Identification of anthropogenic and geogenic contributions of trace metal(loid)s in coastal sediments through a geochemical normalisation approach based on regional background values (Gulf of Trieste, northern Adriatic Sea)

Metal(loid) concentrations in coastal sediments are often due to the contribution of anthropogenic inputs to the site-specific natural geochemical characteristics of the investigated area. The aim of this study was to assess the origin (geogenic or anthropogenic) of metal(loid)s in the surface sediments of the eastern sector of the Gulf of Trieste. Aluminum was identified as an ideal proxy element due to its excellent correlation with the muddy fraction. The common origin of the elements was confirmed by the positive correlation found among correlated groups of elements such as Pb, Cu, Zn and Mo, on the one hand, and Tl, Li, Cr, Mn and Ni on the other. The comparison with the environmental quality standards (SQA-MA) defined by current legislation (Legislative Decree 172/2015) showed that threshold levels were exceeded for Cr (100%), Ni (95%), Hg (87%), Pb (84%), As (74%) and Cd (5%). Since the SQA-MA values do not take into account the natural site-specific geochemical features of sediments, a normalisation procedure was applied to quantify the real anomaly. Regional functions for metal(loid)s were determined using sediments from a core collected in the central sector of the Gulf as a baseline [Covelli et al., 2006] in order to subsequently calculate enrichment factors (EFs). Negligible or low contamination was found for sediments far from the urbanised and industrialised areas, whereas the EFs were higher near the coast, revealing, in some cases, significant isolated anomalies; the highest enrichment was found for Hg, whose average EF value was 14 times higher than the local baseline. The highest EFs were found for Cu (10.7), Cd (8.9), Mo (7.8), Pb (7.2) and Zn (5.6). Chromium and Ni, on the contrary, which show high concentrations, are actually not enriched. This is due to the effectively lithogenic nature of these two elements, which are present in the sediments of the Gulf of Trieste in relation to the peculiar mineralogical characteristics of the area.