Restoration of Water Bodies Impacted by Mine Drainage - the GeoMaTre Project. Findings for Mercury Assessment at Caveira Mine, Portugal

Evaluating the effectiveness of geomaterials in retaining potentially toxic elements of mine effluents is a key issue for the environmental remediation of former mining areas. The project GeoMaTre (Institute of Castelo Branco and the University of Évora, Portugal), aims at finding low-cost solutions for water and sediments rehabilitation using raw geomaterials, on abandoned mines from the Iberian Pyritic Belt, a metallogenic province in SW Portugal and Spain, hosting the largest concentration of massive sulphide deposits worldwide. One of the case studies is the Caveira mine in southwestern Portugal. Large piles of mining wastes containing significant quantities of metals, record the long history of its exploitation, which began in Roman times with the extraction of Au and Ag, having focused after the exhaustion of its reserves, on the extraction of the remaining metals (Cu, Pb, Zn) and S, until the date of its abandonment in the 1960s. These waste piles represent the main sources of metals in the streams, some with very high toxicity, such as Hg, resulting from the mixing with the gold-containing ore, widely used in the past in gold exploration. The design of the best remediation technique using the most suitable geomaterials for retaining pollutant metals started with the study and characterization of the spatial distribution of Hg in stream sediments, given its environmental hazardousness and geochemical behaviour.