Les eaux souterraines sont la principale source d'approvisionnement en eau des populations dans le bassin versant de la Baya. Toutefois, les activités anthropiques sur ce Bassin pourraient dégrader la qualité chimique de cette ressource. Afin d'évaluer la qualité physicochimique de ces eaux et déterminer les phénomènes à l'origine de leur minéralisation, des analyses physico-chimiques et traitements statistiques des données hydrochimiques ont été effectuées sur 13 forages et 12 puits. Il ressort que les eaux souterraines du bassin de la Baya sont dans l'ensemble acides avec un pH moyen de 5,98 et faiblement minéralisées, à l'exception de celles des localités d'Apkokro (1236 µS.cm-1) et Tiédo (1092 µS.cm-1). Les eaux sont marquées par des faciès chloruré-sulfaté calcique et hydrogénocarbonaté calcique et magnésien. Les teneurs en nitrate, chlorure, silice, manganèse et fer sont parfois élevées et dépassent les valeurs de la directive OMS de potabilité des eaux de consommation. L'Analyse en Composantes Principales (ACP) a mis en évidence deux phénomènes dans l'acquisition du chimisme des eaux : la dissolution de la roche encaissante au contact des eaux et une minéralisation liée à la pollution par apports superficiels des eaux d'infiltration et de lessivage des sols. Dans l'ensemble, la qualité des eaux souterraines est bonne mais nécessite dans certains cas, un traitement spécifique avant consommation.
In this study, fourteen physicochemical parameters were monitored at 61 groundwater sampling points located in the southern part of Abidjan District. Each water sampling location was sampled in March and July 2007, representing respectively the long dry season and the long wet season. Groundwater evaluation index and statistical methods were used to assess Quaternary groundwater physicochemical quality in term of drinking and irrigation purposes. The groundwater samples irrespective of seasons fell in mixed K-Na-HCO 3 and K-Na-Cl types. Groundwater quality index values varied from 14.01 to 593.44 and from 20.88 to 567.44, in dry and rainy seasons, respectively. In both seasons, 9.83% exhibited poor water quality and 3.27% indicated unsuitable water for drinking purposes. The studied water wells were heavily contaminated with nitrate, nitrite and ammonium in the strongly urbanized areas at Treichville, Koumassi, Marcory, Port-Bouët and Grand-Bassam. The degree of correlation between physicochemical parameters suggested that groundwater quality was mainly related to geogenic (rock-water interaction) and anthropogenic sources (domestic sewage), and intrusion of marine and lagoon waters in the study area. Sodium adsorption ratio was generally low during study period, indicating most groundwater sampled was suitable for irrigation purposes.
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.
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