Historical sedimentary trends of mercury and other trace elements from two saltmarshes of the Marano and Grado lagoon (northern Adriatic Sea)

Purpose: Previous research conducted in the Marano and Grado Lagoon (northern Adriatic Sea) has shown that this environment has been affected by trace metal contamination, especially by mercury (Hg), from both industrial (the chlor-alkali plant) and mining activities (Idrija mine, Slovenia). Sediment cores were collected from two different saltmarshes of this lagoon environment to evaluate the degree of the anthropogenic enrichments and the historical geochronology of Hg. Materials and methods: Core subsampling was performed by cutting 1-cm-thick slices at discrete intervals. Mercury determination was done differently from the other parameters in that the two long cores were subsampled at 1-cm intervals to obtain continuous concentration profiles. Samples were completely decomposed, using a mixture of mineral acids in a closed microwave system before being analysed for trace metal content using ICP-OES. Total Hg content in the solid-phase was determined by DMA-80. 137Cs was measured via gamma spectrometry. 210Pb activity was measured via alpha-counting of its daughter, 210Po, assuming secular equilibrium between the two isotopes. Results and discussion: In saltmarsh sediments, Fe, Co, Li, Sc and V show no enrichment at both sites thus suggesting that they are essentially lithogenic elements. Conversely, enrichments are minimal (EF= <2) for As, Cd, Cr, Ni, Pb and Zn and moderate (EF= 2-5) for Cu and Mn and also for Pb and Zn but only in some levels of the sedimentary sequences. An exception is Hg, as expected due to the long-term input proceeding from the historical mining activity which has especially affected the eastern sector of the lagoon. Conclusions: The core collected from the saltmarsh in the eastern lagoon (Grado) displays a better time resolution during the last century although the Hg background level has not been reached. Conversely, the core collected in the western sector (Marano) has recorded a longer and more complete history of Hg contamination, from the beginning of the height of Hg extraction activity at the Idrija mine (1850). Both salt marshes still receive Hg inputs and the sediment accumulation rates in the upper section appear to have increased over the last 10-20 years (from 0.30 to 0.45 cm y-1 at Marano and from 0.30 to 0.74 cm y-1 at Grado). Many of these morphological structures suffer erosive processes thus representing a potential source of contaminants associated with sediments, in particular Hg. Conservation and monitoring of saltmarshes should be taken into consideration also from this environmental point of view.