Lake Chad is located in the center of an extremely large sedimentary basin (2 million km2), which is filled with neogenous sandy-clay sediments and contains several huge aquifers of regional importance. This study covers the northern part of this basin, an area of 200,000 km2 located in Chad.
Although this is a very arid area (50 to 400mm of rainfall a year), it is inhabited by different population groups (2,500,000 inhabitants), whose main source of income is predominantly pastoral livestock farming (the livestock population includes more than 30 million ruminants).
The only surface water resource is Lake Chad and most of the population therefore depends on groundwater for both drinking water and water for their livestock.
Rapid population growth (2%/year) and the rise in sedentary lifestyles are increasing the demand for drinking water. In addition, growing insecurity in the Lake Chad area (Boko Haram) has led part of the Lake Province population to migrate further to the North, where there is no surface water resource.
The plio-quaternary aquifer offers a tremendous opportunity for meeting the growing water demand, as it is shallow, extensive and highly productive (the drilling success rate exceeds 95%). The relatively easy access to this resource makes it vulnerable to overexploitation in adverse conditions: climate change models anticipate a decrease in precipitation and, therefore, in groundwater recharge.
To assess this risk and map this resource, we have used the results of the 1,000 boreholes drilled over the past ten years. We have built hydrogeological maps (piezometry, aquifer productivity, drilling success rates and water quality), and produced economic maps (production costs, operating costs) as these constitute important decision-making criteria in poor regions such as this.
On a regional scale, groundwater flows from south to north, where the aquifer is exhausted when it reaches the wide Bodole depression. The piezometric surface is marked by a series of domes and depressions whose origin, based on their chemical tracers, is discussed in this paper.
For the time being, aquifer recharge by rainfall far exceeds the abstraction rate. However, an increase in aridity and the progressive concentration of people in urban centers, which leads to local aquifer depletion and groundwater pollution, could upset this balance.
The chemical quality of the groundwater varies greatly from one area to another. The south-north salinity gradient has long been recognized and is connected to aridity. Consolidation of the drilling data highlights two other areas with excessive salinity: the western part of Lake Province and the eastern part of Hadjer Lamis Province.