Occurrence, leaching, and mobility of major and trace elements in a coal mining waste dump: The case of Douro Coalfield, Portugal

Abstract

The coal mining residues from past mining activities in the Douro Coalfield are emplaced in many dumps near the mining areas, causing substantial environmental impacts. This study aims to contribute to the investigation of environmental impacts caused by the disposal of the residue in the Douro Coalfield through the determination of the geochemical composition of waste materials collected at 3 e 15 m depth, and of the water-leachable fraction produced by percolation in these materials inside the dump. The elements with higher concentration in the leachates are Ca, Cd, Co, Mg, Mn, Na, Ni, S, and Zn, indicating that they are the most easily leached and accessible elements for percolation and mobilization into the environment. The presence of these elements in the leachates is principally attributed to their occurrence as water-soluble ions in clays and sulphides affected by natural weathering conditions, which promotes their breakdown and occurrence as exchangeable ions. The leachates are classified as acid high-metal. Natural weathering and leaching in the S~ao Pedro da Cova waste dump are the principal paths for chemical elements to be percolated and mobilized to environment, posing risk to the soils, waters and ecosystems in the surrounding areas.