This study focuses on the assessment of the level of contamination in metallic trace elements (cadmium, copper, zinc and lead) in the waters of lake Azili in the municipality of Zangnanado. The methodology adopted to achieve these results is the method of molecular absorption spectrophotometry. Four sites were sampled at the beginning of November 2020 (period of the long dry season). The results obtained show that the copper content varies between 1.04 mg/l and 1.67 mg/l. The zinc content between 0.2 mg/l to 0.62 mg/l and those of lead recorded are all above 0.05 mg/l. As for their cadmium concentration, they are all above 0.001 mg/l. These results from water analysis of Azili lake show that the sources of this type contamination of the lake are due on the one hand to human activities (dumps of household waste, use of motorized boats) and others other hand to the contributions of the eff luents of the lake carrying pesticides. The water quality of Lake Azili is therefore influenced by these various metallic trace elements found there. These results show that the waters of lake Azili are at their beginning of pollution even if it is not yet alarming at the time of our study. Under these conditions, if nothing is done to mitigate it, the pollution will be critical over time.
Life on earth is possible thanks to the existence of certain vital resources, including water, a commodity of great importance for living beings (Houssou, 2010). It is an essential element for life and for the real and sustainable socio-economic development of a country, it is therefore necessary to have a better knowledge of the quality of existing water resources. However, the problems related to groundwater pollution and anthropogenic pressure on highly vulnerable aquatic ecosystems require permanent monitoring of their physico-chemical, hydrodynamic and biological properties. Generally, the assessment of surface water quality is based on the measurement of physico-chemical parameters as well as the presence or absence of aquatic organisms and microorganisms, indicators of water quality (Ballouki, 2012).
The demographic expansion and economic growth experienced by the countries of the world without obscuring those of Africa, in particular Benin, have consequences on the environment and on water bodies (Hawa, 2011).
Thus, water resources are under strong pressure from human activity (agriculture, industry, livestock, fishing, domestic, hospitals, etc.). Human activities of a socio-economic nature coupled with those of natural processes (soil erosion, precipitation, evaporation, runoff of river water) accelerate the degradation of surface water resources (Hawa, 2011). They cause disturbances of the natural balance and increase the organic load of water and sediments, then the congestion of water resources, with problems of eutrophication, asphyxiation of the aquatic environment as well as the health problems of populations (Babadjidé, 2011).
For more than ten years, surface water (river, lake, pond, sea, river, lagoon) was used for watering market gardening and agricultural products. Thus, the source of life water can become a danger for the environment and for users if it is not of acceptable quality (Agassounon et al., 2014). In addition, surface water is exploited and treated to supply the population with drinking water (Lamizana Diallo et al., 2008).
Thus, among other things, the poor management of waste, the anarchic installation of latrines influences the quality of water resources. Similarly, Dégbey (2008) discussed the quality of well water in the municipality of Abomey-Calavi in Benin. He concluded that the well waters studied present bacteriological pollution. Lake Azili is not faced with these environmental problems. Water degradation is observed due to the overexploitation of fishery resources, the use of fishing gear that does not comply with the regulations in force and the discharge of liquid and solid waste by local populations (Golf Expertises, 2008). This study’s objective is then to assess the level of contamination in metallic trace elements in the waters of Lake Azili.
