Mercury isotope ratio measurements in sediments of Marano and Grado Lagoon to trace contamination sources and history

In recent years, the development of specific analytical methods for the determination of non-traditional stable isotopes by multiple-collector inductively coupled plasma-mass spectrometry (MC-ICP/MS) allowed for large advances in geochemical as well as environmental research. Non-traditional stable isotopes are a powerful tool for tracing and investigating sources of contamination in the natural environment. Mercury (Hg) experiences complex biogeochemical transformations in the environment and shows different isotopic fractionation processes of both mass dependant (MDF) and mass independent (MIF) nature. In this study, we exploited the accuracy of relatively recent analytic techniques in Hg isotopic ratio measurements to trace the sources of this heavy metal in a northern Adriatic lagoon environment. The isotopic approach was applied to sediment samples collected along an ideal transect extending from the Idrija Hg mine district (western Slovenia) through the Isonzo River and the Gulf of Trieste to the Aussa-Corno river system where Hg was deliberately discharged from a chlor-alkali plant and it finally flowed into the Marano and Grado Lagoon. The differences in Hg isotopic ratios observed between the Idrijca and Isonzo rivers reflect the history of Hg extraction processes, mainly cinnabar ore, in the Idrija mining region. Conversely, the very light isotopic signatures of offshore samples of the Gulf of Trieste and the Northern Adriatic Sea underline that this area was not affected by Hg contamination. In the Grado and Marano Lagoon, Hg isotopic signatures are the consequence of the mixing of two Hg contamination sources: fluvial sediments coming from the Idrija mine, transported by the Isonzo River and carried by tidal currents into the Lagoon and Hg discharged by the Torviscosa chlor-alkali plant. Similar Hg isotopic ratios observed among samples collected from the industrial channel connecting the Torviscosa chlor-alkali plant to the Aussa River and the river itself thus confirm that contamination in this area seems to be directly related and restricted to past industrial activities. Results showed that the isotopic approach can be used as a useful tracing tool in a complex contaminated natural environment