ALBERT

Description: The tailings of Zeida's abandoned mine are found near the city of Midelt, in the middle of the high Moulouya watershed between the Middle and the High Atlas of Morocco. The tailings occupy an area of about 100 ha and are stored either in large mining pit lakes with clay-marl substratum or directly on a heavily fractured granite bedrock. The high contents of lead and arsenic in these tailings have transformed them into sources of pollution that disperse by wind, runoff, and seepage to the aquifer through faults and fractures. In this work, the main goal is to identify the pathways of contaminated water with heavy metals and arsenic to the local aquifers, water ponds, and Moulouya River. For this reason, geophysical surveys including electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and very low-frequency electromagnetic (VLF-EM) methods were carried out over the tailings, and directly on the substratum outside the tailings. The result obtained from combining these methods has shown that pollutants were funneled through fractures, faults, and subsurface paleochannels and contaminated the hydrological system connecting groundwater, ponds, and the river. The ERT profiles have successfully shown the location of fractures, some of which extend throughout the upper formation to depths reaching the granite. The ERT was not successful in identifying fractures directly beneath the tailings due to their low resistivity which inhibits electrical current from propagating deeper. The seismic refraction surveys have provided valuable details on the local geology, and clearly identified the thickness of the tailings and explicitly marked the boundary between the Triassic formation and the granite. It also aided in the identification of paleochannels. The tailings materials were easily identified by both their low resistivity and low P-wave velocity values. Also, both resistivity and seismic velocity values rapidly increased beneath the tailings due to the compaction of the material and lack of moisture and have proven to be effective in identifying the upper limit of the granite. Faults were found to lie along the bottom of paleochannels, which suggest that the locations of these channels were caused by these same faults. The VLF-EM surveys have shown tilt angle anomalies over fractured areas which were also evinced by low resistivity area in ERT profiles. Finally, this study showed that the three geophysical methods were complementary and in good agreement in revealing the pathways of contamination from the tailings to the local aquifer, nearby ponds and Moulouya River.

Description: 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.