Boliden's tailings dam burst above Doñana; the €45m fine became 25 years in court

In the early hours of 25 April 1998 the separation dam of the Los Frailes tailings pond — run by Boliden Apirsa, Spanish subsidiary of Sweden's Boliden — failed on its foundations. A soil slab slid a metre, the wall cracked and collapsed, and 4–5 million cubic metres of acidic slurry carrying zinc, lead, cadmium and arsenic ran down the Agrio into the Guadiamar, travelling 40 kilometres across 4,600 hectares of Doñana's buffer farmland. The park core was spared, but nearly 2,000 birds and 37 tonnes of fish died in Spain's worst environmental disaster.

The failure had been briefed in advance. In June 1996, two years before the break, Geocisa — Boliden's own geotechnical contractor — reported sliding surfaces forming in the marl beneath the dam. The inclinometers deformed in 1997; one had been out of service since May 1997 and the other three were in poor condition. Consultants later found the tailings' weight had displaced clay some 14 metres down, and experts hypothesised the pond's acidic seepage was dissolving the calcium carbonate out of the marl — a foundation quietly dissolving under a dam whose walls had been raised again and again.

The aftermath became a masterclass in deflection: Boliden first implied force majeure, admitted in February 1999 the dam was ill-designed, blamed contractor Dragados, and countersued it for about €115m. Cedex found the marl was impermeable, not semi-permeable as designed for; court-appointed experts called the construction negligent. Criminal liability was ruled out, Boliden Apirsa went insolvent in 2000, the mine closed in 2001 dismissing 425 workers, and in 2002 Spain imposed a record €45m fine — a sixth of the €276m cleanup — which Boliden said it would not pay a cent of.

No human deaths, but 37 tonnes of dead fish and about 2,000 birds. Cleanup €276m by 2002; record €45m fine; €90m civil claim reached court only in 2023; 425 jobs lost.

The pond sat on — and was repeatedly raised over — blue marl whose mechanical properties the design misread; the Eptisa report found clay displacement 14 metres below under the tailings' weight.

Boliden's own contractor documented sliding surfaces in the foundation marl in June 1996, two years before the collapse; the finding produced no redesign.

Monitoring existed but had no consequence: inclinometers showed deformation in 1997, with one instrument broken since May 1997 and the others degraded.

Acidic seepage, pumped and returned at the dam's perimeter, is the leading hypothesis for dissolving the calcium carbonate that gave the marl its strength.

Liability leaked away by structure: the Spanish subsidiary went insolvent, criminal charges were dropped, and the parent treated the record €45m fine as optional.

A dam is only as good as the ground beneath it. The operator's own contractor described the weak marl two years ahead, and the instruments moved a year ahead. Ignoring written warnings cost €276m.

The Guadiamar was cleaned and replanted as a 'Green Corridor'; Spain committed a further €360m, some of it EU money. The mine has stayed shut since 2001. Andalusia moved to reopen Los Frailes in 2014 under rules barring Boliden Apirsa from bidding and banning wet tailings, while compensation claims stayed deadlocked. Only in July 2023 did the Junta de Andalucía finally get Boliden Apirsa, Boliden AB and Boliden BV into a Seville courtroom for six sessions, claiming €90m in cleanup costs — a quarter century after the spill, with earlier attempts all dismissed and the outcome pending.