Los Frailes Tailings Pond Dam Ruptures in Spain

The failure of Los Frailes tailings pond dam unleashed one of the greatest environmental disasters in the history of Spain, threatening the Doñana National Park ecosystem and sparking lawsuits and fines related to the cleanup efforts.

Summary of Event

At 3:00 a.m. on April 25, 1998, a dike of the tailings pond (a pond holding residues produced in the mining operation) for Los Frailes lead-zinc mine slipped toward the Agrio River and rotated, opening a 50-meter (164-foot) breach that allowed 5.5 million cubic meters (about 194 million cubic feet) of acid sludge and metal-laden water to surge down the Agrio and Guadiamar rivers. Within days, emergency dikes were completed around Doñana National Park, Doñana National Park protecting that area from the worst of the toxic spill, but a great deal of farmland was flooded. In the period following the disaster, high cleanup costs led to mutual recriminations among the parties involved. Los Frailes mine spill
Ecological disasters
[kw]Los Frailes Tailings Pond Dam Ruptures in Spain (Apr. 25, 1998)
[kw]Dam Ruptures in Spain, Los Frailes Tailings Pond (Apr. 25, 1998)
[kw]Spain, Los Frailes Tailings Pond Dam Ruptures in (Apr. 25, 1998)
Los Frailes mine spill
Ecological disasters
[g]Europe;Apr. 25, 1998: Los Frailes Tailings Pond Dam Ruptures in Spain[09980]
[g]Spain;Apr. 25, 1998: Los Frailes Tailings Pond Dam Ruptures in Spain[09980]
[c]Disasters;Apr. 25, 1998: Los Frailes Tailings Pond Dam Ruptures in Spain[09980]
[c]Environmental issues;Apr. 25, 1998: Los Frailes Tailings Pond Dam Ruptures in Spain[09980]

The Iberian Pyrite Belt, Iberian Pyrite Belt a massive sulfide deposit extending from south of Lisbon, Portugal, to the Spanish province of Sevilla, has been mined since pre-Roman times. Open-pit mining began in Aznalcóllar in Sevilla in 1976, and Boliden Apirsa, Boliden Apirsa a Spanish subsidiary of Boliden Ltd., a Canadian mining firm (now headquartered in Sweden, where it is known as Boliden AB), purchased the property in 1987. After the Aznalcóllar pit was exhausted in 1996, the nearby Los Frailes open pit was started, with its waste going to the same 160-hectare (395-acre) tailings pond used previously, located along the bank of the Agrio River.

In 1995 and 1996, a retired Boliden engineer and a Spanish environmental group warned of inadequate treatment of mine water seepage of toxic heavy metals through the dam. In 1996, Boliden contracted with Geocisa, a Spanish civil engineering firm, to study the stability of the dam. Although Geocisa Geocisa reported sliding surfaces in the underlying marl formation, it raised no concerns that the dam was therefore unstable. Afterward, monitors (inclinometers) were installed to detect movement in the dam, and greater efforts were made to contain seepage. Despite the fact that the inclinometers indicated dam deformation in 1997, authorities permitted Boliden to raise the dam to facilitate increased production.

The tailings facility was 2 kilometers (1.24 miles) long by 1 kilometer (0.62 miles) wide and was bounded by natural topography on the west and embankments of bentonite plugs and waste rock on the north, east, and south. At its highest elevation, the dam was about 30 meters (almost 100 feet) above ground level; the pond was less than half full when the dam broke. Probable cause of the rupture on April 25 was slippage in the marl formation of about 1 meter (a little more than 3 feet) along a bedding plane some 14 meters (about 50 feet) below the surface. The slippage moved the central partitioning wall forward and rotated the southern section of the eastern embankment, creating a 50-meter gap through which 5.5 million cubic meters of acid sludge and metal-laden water surged down the Agrio and Guadiamar rivers. Transported in the flood were between 1.3 and 1.9 million tons of tailings, about 20 percent of the tailings contained in the reservoir.

The spill caused the Agrio, Guadiamar, and Los Frailes (upstream of the dam) rivers to overflow their banks. The flood surge reached the marshlands on the eastern perimeter of Doñana National Park within eight hours. Because of concern about the possibility of heavy rains, the breach was closed within thirty-six hours. Emergency dikes were constructed along the Entremuros canal to protect the park. These failed, but another dike was raised on the border between Entremuros and the park. There was great concern about Doñana because it is Spain’s largest national park, a UNESCO World Heritage Site and Biosphere Reserve, and a Ramsar Convention wetland. Located on the delta of the Guadalquivir River, Doñana is the last great lowland wilderness in southern Europe and is internationally important for its waterbirds.

The spill affected more than 4,600 hectares (11,367 acres) of land and covered 2,600 hectares (6,425 acres) with tailings, ranging in depth from 4 meters (about 13 feet) near the dam to a few millimeters near Entremuros. The tailings included pyrite, zinc, lead, arsenic, copper, cobalt, thallium, bismuth, cadmium, and mercury. Initially, the pH level of the Guadiamar River was lowered to below 4.0 as far as 20 kilometers (a little more than 12 miles) below the dam. The tailings and acid water were largely prevented from entering Doñana by the efforts made to seal off the park and divert the toxic waters into the Guadalquivir River about 20 kilometers upstream from its mouth.

