Hydrogeophysical Characterization to Inform Future Mine Dewatering Strategies: Case Study of the Beni Amir Phosphate Deposit, Morocco

Abstract

Groundwater management presents a challenge for open-cast phosphate mining within the Beni Amir deposit, Oulad Abdoun Basin, Morocco, where future mining operations are progressively reaching deeper saturated phosphate layers. This study integrates six Electrical Resistivity Tomography (ERT) profiles, eleven Magnetic Resonance Sounding (MRS) measurements, hydrostratigraphic modeling, 690 borehole logs, 580 piezometric measurements, and pumping-test results from 14 production wells to characterize the multilayer aquifer system. The comprehensive interpretation identifies three water-bearing zones: a shallow Eocene aquifer located within the Lutetian–Ypresian succession at depths approximately between 2 and 20 m, an intermediate Danian–Thanetian to Maastrichtian phosphate-bearing aquifer situated at depths ranging from 20 to 60 m, and a deeper Senonian marly limestone aquifer occurring below 60 m. Saturated zones are generally associated with low resistivity values, commonly below 28 Ω·m, and MRS-derived mobile water contents ranging from 0.3% to 4.4%. The hydraulic conductivity derived from MRS data exhibits a range of one and a half orders of magnitude across the majority of soundings, from 1.5 × 10−6 to 5 × 10−5 m/s. A comparative analysis with pumping tests shows that MRS-derived hydraulic conductivity is generally greater than well-test estimates, with localized discrepancies that may reflect scale effects, lithological heterogeneity, inversion uncertainties, and the necessity for localized calibration. As a working hypothesis, areas spatially associated with ephemeral streambed channels may represent potential zones of localized recharge or inflow, possibly where erosional truncation reduces the continuity of confining layers. The resultant hydrostratigraphic framework constrains saturated zones and hydraulic heterogeneity, thereby establishing a foundation for forthcoming groundwater flow modeling and mine dewatering evaluations.