The Commune of Zagnanado is located on the Zagnanado plateau, the smallest of the plateaus north of the Lama depression between 7˚ and 7˚30' north latitude and between 2˚15' and 2˚30' east longitude (Africa Conseil, 2010). It occupies a total area of 750 km2 and is bounded to the north by the Commune of Dassa-Zoumè, to the south by the Communes of Ouinhi and Zogbodomey, to the east by the Communes of Kétou, Adja-Ouèrè and to the west by the Communes of Covè, Zakpota and Djidja. The Municipality of Zagnanado is subdivided into six (6) districts which are Zagnanado, Agonlin-Houégbo, Banamè, Kpédékpo, Dovi and Don-Tan. These districts are subdivided into 27 villages and 7 city districts. It is located 47 km from the municipality of Abomey, the capital of the Department of Zou. As shown in
| Materials | Roles |
|---|---|
| GPS | to take the coordinates of the various sampling sites |
| 1 L plastic bottle | allowed water to be sampled for physicochemical analysis |
| a cooler with ice packs ±5˚C temperature | allowed to keep the sample intact until the laboratory |
| A multi-parameter | to measure in-situ parameters such as: pH, temperature, conductivity |
| A digital camera | allowed to take pictures |
| A marker and adhesive paper | to label samples |
| A pen and a notebook | to record in situ values |
| A motorized boat | for trips on the lake |
The documentary research made it possible to identify the problem and to take stock of the knowledge. This information was obtained from the libraries of the entities of the University of Abomey-Calavi (polytechnic school of abomey calavi, Faculty of Agronomic Sciences). The nature of these documents concerns is Master’s theses, DEA’s theses, doctoral theses, articles, books, journals, etc. The information sought relates to subjects that address issues related to the pollution of surface waters with trace elements.
Surveys are methodical research based on questions and testimonies. They were made for women, fishermen, farmers. They were mainly focused on the way of life of the local population about the body of water and the environment.
As shown in
The physico-chemical parameters measured in situ in the water samples taken are pH, temperature, TDS and conductivity. These parameters were measured in situ with a Combo by HANNA multi-parameter probe mark. During this measurement, the electrode is immersed in the sample and the values of the various parameters are read directly on the screen of the device.
The method used to measure the chemical parameters (copper, zinc, lead and cadmium) is method by absorption spectrometry.
The results are those coming from information based on: The survey of the local population about the way of life regarding the environment; Analyzes of the physico-chemical parameters of the waters; Statistical processing of data.
This survey was conducted with users (fishermen, motorized boat drivers, farmers), local authorities and in health structures. The table below (
| yes | No | |
|---|---|---|
| Open defecation | 80% | 20% |
| Dumping of waste in nature | 100% | 0% |
| Use of chemicals for market gardening | 4% | 96% |
The results of the analysis of the physico-chemical parameters are summarized in Figures 3-10.
Figures 3-6 provide information on the concentrations of physical parameters in the lake water. For all four sampled sites we note an average of 35.05˚C for the temperature, an average of 5.55 for the pH, an average of 58.75 µS/cm for the electrical conductivity and an average of 29.75 mg/l for the TDS.
Figures 7-10 tell us about the metallic trace elements in the water of Lake Azili. Thus, the average copper content is 1.35 mg/L with an elevation downstream (site 1) and upstream (site 2); the average zinc content is 0.33 mg/L with an upstream elevation (site 2); the average lead content is 0.25 mg/L with a downstream elevation (site 1) and the average cadmium content is 0.015 mg/L.
