Hazard and persistence evaluation of Acid Mine Drainage (AMD) process in Valle Giove mining area (Rio Marina district, Elba Island)

Elba island mining activity has been documented since Roman times and was related mainly to iron exploration and exploitation in three districts: Rio Marina, Rio Albano and Capoliveri. Valle Giove is the greatest open pit of the Rio Marina district with 0.5 km2 and is characterized by a hematite + pyrite ± magnetite ± sphalerite ± chalcopyrite ± galena ± bismuthinite ± anglesite ± cerussite ± native sulfur ore association (Tanelli et al., 2001) occurring in strata, lens or vein deposits (Zuffardi, 1990). The effect of open pit mining and other mining-related activities is that sulfides previously isolated from the atmosphere are exposed to oxygen. The oxidation of sulfides acidifies flowing waters that leach ore minerals and dissolve elements such as arsenic, copper, lead, manganese, iron and zinc (Blowes et al., 2003). All these steps, leading to environmental pollution, are known as Acid Mine Drainage (AMD). At Valle Giove AMD process occurs as testified by all the sampled waters (Giove stream, drainage channels, surficial pools and settling basin) which have pH values ranging from 2.08 to 3.35 and an heavy metals content that reaches 903.16 mg/l for Fe, 45.02 mg/l for Mn, 10.08 mg/l for Zn and 1.75 mg/l for Cu. For a preliminary AMD hazard evaluation a central aligned 100 m mesh square grid sampling (EPA, 2002) was performed and fifty-four different terrain samples (residual soils, waste rocks or debris materials) were collected. They were analysed for major elements by XRF and for trace elements by ICP-AES. Net Acid Producing Potential (NAPP) of terrains was determined following AMIRA procedure (IWRI & EGI, 2002). In addition, spatial distribution of heavy metals and hazard assessment of AMD process were investigated using statistical and geostatistical techniques. Moreover, 3-D Digital Terrain Model (DTM) of the mining area at the beginning (1954) and close to the end (1979) of exploitation were developed using AutoCAD 2004 and SURFER 8 software. The comparison between the two periods shows the distribution of topographical positive (mining waste disposals) and negative variations (quarrying areas) and leads to estimate the volume of the material removed from the mining area in the order of 1.38*106 m3 and the volume of material discharged in the same area in the order of 1.57*103 m3. The latter value must be adjusted with additional investigations since the first-mined open pits next to Valle Giove were used as mine waste dumps (Servida et al., in progress). According to the obtained data three main hazard classes were established and their distribution in the mining area was assessed. About 46% of the mining area surface belongs to class I, where AMD process occurs (NAPP>0 and heavy metals content exceeds law limits), about 52% belongs to class II, where AMD process could occur only if some conditions are satisfied (NAPP>0 and heavy metals content is lower than law limits or NAPP<0 and heavy metals content exceeds law limits), only 2% belongs to class III, where AMD process cannot occurs (NAPP<0 and heavy metals content under law limits). Finally, the persistence of AMD process at Valle Giove was evaluated starting from yearly rainfall, mining waters pH and NAPP values. A complete leaching of the first 0.25 m of the terrains can retain the current environmental conditions for several centuries.