Infiltration and recharge dynamics in the Nubian Sandstone Aquifer System of northern Chad

The Nubian Sandstone Aquifer System (NSAS) is one of the world’s largest fossil groundwater resources. In northern Chad, notably in the areas of the Tibesti and Ennedi mountains, precipitation occurs seasonally with rates up to 150 mm year –1 . This precipitation could lead to diffuse recharge, as well as concentrated recharge along the episodically flooded wadis. Although it is clear that infiltration occurs under flooded areas, it is unknown if and to what extent the infiltration can recharge groundwater. This study combines remote sensing data on precipitation, evapotranspiration, and the temporal and spatial dynamics of the flooded areas with chemical and stable isotopic data from groundwater and surface water sampled between 2013 and 2016. The combination of these data shows that (1) the only area where diffuse recharge occurs is in the southern area of the Ennedi mountains, where concentrated recharge through the wadis occurs concurrently during the month of August, and (2) southeast of the Tibesti and north of the Ennedi mountains, only concentrated recharge occurs. The length of the flooded areas and thus the spatial extent of concentrated recharge varies significantly from year to year and can last up to 3 months. The study has shown that modern recharge does occur in northern Chad, but to a very limited extent, both in space and time. This means that achieving sustainable management of this renewable resource can only be considered through rigorous quantitative assessments. Furthermore, these findings have important implications for future studies on the regional dynamics of the NSAS.