Spain's Andalucían regional government is re-opening the controversial Los Frailes mine in Aznalcóllar, near Seville.
In April 1998, a holding dam at the mine burst releasing around five million m³ of acidic mine tailings containing dangerous levels of several heavy metals.
Toxic sludge reached the nearby River Agrio and then its affluent, the River Guadiamar, travelling around 40km along the waterways. Total tailings spilled was estimated to be between 1.3 million tonnes (Mt) and 1.9Mt.
The spill flooded the banks along the rivers Los Frailes, Agrio and Guadiamar, from a point 300m upstream of the tailings pond in the river Los Frailes, down to Entremurosmine.
Following the incident, 4,634ha of land was affected, with 2,600ha being covered by tailings. Remediation work took three years and reportedly cost about €240m.
Three separate investigations were held to assess the Los Frailes mine accident, the first commissioned by Boliden Apirsa, the second by regional authorities and the final one called for by the judge leading the legal procedures.
The Spanish Government claims that re-opening Los Frailes mine will create 450 jobs the area, which has the highest unemployment rates in Spain.
Conservationists are arguing that the mine poses environmental risks to the Guadiamar river, which is the primary water source for Doñana national park, a Unesco world heritage site, according to the Guardian.
Los Frailes mine is owned by Boliden-Apirsa. The mining contract has been awarded to a consortium led by Grupo México, which was fined $150m (£98m) to clean-up an accident at its mine in Sonora, Mexico, last August.
Spanish officials insist that Grupo México has guaranteed its Aznalcóllar mining activities will not affect protected zones.