Impacts of climate variations and human influences on groundwater recharge in the semi-arid Trarza aquifer (SW Mauritania)

         Even in regions with very sparse population, hydrological processes are rapidly changing in semi-arid areas under the double constraint of climate change and anthropization. This is visible in the Trarza aquifer in south-western Mauritania where we investigated changes in groundwater recharge at different scales of time and space. This flat region of about 40,000 km² is mainly made of a succession of Quaternary sandy dunes and interdune depressions where rainfall varies between 100 and 250 mm.a^-1 from north to south. The apparent data scarcity, because of a very low demographic density and a limited hydraulic infrastructure, forced to search for all sources of hydrodynamic information, recent hydrogeological surveys as well as much older field campaigns. Out of this time scale of 60 years, the geochemical information shows the importance of Holocene eustatic variations in the present groundwater mineralization.

            While rainfall has varied a lot in the last decades (e.g. the dramatic droughts of the 1970s and 1980s), its infiltration in interdune depressions did not vary significantly as suggested by the piezometric stability over the last 50 years. This component of the groundwater recharge is estimated at about 4 mm.a^-1.

            Since 1989, the big Diama dam has raised the mean level of the Senegal River, which is now always higher than the neighbouring groundwater. The additional inflow from the river to the alluvium and to the Trarza aquifer is estimated at about 34 hm³.a^-1. Groundwater recharge from the river and rainfall infiltration over the whole area have the same order of magnitude. An even more recent form of anthropization is the new supply of water to the capital (45 hm³.a^-1), pumped from the Senegal River. The increased injection of urban waste waters in the aquifer has led to a groundwater rise in the Nouakchott area, where several urban districts are now submerged.

            The Trarza aquifer appears then as to be presently much more influenced by human pressures than by climatic fluctuations, but the long term rise of sea level may have heavy consequences before the end of the 21^th century in this very flat region.