Research article Environmental Geology 40 (4-5) February 2001 7 Q Springer-Verlag 417 Hydrological and geomorphological criteria to evaluate the dispersion risk of waste sludge generated by the Aznalcollar mine spill (SW Spain) Gerardo Benito 7 Alfonso Benito-Calvo 7 Francesc Gallart 7 Juan Pedro Martín-Vide David Regües 7 Ernest Bladé Received: 18 January 2000 7 Accepted: 8 March 2000 G. Benito (Y) Centro de Ciencias Medioambientales, CSIC, Serrano 115bis, 28006 Madrid, Spain A. Benito-Calvo Departamento de Geodinámica, Facultad de Ciencias Geológicas, U.C.M., Ciudad Universitaria s/n, 28040 Madrid, Spain F. Gallart, D. Regües Institut des Ciènces de la Terra ‘Jaume Almera’, CSIC, Solé Sabaris s/n, 08028 Barcelona, Spain J. Pedro Martín-Vide, E. Bladé Departament Enginyeria Hidraùlica, Marítima i Ambiental, ETSECCP, UPC, Gran Capità s/n, 08028 Barcelona, Spain Abstract Following the Aznalcóllar pyrite mine disaster (Seville, Spain) which caused the spilling of some 4.5 hm 3 of acid water, the floodplains of the rivers Agrio and Guadiamar were rapidly cleaned of waste sludge. However, despite the efficiency of cleaning activities, there is still evidence of a fine superficial layer of sludge and some soil contami- nation, with the consequent risk of remobilisation of the pollutants by water erosion. There is much concern that these contaminated sediments may af- fect the precious ecosystems of the Don˜ana Nation- al Park and the Guadalquivir marshlands. This re- port describes the evaluation of the risk of mobilis- ation of the waste sludge through (1) detailed geo- morphological analysis, indicating potential areas of erosion-sedimentation on the floodplains of the rivers Agrio and Guadiamar, and predicted dynam- ics of the waste sludge, and (2) evaluation of the potential dispersion of the waste sludge provoked by future flood events, including hydraulic calcula- tions to model channel flow and the analysis of the texture of the sludge to obtain critical transport and sedimentation values. Findings suggest that the waste sludge is likely to be transported and deposi- ted within the Don˜ana National Park during future flood events. Keywords Heavy metals 7 Mine spill 7 Sludge 7 Spain Introduction On 25 April 1998, the rupture of the dam containing the tailings reservoir of the pyrite mine of Aznalcóllar caused the spilling of 4.5 hm 3 of acid water and pyrite ore. The resulting sludge, containing high concentrations of heavy metals (mean particle size 12 mm), is of a solid content of some 0.88 hm 3 and 2.97!10 3 Mg mass (personal commu- nication at the Junta de Andalucía 1998). The accident provoked the inundation of the floodplains of the rivers Agrio and Guadiamar and was close to affecting the Guadalquivir marshlands posing a substantial threat to the ecosystems of the Don˜ana Natural and National Parks. The tailings flood led to the sedimentation of a layer of sludge in the reach and floodplain of the Agrio- Guadiamar system, covering 2616 ha from the failured dam to the start of the Guadalquivir marshlands. This corresponds to a total volume of 1,981,844 m 3 of pyrite- contaminated sludge (ITGE. Cartografía y cubicación de lodos mineros en la cuenca del río Guadiamar. Aznalcól- lar-Entremuros, unpublished report, 1998). The estimated amounts of metals of environmental concern in the toxic mud released by the Aznalcóllar mine tailings spill were in the order of 16,000 tons of zinc and lead, 10,000 tons of arsenic, 4,000 tons of copper, 1,000 tons of antimony, 120 tons of cobalt, 1,000 tons of thallium and bismuth, 50 tons of cadmium and silver, 30 tons of mercury, 20 tons of selenium and other metals (Grimalt and others 1999). Heavy metal pollution mostly affected the superfi- cial soil layers (0–20 cm), although in some coarser soils pollution may penetrate down to at least 50–80 cm (Ca- brera and others 1999). The principal soil pollutants were Zn, Pb and Cd, which penetrated the soil in the solution