Preliminary results of GEM concentration in interstitial soil and free gases from Mt. Etna, Sicily

Tripodi F.1, Brugnone F.1, D'Alessandro W.2, Parello F.1, Pecoraino G.2, Giammanco S.3, Stagno V.2,4, Calabrese S.1, 2 (1) Università degli Studi di Palermo, Dipartimento di Scienze della Terra e del Mare (DiSTeM), Palermo, Italia. francesco.tripodi01@unipa.it (2) Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Palermo, Palermo, Italia (3) Istituto Nazionale di Geofisica e Vulcanologia – Osservatorio Etneo (INGV-OE), Catania, Italia (4) Sapienza Università di Roma, Dipartimento di Scienze della Terra, Roma, Italia. Introduction Mercury (Hg) is one of the most hazardous elements released into the atmosphere from both natural and human-made sources. It exists in three oxidation states: gaseous elemental mercury (GEM), monovalent mercury, and divalent mercury. Volcanic emissions represent a significant natural source of mercury to the environment and over the last 40 years several studies have been focused on estimating mercury fluxes (Varekamp & Buseck, 1986; Pyle & Mather, 2003; Nriagu & Becker, 2003). Understanding the pathways of mercury once it is released into the atmosphere is crucial, given its potential toxicity and long atmospheric residence time (approximately one year) that allows for efficient global distribution compared to many other volcanogenic elements. This research focuses on the biogeochemical study of mercury at Mt. Etna volcano, where both passive degassing and volcanic eruptions represent important sources of Hg input into the environment, with total annual fluxes between 2.1 to 27.3 t yr-1 (Bagnato et al., 2007; Edwards et al., 2021; Ferrara et al., 2000; Ferrara & Maserti, 1990; Dedeurwaerder et al., 1982; Buat-Ménard & Arnold, 1978). The specific objectives are: (i) to determine the concentrations of mercury in soils and interstitial gases within these soils; (ii) to characterize the microbiota associated with the mercury cycle in the soils; (iii) to quantify the mercury emissions from the volcano's peripheral sources; and (iv) to measure the atmospheric deposition fluxes of mercury. Our preliminary results focused on the gaseous elemental mercury concentrations both in free and in soil interstitial gases on Mt. Etna. Methods The levels of gaseous elemental mercury (GEM) were measured in real-time using the Lumex RA-915M, an atomic absorption spectrometer that utilizes the Zeeman effect and high-frequency modulation of light polarization (ZAAS-HFMLP). This instrument isolates spectral lines at a wavelength of 254 nm with the help of a permanent external magnetic field, using a mercury (Hg) lamp as the radiation source. It operates at a flow rate of 10 L min-1 and has a detection limit of 0.5 ng m-³, with an accuracy of 20% for concentrations ranging from 2 to 50,000 ng m-³. During several field campaigns, we measured GEM at different sites of Mt. Etna (Fig. 1) in atmosphere, in diffuse degassing areas and in the interstitial soil gases. To determine GEM in interstitial soil gases, gas samples were collected from the soil at a depth of approximately 50 cm using a rigid Teflon tube embedded in the ground, connected with a 100 mL syringe and then injected into the instrument's gas inlet (Tassi et al., 2016). In case of free gases (fumaroles and bubbling pools), GEM was collected using a three-way valve connected to an inverted funnel placed inside the degassing pools. Results Atmospheric gaseous elemental mercury (GEM) concentrations were measured at various locations around Mt. Etna (Fig. 1). The results revealed concentrations ranging from 0.5 to 5 ng m-³. Our preliminary findings indicate that the median Hg concentration at sampling points in the east, northeast, west, southwest, and southeast sectors aligns with the global background level of 2 ng m⁻³ (Fig. 2a). Based on this data, it appears that the volcanic plume does not significantly deliver Hg at downwind sites, at least at ground level. In contrast, in some degassing areas on the volcano’s flanks (e.g. low-temperature fumaroles or degassing bubbling pools) high Hg levels were measured, ranging from 2 to 200 ng m⁻³. In particular, at the Aqua Grassa site GEM concentrations exceeded 1,000 ng m⁻³, which is the WHO average annual concentration limit for mercury vapor inhalation (Fig. 2b). In addition to atmospheric GEM measurements, interstitial soil GEM concentrations were measured. GEM concentrations were recalculated considering the dimensions of the areas of the Hg-spikes in the instrumental output, with respect to the standard. The sampling points were located near low-temperature fumaroles, namely CRT1 and BOT sites, where soil temperature varied from 29.5 to 59 °C and from 12.5 to 90.2 °C, respectively. At the CRT01 site, the measured mercury (Hg) concentrations ranged from 146 to 9615 ng m-3, indicating significant variations in Hg levels across the study area. Meanwhile, the BOT site showed a gaseous elemental mercury (GEM) concentration ranging from 114 to 3109 ng m-3. The binary diagrams of Figure 3 demonstrated a strong correlation between GEM concentration and soil temperature at the BOT site (R2 = 0.68). In contrast, no correlation was observed at the CRT01 site (R2 = 0.05). Conclusions This study highlights that peripheral volcanic emissions significantly contribute to atmospheric mercury levels. Field measurements at Mt. Etna and its surrounding areas demonstrated substantial outgassing of mercury, with GEM concentrations far exceeding median global atmospheric levels. Free gas emissions (e.g. fumaroles, bubbling pools), as well as interstitial soil gases, can reach high concentrations of mercury, thus underscoring the importance of volcanic sources of mercury. Notably, GEM concentrations had a strong correlation with soil temperature at certain sites, like BOT, but not uniformly across all locations, such as CRT01. These preliminary findings emphasize the necessity of further research to better understand the biogeochemical cycles of mercury, especially regarding its peripheral emissions, and their broader environmental impact. References Bagnato, E., Aiuppa, A., Parello, F., Calabrese, S., D’Alessandro, W., Mather, T.A., McGonigle, A.J.S., Pyle, D.M., Wängberg, I., 2007. Degassing of gaseous (elemental and reactive) and particulate mercury from Mount Etna volcano (Southern Italy). [Atmospheric Environment 41(35), 7377–7388. https://doi.org/10.1016/j.atmosenv.2007.05.060]. Buat-Ménard, P., Arnold, M., 1978. 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