Mobility of trace elements at the sediment-water interface in two tourist port areas of the Gulf of Trieste (northern Adriatic Sea)

One of the main environmental issues affecting the coastal marine environments is the accumulation of contaminants in sediments and their potential mobility. The sediment acts as a sink for many chemical species (metal(oid)s, PAHs, PCBs etc.). However, this situation can be only temporary since early diagenesis biogeochemical processes can recycle these contaminants which may subsequently accumulate along the aquatic trophic chain. In this work, in situ experiments were conducted at two tourist ports (marinas) located in the Gulf of Trieste (northern Adriatic Sea), one in Slovenia and one in Italy. The latter is well known to be a site contaminated by long-term shipbuilding activities. The main aim was to understand if and where recycling at the sediment-water interface (SWI) may affect trace metal(oid)s. A benthic chamber, successfully used in other works in the same Gulf 1,2, was employed. Water sampling was performed during a 8-hour period to measure trace metal(oid) concentrations with time. Short sediment cores were collected near the chamber to investigate the solid (sediments) and dissolved phases (porewaters). Both diffusive and benthic fluxes were estimated to elucidate the release of trace metal(oid)s at the SWI. Total trace element concentrations were determined following a complete mineralization of the sediment (aqua regia + HF), whereas a weak extraction with diluted HCl (0.5 M) was carried out to quantify their potential “bioavailable fraction”. Quantitative analyses were performed by ICP–OES on sediment mineralized solutions and by ICP–MS on porewater and benthic water samples. Total element contents in sediments, resulted up to two orders of magnitude higher in the Italian marina (0.4 %Pb, 0.15% As and Cu, 0.25% Zn) than in the Slovenian one, confirming the high contamination level of this site. Despite this heavy contamination, the “bioavailable fraction” of metal(oid)s extracted was scarce or null. Conversely, sediments from the Slovenian site showed high percentages of the extractable fraction from 4% (Cr) to 82% (Cu) of the total content. Trace metal vertical profiles in porewaters showed a clear diagenetic sequence. The suboxic/anoxic conditions of sediments allowed the dissolution of Fe and Mn oxy-hydroxides, thus enhancing the concentrations in porewaters of Fe, Mn and other associated trace metal(oid)s. However, the diagenetic sequence and the suboxic/anoxic transition were highly dependent on the characteristics of each site. Maximum concentrations of dissolved Fe, Mn, Pb and Zn were found in the first centimeters of one site of the Italian marina where sediments became quickly anoxic with depth. Conversely, in the Slovenian marina and in the second Italian site, the highest concentrations were recovered from the deepest porewater layers, especially for Fe (up to 2.5 and 7.7 mg L-1, respectively). Positive effluxes were observed for Mn, Pb, Zn and V in the Italian marina whereas for Cu and Ni were most notably in the Slovenian marina and for as at both sites. Hg fluxes were low in spite of its high concentrations (up to 1000 mg kg-1) in sediments of the Italian site. In general, benthic fluxes of metal(oid)s at the SWI can be partially explained by considering the dissolution of Fe/Mn oxy-hydroxides and metal speciation in bottom sediments. The results suggest that although the bottom sediments of the Italian marina exhibit the highest trace metal(oid) concentrations, they are scarcely remobilizable. On the contrary, in the Slovenian marina, sediments seem to be comparatively a significant source of trace elements (except for Fe, Pb and V) in the water column as consequence of diagenetic processes at the related interface.