Integration of Multi-geophysical Approaches to Identify Potential Pathways of Heavy Metals Contamination - A Case Study in Zeida, Morocco

Abstract

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.

A Review on Applications of Time-Lapse Electrical Resistivity Tomography Over the Last 30 Years : Perspectives for Mining Waste Monitoring

“… 2016 ), eolian transport (Lachhab et al. 2020 ) or leaks from sealing layers (Cortada et al. 2017 ; Rey et al.…”

“…( 2020 ) Brazil Minas Gerais WRP pseudo-3D ERT 600m x 900m Large-scale 3D mapping of AMD affected water Design of future reclamation Lachhab et al. ( 2020 ) Morocco Drâa-Tafilalet TSF 2D ERT lines (x3) 630m to 790m ERT combined with seismic to identify AMD pathways from TSF through fractures Martín-Crespo et al. ( 2020 ) Spain Murcia WRP/TSF 2D ERT lines (x2) 120m Combination of aerial photogrammetry and ERT to map AMD occurrence Moreira et al.…”

Mapping Leachate Pathways in Aging Mining Tailings Pond Using Electrical Resistivity Tomography

“…Guireli Netto et al conducted a geophysical investigation in a cemetery using the DC resistivity method with different electrode arrays, confirming the presence of low resistivity anomalies related to the presence of ionic compounds [42]. Also, ERT has proven successful in detecting leachate pathways in mining tailings [43][44][45][46]. It plays a crucial role in characterizing and monitoring tailings ponds, offering valuable insights into the spatial and temporal variations of subsurface electrical resistivity values, which are influenced by the site's physical and chemical properties [20].…”