Almaden Mine, Spain

The Almadén mine was active for more than 2,000 years, it is said that one third of all the mercury used by mankind has been extracted from it. Mainly due to the regulations set by the European Union due to the risk of mercury poisoning, as well as a major collapse in 2001, led to the mine’s closure and subsequent abandonment in 2003.

In order to minimize the environmental impact on the surroundings caused by the waste from when it was active, Minas de Almadén carried out in 2004 the restoration of its Cerco San Teodoro tailings dump. For centuries, this dump received both tailings from mining operations and slag from metallurgical processes. Reaching a size close to 3.5 million tons and covering an area of 10 hectares. After the study carried out, the dump was characterized as hazardous material due to its high mercury content and it was determined that the permeability of the substrate on which it rested was very low, with no fractures that could constitute preferential drainage routes. The influence of the dump on groundwater was therefore identified as local and due to the low mobility of the mercury its impact was concluded to be limited to the immediate surroundings of the dump.

As an action measure, it was decided to restore the dump with an encapsulation guaranteeing the upper waterproofing of the dump, avoiding the recharge of the dump and therefore minimizing the effects on ground and surface water in the area. It also reduces the dispersion of the material deposited in the dump that could affect the surrounding soils.

San Teodoro tailings disposal site in May 2005

Solution

To carry out this action, the following encapsulation project was designed and constructed, divided into a series of phases:

• Softening of the existing slopes. Whose average slope was around 36º, down to a slope of 25º, more in line with the natural slopes existing in the surroundings. A horizontal upper platform was also created.
• Sealing with geosynthetics package. In order to prevent water from entering the dump with the consequent generation of leachate and dispersion of materials, a complete package of geosynthetics was designed, including 2.00 mm Atarfil HD sheet due to its high durability, as well as geotextiles, geodrains, geogrids, reinforcement geogrids and geocells.
• Installation of a system for the collection, circulation and evacuation of rainwater by means of perimeter ditches, berm ditches and downspouts.
• Restoration of the encapsulation with vegetative cover using borrow material, mechanical seeding on the upper platform, hydroseeding in the slope areas and planting of shrub species.

Finally, after the correct construction and installation of the geosynthetics package, an Environmental Monitoring Plan was implemented to ensure the proper functioning of the project for 50 years.

Encapsulation of the Cerco de San Teodoro landfill site