The geochemical aspects of toxic waters retained in the Entremuros area (Spain)

Two walls were built, in order to retain approximately 2 Hm(3) of toxic water from the Aznalcollar mine-tailing dam. This water accumulated in a channel (approx. 10 km long and 1 km wide) in the lower part of the Guadiamar River (Entremuros) and was sampled during the first week of June 1998. The results indicate that the chemistry of the samples located at the northern part of the channel is controlled by a mixture of two processes - dilution of polluted water by the Guadiamar River and precipitation; consequently, medium or low concentrations of toxic elements were measured. The solid phases that determine the chemical equilibrium are calcite, smithsonite and gypsum. On the other hand, the most polluted zone is located to the south of the area studied. These waters are more acidic, and hence, have a higher content of dissolved heavy metals. For example, the Zn concentration is three orders of magnitude greater than in unpolluted samples. Prior to their flushing to the Guadalquivir River, the Entremuros waters were chemically neutralised with Na2CO3 and NaOH. This neutralisation process was modelled and the results were compared with the analytical data obtained during the sampling campaign. The natural removal of heavy metals, which was observed in the waters, was produced by an increase of pH due to a strong evaporation in the zone. This removal produced the precipitation of hydroxides and carbonates of heavy metals, a process, which was also clearly shown in the modelling. (C) 1999 Elsevier Science B.V. All rights reserved.