Evaluation of metal contamination in a changed sedimentary environment: Ghar El Melh Lagoon, Tunisia

Abstract

Three cores were collected in the lagoon of Ghar El Melh (Northern Tunisia) and the sediments were analysed for trace and major elements, sulfides acid volatile sulfides (AVS) and pyrite and total organic carbon (TOC). The sediments are composed of black-mud in the upper layer (0–10cm depth) and grey-mud underneath. Based on the lagoon history, it was believed that the black-mud is endogenic, while the grey-mud is exogenic (derived from old contributions of the Mejerda River before its diversion). The concentrations of TOC and AVS decrease with depth, while the redox potential (Eh) is negative in the black-mud and positive in the grey-mud. The Eh measurements thus revealed the singularity of the sedimentation mode in this lagoon. The North American Composite Shale (NASC) normalisation indicated that Cd, Zn and Pb were enriched through the entire profiles, indicating that this sediment was contaminated for a long time by mining activities and human pollution. Trace metal profiles of Fe, Cd, and Cu approximated that of TOC, while the profiles of Mn, Co, Pb, Ni and Zn followed the Eh. These results, confirmed by the principal component analysis (PCA), suggested that some metals can accumulate in the reduced sediment, while others accumulate in the sub-oxic sediment. Such inference is supported by the metal chemical speciation, which showed these metal–sediment component associations: Mn, Co, Ni, Pb and Zn to the Mn-oxi-hydroxide fraction, Fe to the residual and organic–sulfide-fractions, Cu to the organic–sulfide fraction and Cd to carbonates and sulfides.