Los Frailes Aftermath

The following article was published in the July 1998 issue of Mining Environmental Management.  We are grateful to the magazine for allowing us to reproduce the text here.

In the early hours of Saturday, April 26, the tailings impoundment failed at Boliden’s Los Frailes mine, releasing a flood of slurry into the nearby Rio Agrio. The torrent of tailings flowed down the natural river system, inundating large areas of river bank and farm land and threatening the Doñana National Park, situated 50 km to the south.

Boliden has temporarily stopped production at the project, located at Aznalcóllar in Spain, and the company has declared force majeure with respect to its obligations to customers and suppliers. An investigation into the causes of the failure and a clean-up operation started three days after the failure. It is anticipated that the majority of the discharged tailings will have been removed from the river banks and farm land by the end of September, before the inception of the rainy season, and that Boliden will be in a position to re-start the project at this time.

The History
The Tailings
The Surrounding Area
The Failure
The Aftermath
The Finances
The Ramifications

The History
Aznalcóllar is located at the eastern-most end of the Iberian pyrite belt, 45 km northwest of Seville, in southern Spain. The area has a long history of mining, dating back to at least the Roman period. More recently, from 1876 to 1942, five orebodies at Aznalcóllar were exploited for copper by Seville Sulphur and Copper Ltd and then drilling by Peñarroya España in 1956 located a substantial zone of complex pyritic zinc-lead-copper mineralisation. There were two main types of ore, a pyritic chalcopyrite (pyroclast) and a polymetallic sulphide (pyrite).

In 1960, Andaluza de Piritas, SA (APIRSA) was formed as part of the Banco Central SA industrial group (now called Banco Hispano Americano), in order to acquire and exploit the deposit. Overburden stripping started in 1975, the tailings dam was constructed in 1978 and processing began in 1979 at a rate of 3.5 Mt/y with the production of zinc, lead and copper concentrates. The tailings dam was designed and built by GEOCISA, a company owned by Banco Central.

In 1987, APIRSA was acquired by the Swedish group, Boliden, which continued extraction from the Aznalcóllar open pit until 1996. By this time, the company had located another orebody, called Los Frailes, in the same area as Aznalcóllar and production from this deposit started in 1997, with a capacity of 4 Mt/y, using the same processing facilities and tailings impoundment as Aznalcóllar. The project is now operated by Boliden APIRSA SL, the Spanish subsidiary of Boliden Ltd.

The Tailings
The tailings impoundment is located in a valley, just above the conjunction of the Rio de los Frailes to the east and the Rio Crispinejo Agrio to the west. The Rio Agrio flows into the Rio Guadiamar 2 km below the project site, which then flows into the Rio Guadalquivir. The Rio Agrio originally flowed through the site of the Aznalcóllar pit but was diverted by means of a 20 million m³ upstream dam and a 2 km tunnel. The geology of the valley consists of a 10 m layer of alluvial gravel overlying 30 m of Miocene marl beds.

The tailings facility is 2 km long and 1 km wide, and abuts the natural topography to the west, with embankments to the north, east and south. The embankments are constructed of a bentonite plug that extends through the alluvial sediments and 1.5 m into the clay bedrock, and are built up with waste rock. At its highest point, the embankment is currently 30 m above ground level and is raised annually by 1 m. The tailings are discharged from a single point which is moved regularly to create a beach.

The impoundment is divided into two sections and, originally, the tailings from the pyroclast processing were discharged separately to the tailings from the pyrite. Since the start up of Los Frailes there is only one type of ore and therefore only a single tailings discharge stream, with both sections of the impoundment being used. The impoundment had a designed capacity for 70 Mt of tailings, equivalent to a volume of 32.6 million m³. It is estimated that about 14 million m³ of tailings were discharged from the processing of the Aznalcóllar ore and a further 1 million m³ from the Los Frailes deposit with the impoundment containing a total of 15 million m³.

The supernatant liquid in the tailings impoundment has a pH of between 2 and 4, and contains elevated levels of copper, lead, zinc and iron. This liquid is pumped from the tailings pond to the water treatment plant from where it is recycled to the processing plant. Any surplus water is discharged to the Rio Agrio. The plant also treats water that is pumped from a series of seepage collection systems, including wells, a covered drain and an open ditch, around the tailings impoundment. Prior to the failure, an estimated 15 m³/h was collected from these seepage systems.

