SUITMA 2005 Cairo - Biogeochemical cycle of trace elements on a forested technosol developed on a flotation pond
SUITMA 2005 Cairo
| This abstract is about one of the papers of the Consumed mining and military areas theme of the SUITMA 2005 symposium. |
| J. Muller,i | Louis Florentin,i |
| Christophe Schwartz,i | Guillaume Echevarria.i |
- i - Laboratoire sols et environnement (LSE), ENSAIA, Université de Lorraine - INRA, Nancy, France.
The Region Lorraine (north-eastern France) is historically industrialised with coal and steel industries. It shows more than 6000 hectares of industrial brownfields deriving from those activities and is especially concerned by their requalification. The aim of this work is to better understand the biogeochemical cycle of selected metals (Cd, Ni, and Zn) on a flotation pond on which a spontaneous forest has been growing for more than 50 years. Therefore, the distribution of total and bioavailable trace elements in the organic surface horizons and their transfer to the vegetation was investigated. The 20-ha flotation pond received and accumulated liquid wastes from different processes of steel industry over many years before abandon. During the last 50 years, the technic materials have evolved along with the growth of trees and herbaceous species. Materials from the 3 upper layers of the technosols were sampled on 17 points according to a 10m x 10m grid. A pit was opened and described at one of the 17 locations. The distinct layers were described and corresponded to the several former deposits and, in the upper part, to the accumulated humus layer. The soil samples were thoroughly characterized for their pedological properties. Total metal contents in soils were measured after HF digestion and their available pools were estimated with selective chemical extractions (DTPA, CaCl2, ultra-pure water). We also collected and identified all the plant species. Shoot biomass was analysed for total heavy metal content. The soils were homogeneously distributed over the pond and were all alkaline and strongly polluted with Cd, Pb and Zn (respectively up to 180, 53 000 and 60 000 mg kg-1). The main difference concerned the particle size distribution of the samples. Results show that total concentration of metals in the technosols profiles are increasing with depth whilst extractable Cd, Ni, and Zn contents are decreasing. Surprisingly, no phytotoxicity symptoms were recorded. The content of heavy metals in the shoots of harvested plants was lower than reported toxic levels except for five of the 49 species. This can be explained by the fact that available metals are mainly located in the humus layer whereas the root systems explore the deeper mineral horizons. The increase of the availability of metals in the organic surface horizon could have a strong effect on the further evolution of this technosol, inducing an inhibition of the degradation of the fresh organic matter from the litter and an accumulation of a less carbonated humus layer. Metals can have a key role in the pedogenesis of young technosols by influencing the organic matter turnover. Plants may be used as a tool for the containment of metals in such highly contaminated soils. Phytostabilisation is an alternative remediation technique provided that soil evolution and mobility of metals are monitored.