Although no human lives or livestock were lost, according to the Spanish government, the tailings floodwaters killed almost 30,000 kilograms (about 66,000 pounds) of fish, 240 kilograms (529 pounds) of crabs, twenty-three birds, four rabbits, and one water rat. Hundreds of birds and thousands of eggs from bird nests on the ground had to be rescued. Thanks to the emergency dikes and the floodwater diversion to the Guadalquivir, there was no immediate impact on Doñana; however, the aquifer underlying most of the area may have been contaminated.

Between Los Frailes and Doñana, more than fifty irrigation wells were contaminated, and the mayors of seven communities warned residents not to drink water from wells. The lands affected included riverbanks, farms, pastures, and groves and orchards. Because of fear of contamination, rice paddies were not planted in 1998, and Boliden Apirsa bought the entire fruit harvest that year from the affected area. Nevertheless, it remained to be seen whether toxic metals remaining in the food chain would eventually affect humans.

The cost of the cleanup—more than $330 million— was shared by Boliden (which provided only $25 million) and the Spanish government. Hundreds of trucks and excavators collected and loaded the spilled tailings and hauled them to the Aznalcóllar open pit for storage. Vegetation, except for some large trees, was cleared, and wells were limed and then drained with pumps. By December 1, 1998, when the initial cleanup was deemed complete, ten million tons of tailings and soil had been hauled to the open pit. Denuded areas were later revegetated, and the surface water, groundwater, soils, vegetation, and wildlife were monitored for long-term effects from the heavy-metal contaminants. The greatest concern that remained was that the tailings and water in the open pit might someday enter the Nieblas-Posadas aquifer, which supplies the city of Seville and contributes to Doñana’s wetlands.

Boliden closed and decommissioned the tailings pond through a series of environmental remediation measures. The mine reopened in 1999, but, because of technical problems with ore concentration, it closed again in September, 2001. Later that year, an appeals court in Seville ruled that there was no penal responsibility on the part of Boliden in the dam failure.


The Los Frailes mine spill was one of Spain’s most devastating environmental disasters. This event highlighted the fact that modern large-scale mining is a very complex undertaking, and the ultimate responsibility for ensuring that sound technical practices and sufficient environmental protections are observed must rest with government. Public officials must be able to apply critical evaluation to mining operation proposals and reports, and government must be prepared to resolve any unanswered questions. In the case of Los Frailes, there is no doubt that some risk was inherent in anchoring a large dam in a marl formation.

Both the Spanish government and Boliden suffered significant economic losses from the cleanup. Boliden also spent nearly $50 million on closing the tailing pond and buying the 1998 fruit harvest. The costs of the criminal and civil suits that came out of the disaster were also substantial. The Spanish government tried to recuperate its costs through civil suits and administrative fines against Boliden. In 2004, Boliden Apirsa filed a final claim of 248 million euros against Dragados y Construcciones S.A. (later ACS), the Spanish company that built the dam; its associated engineering firms, Geocisa and Itecsa; and Banco Vitalicio y Zurich. In November, 2006, a Madrid judge rejected that claim, and Boliden Apirsa filed an appeal.

Over time, the environment in the area of the spill visibly recovered, thanks to timely human intervention and the resilience of nature. Monitoring of the Aznalcóllar pit has continued into the twenty-first century, however, to ensure that there is no seepage of the drainage water accumulating above the heavy-metal mud transferred there from the reclaimed area and the site of the former tailings pond. Los Frailes mine spill
Ecological disasters

Further Reading

  • Achterberg, Eric P., et al. “Impact of Los Frailes Mine Spill on Riverine Estuarine and Coastal Waters in Southern Spain.” Water Research 33, no. 16 (1999): 3387-3394. Technical article presents results of a study of the heavy metals in the Guadiamar River after the dam failure. Notes that no evidence of the spill was found in the Guadalquivir River plume discharging into the Gulf of Cádiz and explains the lack of impact on coastal waters as the result of human intervention, natural removal processes, and near drought conditions.
  • Eriksson, N., and P. Adamek. “The Tailings Pond Failure at the Aznalcóllar Mine.” In International Conference on Environmental Issues and Management of Waste in Energy and Mineral Production, edited by Raj K. Singhal and Anil K. Mehrotra. Brookfield, Vt.: A. A. Balkema, 2000. Discusses the cleanup, environmental impact assessments, closure of the tailings pond, and the brief restart of mining, as well as the trauma suffered by Boliden employees and consultants.
  • Leslie, Jacques. Deep Water: The Epic Struggle over Dams, Displaced People, and the Environment. New York: Farrar, Straus and Giroux, 2005. Presents three extensive essays about dams in India, Southern Africa, and Australia. Addresses the environmental, economic, political, and social issues surrounding the construction of dams.
  • Van Geen, A., and Z. Chase. “Recent Mine Spill Adds to Contamination of Southern Spain.” Eos, Transactions, American Geophysical Union 79 (September 22, 1998): 449, 455. Presents a technical analysis of samples of river sediment collected from the Guadiamar, Guadalquivir, and Tinto rivers after the spill. Notes that pollution of the first two rivers was comparable with that of the long-contaminated Tinto.

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