Hydrogeophysical investigations of El Azrag aquifer, Zouerate State, Northern Mauritania

Abstract

Mauritania, a desert country with high temperatures and minimal annual rainfall (<70 mm), faces critical water scarcity, making groundwater a vital resource for all sectors. Securing permanent water supplies is a significant challenge due to the absence of perennial rivers. To ensure sustainable long-term use, evaluating aquifer hydraulic parameters through effective tools like pumping tests is essential. This study investigates the hydrogeological characteristics of the El-Azrag aquifer in Zouerate State, northern Mauritania. We employed an integrated approach, combining step-drawdown, constant discharge, and recovery pumping tests from four wells with complementary electrical resistivity surveys (VES and ERT). Key hydraulic parameters, including formation loss coefficient (B), well loss coefficient (C), transmissivity, hydraulic conductivity, and aquifer potentiality, were determined. The formation loss coefficient (B) ranged from 3.25 × 10⁻⁴ to 5.14 × 10⁻³, and the well loss coefficient (C) from 9.12 × 10⁻⁷ to 1.97 × 10⁻⁵, indicating properly developed and well-designed pumped wells. Transmissivity values from constant rate and recovery tests (Tp and Tr) revealed high aquifer potential, particularly around wells AOE19 and AOE30 (exceeding 500 m²/day). The resistivity surveys delineated lithological boundaries, fracture zones, and groundwater saturation patterns, confirming the fractured Azougui Formation as the primary aquifer despite its moderately saline water (6-14 g/L). Resistivity data further revealed aquifer compartmentalization due to NE-SW trending faults, aligning with observed hydraulic behavior and highlighting the critical dependence on secondary porosity through fractures. This integrated hydrogeological and geophysical approach provides crucial data for informed and sustainable groundwater resource planning in this water-stressed environment.