Spatial and temporal variation of traces elements in surface water in the Agneby watershed (Côte d’Ivoire, West Africa).

Water is vital for human beings. Preserve its quality and quantity is essential to ensure welfare to populations. The aim of this work is to determine the quality of surface water catchment of Agneby in Agboville. Investigations consisted in evaluating physicochemical parameters and trace metals of surface water in the basin. Indeed, the surface water samples were collected in five stations and during four different seasons. Physicochemical parameters (Temperature, potential Hydrogen, Electric Conductivity and Total Dissolved Solids) were measured in situ using a multiparameter Hanna (HI 991001 pH meter and HI 99300 conductmeter) while lead, zinc and copper were analyzed by molecular absorption spectrophotometer HACH DR 2800. The Principal Component Analysis and Hierarchical Ascending Classification showed the existence of two groups of water. The first included waters of the long dry season, highly mineralized with high contents of zinc (3.14 mg/L in Agboville and 6.49 mg/L to Afféry), copper (0.45 mg/L to Afféry and 3.3 mg/L to Kotchimpo). The second, characterized by high levels of lead (0.094 mg/L in Agboville and 0.19 mg/L in Copa), gathers waters of the long rainy and short seasons. Results of surface waters analysis revealed good mineralization (117 μS/cm in Afféry and 389 μS/cm in Kotchimpo during dried season) and the acidity of Kotchimpo waters (6.39 0.01 mg/L). Mean concentrations of Cu registered were highly pollutant in Kotchimpo (1.25 mg/L), Agbossou (1.08 mg/L) and Copa (1.03 mg/L)

Characterization of the vegetation cover and water erosion dynamics in the Aghien lagoon catchment

Soil erosion affects land quality and water resources. The present research aimed to estimate spatio-temporal changes in land-use/land-cover pattern and soil erosion in the lagoon Aghien watershed in Côte d'Ivoire. This study was carried out by using Landsat imageries of 2016 and 2020. Images were classified into categories using supervised classification by the maximum likelihood algorithm. Universal Soil Loss Equation modeling was applied in a GIS environment to quantify the potential soil erosion risk. The area of bare soil/Habitats and crops/Fallow increased by 2981 ha (37.8%) and 2642 ha (17.58%) during 2016–2020. The high soil losses are located on the slopes of the rivers and valleys adjacent to the Aghien lagoon, which are also naturally favored by the steepness of the slopes and their length and inclination. However, the average soil loss values were 60.65% in 2016 and 47.64% in 2020. However, the very low and low soil loss values are scattered over the watershed for an area of 34441.52 ha corresponding to a rate of 94.36% in 2016, in 2020 they occupy an area of 34956.76 ha with a rate of 95.77%. On the other hand, high and very high soil losses are insignificant, corresponding to rates of 0.95% and 0.60% in 2016 and 2020 respectively. However, most of the soil loss in the watershed is due to moderate erosion, occupying areas of 1712.19 ha (4.69%) and 1305.77 ha (3.58%) also in 2016 and 2020

Fonctionnement hydrologique et origine des écoulements sur un bassin versant en milieu tropical de socle au Bénin : bassin versant de la Donga (haute vallée de l'Ouémé)

Within the international AMMA project (African Monsoon Multidisciplinary Analysis), the Upper Oueme catchment has been selected as the southermost site of the AMMA-Catch Observatory in Environment. Raingauge, limnigraph and piezometric networks have been gradually set-up since 1997 over the catchment (10 000 km2) and particularly on the Donga sub-catchment (586 km2).The purposes of this work are (i) to identify the prominent hydrological processes on the Donga catchment by comparison of precipitation, streamflow, water table and potential evapotranspiration data and (ii) to determine and quantify the streamflow components by geochemical field surveys. A coherent hydrological scheme has been deduced from hydrodynamic and hydrogeochemical information. The seasonal character of surface water, the lag between the river drying up and the low groundwater level, the low streamflow mineralization lead to consider a poor connection between the deep groundwater and the hydrological network.Based on this result, a two components (effective runoff and subsurface flow) hydrograph separation has been implemented for two hydrological-contrasted years (2003 and 2004). In each case, a major contribution of superficial groundwater to streamflow has been found.Dans le cadre du programme international AMMA (Analyse Multidisciplinaire de la Mousson Africaine), la haute vallée de l'Ouémé (centre ouest du Bénin), d'une superficie de 10.000 km2, est instrumentée (pluie, débit) depuis 1997. Le sous-bassin de la Donga (586 km2) est le cadre d'observations intensives précisant les processus hydrologiques essentiels et quantifiant les termes du bilan.La recherche entreprise vise (i) à déterminer le fonctionnement hydrologique du bassin de la Donga par confrontation des données de précipitation, d'écoulement de surface, de nappe et d'évapotranspiration potentielle, (ii) à caractériser les écoulements par traçage naturel géochimique (paramètres physiques, éléments majeurs et traces) et isotopique (isotopes stables de l'eau : d18O et d2H) afin de hiérarchiser les processus majeurs mis en jeu. Le croisement des informations hydrodynamiques et hydrogéochimiques permet de reconstituer un schéma cohérent du fonctionnement hydrologique sur le bassin. Le caractère temporaire des eaux de surface, l'asynchronisme entre le tarissement des rivières et la vidange des nappes ainsi que la très faible minéralisation des écoulements traduisent une origine superficielle des débits et une déconnexion apparente de la nappe libre du réseau hydrographique.En accord avec les processus mis en évidence, une décomposition de l'hydrogramme en deux composantes (ruissellement rapide sur surface saturée et flux de subsurface) a été retenue pour 2003 et 2004, deux années à pluviométrie et écoulement contrastés

