The Nubian Sandstone Aquifer System (NSAS) is a transboundary aquifer shared between Libya, Egypt, Sudan and Chad. The NSAS is one of the largest aquifers of the world. Groundwater is the only available water resource for most of the population living within its boundaries, given the extreme aridity that characterizes the region. The NSAS of Northern Chad might correspond to the least exploited and to the least investigated part of the system. The region is characterized by two mountainous regions, the Ennedi and Tibesti Mountains, experiencing increased precipitation rates (approx. 100 mmy-1) compared to the mean regional rates (< 5 mmy-1). Albeit small in quantity, recharge occurring in these mountainous regions might constitute an important, renewable resource. Previous authors studying the hydrodynamic properties of the NSAS have also concluded that modern recharge might play a role in maintaining higher hydraulic heads and gradients, thus slowing down the depletion of the NSAS. In view of a likely increase of exploitation of this aquifer in Chad, it is essential to characterize the temporal and spatial dynamics of modern recharge and to assess its influence to the regional aquifer system.
Key challenges of this project were the remoteness of the region and the lack of ground-based monitoring climatic and meteorological data. In the framework of the ResEau project, several fieldwork campaigns were undertaken between 2013 and 2016, covering approximately an area of 100,000 km2. Hydraulic heads measurements, physico-chemical groundwater characterization and water probe sampling were performed for 185 water points. Water samples were analyzed for their major anions and cations concentrations and stable isotopic ratios (δ18O and δ2H). Remote sensing products (RFE2.0, LandSat8OLI) were employed to visualize the temporal and regional dynamics of precipitation, runoff and evapotranspiration and to quantify the system gains and losses. The period of observation was set between years 2013 and 2016.
The combined use of remote sensing products, fieldwork and the groundwater chemical and stable isotopic characterization enabled to qualitatively assess the relative importance of diffuse and concentrated recharge within the mountainous regions of Northern Chad. A groundwater flow conceptual model was developed in order to formulate hypothesis on the relationship between the zones experiencing modern recharge and the regional aquifer system. Groundwater’s hydro chemical and stable isotopic compositions shows distinctive characteristics whether if recharged during the current climatic period or corresponding to paleo-groundwater recharged during the previous humid but cooler periods of the late Pleistocene to Holocene. This distinction has important implication for water management.