An application of electrical resistivity tomography to investigate heavy metals pathways

The abandoned lead mine of Zeida is located at the center of the High Moulouya watershed between the Middle and the High Atlas Mountains of Morocco. Zeida has produced a total of 640,000 tons of concentrated lead during the 14 years of its activity (1972'1985). Three large tailings were left at the center of the mine on both sides of Moulouya River without any risk reduction measures or monitoring and there is a concern regarding heavy metal contamination of local groundwater. Samples taken from and around the tailings were found to contain average lead and zinc concentrations of 3,000 ppm and 140 ppm, respectively, primarily in the form of galena and barite. Prior studies have also found high concentrations of lead and zinc in both local wells near the town of Zeida and along the banks of Moulouya River. In this study, five electrical resistivity imaging surveys were performed to identify the risk of pollution and trace the pathways of mine-based contaminants to groundwater and to the Moulouya River. The analysis of electrical resistivity data has provided new insights showing: 1) an average tailings thickness of 15 m; 2) rounded structures with high resistivity values at the center and gradually decreasing toward their edges that are assumed to be granite, with fractures and a weathered zone; and 3) the potential pathways of heavy metal occur predominantly along these fractures and in the thick layers of the sandstone overlaying granitic bedrock.