As shown in
| Variables | T (˚C) | pH | CE | TDS | Cu | Zn | Pb |
|---|---|---|---|---|---|---|---|
| pH | 0.83 | ||||||
| CE | −0.55 | −0.44 | |||||
| TDS | −0.81 | −0.64 | 0.94 | ||||
| Cu | −0.43 | −0.76 | 0.66 | 0.61 | |||
| Zn | 0.43 | −0.13 | −0.39 | −0.50 | 0.38 | ||
| Pb | −0.93 | −0.95 | 0.65 | 0.83 | 0.73 | −0.14 | |
| Cd | 0.44 | 0.83 | 0.03 | −0.13 | −0.65 | −0.61 | −0.63 |
| Variables | T (˚C) | pH | CE | TDS | Cu | Zn | Pb |
|---|---|---|---|---|---|---|---|
| pH | 0.1662 | ||||||
| CE | 0.4486 | 0.5565 | |||||
| TDS | 0.1929 | 0.3635 | 0.0639 | ||||
| Cu | 0.5736 | 0.2416 | 0.3408 | 0.3905 | |||
| Zn | 0.5702 | 0.8676 | 0.6051 | 0.5016 | 0.6190 | ||
| Pb | 0.0719 | 0.0467 | 0.3465 | 0.1659 | 0.2701 | 0.8614 | |
| Cd | 0.5625 | 0.1656 | 0.9730 | 0.8742 | 0.3487 | 0.3858 | 0.3704 |
It is necessary to make an in-depth analysis of the parameters studied in order to assess the quality and environmental terms of the waters of Lake Azili. Thus, as far as the physical parameters are concerned, all the temperature values recorded do not comply with the standard in force; this could be explained by several factors: the time at which the measurements were taken; the change in elevation and slope of the watercourse. The pH results show that the pH value does not belong to the standard range 6.5 - 8.5 but below it. This shows that the waters of the lake are acidic. Indeed, this acidity of the waters depends largely on the nature of the land crossed and the discharges transported. This may also be due to a supply of humic and fulvic acids resulting from soil leaching by precipitation (Merhabi et al., 2019). The results obtained at the electrical conductivity level show an elevation downstream. This elevation could be related to the geology of the land rich in mineral salts inducing a strong mineralization
These results confirm those of Amine and Abdelmalek (2012). The results of the analysis of the metals studied show us that the waters of Lake Azili are polluted with metallic elements (Cu, Pb, Cd) because all the averages greatly exceed the current standards. That is 1.35 mg/l for copper, 0.015 mg/l for cadmium and 0.25 mg/l for lead. These results are consistent with those of Senouvo (2002) and Dedjiho (2014) who deduced in this case that the waters of the lake are then confronted with the problems of pollution by domestic wastewater as well as those resulting from the leaching of agricultural land. The distribution of these pollutants in the lake (
Moreover, a strong negative correlation is observed between Pb and pH (−0.95). This correlation between pH and Pb shows that the evolution of the pH concentration reduces the Pb concentration. Contact between water and metallic lead inevitably gives rise to corrosion phenomena, the extent of which depends both on the physico-chemical properties of the fluid. An increase in pH therefore reduces the concentration of lead in water (Michel, 2003).
Overall, these results confirm those of the firm Golf Expertises (2008) on the management plan of the lake Azili which revealed a degradation of water due to the overexploitation of fishery resources and the use of gear. Fishing that does not comply with the regulations in force. It also revealed that the liquid and solid waste discharged by the local populations contributes to the pollution of the waters of Lake Azili.
This work allowed the evaluation of the level of contamination in metallic trace elements (Cadmium, copper, zinc and Lead) in the waters of Lake Azili in the commune of Zangnanado. For a total number of four samples from different locations in the lake, interpretive results emerge.
Thus, the waters of the various sampling sites presented levels of metallic trace elements (Cadmium, copper and Lead) above the thresholds recommended by the Beninese standard in force. As a result, the waters are not of good quality and therefore polluted. This pollution is observed more when moving from upstream to downstream with respect to the parameters (copper and lead). We note that the way of life of the local population (disposal of household waste, defecation in the open air, use of chemical fertilizers, etc.) could be on the one hand the cause of this pollution. The overall state of degradation of the lake is not as alarming but requires constant vigilance and awareness to avoid the worst in the years to come. Future research work will take into account the two rainy seasons (low water and high water) by broadening the field of analysis of chemical parameters.
The authors declare no conflicts of interest regarding the publication of this paper.
Dovonou, F. E., Godonou, J. L., Kindoho, M., Alladassivo, E. M., Dossou, P. M., Hounsou, M. B., & Yalo, N. (2022). Evaluation of Metallic Trace Elements Content (Cadmium, Copper, Zinc and Lead) in the Water of Lake Azili in the Municipality of Zangnanado (Centre-Benin). Journal of Geoscience and Environment Protection, 10, 186-197. https://doi.org/10.4236/gep.2022.107012