Rodriguez-Valdés, E, Blanco, J, Alba, R, Forján, R, Baragaño, D & Gallego, JLR 2025, 'A nature-based approach for remediating severe arsenic and mercury pollution 
at La Soterraña, Asturias, Spain', in S Knutsson, AB Fourie & M Tibbett (eds), Mine Closure 2025: Proceedings of the 18th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-14, https://doi.org/10.36487/ACG_repo/2515_101
(https://papers.acg.uwa.edu.au/p/2515_101_Rodriguez-Valdes/)
Abstract: La Soterraña, a former mercury mine in Asturias, northern Spain, is an orphan site considered one of the most contaminated mining areas in Europe, with severe arsenic (As) and mercury (Hg) pollution affecting soil, water, and sediments. The site contains a large-scale spoil heap exceeding 80,000 tonnes of mining and metallurgical waste, spanning over 5 hectares, and continues to release toxic leachates, with dissolved arsenic concentrations reaching 40 mg/L. Both mineral extraction and pyrometallurgy were carried out on site, leading to the widespread dispersion of arsenic- and mercury-rich waste, impacting an area of more than 3 km². The high mobility of fine-grained metallurgical residues (up to 100,000 mg/kg of As) has not only resulted in persistent contamination but has also facilitated the dispersal of pollutants through wind and surface runoff.
The current restoration project involves encapsulation, sealing, and extensive civil works, with over 50,000 m³ of material being relocated and re-profiled to stabilise the spoil heap. In addition, it is necessary to implement a passive, cost-effective, and long-lasting leachate treatment strategy. This study presents the design and early implementation of a hybrid remediation system, combining adsorptive filtration media, organic amendments, and phytoremediation within a green filter-infiltration trench. The objective is to reduce arsenic loads, stabilise contaminants in situ, and establish a self-sustaining system with minimal maintenance requirements.
This approach highlights the scientific and technical challenges of addressing extreme arsenic and mercury contamination without energy-intensive treatments. By integrating nature-based solutions at a site with severe pollution loads, this study contributes to the development of scalable, cost-effective remediation strategies for legacy mining sites where conventional treatment methods are unfeasible.

Keywords: nature-based solutions, arsenic, mercury, passive treatment, abandoned mine