The flooded pit at Aznalcóllar (Seville, Spain) and its use as a dumping site for mine waste
DOI:
https://doi.org/10.21701/bolgeomin.122.2.008Keywords:
acidic mine drainage, Iberian Pyrite Belt, mine waste, pit lakeAbstract
Since its closure in 1995 and the subsequent failure of the slurry deposit dam in 1998, the Aznalcóllar pit has been used as a disposal site for diverse metal-rich materials, such as the polluted soils removed during the cleaning-up of areas along the Guadiamar river together with waste rock, pyrite sludge and ashes deriving from pyrite roasting. At present the pit is partly flooded with a highly acidic (pH 2.7) lake holding some 6 Mm3 of metal- and sulphate-rich water. Detailed research undertaken in the area during recent years has proved that the dumping of wastes and the inflow of acidic mine waters are having a dramatic effect on the quality of the water in the lake. The greatest effect upon the Aznalcóllar lake water was due to the dumping of 1.4 Mm3 of pyritic wastes during 2005 and 2006. The oxidative dissolution of this mineral has resulted firstly in the total consumption of dissolved oxygen, secondly, a notable increase in electric conductivity (from 8.6 to 12 mS/cm), thirdly, considerable acidification (from pH 4.2 to 2.7) and finally, heating due to the exothermic character of pyrite oxidation. Despite the almost total lack of oxidizing agents such as O2 and Fe(III) (Fe(III)<5% Fetotal) in the pit lake, pyrite continues to oxidize, producing a concomitant increase in SO42- and Fe.
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