Evolution of pyrite mud weathering and mobility of heavy metals in the Guadiamar valley after the Aznalcóllar spill, south-west Spain

Abstract

This study focuses on the levels and mobility of the potentially toxic heavy metals in the pyrite slurry which flooded the Guadiamar valley as a result of the Aznalcóllar spill. Chemically, the pyrite slurry was characterised by high concentrations of several potentially toxic elements such as Zn, Pb, As, Cu, Sb, Co, Tl, Bi, Cd, Ag, Hg and Se. The variation in their concentrations along the Guadiamar river was very small, with the exception of Pb, Ag, Sb, and Bi, for which the concentration progressively increased downstream. The partial oxidation of sulphides, as a result of chemical and bacterial processes, led to the formation of an extensive white sulphate crust due to the intense evaporation of interstitial fluids flowing upwards by capillary action. This crust contained high Fe, Zn, Cd and Cu concentrations and low As and Pb levels. The former group of elements occurred as highly soluble sulphates which resulted in the formation of the oxidation crust (70–98% of its bulk content was made up of these sulphates). During summer, a large part of the sulphate crust disappeared, probably because of wind erosion. Subsequently, mud cracking developed, resulting in the formation of low-permeability pyrite blocks. At the end of September (1998) a wet period led to the partial dissolution of the sulphates in areas where the soil reclamation activities had not been completed. The presence of these weathering crusts have been noticed in the Rio Tinto and Odiel valleys. The conclusions obtained in this study may have implications for the understanding of the mobilisation and distribution of contamination along these valleys for centuries. Throughout the study period (5 months) bulk concentrations of major and minor pyrite-related elements did not change significantly. This indicated that these elements were not extracted from the slurry in an appreciable rate. The low permeability of the slurry and the low precipitation probably accounted for this low leaching rate. However, the mobility of most of the heavy metals increased considerably throughout the study period.