Spatial distribution and assessment of potentially toxic elements in an Iberian Pyrite Belt Mine: the case-study of São Domingos (Southern Portugal)

Mining activities generate significant soil contamination, leaving behind a persistent legacy of potentially toxic elements (PTEs) in the environment. At the inactive São Domingos mine in southern Portugal, the legacy of mining is marked by acid mine drainage (AMD), which facilitates the release and dispersion of PTEs, creating ongoing risks to ecosystem integrity and public health. In this context, the present study aims to characterize and evaluate the risks of soil contamination by PTEs in a selected area of São Domingos, where 11 topsoil samples (0-20 cm; A2 to A12) were collected. The PTE analyzed were cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), zinc (Zn), and arsenic (As). Quantification was performed by inductively coupled plasma mass spectrometry (ICP-MS) after microwave-assisted digestion, according to the USEPA (2007) method. PTE concentrations were compared with Portuguese reference values for agricultural and industrial soils, respectively (Cd: 1.0/1.9 mg/kg; Cr: 160 mg/kg; Ni: 130/340 mg/kg; Pb: 45/120 mg/kg; Cu: 180/300 mg/kg; Zn: 340 mg/kg; As: 18 mg/kg; APA, 2019) and Canadian soil quality guidelines for agricultural and industrial soils, respectively (Cd: 1.4/22 mg/kg; Cr: 64/87 mg/kg; Ni: 45/200 mg/kg; Pb: 70/600 mg/kg; Cu: 63/91 mg/kg; Zn: 250/410 mg/kg; As: 12 mg/kg; CCME, 2018). Cadmium (0.29–0.43 mg/kg) and chromium (25.31–67.27 mg/kg) showed low concentrations, remaining below guideline values for agricultural and industrial soils. The Ni concentrations (Ni; 36.34–163.29 mg/kg) exceeded agricultural thresholds but remained below limits established for industrial land use at some sampled locations. In contrast, As (324.72–2612.97 mg/kg), Pb (153.51–5321.22 mg/kg), Cu (197.50–1307.09 mg/kg) and Zn (61.17–2743.12 mg/kg), exhibited the highest concentrations, largely exceeding both national and international guideline values for agricultural and industrial soils. The results revealed high spatial variability in PTE concentrations across the study area, a characteristic feature of mining-impacted environments, with the identification of located hotspots representing critical zones of environmental risk.  The results highlighted distinct contamination patterns, identifying As, Pb, Cu, and Zn, as the primary contaminants. These contaminants are primarily associated with historical mining activities and acid mine drainage processes. These elements are characterized by high toxicity and persistence in soils and therefore constitute the main contributors to the environmental risk identified in the study area. These findings support the INCOME project’s environmental management framework and inform the development of integrated and sustainable strategies for the remediation of abandoned mining areas.

Funding: The work is supported by the Promove Program of the “la Caixa” Foundation, in partnership with BPI and the Foundation for Science and Technology (FCT), in the scope of the project INCOME – Inputs para uma região mais sustentável: Instrumentos para a gestão de zonas contaminadas por metais (Inputs for a more sustainable region: Instruments for managing metal-contaminated areas), PD23-00013, and by national funds through FCT, in the framework of the UID/06107/2025 – Centro de Investigação em Ciência e Tecnologia para o Sistema Terra e Energia (CREATE – University of Évora), and in the frame of UID/00073/2025.