Bioavailability and assessment of heavy metal pollution in sediment cores off the Mejerda River Delta (Gulf of Tunis): How useful is a multiproxy approach?

•Geochemical analysis, pollution and toxicity determined in surface sediment•Enrichment levels of heavy metals are recorded in each sediment core.•Sediment profiles of heavy metals and EF and I-geo were investigated.•Chemical speciation and SEM/AVS used to assess toxicity in deeper sediments

Three core samples were taken from zones offshore from the Mejerda River Delta (Tunisia) and analyzed for major and trace elements to assess their relationships with organic matter, monosulfides and carbonates, as well as for pollution and bioavailability. Chemical speciation, ∑ SEM/AVS, the enrichment factor (EF) and the geo-accumulation index (I-geo) were used. Iron, cadmium, lead and zinc – the most frequently mined metals in the Mejerda catchment – were found as contaminants in the offshore areas. Estimations of trace element accumulation using the EF and the I-geo index show that lead, and to a lesser extent zinc, are the most polluting metals off the Mejerda outlet. According to their bioavailability, these metals are also the most toxic. Only cadmium is heavily present in delta sediment (EF > 100) though deeply sequestrated (100% bound to the residual fraction) and thus presents no toxicity.