Fonctionnement hydrologique et origine des écoulements en milieu tropical de socle au Bénin (cas du bassin versant de la Donga (haute vallée de l'Ouémé))

Dans le cadre du programme international AMMA dont l un des objectifs est d identifier et analyser les processus physiques de surface entrant en jeu dans les mécanismes de la mousson ouest africaine, le haut bassin de l Ouémé (Bénin) a été retenu comme le site le plus méridional de l Observatoire de recherche en environnement (ORE) AMMA-Catch. D une superficie de 10.000 km2, ce bassin est instrumenté (pluie, débit) depuis 1997. Un sous-bassin de 586 km2, le bassin de la Donga, est le cadre d observations intensives qui ont pour objectif l évaluation des termes du bilan hydrologique. Le travail vise (i) à déterminer le fonctionnement hydrologique du bassin par confrontation des données de précipitation, d écoulement, de nappe et d évapotranspiration potentielle, (ii) à caractériser les écoulements par traçage naturel géochimique et isotopique afin de hiérarchiser les processus mis en jeu. Le croisement des informations hydrodynamique et hydrogéochimique a permis de reconstituer un schéma cohérent du fonctionnement hydrologique sur le bassin. Le caractère temporaire des eaux de surface, l asynchronisme entre le tarissement des rivières et la vidange des nappes ainsi que la très faible minéralisation des écoulements traduisent une origine superficielle des débits et une déconnexion apparente de la nappe libre du réseau hydrographique. En accord avec ce processus mis en évidence, une déconvolution de l hydrogramme en deux composantes (ruissellement rapide sur surface saturée et flux de subsurface) a été retenue pour les années 2003 et 2004. Cette étude, outre la compréhension du fonctionnement hydrogéochimique du bassin, s insère dans une perspective d amélioration des modèles hydrologiques par l implémentation des processus majeurs mis en évidence. Discipline : Météorologie, océanographie physique et physique de l'environnement Mot-clés : Bénin, aquifère libre, hydrochimie, hydrodynamique, rivière tropicale et déconvolution de l hydrogrammeWithin the international program AMMA, the upper Oueme River was retained as the most meridian site of the Research Observatory in Environment (ROE) Amma-Catch. One of the main objectives is to identify and analyse the surface physical processes at stake in the West African monsoon mechanisms. Since 1997 this 10.000 km basin is equipped, with especially rain and discharge measurement systems. An intensive observation on the under basin of the Donga (586 km ) aims at evaluating the terms of the water budget. The purposes of this work are (i) to determine the hydrological functioning of the basin by confrontation of precipitation, discharge, water table and potential evapotranspiration data and (ii) to characterise the flows by natural geochemical and isotopic follow-up in order to structure the processes at stake. The consideration of the hydrodynamic and hydrogeochemical information has permitted to develop a coherent schema of the hydrological processes on the basin. The temporary character of the surface waters, the absence of synchronism of the stream drying up with the water table draining and the very little flow mineralization lead to think that the flows are superficially originated and that there is an apparent disconnection between the underground water and the hydrological network. From this conclusion we implemented a hygrograph separation in two components - effective runoff on saturated area and subsurface flow - for the years 2003 and 2004. Focused on a better understanding of the hydrological functioning of the basin, this study leads to a perspective of improvement of the hydrological models with the implementation of the major processes pointed out. Key-words: Benin, water table, hydrochemistry, hydrodynamic, tropical streams and hydrograph separationMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Multifrequency electromagnetic method for the hydrogeophysical characterization of hard-rock aquifers: the case of the upstream watershed of White Bandama (northern Ivory Coast)

