Environmental Management of Aznalcóllar Mine and Its Influence in the Hydrogeochemical of the Pit lake

Abstract

Since its 1995 closure and the subsequent 1998 accident, the Aznalcóllar mine pit has been used as a disposal site for diverse metal-rich materials, such as the polluted soils removed during the cleaning of areas along the Guadiamar River, waste rock, pyrite sludge, and ashes deriving from pyrite roasting. At present, the mining pit is partly flooded and contains a highly acidic (pH 2.7) pit lake with approximately 6 Mm3 of metal and sulphate-rich water. Recent detailed research performed in the area has proven that the reaction of the lake water with both the dumping mining wastes and the inflow of acid mine waters are strongly modifying the water quality of the pit lake. A major change was identified in the water quality that resulted from the dumping of 1.4 Mm3 of pyritic wastes during 2005 and 2006. The oxidative dissolution of this mineral has resulted in: (1) a total consumption of dissolved oxygen, (2) a notable increase of electric conductivity (from 8.6 to 12 mS/cm) and (3) a strong acidification (pH from 4.2 to 2.7) and an important heating that was observed as a result of the exothermic character of pyrite oxidation. Despite the total lack of oxidizing agents-such as 02 and Fe(III) (Fe[III] < 5% total Fe)-in the pit lake, pyrite is being oxidized with a continuous increase of the SO(4)2- and Fe concentrations.