Recharge and groundwater dynamics in the Tamanrasset alluvial aquifer (Algerian Sahara)

            Social, economic and environmental changes are deeply affecting the entire Earth. Population growth and agricultural intensification are ever more demanding in natural resources, and especially water that is now threatened in both quality and quantity. This degradation is particularly visible in semi-arid and arid areas. The Sahara desert is typical of these multiple constraints and rapid changes where the sustainable management of water resources is a major issue. In Southern Algeria (P < 80 mm.a^-1, PET > 2000 mm.a^-1), perennial water reserves are groundwater with a small to very small recharge: the biggest regional aquifers are considered as having nearly fossil water. In contrast, the alluvial areas associated with the currently active wadis represent small aquifers which are renewed by a few floods each year. In the case of Wadi Tamanrasset, the alluvial aquifer has an average thickness of 20 m and is exploited by the inhabitants because of its accessibility and its good chemical quality. Rare recharging flood events occur mainly between July and September.

            We studied the physical conditions commanding flow and recharge of groundwater, and especially the transfer from surface to groundwater by combining hydrodynamics and isotope tracing of water (¹⁸O, ²H). Monthly monitoring of groundwater level was carried out in around thirty wells and was complemented by automatic recorders with hourly time steps installed since March 2016 in three wells, along a 600 m transect perpendicular to the wadi bed. The part of flood water recharging the alluvial aquifer could then be estimated. The response of the water table is slow and continuous, and becomes visible only two months after the first flood; its maximum rise was 1 m in 2016. Isotopic analyses of rain water on a daily time scale, of the wadi water on a 1-hour time scale during runoff and of groundwater (two complete campaigns in the dry and wet seasons and a specific monitoring of the transect every month) have provided additional information on surface-groundwater transfer.