Phytomanagement of a contaminated dredged sediment landfill site with a focus on trace element mobility and accumulation by plants
Résumé
Aided phytostabilisation is a management option which aims at reducing environmental risk, by the action of plants and amendments. Reported as a sustainable alternative to conventional techniques of soil management, aided phytostabilisation is an attractive strategy but rarely applied by soil management professionals due to the lack of feedbacks. Demonstrations at a field scale are still needed to evaluate its long term performance. Therefore, a field trial of 1 ha was set up at the end of 2011, on a contaminated landfill site of dredged sediments in Fresnes-sur-Escaut (Hauts-de-France, France). The site was contaminated with Cd (9.4 ± 0.9 mg kg-1), Cu (110 ± 7.2 mg kg-1), Pb (956 ± 89 mg kg-1) and Zn (6089 ± 825 mg kg-1) and presented a spatial heterogeneity in TE (trace element) distribution in topsoil. Tufted Hairgrass (Deschampsia cespitosa) was selected to cover the sediment and immobilize TE, and was combined with the incorporation of a basic mineral amendment in soil. In addition, with a perspective of biomass valorisation via energy production, two willow clones (Inger and Tordis) were planted in very short rotation coppices (VSRC). We investigated the influence of soil conditions and treatments on TE mobility, as well as their accumulation in leaves, three year after the establishment of the field trial. Ca(NO3)2 extractions (0.001M) were performed on 126 sediment samples to assess TE extractability. In parallel, willow and grass leaves were collected at each sampled points to study TE accumulation. Sequential extractions (Quevauviller, 1997) were carried out to identify TE speciation in contrasted plots (moderately or highly contaminated with TE). Since the set up of the trial, Deschampsia cespitosa did not show any toxicity symptoms, suggesting a good tolerance of the plant to the site conditions. Physiological concentrations of TE were found in Deschampsia cespitosa leaves, confirming its relevance for aided phytostabilisation. Although willows are considered as tolerant species to a wide range of soil and climate conditions, chlorosis affected willow leaves, in most of the plots. Survival rates varied significantly between plots that had different levels of contamination, suggesting that TE induced phytoxicity in willow clones. Conversely, foliar Cd and Zn concentrations were far above physiological values despite the presence of the basic mineral amendment. Extractable concentrations of Cd and Zn were generally low, and lower in some plots, suggesting a potential influence of the amendment. However, no correlations were found between TE extractable concentrations in soil and concentrations in willow leaves. The mineral amendment did not affect TE speciation but Cd and Zn were mostly bound to carbonates and Fe and Mn oxides in all tested plots. Cd and Zn could be dissolved easily under acidic conditions induced by exudates, enhancing TE accumulation in trees. Otherwise Cr, Cu and Pb were mostly bound to the residual fraction and organic matter. As extractable fractions of TE were not correlated to the TE contents in plants, sequential extractions seem to be more relevant to assess TE mobility and transfer to plants.