Hydrogeochemical Evolution of the Aznalcóllar Pit Lake during the Spill of a Pyritic Waste Pile

Since its closure in 1995 and the subsequent accident happened in 1998, the Aznalcóllar mine pit has been used as a disposal site for diverse metal-rich materials removed during the rehabilitation of polluted areas along the Guadiamar river and around the former ore processing facilities. At present, the mine pit is partly flooded and contains a highly acidic (pH 2.7) pit lake with ~6 Hm of metal and sulphate-rich water. Detailed research performed in the area during recent years (November 2002 to present-day) has shown that the reaction of the lake water with the spilled wastes, in addition to mixing with two inflows of acid mine waters coming from a waste pile and a pool which collects acidic leachates generated in the mine area, are strongly modifying the water quality of the pit lake. A major and dramatic highlight in the water quality of the pit lake has been the spill of 1.4 Hm of pyritic wastes from an adjacent waste pile during the period from November 2005 to November 2006. The oxidative dissolution of this pyrite has resulted in (i) a total consumption of dissolved oxygen, (ii) a notable increase of total dissolved solids (conductivity 8.6 to 12 mS/cm), and (iii) a strong acidification (pH 4.2 to 2.7). In addition, from January to October 2006 an important heating of the hypolimnion was observed. This heating is due to the exothermic nature of pyrite oxidation, and is three times greater than the one observed during a similar period in 2005 (∆T=5.3oC vs. 1.6oC, respectively). Despite the virtually total lack of oxidizing agents such as O2 and Fe(III) (Fe(III)<5% Fetotal) in the pit lake, pyrite is being oxidized and dissolved at high rates, as evidenced by the continuous increase of the SO4 and Fe concentrations. The aim of this work was to investigate the hydrochemical evolution of the Aznalcóllar pit lake during this time interval, as well as to recognize the different geochemical reactions which are taking place in the water column.