In Sardinia (Italy), Pb-Zn vein ores hosted in Palaeozoic silicate-dominant rocks of the Arburese mining district have been exploited in a system of overlapping galleries for a depth of 600 m below ground level. To keep the pits dry, a total flow in the range of 55 to 70 liters per second (L/s) of water was pumped out of the mines. In 1973 (i.e. under active mining), the pumped water at the Casargiu mine was circumneutral (pH 7.5) and concentrations of sulfate (1400 mg/L)and metals (e.g. 70 mg/L Zn, 1.2 mg/L Cd) were high. The mine closure in the 1980s implied the shutdown of pumping systems, thereafter drainage flowing out of the Casargiu shafts was observed since 1996. At the beginning, the mine water showed a pH of 6.0 and dissolved concentrations of sulfate (5000 mg/L) and metals (e.g. 1000 mg/L Zn, 2 mg/L Cd) much higher than those observed under dewatering conditions. These extreme concentrations can be explained taking into account that the underground workings were kept dry during exploitation; flooding allowed the contact of water with sulfide minerals, promoting their oxidation and mobilization of metals, and the dissolution of efflorescent salts. The acidity produced by the oxidation of sulfide minerals (especially pyrite) has been in part buffered by the occurrence of calcite and ankerite minerals in the Casargiu ore. As compared with values recorded at the first stages of rebound at Casargiu, a significant decrease (about 50%) in sulfate and metal concentrations was observed in 2009; nevertheless, a very high contamination level still persists at this site after 14 years of flushing. Mine drainages from Casargiu (20-70 L/s) flow into the Rio Irvi stream. A decrease in dissolved metals occurs downstream by precipitation of solid phases. In the reach of the Rio Irvi stream (pH 6-7), low crystalline goethite as fresh and more consolidated precipitates, and “green rust” as very thin layers in the ochreous deposits and suspended colloidal particles have been observed. Despite the load of dissolved contaminants decreases from the mine site downstream, the dispersion of toxic elements extends about 5 km westwards to the Mediterranean Sea. The estimated amount of dissolved metals discharged into the sea is relevant (e.g. 900 kg/day Zn, 1.4 kg/day Cd, 0.8 kg/day Pb).
Impact of the Casargiu mine drainage on the Mediterranean Sea
CIDU, ROSA;
2009-01-01
Abstract
In Sardinia (Italy), Pb-Zn vein ores hosted in Palaeozoic silicate-dominant rocks of the Arburese mining district have been exploited in a system of overlapping galleries for a depth of 600 m below ground level. To keep the pits dry, a total flow in the range of 55 to 70 liters per second (L/s) of water was pumped out of the mines. In 1973 (i.e. under active mining), the pumped water at the Casargiu mine was circumneutral (pH 7.5) and concentrations of sulfate (1400 mg/L)and metals (e.g. 70 mg/L Zn, 1.2 mg/L Cd) were high. The mine closure in the 1980s implied the shutdown of pumping systems, thereafter drainage flowing out of the Casargiu shafts was observed since 1996. At the beginning, the mine water showed a pH of 6.0 and dissolved concentrations of sulfate (5000 mg/L) and metals (e.g. 1000 mg/L Zn, 2 mg/L Cd) much higher than those observed under dewatering conditions. These extreme concentrations can be explained taking into account that the underground workings were kept dry during exploitation; flooding allowed the contact of water with sulfide minerals, promoting their oxidation and mobilization of metals, and the dissolution of efflorescent salts. The acidity produced by the oxidation of sulfide minerals (especially pyrite) has been in part buffered by the occurrence of calcite and ankerite minerals in the Casargiu ore. As compared with values recorded at the first stages of rebound at Casargiu, a significant decrease (about 50%) in sulfate and metal concentrations was observed in 2009; nevertheless, a very high contamination level still persists at this site after 14 years of flushing. Mine drainages from Casargiu (20-70 L/s) flow into the Rio Irvi stream. A decrease in dissolved metals occurs downstream by precipitation of solid phases. In the reach of the Rio Irvi stream (pH 6-7), low crystalline goethite as fresh and more consolidated precipitates, and “green rust” as very thin layers in the ochreous deposits and suspended colloidal particles have been observed. Despite the load of dissolved contaminants decreases from the mine site downstream, the dispersion of toxic elements extends about 5 km westwards to the Mediterranean Sea. The estimated amount of dissolved metals discharged into the sea is relevant (e.g. 900 kg/day Zn, 1.4 kg/day Cd, 0.8 kg/day Pb).
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