Geophysical Characterization of Subsurface Pollution by Acid Mine Drainage (AMD) From Artisanal Gold Mining in Perma, Benin

This study was conducted in the district of Perma, in northwestern Benin, in the department of Atacora. This region of Precambrian crystalline bedrock is known for its gold potential, which is exploited using artisanal and semi‐industrial methods. The use of mercury and cyanide has serious repercussions on soil and water, mainly due to acid mine drainage (AMD). The main objective of this study is to characterize AMD associated with gold mining in Perma using magnetic methods and electrical resistivity tomography. Four mining sites were studied, including two alluvial sites (Perma‐Centre and Perma‐Koka) and two vein sites (Gnagnamou and Koussigou Montagne). The magnetic data show that AMD is stored in the bed of the Perma River at the alluvial sites, with a total magnetic field intensity varying between 33,337.0 and 33,360.8 nT at Perma‐Centre and 33,429.2 and 33,443.5 nT at Perma‐Koka. At the vein sites, the AMD is stored in fractures and galleries created by gold miners, with magnetic intensity varying between 33,434.7 and 33,446.2 nT at Gnagnamou and 33,437.4 and 33,470.7 nT at Koussigou Montagne. These magnetic data guided the choice of profiles for ERT section acquisition. ERT data show that resistivities at the alluvial sites range from 114 to 10,121 Ω m at Perma‐Centre and from 45.4 to 7967 Ω m at Perma‐Koka. The AMD, with resistivity ranging from 199 to 402 Ω m, infiltrates the riverbed to a depth of 30 m at Perma‐Centre and 15 m at Perma‐Koka and circulates in the gravel. At the vein sites, resistivities vary between 655 and 47,180 Ω m at Koussigou Montagne and between 2181 and 134,398 Ω m at Gnagnamou. The AMD flows toward depressions and accumulates in fractures, sometimes reaching depths of more than 40 m. These variations in resistivity confirm the differentiated impact of the AMD.