Assessment of aquifer geometry and groundwater storage using three-dimensional hydrostratigraphic modeling and geophysical survey: The case study of Nefza massive dunes (Northern Tunisia)

Abstract The aquifer system of Nefza in northern Tunisia has been exploited for decades in its southern part to meet the increasing need for drinking water. The drop in groundwater levels observed since the 1980s in this exploited area has led water resource managers to focus on the depletion of the groundwater resource, while the availability of groundwater in the entire aquifer system is not well known. To date, the total amount of groundwater stored in the aquifer system has never been fully estimated. That is the purpose of this paper. The methodology includes (1) creating a comprehensive database combining all available lithological and geophysical data (2) designing a three-dimensional model of the aquifer system (3) incorporating hydrodynamic parameters into the model to estimate groundwater storage. The developed model illustrates the spatial distribution of groundwater storage, improves mapping of aquifer geometry, and delineates major aquifers and aquitards. Over 45 km2 of the modeled area, the calculated groundwater storage in 2019 was approximately 3 × 109 m3 with a highly variable spatial distribution. It was found that the area currently exploited for drinking water supply represents low groundwater storage between 97 and 377 m3/voxel (voxel size =1000 m x 1000 m x 5 m) while the unexploited area has excellent groundwater storage up to 798 m3/voxel. Furthermore, over the past 52 years (1976-2019), only 0.2% of the groundwater storage in the aquifer has been depleted. This paper suggests that water management challenges should focus on accessing non-exploited areas of the aquifer covered by forests, even though the cost of the land development and the installation of water pipelines is burdensome. In addition, the water balance of the aquifer is poorly known and groundwater flow in this region needs to be better assessed by multiple methods such as geochemistry and hydrodynamic modeling.