## 1 Introduction  
Acid generation and metal release from oxidative dissolution of sulphide minerals, primarily pyrite, is one of the most serious environmental concerns in many historic mining districts worldwide [1; 2]. Contamination arising from acid mine drainage (AMD) not only adversely affects the water quality of downstream waterbodies [3; 4; 5], but also may cause soil degradation and hazards to human health and the environment around the mine sites [6; 7; 8; 9]. This situation becomes more challenging and harder to manage when ecologically sensitive wetlands appear to be impacted by AMD effects related to improper disposal of hazardous mining waste. The weathering processes may be similar in terms of mineral oxidation and dissolution; however, the hydrologic regime, the rates of reaction, and the environmental consequences can be quite different [10; 11; 12]. Geochemical reactions and mineral transformations resulting from direct interaction of AMD with coastal wetlands highly vulnerable to contamination are not yet fully understood and require further research to address their environmental implications. The estuary of Huelva, on the southwestern Spanish coast, provides the opportunity to assess land degradation processes driven by weathering of abandoned sulphide-rich wastes in a coastal wetland. Past mineral-processing operations have left a legacy of large amounts of pyrite concentrates stockpiled on the banks of the salt marsh (Figure 1). The ores extracted from the Tharsis and La Zarza mines in the Iberian Pyrite Belt (IPB) were transported by railway to the estuary to be shipped overseas. Prior to shipment, the ore was crushed, ground, and screened in processing plants located at the railway terminal of Corrales near the loading dock, facing the city of Huelva (Figure 2). The last mining operation closed at the end of December 2000 for economic reasons. Consequently, the pyrite concentrates stored in piles at the processing facilities were left without any remediation plan to mitigate the environmental impacts of the abandoned stockpiles on the adjacent wetland ecosystem. During the operational phase, and especially since the closure of the ore processing activity, the pollutants have been and still are being transferred from the sulphide heaps to nearby marsh soils, resulting in the deterioration of environmental quality [13]. It is particularly concerning that a considerable portion of the wasteland was converted for urban development. From the entry into force of the contaminated land regime in Spain [14], the parcels that had been re-classified for residential use were declared as polluted in 2007 because the site posed unacceptable risks to human health by exposure to potentially toxic trace elements (PTEs). In 2009, the sulphide wastes were removed from the private parcels and the underlying soil was lime-treated to neutralize acidity and sealed with compacted clay to prevent seepage and the release of pollutants. However, the stockpiles within the maritime-terrestrial public domain, occupying an area of about 15,000 m\({}^{2}\), still remain unrealaimed and continue to pose a risk to human health and the environment [15]. Figure 1: Field pictures of the degraded land area showing the abandoned stockpiles of crushed pyrite ore (**a**) and the salt marsh soil affected by the acid drainage (**b**). Figure 2: Geographical setting of the Huelva estuary showing the location of the study area (**a**) and the salt marsh soil impacted by past mineral handling activities (**b**). Knowledge of the geochemical and mineralogical variability and the extent or degree of contamination is crucial in developing the remediation strategies that are currently being considered by the regional authorities, because AMD minerals usually carry varying amounts of PTEs and are responsible for the production of acid upon dissolution [16].