Two landslides occurred on the sides of a 600,000-tonne retention basin for ultimate waste from a former gold mine, upstream of the GOURG PEYRIS River, a tributary of the RIEUSSEC, which empties into the ORBIEL. The retention dike was ripped open over a 25-m length in two spots, leaving solid materials containing arsenic, cyanide, lead and other heavy metals exposed to the open air.
Since the basin had been fitted with an impermeable bottom (geotextile), mining residue (covered with topsoil in order to avoid spreading by the wind) was saturated with water during the heavy rains. Basin contents became heavier, reaching a point of exceeding the basin resistance capacity and causing the failure.
Over the last years of mining operations, the basin had been raised several metres above its original level. A dike was also built down-slope to limit eventual landslides; the dike then had to be expanded following ground movement.
Operations at the ore extraction and processing facility were shut down definitively in 2004. An agreement signed in July 2010 between the site operator and the French government handed ownership of the most polluted parcels over to the government, in addition to assigning responsibility for site cleanup, in exchange for a substantial contribution from the operator. Site restoration was overseen by the ADEME Agency between 1999 and 2008 for an amount in the range of 50 million.
80 years of onsite mining activity had generated long-lasting arsenic pollution (ARIA 4446, 25267) of the soils and the ORBIEL River, whose water was not fit for consumption (affecting a total of 20 municipalities). The sale of thyme and leafy vegetables was also prohibited over an area encompassing 5 towns.