The Aznalcóllar Tailings Dam Accident - A Case Study

Abstract

The Aznalcollar tailings dam owned by Boliden Apirsa failed on April 25, 1998, releasing approximately 1.3 million cubic metres of fine pyrite tailings and 5.5 million cubic metres of tailings water into the nearby river systems. Approximately 2,600 hectares of river banks and agricultural land were affected by tailings deposition and an additional 2,000 hectares were affected by tailings water. Apirsa closed the breach in the dam within one day and within a week commenced tailings removal. Responsibility for the clean-up was divided between Apirsa and the Spanish authorities. By the end of November 1998, Apirsa had completed the clean-up in its sector, an area containing 80% of the discharged tailings. A consultant engaged by Apirsa, Eptisa, concluded that the failure occurred as a result of a 60 metre lateral displacement along a bedding plane in a blue marl clay formation some 14 metres below the dam. Another Apirsa consultant, Principia-EQE, concluded that several deficiencies in the work carried out by the designers of the dam in 1978 and by the independent consultants who studied the stability of the dam in 1996 contributed to the failure. In responding to the accident, Apirsa positioned itself as part of the solution, taking an open and proactive posture in its dealings with the media, the general public and the Spanish authorities. Apirsa built support for its environmental assessment and remediation activities by subjecting its plans to external scientific peer review and applying principles of continuous improvement to their implementation. Apirsa's response to the accident can usefully serve as a model for others faced with adverse circumstances of the magnitude of the failure of the Aznalcollar tailings dam.