Heavy metal contamination from mining sites in South Morocco: 1. Use of a biotest to assess metal toxicity of tailings and soils

Our work was conducted to investigate the heavy metal toxicity of tailings and soils collected from five metal mines located in the south of Morocco. We used the MetPAD™ biotest Kit which detects the toxicity specifically due to the heavy metals in environmental samples. This biotest initially developed to assess the toxicity of aquatic samples was adapted to the heterogeneous physico-chemical conditions of anthropogenic soils. Contrasted industrial soils were collected from four abandoned mines (A, B, C and E) and one mine (D) still active. The toxicity test was run concurrently with chemical analyses on the aqueous extracts of tailings materials and soils in order to assess the potential availability of heavy metals. Soil pH was variable, ranging from very acidic (pH 2.6) to alkaline values (pH 8.0–8.8). The tailings from polymetallic mines (B and D) contained very high concentrations of Zn (38 000–108 000 mg kg−1), Pb (20 412–30 100 mg kg−1), Cu (2019–8635 mg kg−1) and Cd (148–228 mg kg−1). Water-extractable metal concentrations (i.e., soil extracts) were much lower but were highly toxic as shown by the MetPAD™ test, except for soils from mines A, E and site C3 from mine C. The soil extracts from mine D were the most toxic amongst all the soils tested. On this site, the toxicity of soil water extracts was mainly due to high concentrations of Zn (785–1753 mg l−1), Cu (1.8–82 mg l−1) and Cd (2.0–2.7 mg l−1). The general trend observed was an increase in metal toxicity measured by the biotest with increasing available metal contents in tailings materials and soils. Therefore, the MetPAD™ test can be used as a rapid and sensitive predictive tool to assess the heavy metal availability in soils highly contaminated by mining activities.