Study Uncovers Trace Metal Contamination Risks from Mine Tailings

By Dr. Noopur JainReviewed by Laura ThomsonJul 24 2024

In a recent article published in Hydrology, researchers from Belgium and Morocco investigated the complex interactions between mine tailings and trace metal contamination in the Upper Moulouya Watershed of Morocco. The study highlighted the significance of understanding trace metal fluxes due to the environmental and health risks associated with mining activities, particularly in semi-arid regions where soil erosion exacerbates the issue.

Background

Mining operations often lead to the accumulation of tailings, which can release harmful trace metals into surrounding ecosystems. The Zaida Mine, located in a semi-arid area, presents a unique case for studying these dynamics. Previous research has indicated that soil erosion can significantly influence the transport of contaminants, making it crucial to assess the extent of trace metal fluxes from mine tailings to nearby water bodies. This study aims to fill the knowledge gap regarding quantifying and assessing these fluxes.

The Current Study

The research was conducted in the Upper Moulouya Watershed, specifically focusing on the abandoned Zaida Mine in northeastern Morocco. The study employed the Revised Universal Soil Loss Equation (RUSLE) to estimate soil erosion rates. This widely recognized empirical model predicts average annual soil loss based on some key factors.

The Rainfall Erosivity (R) data were obtained from local meteorological stations to calculate the erosivity factor. The Soil Erodibility (K) factor was calculated by collecting soil samples from various locations within the watershed to determine their physical and chemical properties. Slope Length and Steepness (LS) were derived using digital elevation models (DEMs).

Cover Management (C) data were gathered through remote sensing techniques and field surveys to assess vegetation cover and its impact on soil erosion. Erosion calculation was done using the RUSLE equation to estimate soil loss across the watershed, allowing for the identification of areas with high erosion potential.

To assess trace metal concentrations, soil samples were collected from the top 15 cm of the soil profile, as this layer is most susceptible to erosion and contamination. Sampling points were strategically chosen based on proximity to the mine and hydrological flow patterns.

Trace metal concentrations (Pb, Cu, Cd, Zn) were determined using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) following acid digestion with a mixture of nitric and hydrochloric acids.  Several contamination indices were calculated to evaluate the metal pollution level in the sediments. These included the Geo-accumulation Index (Igeo), Pollution Load Index (PLI), and Contamination Factor (CF).

Results and Discussion

The application of the RUSLE model revealed an average annual soil erosion rate of 9.1 tons per hectare per year (t/ha/yr) across the Upper Moulouya Watershed. This rate indicates a relatively low level of erosion compared to other regions. Yet, it remains significant given the semi-arid context and the potential for increased erosion during heavy rainfall events.

The spatial distribution of erosion rates highlighted areas with higher susceptibility, particularly those adjacent to the mine tailings, where the combination of steep slopes and disturbed soil conditions contributed to increased erosion.

Analysis of the soil samples indicated that lead (Pb) was the predominant contaminant, with concentrations exceeding 200 mg/kg in areas directly downstream of the mine tailings. Other trace metals, including copper (Cu), cadmium (Cd), and zinc (Zn), were also detected, albeit at lower concentrations.

The highest contamination levels were found within a 14 km radius of the tailings, suggesting that the prevailing wind patterns and hydrological transport mechanisms play a crucial role in dispersing these contaminants throughout the watershed. The results correlate soil erosion rates and trace metal concentrations.

The Geo-accumulation Index (Igeo) indicated that the sediments in the Moulouya River were moderately to heavily contaminated with Pb and Cu, while Cd and Zn showed lower levels of contamination. The Pollution Load Index (PLI) confirmed these findings, revealing that the river sediments acted as a sink for the trace metals mobilized from the surrounding contaminated soils.

The CF analysis highlighted that Pb posed the greatest risk to the aquatic ecosystem, with concentrations significantly exceeding the threshold limits established for sediment quality. This elevated level of contamination raises concerns regarding the potential ecological impacts, including toxicity to aquatic organisms and bioaccumulation in the food chain. Additionally, soil pH and organic matter content were found to influence the availability and mobility of trace metals, with lower pH levels associated with higher metal solubility.

Conclusion

In conclusion, the study shows that mine tailings in the Upper Moulouya Watershed are a significant source of trace metal contamination driven by soil erosion processes. The integrated approach in this research provides valuable insights into the dynamics of metal fluxes in semi-arid environments. The authors emphasize the need for ongoing monitoring and the implementation of remediation strategies to address the environmental challenges posed by mining activities in such vulnerable regions.