The Lake Chad Basin is characterized by variable climatic and physical conditions. Its southern part receives increased rainfall, mainly collected by the Chari-Logone Rivers, while the northern part is dominated by more arid conditions. Lake Fitri, in the Central Lake Chad Basin, is a miniature of Lake Chad at the western tip of the Batha River. Both lake and river water are available water resources, but populations are mainly dependent on groundwater for their daily needs. However, groundwater quality is often poor in this part of the basin. In this study, we focus on the Quaternary Aquifer north of Lake Fitri in order to characterize the groundwater types and to assess the origin of salinity measured through its Electrical Conductivity (EC). The study is conducted within the framework of the ResEau Project through its support to the GIS-Hydrogeology Program (Master HydroSIG) of the University of N'Djaména (Chad). A fieldwork, carried out in November 2017, resulted in the collection of groundwater samples and subsequent analysis of cationic and anionic contents, as well as δ2H and δ18O isotopic compositions.
This field-based investigation reveals the presence of a shallow aquifer, characterized by hydraulic heads above 280 masl, low EC values (average 1343 µScm-1) and a calcium-bicarbonate composition. At lower hydraulic heads (below 270 masl), groundwater is distinctively different, characterized by higher EC values (up to 20,600 µScm-1) and a sodium-sulfate composition. Strong correlations among SO4, Na and K, Na/Cl ratios of the analyzed samples and the occurrence of gypsum layers suggest that this chemical composition is mainly due to interaction of Ca-rich groundwater with K-Na sulfates, such as Hanksite (17Na2SO4.4Na2CO3.K2SO4.2NaCl). The isotopic compositions of the analyzed samples are in accordance with the hydrochemical patterns. Calcium-bicarbonate samples plot along a line δ2H = 5.84 x δ18O – 3.84 (R2 = 0.96) while sodium-sulfate samples plot along a line (δ2H = 5.58 x δ18O – 14.13; R2 = 0.96). The shift of the stable isotopic composition of the sodium-sulfate groundwater is in agreement with fractionation resulting from the interaction with evaporites characterized by heavy O isotopic compositions.
In addition to the identification of the origin of groundwater salinity in the Lake Fitri area, the characterization of the chemical and stable isotopic composition of groundwater is used to geographically locate sources of modern recharge. Such characterization is essential to develop an integrated water resource management plan for this region, particularly sensitive to climatic change. However, a more precise quantification of the resource is required to plan its sustainable use for agricultural, industrial and domestic purposes.