Conflicting outcomes of an integrated approach to sediment quality assessment in a Sardinian coastal area subjected to mining activities

Purpose Multidisciplinary research was carried out to verify the best approach for a reliable assessment of sediment quality in marine areas affected by strong metal enrichment. The study was carried out on Sulcis-Iglesiente marine coastal areas (Sardinia, Italy), where a long-lasting past mining operation strongly contaminated marine sediments with heavy metals (Cd, Pb and Zn). Materials and methods Surface sediments were analysed for grain size and mineralogical content, total organic carbon, total metal concentrations and sequential extractions. Additionally, ecotoxicological bioassays were performed using Vibrio fischeri (Microtox®) on the solid phase and elutriate, Dunaliella tertiolecta on elutriate and Brachionus plicatilis on elutriate. Results and discussion Analysed sediments were nearly exclusively sand (98.5–100%) with a very low TOC content; their texture and mineralogy indicated a N-S drift affecting the dispersion of mining contributions. The total concentrations of Cd, Pb and Zn, which are positively correlated, exceeded background and Environmental Quality Standards (EQS) values by up to two orders of magnitude, while sequential extractions revealed their high mobility because Cd and Pb were mainly extracted in the first phase (up to 77 and 82%, respectively) and Zn in the second phase (up to 99%). In spite of this, bioassays recorded the absence of toxicity in all the samples. Conclusions These apparently conflicting results suggest that ecotoxicological tests cannot replace chemical analyses in sediment quality assessments because they could fail to provide reliable information on the bioavailability of contaminants over time.