Soils chemical and biological characterization tools for managing metal-contaminated areas: case-study São Domingos mine (South of Portugal)

According to the European Union Soil Strategy for 2030, it is crucial to address contamination from mining areas, and its impact on watercourses. to achieve these goals, it is essential developing methodologies for identifying and monitoring contaminated areas, and implementing sustainable solutions for their recovery to protect soil health and ensure sustainable land use. In Portugal, soil and water contamination in former mining areas, is a significant environmental challenge, especially due to the presence of potentially toxic metals that can affect human health and ecosystems. São Domingos mine, located in the Iberian Pyrite Belt, is an open-pit mine, submerged in acidic drainage water, resulting from mining extraction activities carried out until the middle of the 20^th century. In this sense, the objective of this study was to analyze the chemical and biological characterization of the soils of the São Domingos Mine, contributing to the development of an environmental management model for abandoned mining areas. To achieve this purpose, 11 topsoil (0-20cm) samples (A2 to A12) were collected in São Domingos mine, and the following parameters were analyzed: (i) chemical: pH (deionized water suspension of 1:2.5 (w/v)); electrical conductivity (EC) (deionized water suspension of 1:2 (w/v)); phosphorus (P) and potassium (K) (Egner-Riehm Method); total nitrogen (N) (Kjeldah method); organic matter (OM) (Walkley & Black method); (ii) biological(enzymatic parameters): dehydrogenase activity, acid phosphatase activity and β-glucosidase activity. The results evidenced pH ranged from 3 to 4 (very acidic). The EC, ranging from 115 to 5043 µS/cm, with most of the samples classified as non-saline. The percentage of OM was generally low (0.2 to 2.5%). Regarding macronutrients, the results were equally limiting, with the samples showing low levels of N (0.05 to 0.17%), P (1 to 6 mg P₂O₅ kg^-1) and K (3 to 30 mg K₂O kg^-1). Analysis of enzyme parameters revealed low enzymatic activity frequently lower than the detection limit of the technique. An exception to β-glucosidase that generally had low values, (0.01 to 0.40 µmol PNP g^-1 DM h^-1), and phosphatase showing values among 0.27 to 0.96 µmol PNP g^-1 DM h^-1. This can be mainly related to the low values of pH, low percentage of organic matter and nutrients, and high amount of potentially toxic metals. These results will be extremely important in the development of the environmental management model proposed in INCOME project, as they provide essential information on the variability of the contamination in the mine area, essential information for validate the rest of the methodologies applied. Further, this type of model will be applicable to other regions of contamination, contributing to economic and tourist development, public health, and protection of local ecosystems, in line with the Sustainable Development Goals.

Funding: The work was 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 for a more sustainable region: Instruments for managing metal-contaminated areas, PD23-00013. Acknowledgment: CREATE Project (R&D Unit ID 6107).