State of trace element contamination of sediments from the Nangbéto hydroelectric dam lake (Togo)

Abstract

The Nangbéto hydroelectric dam lake, which is a spillway for agricultural input residues, urban and industrial effluents present in the Mono basin, is exposed to all forms of pollution. The aim of this work is to estimate the content of seven trace elements present in the sediments of the aforementioned lake in order to assess the ecological risk incurred by the aquatic biocenosis and therefore by man. Forty-two sediment samples (21 per season) are realized in different areas of the Lake. Cd, Pb, Ni, Cu, Cr, As and Hg are measured out in the sediments by the flame atomic absorption spectrophotometer in these pre-mineralized samples. The results show average contents varying from the lowest level 0.04 mg / kg for Hg to the highest 12.15 mg / kg for Cu in the rainy season and from 0.03 mg / kg for Hg and for As to 9.14 mg/kg for Cu in the dry season. All of these values are below Canadian sediment quality standards. The strongest average geoaccumulation indices (Igeo) are obtained for Pb with values less than 2. The mean values of the contamination factor (FC) are less than 1. The mean indices of contamination (Im) are less than 2, threshold d 'an onset of polymetallic contamination. The ecological risk (Ri) in relation to the contamination of the sediments of the Lake with trace elements is low in any season (Ri <40) and this implies a low risk of contamination of the biocenosis. However, the present study only deals with metallic trace elements. It does not take into account other pollutants, nor the potential "cocktail" effects between them. Additional work should therefore be continued in this direction to consolidate the low risk recorded for the biocenosis. Le Lac du barrage hydroélectrique de Nangbéto qui constitue un déversoir des résidus des intrants agricoles, des effluents urbains et industriels présent dans le bassin du Mono est exposé à toutes formes de pollution. Le but de ce travail est de quantifier les teneurs de sept (7) éléments traces dans les sédiments dudit Lac afin d’évaluer le risque écologique encouru par la biocénose aquatique et partant par l’homme.Quarante-deux (42) échantillons de sédiments (21 par saison) sont échantillonnés dans différentes zones du Lac. Le Cd, Pb, Ni, Cu, Cr, As et Hg sont dosés dans les sédiments par le spectrophotomètre d’absorption atomique à flamme dans ces échantillons minéralisés. Les résultats montrent des teneurs moyennes variant de la plus faible teneur 0,04mg/kg pour le Hg à la plus forte (12,15mg/kg) pour le Cu en saison pluvieuse et de 0,03 mg/kg pour le Hg et pour l’As à la plus forte (9,14 mg/kg) pour le Cu en saison sèche. Toutes ces valeurs sont inférieures aux normes canadiennes de qualité des sédiments. Les plus forts indices de géoaccumulation (Igeo) moyens sont obtenus chez le Pb avec des valeurs inférieures à 2. Les valeurs moyennes de facteur de contamination (FC) sont inférieures à 1. Les indices moyens de contamination (Im) sont inférieurs à 2, seuil d’un début de contamination polymétallique. Le risque écologique (Ri) par rapport à la contamination des sédiments du Lac en éléments traces est faible en toute saison (Ri < 40) et cela implique un faible risque de contamination de la biocénose. La présente étude ne porte toutefois que sur les éléments traces métalliques. Elle ne tient compte ni des autres polluants, ni des effets "cocktail" potentiels entre eux. Un travail complémentaire devra donc être poursuivi dans ce sens pour consolider le faible risque enregistré pour la biocénose.

Physicochemical Quality of the Water and Heavy Metal Contamination of the Sediment, Water, and Flesh of Some Fish in the Lower Reaches of the Mono River (Benin, West Africa)

“…Sediment and water samples were collected once per season at each station. Sediments of 0 to 10 cm deep were scraped by divers using polyethylene containers, placed in polyethylene bags, labeled, and kept in a cool box at 4 °C for transport to the laboratory (Adje et al, 2021). Water samples were collected from the water column using sterilized 1 L glass bottles, labeled, and then stored in a cool box at 4 °C for transport to the laboratory.…”

“…The sediment samples were processed following the methods described by Adje et al (2021) to obtain a 100 mL solution which is used for the assay. For the fish samples, pieces of flesh with skin were taken from the dorsal part and were processed using the methods outlined by Ennouri et al (2013) to obtain a 100 mL solution, which is used for the assay.…”

“…The installation of the Nangbéto hydroelectric dam in the middle reaches of the basin and its implementation have had a damaging effect on the Mono River. The retention and sudden release of water from the dam accelerate bank erosion, modify the hydrology of the River (Amoussou et al, 2012), alter the quality of the biotope (Chouti and Hounkpêvi, 2018;Adje et al, 2021), and disrupt the biology and ecology of living organisms in the environment (Lederoun et al, 2021). In addition, there is pollution from agricultural activities (use of pesticides and chemical fertilizers), livestock farming, domestic activities (cooking, washing clothes, and dishes), transport, and tourism (Chouti and Hounkpêvi, 2018;Adje et al, 2021).…”

“…The retention and sudden release of water from the dam accelerate bank erosion, modify the hydrology of the River (Amoussou et al, 2012), alter the quality of the biotope (Chouti and Hounkpêvi, 2018;Adje et al, 2021), and disrupt the biology and ecology of living organisms in the environment (Lederoun et al, 2021). In addition, there is pollution from agricultural activities (use of pesticides and chemical fertilizers), livestock farming, domestic activities (cooking, washing clothes, and dishes), transport, and tourism (Chouti and Hounkpêvi, 2018;Adje et al, 2021). Given these disturbances and threats to this ecosystem, there is a great need for research into the quality of the biotope and the aquatic organisms that are most commonly consumed.…”

“…A study conducted in the lake of the Nangbéto dam on the state of contamination of the biotope revealed contamination of the sediment by heavy metals (cadmium, lead, nickel, copper, chromium, arsenic, and mercury) (Adje et al, 2021). However, there is a lack of data on water and sediment quality in other parts of the basin and on the quality of fishery products throughout the basin.…”