In West Africa, the drinking water supply relies on the hard-rock aquifers. In Ivory Coast, the population growth along with the climate changes make drinking water resources highly vulnerable. The White Bandama upstream watershed in northern Ivory Coast is located on a fissured hard-rock aquifer and is poorly known, both in the geometry of the reservoirs and in the hydrogeological potential of the reserves it contains. Indeed, the heterogeneous subsurface in this region shows high variability in the hydraulic conductivity inducing difficulties in the hydrogeological exploration. The determination of the geometry and hydrodynamic properties of the aquifer are required for a sustainable management of this water resource and for a better choice of future well locations. This study presents a hydrogeophysical approach using the multifrequency electromagnetic device PROMIS®, as well as lithology logs and geological information of a 30 × 30 km zone in the northwestern part of the White Bandama catchment. Our geophysical data are interpreted with 1D multi-layer models consistent with the discontinuities observed in lithology logs and the geology of the site. Results allow to precise the local thicknesses of the 3 main units of our study area down to 50 m, being from top to down, saprolite (which is often indured close to the surface), fissured-rock zone and rock substratum. Between the saprolite and the fissured zone, the main aquifer unit constitutes the interesting target for productive water wells. Its thickness ranges from 15 to 30 m. A detailed knowledge of the local aquifer geometry constitutes the first and crucial step before going further into a complete hydrogeological study

Processus et bilan des flux hydriques d'un bassin versant de milieu tropical de socle au Bénin (Donga, haut Ouémé)

International audienceHydrodynamic, geochemical, and subsurface geophysical investigations, for two consecutive years with contrasting rainfall conditions, were used to characterize the hydrological processes occurring, and the water balance of a 586-km2 watershed in Benin (Africa). The water table's monitoring shows that recharge occurs by direct infiltration of rainfall, and represents between 5 to 24% of the annual rainfall. Both surface water outflow, limited to the rainy season, and water chemistry indicate a weak groundwater contribution to river discharge. This implies that the calculated variations in annual runoff coefficients (of 14 and 28%) are mainly governed by surface and subsurface flows.L'étude d'un bassin versant de 586 km2 au Bénin (Afrique de l'ouest) par des suivis hydrodynamiques et hydrochimiques, ainsi que par reconnaissance géophysique de sub-surface, a permis de déterminer les processus et le bilan des flux hydriques sur deux années à pluviométrie contrastée. La recharge de la nappe phréatique s'effectue par infiltration directe des précipitations et représente de 5 à 24% de la pluviométrie. L'écoulement à l'exutoire, limité à la saison des pluies ainsi que la minéralisation faible des eaux indiquent des flux nappe - rivière négligeables. Cette faible contribution de la nappe implique que les variations interannuelles du coefficient d'écoulement (de 14% à 28%) sont principalement gouvernées par des flux de surface et de sub-surface

Comparative study of physicochemical parameters and dissolved heavy metal dynamics in tropical waters (Bété, Mé, Djibi Rivers and Aghien lagoon) in south of Ivory Coast, West Africa.

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Study of Groundwater-river Interactions Using Hydrochemical Tracers in Fissured Rock: Case of the Lobo Watershed at Nibéhibé (Central-West, Côte d’Ivoire)

Water is a vital resource for all populations. However, there are warning signs that the water from the Lobo River used by SODECI to supply drinking water to the population is declining in quantity during the dry season and its quality is becoming poor due to climate variability and anthropogenic activities. However, the river is able to maintain a certain flow, probably with the contribution of groundwater. It is therefore a question of whether there is really a connection between surface water and groundwater. The aim of this study is to characterize the groundwater-river interactions based on the physico-chemical parameters of the Lobo watershed in Nibéhibé. The approach adopted is a coupled statistical-geochemical approach applied on data from two sampling campaigns (dry and rainy season). This coupled approach consisted, on the one hand, in understanding the chemical specificities within the water classes using the piper diagram and, on the other hand, in classifying the waters according to their physico-chemical similarity and highlighting the phenomena at the origin of the water mineralization using the Kohonen self-organized map (SOM). The results obtained from the piper diagram show that in both the wet and dry seasons, the chemical signature of the waters remains controlled by two main hydrochemical facies: the chlorinated calcium-magnesium nitrate hydrofacies and the bicarbonate calcium-magnesium hydrofacies. Kohonen's self-organized map has established that the mineralization of groundwater, under natural conditions, comes from the nature of the rocks crossed during infiltration and from the contact time between water and minerals. This work provides managers with decision-support tools for planning and searching for groundwater in support of surface water to reinforce the drinking water supply of the populations in this watershed.&nbsp

Using fluorescence spectroscopy EEM to evaluate the efficiency of organic matter removal during coagulation–flocculation of a tropical surface water (Agbo reservoir)

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