Geochemical analysis of the upper shoreface sediments between Livorno and the Serchio River (Tuscany): a focus on heavy metals

This study, conducted in collaboration with the Geologica! office of Province of Pisa, was carried out in order to assess the spatial distribution of some heavy metals of environmental relevance (Cr, N i, Pb, Zn, Cu) in near-surface (1 0-20 cm deep) coastal marine sediments (upper shoreface), collected along the coastline of the province of Pisa, between the Serchio River and Livorno (northern part of Tuscany, ltaly). The research enabled the evaluation of the quality status of the near-surface sediments, allowing the identification of the various sources of heavy metal on the marine basin. The samples were collected along the upper shoreface at 1 m of water depth over 25 km of coast. T o evaluate the possible variation of distribution of the heavy metal with the water depth, the sampling was extended also to six transects normal to the shore up to -10m. Moreover, sediments from the rivers (Serchio, Arno and Scolmatore canal) feeding the studied area were sampled and analyzed. Finally, samples from analogous depositional environment dating back to the Holocene and obtained through continuous cores were included in the study as representative of the natural background values. The geochemical analyses provided for ali samples an evaluation of the total concentration of Cr, Ni, Pb, Zn, Cu by X ray fluorescence spectrometry, that offered information also on other chemical elements (Si02, Ti02, AI20s, Fe20s(tot)• MgO, MnO, CaO, Na20, K20, P205, Se, V, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Th). On some samples (marine, fluvial and borehole), an acqua regia digestion coupled to ICP-AES analysis has been performed. This analytical method allows the direct comparison to the thresholds quality imposed by the legislation currently in force (DM 367/2003). The results suggest that for ali the heavy metals (except Cr) a grain-size control is relevant with the highest concentrations in the samples with more abundant fine fraction. Cr appears at particularly high values (51-484 ppm Cr) in the section between the mouths of the rivers Arno and Serchio due to the selective deposition of heavy minerals. Zinc concentrations are not unusual within sandy sediments (24-56 ppm Zn), with the highest values observed near Tirrenia an d up t o the mouth of the Scolmatore canal. Marine sediments do not evidence particularly high values for Ni (23-58 ppm), Pb (11-28 ppm) and Cu (5-15 ppm) with a limited dispersion of the data. On the contrary, ali the examined metals display high concentrations in the fine-grained fraction of river sediments. However, they are transported and accumulated offshore beyond the isobath of 1Om, not compromising the quality of the analyzed coastal sediments, which are on average composed of fine or very fine sands. Core sediments were used to calculate reference values used in the calculation of the Enrichment Factor (EF=[Me/AI]sample/[Me/AI]reference). The element with more frequent high values (EF> 1.4) is Cr: they occur from the estuary of the Serchio up to Tirrenia and partly at the mouth of Scolmatore, and are believed to be mainly originated by natural processes, such as current dynamics. This leads to an enrichment in Cr-rich heavy metals, that analogously to N i is contained in ultramafic rocks (ophiolites) outcropping along the drainage basins especially of the Arno River. High EF for Zn occur at the mouth of the Scolmatore and the village of Tirrenia and could be related to anthropogenic sources. Furthermore, although data of pseudototal concentrations (ICP-AES analysis) show that Cr total and Ni exceed the quality threshold (respectively of 50ppm and 30ppm) in samples from the beach and from the rivers, the same high value observed in the background samples deriving by continuous cores (average of pseudototal concentrations of Cr is 67ppm and of Ni is 48ppm) suggest the existence of high natural background concentrations for Cr and Ni in the examined area.