Environmental assessment of potential toxic elements in the mining district of Mazarrón (SE, Spain)

Mining areas are highly susceptible to environmental contamination due to the accumulation of potentially toxic elements (PTEs), necessitating regular monitoring to assess environmental risks. In this study, soil samples were collected from two depths (0-15 cm and 15-30 cm) at 74 stations, along with deep waste samples (0-12 m) from three adjacent tailings ponds in the Mazarrón mining area, Spain. Soil/waste physicochemical properties and the Cd, Cu, Zn, and Pb concentrations of total, water-soluble, and bioavailable fractions were analyzed. Pollution indexes, spatial distribution maps and vertical profiles of metal concentrations in the tailings ponds were generated to evaluate contamination degree and distribution patterns. The results indicated highly acidic soil conditions (pH 4), and elevated electrical conductivity (3.8 dSm^-1), particularly in the tailing ponds (pH 2.6 and EC: 8.3 dSm^-1), which enhance metal solubility and mobility, and a significant mine drainage generation. Severe contamination was observed for Cd, Zn, and Pb, with Pb concentrations exceeding 10,000 mg kg^-1 in surface soils and Zn levels surpassing 16,000 mg kg^-1 in mining waste. Cd exhibited the highest bioavailability (up to 99 %), posing substantial environmental and ecological risks, whereas Pb remained largely immobile despite its high total concentration. Statistical analyses identified soil acidity and salinity as key factors controlling metal mobility, while anthropogenic inputs, from mining activities, contributed to Cd and Zn enrichment. The Pollution Load Index and Ecological Risk Index confirmed extreme contamination levels, particularly in the central region of the mining district, with Cd emerging as the primary ecological risk element. These findings highlight the urgent need for remediation strategies, including pH stabilization, containment of highly mobile metals, and continuous environmental monitoring to mitigate long-term ecological impacts.