The Surrounding Area
The Rio Agrio and the Rio Guadiamar flow down through a rich farming area that produces a range of citrus fruit, peaches, sunflowers, wheat, maize and olives, before reaching the marsh lands that surround and make up the Doñana National Park. In this marsh area, a system of dikes and canals has been built to control and re-route the flow of the river away from its original course and into the Brazo de la Torre, which then flows into the Rio Guadalquivir. The main canal, which is known as the Entremuros, separates the reclaimed agricultural land to the west from the rice paddies to the east.

The World Wildlife Fund (WWF) was founded, in 1961, by a group of international ecologists and one of its initial actions was to buy the Coto Doñana (Coto literally means ‘hunting reserve’) in order to preserve the area for the future. In 1964, WWF purchased 6,800 ha and established the Biological Station of Doñana. It was first declared a National Park in 1969, covering an area of 35,720 ha, and was then reclassified in 1978 with a total area of 50,720 ha. In 1989, a further area of Natural Park was declared around the existing park, to act as a zone of protection, and the whole area now covers 100,000 ha and stretches from the Rio Guadalquivir in the east to the El Rocio road in the west.

The protected area includes marshes, sand dunes and ‘virgin’ beaches and is famous for its flora and fauna, particularly the bird-life. The marshes are breeding grounds and winter homes for 250 bird species as well as being located on a number of important migration routes. The area holds 6 million migratory birds each year. The National Park is classified by UNESCO as a World Heritage Site and a Biosphere Reserve and is also designated a Ramsar Convention wetland site.

The Failure
The first indication of a failure of the tailings dam wall was when a technician noticed that one of the seepage collection pumps was not working. On checking, he heard the sound of rushing water, walked out along the embankment to see what was causing the noise and discovered the failure. The preliminary report from the Spanish engineering and consulting company commissioned to investigate the failure, EPTISA Servicios de Ingeneria SA, states that the probable cause was a slippage in a shear zone in the marl formation, 0-20 m below ground surface. The movement rotated the southern section of the eastern embankment, creating a breach both in the main embankment and in the central partitioning wall. The flow of tailings that then escaped through this breach created a 50 m wide gap in the embankment.

The first reports stated that a wall of slurry, 2 m high, flowed down the Rio Agrio and Rio Guadiamar valley (although the bridge at Sanlúcar la Mayor, 13 km downstream, has a tide mark at least 4 m above the river bed, which indicates a much greater flow at some stage). Boliden initially estimated that a total of 4.5 to 5 million m³ of liquid and 1 million m³ of solids had been released from the tailings impoundment. Boliden’s environmental manager, Manfred Lindvall, now feels that this guesstimate was an over-estimation and is currently calculating more accurate figures.

The flow followed the course of the Rio Agrio and Rio Guadiamar and reached the marsh lands on the eastern edge of the Doñana National Park, 60 km to the south, some seven or eight hours later. Here the flow was partially stemmed by a series of dikes that were hastily constructed along the Entremuros by staff from the park with the help of rice farmers in the area.

These original dikes failed to hold but, by the fifth day after the failure, a more permanent structure had been constructed at the southern-most end of the Entremuros, on the border of the National Park. This dam has resulted in the backing up of the river water in the canal and the flooding of large areas of pasture in the Natural Park to the west of the Entremuros.

Spain’s largest farmers union, ASAJA, which represents 95% of the farmers in the area, claims that the tailings have inundated approximately 3,000 ha of farmland with sludge and affected more than 5,000 ha. The areas affected are made up of:

• 710 ha cotton

• 170 ha peaches

• 430 ha citrus (mainly oranges and lemons)

• 410 ha olives

• 1,000 ha pasture

• 1,510 ha sunflower and wheat

• 300 ha maize

• 420 ha forestry (eucalyptus etc.)

Also affected are 1,000 ha of rice paddies which are not being planted for this season’s crops because of the high levels of water in the region of the Entremuros and the risk of contamination.

The Spanish Government has stated that, "the tailings flood killed 29,725 kg of fish, 240 kg freshwater crabs, 23 birds, 4 rabbits and 1 water rat". At least 135 birds were rescued from the wetland area and 1,450 eggs from birds nests on the ground, some of which are now reported to have hatched. The fish and the crabs died from suffocation, but it is not known what caused the death of the other animals. The government claims that there has been no immediate effect on the Doñana National Park, though fears have been expressed by a number of ecological groups that the aquifer that underlies most of the area (Aquifer 27) around the park may have been contaminated.

More recently, Boliden has reassessed the damage and, using satellite images that show the area affected by the discharge, has estimated that the total area that suffered from inundation was 2,100 ha. The company has also estimated that at least 80% of the tailings were deposited in the first 17 km of river, in the Rio Agrio and Rio Guadiamar, between the tailings impoundment and the Sanlúcar la Mayor road bridge.

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