Dwindling Geothermal Groundwater Resources in Tunisian NWSAS, from Global Recovery and Climate Experiment (GRACE) Satellite Gravity Observations

Abstract

The North-Western Sahara Aquifer System (NWSAS), is a geothermal aquifer from a sandstone formation of Lower Cretaceous age, extends over a total area of one million km2. This trans boundary aquifer system is shared by Algeria (700 000 km2) Libya (250 000 km2) and Tunisia (80 000 km2), is most heavily irrigated region in arid zone. The water reserves are estimated at 60 000 billion m3 distributed under two main superposed confined aquifers. These layers are, from the top, the Terminal Complex (TC) and the “Continental Intercalary” (CI). The depth is comprised between 100 and 2500m. The initial production by deep well is about 137 liters per second at a well-head temperature of 59˚ and water salinity of 3g/l. Temporal changes in Earth’s gravity field in this region as recorded by the Global Recovery and Climate Experiment (GRACE) satellite mission reveal a steady, large-scale mass loss that we attribute to excessive extraction of groundwater. Combining the GRACE data with hydrological models to remove natural variability, we conclude the region lost groundwater at a rate of 54 ± 9 km3 /yr between April 2002 (the start of the GRACE mission) and December 2016. The GRACE data variations are very well fitted when compared with in situ observations. This is probably the largest rate of groundwater loss in any comparable-sized region on Earth. This trend, if sustained, will lead