Contaminants in continental shelf sediments, a way to reconstruct a source to sink pathway (Naples Bay, Italy)

In areas characterized by geologic variability and high demographic pressure, seafloor sediment characteristics and the study of contaminants are important to reconstruct the origin and pathway of both contaminants and the sediments from source to sink.

The area off-shore the alluvial Sarno plain (Naples Bay, Eastern Tyrrhenian Sea) is bounded by the Vesuvius volcano in the northern part and by the carbonates relief of the Sorrento Peninsula in the southern part, and it is affected by metals contamination due the outflow of industrial vast.

A Geochemical and physical parameters of the sediments were analysed along a transect moving from the coast until the 100 m of water depth with the aim to explore how the onshore documented contamination affect the offshore counterpart. Surface sediment samples collected from the offshore Sarno plain, were analysed for grain size, nutrients (TOC, TN, TP) and heavy metals (Hg, Cd, As, Cr, Ni, Cu, Zn, and Pb) to evaluate the contamination status, and processed using multivariate statistical analysis. A sediment survey along the transect has been used to evaluate: 1) the relative influences of parent lithology and anthropogenic effects offshore the Sarno river; and 2) the extension of the influence of the river in the submarine area.

Four clusters are identified through PCA analysis: 1) the first resulted associated to the presence of As and Fe, low TOC content and prevalence of sandy fraction reflecting a geogenic contribution from Vesuvius Plan; 2) the second mainly include Cr, Cu, Zn, Pb and partially Cd and Hg, high TOC content and finest granulometry, reflecting the influence of the Sarno River discharges in the marine area; 3) the third include a variability in the Mn, Fe and TOC content. This area might reflect the Sorrento-Peninsula influence; 4) the forth include samples of the distal area in which a low contamination rate is displayed and irregular Hg and Cd pattern are verified, probably due to diffuse contamination origin and other coupling factors

Results indicate that 1) the area offshore Vesuvius displays physical and geochemical association mainly related to the natural origin of volcanoclastic sediments; 2) in the central area, the association of contaminants suggests their anthropogenic origin from the Sarno Plain, whereas the distal area, characterized by low rate of contamination, are mainly influenced by sediment from Sorrento Peninsula. Finally based on the contaminant and nutrient distribution it is possible to individuate the distribution of terrigenous sediments and organic matter of the Sarno delta deposits. The results show that the river should account as one of the main contribution sources of anthropogenic contaminants.  Some metals contamination anthropogenic in origin and TOC in general decreased gradually with distance from the coast and in particular is limited to the area of deposition of the river discharge.

Individuation of several marine sector with different geochemical associations permitted the reconstruction of source to sink contaminants pathway on the continental shelf.