the river. The PCA also shows that in the Kessounou, Dannou, Gbodjè, Bembè 1 and Hêtin-Sota localities the river water has low concentrations of BOD5, COD, SO42-, TDS and EC. The water pollution of the river in these localities is less important than in the other localities (Avrankanmè, Gboa, Sotinkpékon, Gouké, junctions of Lake Nokoué and the Porto-Novo lagoon). Finally, the anthropogenic pollution of the river is linked to the discharge of household waste and sewage, bathing, laundry, transport of petroleum products on the river, fishing, leaching of agricultural fields and decomposition of aquatic plants.
Conclusion. The results of this study revealed that the waters of the Ouémé River in its lower valley are polluted by nitrogen (nitrite, nitrate and ammonium) and phosphate pollutants. The results pointed out that the quality of the surface water is rather of an average level. These pollutants come mainly from the use of chemical fertilizers, domestic wastewater discharges and unhygienic behaviour. The statistical study (Principal Components Analysis) reveals that these pollutants emanate from an anthropic influence of agricultural, domestic and even fishery origin polluting the water in the lower valley of the Ouémé. This pollution contributes to the deterioration of the water quality of the river and impacts the quality of fish prod-ucts.There is a health risk for the population consuming these fish products. Moreover populations consume directly this water polluted by nitrogen and phosphate pollutants for they daily needs (drinking, cooking, washing, etc.) with a potential risk of various chemical intoxications. Taking into account all these problems, it's obvious that the aquatic ecosystem of the Ouémé River deserves special attention to insure better fish products and sanitary safe river waterto the population for their dailywater consumption.Therefore, there is an urgent need to monitor the quality of the water and to raise public awareness of better management of this natural resource.
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