Geochemistry of Soils from the Surrounding Area of a Coal Mine Waste Pile Affected by Self-Burning (Northern Portugal)

Coal mining can generate organic and inorganic contaminants that can be disseminated in the surrounding soils by leaching and/or aerial deposition. This study aims to identify and characterize the physicochemical and geochemical changes promoted in soils from the surrounding area of a self-burning waste pile in an abandoned coal mine. A soil sampling campaign was conducted bordering the waste pile, comprising the main drainage areas as well as the areas uphill. The soils were characterized geochemically for major and trace elements and multivariate statistics was used in combination with geostatistical methodologies to study the statistical and spatial relations of the different elements and infer their Potentially Toxic Elements (PTEs) sources. The 16 priority Polycyclic Aromatic Hydrocarbons (PAHs) were identified and quantified in soils according to their spatial distribution, and their pyrogenic/petrogenic sources were inferred. Different sources were identified as contributing to the soil geochemical signature, considering not only the mine but also anthropogenic urban contamination or naturally enhanced regional geochemical background in multiple PTEs. PAHs tend to concentrate downstream of the waste pile, along the runoff areas, presenting a greater variety of the 16 priority PAHs and an increase of High Molecular Weight (HMW) PAHs pointing to its pyrogenic origin, possibly related to the self-combustion phenomenon occurring in the waste pile.