Etude dynamique des eaux souterraines de la localité d’abeche (Est du Tchad)

Abstract

The locality of Abéché (eastern Chad) is located in the center of the ouadi chao watershed, between the parallels 13°48' and 13°50' north latitude and the meridians 20°48' and 20°52' east longitude. Its surface area is 3600 ha and its average altitude is 615 m. The field and laboratory work carried out jointly consist of the identification and piezometric monitoring of structures (wells), soil and water sampling for analyses of particle size, Atterberg boundaries, physical characteristics and physicochemical parameters. At the end of this work, the particle size analysis of the soils in the locality shows a high proportion of soils in gravelly fractions but also a small proportion of soils in fine fractions. These materials have plasticity index values that vary from 2.7 to 10.26 and consistency indices that are greater than unity. This high proportion of gravelly fractions gives the soil a coarse behaviour characterised by a high permeability of between 4.3.10 and 10 m/s. Static level measurements vary between 2.60 and 33 m deep and piezometric levels vary between 513.6 and 540.88 m. The fluctuations range from 0.16 to 21 m. The small fluctuations are observed in the shallows, while the strongest are observed at the top of hills. The analysis of the piezometric maps shows two types of groundwater flow (convergent and divergent). The maximum piezometric map produced using the Surfer 12 softwares made it possible to locate the supply areas and storage areas as well as the direction of water flow, which is from NE to SW. The water balance established shows that the water table is recharged in August by the action of excess rainfall. Thus, the hydrodynamic behaviour of the aquifer is closely linked to rainfall fluctuations. Physico-chemical analyses show that the waters are generally neutral to slightly basic (7.06 ≤ pH ≤ 8.17) with low to excessive mineralisation (225 μS/cm ≤ EC ≤ 4590 μS/cm). All chemical elements analysed are within the range defined by WHO, with the exception of potassium (K) in the BGP44 sample, which has a concentration higher than the WHO standard. The representation of the major ions in the Piper diagram shows the dominant chemical facies which are of the chloride and sulphate calcium and/or magnesium type. The representation of the different samples in the Wilcox diagram shows that the water in the area is of excellent quality and good for irrigation, except for that from wells KMP36 and BGP44 which are mediocre and bad. The assessment of the quality of water for human consumption is carried out by comparing the values of the various physico-chemical parameters of the local water with those accepted in the standards of the World Health Organization (WHO). As a result, it can be noted that the waters of the area are good for human consumption according to WHO standards, but also require treatment and are suitable for irrigation.