Evaluating mercury contamination in the context of sustainable sediment management of a coastal lagoon: evidence from the Marano and Grado Lagoon saltmarshes (Northern Adriatic Sea)

Saltmarshes are among the most valuable coastal environments, providing several ecosystem services, as well as a fundamental habitat for waterbirds and halophytes. They are also vulnerable to the effects of anthropogenic and natural pressures. Erosive processes, as well as removal by the periodical dredging of navigation channels cause the loss of sediments from the overall lagoon sedimentary budget, with negative effects including the disappearance of saltmarshes. A sustainable management of coastal lagoon sediments should provide for a positive balance between erosion and accretion processes, promoting the construction of peculiar and valuable morphologies such as saltmarshes. On the other hand, several coastal environments suffer the impacts of contamination both from organic and inorganic pollutants. The level of contamination determines the allowed sediment end use and its final destination, such as its disposal into coastal landfills, on the basis of risk assessment and cost-benefit analysis. As such, the geochemical characteristics of sediments are an important parameter informing the management and decision processes. The Marano and Grado Lagoon (Northern Adriatic Sea) has been affected by severe mercury (Hg) contamination from both industrial (a chlor-alkali plant) and long-term mining activities (the Idrija Hg mine in Slovenia), with total sedimentary contents up to 12 μg g-1. There has been evidence of a significant decline in the total saltmarsh area of the lagoon, with direct human impact playing a primary role through dredging and land reclamation. In order to provide evidence for the reuse of dredged sediments for saltmarsh re-construction as a possible alternative in the management of the Marano and Grado Lagoon sediments, we carried out a preliminary evaluation on the content and biogeochemical cycling of Hg in saltmarshes. Mercury content was determined in sediments and in plants from selected saltmarshes located in the Marano and Grado Lagoon. In sediments, Hg content exceeded the estimated background value for the area (0.13 μg g-1), showing the highest concentrations in the eastern sector (Grado Lagoon), the most affected by cinnabar ore extraction. Mercury was also detected in roots, shoots and leaves of the halophytes Sarcocornia fruticosa, Limonium vulgare and Halimione portulacoides, with various degrees of contamination depending on the location, species and depth. Data showed Hg enrichment in the roots, compared to the saltmarsh sediment, as well as hints of translocation of the metal to the aboveground biomass. The results enabled us to estimate the role of saltmarshes as sources and/or sinks of Hg to the lagoon environment and to examine the hydrologic and biogeochemical controls on Hg fate and transport in these environments.