Los Frailes Aftermath

The following article is due to be published shortly in the November issue of Mining Environmental Management.  We are grateful to the magazine for allowing us to reproduce the text here ahead of its publication.

Following the tailings impoundment failure at Boliden’s Los Frailes mine in Spain in April of this year (MEM, July 1998, p.8) the company and government have spent the summer cleaning up the tailings, monitoring the water quality and impacted soils, establishing the causes for the failure and exploring ways to re-open the mine. Latest figures estimate that a total of 6.8 million m³, comprising 1.3 million m³ of solids and 5.5 million m³ of water were released from the breach inundating the Rio Agrio, the Rio Guadiamar and surrounding agricultural land. Spain’s agriculture ministry estimates that up to 3,600 ha of farmland were effected by the spill and a Spanish farmers association estimated that the clean up costs would total more than US$ 120 million. Boliden estimates that a total of 2,800 ha were inundated by the tailings of which 2,000 ha are farmland. It has reserved US$ 34 million towards the clean-up, rehabilitation and business interruption costs caused by the accident, net of amounts covered by insurance which totals $79 million, but has not accepted liability.

According to Boliden, the clean up operation has progressed smoothly, aided by the dry summer, and at the end of October was 99.8% complete for the whole river system. However, Greenpeace claim that official reports from the Andalucian government show that in some ‘clean’ areas 75% of the pollution still remains. Most of the material removed from the fields and river banks, which in many places included the topsoil and totals an estimated 10 mt, has been disposed of in the Aznalcóllar open pit though some is still placed in stockpiles by the river waiting for removal. In addition, the company has been working on smoothing out and restoring the river bed to a more natural stable shape and have hydro-seeded the river banks down as far as the Sanlścar la Mayor bridge.

In July the government arranged for a purification plant to be installed in the Entremuros area, to the east of the Dońana National Park, and the polluted water that was trapped by the temporary dikes has been treated and discharged into the Rio Guadalquivir. Monitoring has shown that the dissolved metals levels in the river system have returned to normal and very little to no metals have leached from the pyrite tailings into the soil. The soil contains a large percentage of calcite and this helped to neutralise the waste and to precipitate the heavy metals in the top few centimetres, which has been removed during the clean-up operation. Monitoring of both the water and the soils is still continuing but, in spite of alarmist newspaper reports to the contrary, the company is confident that the onset of the rains will not mean a massive surge in pollution levels downstream.

A report prepared for Boliden on the tailings embankment failure has identified a slip in a clay formation 14 m below surface as the reason for the breach. The EPTISA engineering firm concluded that increased pressure in the interstitial water within the clays, caused by the weight of the dam and tailings deposited, caused a slip along a bedding plane of the clay unit of over 60 m lateral movement. Boliden states that neither of two reports on the dam, in 1977 and 1996, had apparently set out parameters which, according to EPTISA, are of basic importance in the behaviour of the sub-soil that caused the failure. Boliden has commissioned a technical study to compare EPTISA's report with the previous studies, to try to find out if errors exist in the previous reports, and if so, to demand compensation.

Boliden expects mining activities to re-start at the end of October with the excavation of overburden in the Los Frailes pit. Ore mining is due to start the first week in December with production resumed by the end of the second week. At present the company is still hoping to be granted permission by the Spanish government to dispose of the tailings and some of the waste rock from continued mining operations in the Aznalcóllar open pit. The pit has a capacity of approximately 70 million m3 of which the tailings and the waste will occupy some 10% allowing for the remaining 9 year life span of the Los Frailes deposit. The tailings will be isolated from the ground water as the level of deposition will remain below the existing ground water table. It is also expected that the tailings pond in the pit will have a higher pH than that in the previous impoundment, due to the more limited contact with the atmosphere.

The studies for this option are being conducted by the regional government of Andalucia, Spain’s Hydrographic confederation and Golder Associates. Two holes were initially drilled to establish the geotechnical nature of the bedrock and, although the findings were reported to be positive, the authorities requested that a third investigatory hole be drilled and analysed. This work was carried out in October and the final report is due in mid-November. Golder are also carrying out the decommissioning and rehabilitation of the original tailings impoundment.

In January 1999 the European Scientific Foundation (ESF), part of the EU, are sponsoring a workshop on the ‘Scientific basis for the remediation of the toxic spill of the Aznalcóllar mine’, which will take place in Seville. Attendance of the workshop, which is being organised by the Life Sciences and Environment Standing Committee, will be restricted to 25-30 invited speakers with a few additional attendants.

• For further information, contact Mining Environmental Management.
  

• For a detailed investigation into the Los Frailes incident, got to 'Los Frailes Aftermath'.
  

• A report on the technical aspects of the incident and the media coverage given to it has been received from the Mining and Environment Research Network.
  

• For additional links, click here.
  

• To find more information about tailings, click here.