Rhizostabilisation of highly heavy metal contaminated mine spoils by using METALlicolous plants associated with microbial SYMbionts – SyMetal

Numerous degraded land areas resulting from industrial or mining activity are still remaining in France and in Europe. Most of those areas were not restored before being abandoned and constitute "hot spots" of metal pollution. Moreover, under the action of water and wind erosion, the metal pollution is being dispersed and contaminate in a diffuse and continuous way soils, cultures and river sediments. Restoration of such surfaces by physical or chemical methods would be very delicate and extremely expensive. Therefore treatments such as phytoremediation, a technology that uses selected tolerant plants to immobilize metals in contaminated soils, appears the most economic and effective solution for a long-lasting restoration. However, ecological engineering for phytoremediation of mining areas remains inefficient due to the very low level of fertility and the strong toxicity linked to high metal contents in those soils, limiting the development of a plant cover. The success of the phytostablisation of the mining clearings cannot be thus envisaged without the consideration of these constraints. Therefore, the study of the biological resources including native plants and bacteria of mine areas and the elaboration of a rational approach in order to improve both the development of plants and associated microorganisms to increase vegetation cover and soil rhizostabilisation, are thus indispensable to develop effective strategies of ecological engineering adapted to the management of former mining sites.
The area selected for this study proposed within the framework of the project SyMetal is the old mine of Les Avinières. The site is located in the mining district of the Malines within the region of Saint-Laurent-le-Minier (Gard) which constituted the most important zone of lead and zinc exploitation in France in the XIX-XXth centuries. The contaminated area covers a surface of about fifteen ha and it is an example of strong metallic pollution in Mediterranean region. Mine soils are very unfertile with a low organic matter content and a shortage of major plant nutrients. Total metal concentrations in mine tailings located in the tailing ponds located just near the river, La Vis, are extremely high (total Zn: 161 000 mg kg-1, Pb: 92 700 mg kg-1 and Cd: 1382 mg kg-1) and is to a large extent, one of the most polluted sites in Europe. High concentrations in Zn and Pb than 1000 mg kg-1 were also observed in an area of 4,5 km downstream the mine.
The aim of the SyMetal project is to utilize heavy metal tolerant plants and associated microorganisms growing in these ecosystems for the development of innovative environmental engineering with the aims to immobilize in situ soil metallic pollutants of highly polluted areas.
The SyMetal Project is based on: (i) the identification and selection of symbiotic microorganisms such as mycorrhizal fungi adapted to highly metal polluted soils and increasing the diversity of available plants (ii) the selection of metal tolerant plant species, especially Fabaceae, associated with their symbiotic legume bacteria in order to improve ecosystem development by naturally increasing the nitrogen content in the soil and thus raising soil fertility (iii) a specific study of the influence of rhizostabilisation on the metal immobilisation and speciation in the soil under plant cover. In this context we propose an integrated and multidisciplinary approach to promote innovative environmental engineering with the aims to phytostabilize metal contaminated soils and minimize the dispersion of contaminants in the environment.