## 3 Case Study  
The Douro Coalfield (NW of Portugal) represents the most important coal-bearing deposit in Portugal (Upper Pennsylvania), with a NW-SE alignment, a variable width (30-250 m) and approximately 53 km in length [36]. The Douro Coalfield sedimentary record corresponds to an intramontane basin with alluvial-palustrine/lacustrine-fluvial and deltaic deposits containing coal seams [36]. The coal-bearing deposits of anthracite A [37] from the Douro Coalfield were exploited for many years (1795-1994) in two principal centers of mining activity (the S. Pedro da Cova and Pejao mining areas) and in several small mines dispersed in the Douro Coalfield area [38]. The exploited coal was the main fuel supply used for power generation in the Tapada do Outeiro thermal power plant between 1959 and 1997, although it was also used for household, cement, and metallurgical industry [38]. Both coal mining and coal consumption contributed significantly to the economic and energy sectors of Portugal, as well as to the socioeconomic and cultural sectors of the region. Despite the importance of mining in the Douro Coalfield, its exploitation and consumption inevitably impacted the environment. The rejects and overburdened material that resulted from the exploitation of the Douro Coalfield were discarded in at least 28 waste piles placed over the old mine sites and adjacent areas, representing a significant environmental impact associated with the mining activities in the region [24]. Three of these waste piles, namely, S. Pedro da Cova, Midoes, and Lomba, started burning in 2005 after ignition caused by forest fires, further adding to environmental concerns [22]. The S. Pedro da Cova waste pile is still undergoing a self-combustion process and is located close to the oldest center of mining activities in S. Pedro da Cova and near to a population center and social infrastructure. This waste pile has an elongated form, occupying an area of approximately 28,000 m\({}^{2}\), with the burning process occurring only along the south slope. Previous work deals with the comprehensive characterization of the materials that constitute the S. Pedro da Cova waste pile as well as with the identification of combustion byproducts (gases emitted from vents and coal fire gas minerals) [22; 23; 24; 25; 26; 27; 28]. The sampling for previous studies included the collection of solid waste material from burning or already burnt zones and from unburned zones of the waste deposit, neoformed minerals found in the surface of the deposit (coal fire gas minerals), and gaseous emissions. Figure 1 shows some images of the S. Pedro da Cova waste pile and some particular aspects related with the combustion process, such as the gaseous emissions and neoformed minerals. The methodologies applied for comprehensive characterization of the sampled materials from S. Pedro da Cova waste pile included petrographic, mineralogical, and geochemical studies. The petrographic observations of waste materials show that organic and inorganic matter is affected by weathering processes as well as by the combustion process. Figure 2 shows some petrographic aspects that are evidence of the effects caused by the combustion process. The thermally affected waste material shows strong petrographic changes such as development of dark reaction rims (Figure 2A), devolatilization vacuoles, cracks, and reduced anisotropy in organic particles, development of pyrolytic carbon (Figure 2B) and char structures, oxidation of sulfides (e.g., pyrite), and formation of iron oxides (Figure 2C) [22]. The mineralogical phases identified in the burning or burnt waste material (e.g., mullite, cristobalite, and hematite) demonstrated that combustion temperature reached at least 1000 \({}^{\circ}\)C, and may have reached temperatures of 1500 \({}^{\circ}\)C. The magnetic parameters demonstrated a significant magnetic enhancement, which is attributed to the combustion process that was responsible for the formation of iron oxides [28].