Use of hydrochemical and isotopic tracers to investigate the groundwater quality and recharge processes of the shared Iullemeden Aquifer System in the Sahel region (Western Africa)

The management of water resources across many parts of Africa is facing significant challenges in quantitative and qualitative terms. Meeting the increase in water demand, linked to population growth and economic development, is affected by the limited resources due to climate variability, often-inefficient management of water resources, and quality degradation caused by some polluting domestic and agricultural practices. This investigation of the Iullemeden Aquifer System (IAS), which is shared between Niger, Nigeria, Mali, Benin and Algeria, is based on the combined use of hydrochemical and environmental isotope methods. The aim is to improve understanding of the IAS hydrogeological functioning, including groundwater flow patterns, mechanisms of recharge, and vulnerability to pollution, as well as the interaction between surface waters and the most significant aquifer units. Assessment of groundwater recharge in this arid and semiarid area is complex due to the temporal and remarkable spatial variability of rainfall and recharge processes. In the IAS, groundwater recharge was highly heterogeneous, mainly occurring during the late Pleistocene and humid periods of the Holocene, particularly for the Continental Intercalary and Continental Terminal aquifers. The heterogeneity of groundwater residence time is reflected in the considerable spatial variation of δ 18 O, δ 2 H, 3 H, d-excess, 14 C and total dissolved solids (TDS) contents.