Acid mine tailings in southern Spain

Sediment and water samples were collected along the Guadiamar river in southern Spain on 1–3 May and on 24–25 May 1998, following the release of acid sludge from the Los Frailes zinc mine near Aznalcollar on 25 April 1998. A maximum Zn concentration of 12 mg/g in the sediment was measured approximately 10 km south of the mine. The low pH of 3.8 in Guadiamar river water approximately 20 km downstream of the mine contrasts with prevailing alkaline conditions (pH∼8) upstream and in the adjacent Guadalquivir river. The concentration of dissolved sulfate increased from 1 mM upstream from the mine and in a nearby uncontaminated tributary to a maximum of 40 mM approximately 40 km downstream. Dissolved Zn concentrations were particularly sensitive to contamination by the tailings and ranged from as low as 3.6×10⁻⁸ M in a uncontaminated tributary to as high as 6.6×10⁻³ M 20 km downstream of the mine. By 25 May 1998, dissolved Zn concentrations in the most contaminated area had declined by more than an order of magnitude at all but one location. On the basis of a simple mixing model for the tailings, we estimate that 40 000–120 000 tons of Zn were released to the watershed by the accident. This is equivalent to the dissolved Zn flux reaching the adjacent, chronically contaminated, Tinto-Odiel estuary every 0.8–2.4 years.