State of Contamination of the Waters in the Guadiamar Valley Five Years after the AznalcÓllar Spill

In April 1998, the Aznalcóllar mine tailings dam spilled 2 hm³ of slurry and 4 hm³ of acid waters into the Agrio River (affluent of the Guadiamar River). The pollutants reached the proximity of the Doñana National Park, 60 km downstream. The state of contamination of groundwaters and surface waters, from several samplings made subsequent to the spill, is described. Although the Guadiamar valley groundwaters remain contaminated, this situation cannot be attributed to the mining spill, but to the long history of pollution from mining activity at Aznalcóllar. Three zones can be distinguished: the most polluted zone, with pH values close to 4 and very high concentrations of metals (up to 18 mg/l of Al and 7 mg/l of Mn) and sulfates (up to 1263 mg/l); a second zone where pH values are higher (close to neutral) and, as a result, the concentration of metals is significantly lower; and a third zone, with pH exceeding 7, in which the concentration of metals has already fallen to normal levels but sulfate concentrations remain high (above 500 mg/l). The waters of the Agrio River present characteristics similar to those of the most-polluted groundwaters; on mixing with the waters of the Guadiamar River they are neutralized, precipitating firstly Al and then the rest of the metals. These metals remain deposited on the river-bed, where they are readily remobilized in later spates. Analysis of changes in groundwater quality shows a trend toward a lower content of Zn and the other metals in most wells.