A new geochemical approach to estimate the distribution of air pollutants from natural and anthropogenic sources: examples from Solfatara Crater (Campi Flegrei, Southern Italy) and Mt. Amiata Volcano (Siena, Central Italy)

Volcanic and geothermal systems significantly contribute to the input of volatile contaminants, such as mercury and hydrogen sulfide, into the atmosphere. Mercury has a strong environmental impact. In the atmosphere the prevalent elemental form is Hg0 (~98 %), whose main physical-chemical features are: high volatility, low solubility and chemical inertness. Hydrogen sulfide (H2S), one of the most abundant gas compounds in volcanic fluids, is highly poisoning and corrosive and unpleasantly smells of rotten eggs. Measurements of Hg0 and H2S concentrations in air are commonly performed by means of passive samplers. However, real-time measurements, coupled with monitoring of local atmospheric conditions, are strongly recommended for a reliable reconstruction of the dispersion dynamics once such contaminants are discharged in air. In this paper, a new real-time measurement method for Hg0 and H2S is presented. A portable Zeeman atomic absorption spectrometer with high frequency modulation of light polarization (Lumex RA-915M) and a pulsed fluorescence gas analyzer (Thermo Scientific Model 450i) were used for Hg0 and H2S measurements, respectively. These instruments were synchronized and set to high-frequency acquisition. Measurements were carried out along transects at an average speed <10 km/h. GPS data and meteorological parameters (wind direction and intensity) were also recorded. The proposed method was applied in two different sites, characterized by natural (Solfatara Crater, Campi Flegrei, Southern Italy) and anthropogenic (Mt. Amiata Volcano, Siena, Central Italy) emissions. With this highly efficient and effective approach, a reliable and reproducible Hg0 and H2S distribution in air was provided, allowing to identify and characterize the gas sources from such different environments. At Solfatara, the distribution of Hg0 and H2S concentrations, the highest values being measured close to the fumarolized areas (>60 ng/m3 and >2,100 μg/m3, respectively), suggests that these gases were discharged from both fumaroles and diffuse degassing from the crater bottom. At Mt. Amiata, the maximum Hg0 and H2S concentrations (>100 ng/m3 and >35 μg/m3, respectively) were recorded close to the geothermal power plants of Piancastagnaio. According to detailed dot-maps constructed on the basis of the measured values, as expected, wind was the main environmental parameter able to control the behavior and the dispersion halo of the Hg0- and H2S-rich plumes emitted from the contaminant sources

Active real-time analyzers vs. passive/diffusive samplers for hydrogen sulfide (H2S) in air: a critical comparison

Hydrogen sulfide (H2S) is a gas pollutant discharged in air from a large number of natural and anthropogenic sources. Its peculiar rotten-egg smell, causing odor nuisance to neighboring communities, is detectable at concentrations between 0.7 and 42 μg/m3 (Schiffman & Williams, 2005). High H2S concentrations could cause eye irritation, damage to the upper respiratory apparatus and loss of smell. The effects of long-term low level (< 2,800 μg/m3) exposures to H2S are still matter of debate (Bates et al., 2013). Hence, the development of techniques for accurate measurements of H2S in air at a wide range of concentrations is a primary issue in environmental monitoring. Two different approaches are currently used: 1) passive samplers and 2) real-time measurements. The latter are generally expensive and require a power supply. On the contrary, passive samplers are low cost and can be deployed in the field with minimal maintenance. Therefore, passive samplers offer an appealing alternative to real-time measurements, especially for regional-scale monitoring. However, the reliability of passive samplers in outdoor applications strongly depends on several environmental factors, such as temperature, humidity and wind speed (Delgado-Saborit & Esteve-Cano, 2006). In this study a comparison between H2S measurements using diffusive radial-type passive samplers (Radiello) and a real-time gas analyzer (Thermo Scientific Model 450i) based on pulsed fluorescence, is presented. The measurements were carried out in areas affected by both anthropogenic and natural sources using both techniques. The results show substantial differences. The passive samplers systematically produce higher H2S concentrations than those of the active analyzer. The relative error was up to > 1,000% for concentrations < 7 μg/m3 and exposure duration ≥ 2 hours. H2S measurements by Radiello were affected by meteo parameters (wind, rain, humidity, temperature). The efficiency of this method was demonstrated to be also strongly dependent on H2S concentrations. In addition, passive samplers give an average concentration value for the exposure period, but are not able to detect short-term H2S increments. These results show that the use of passive samplers for environmental monitoring should thus be limited to preliminary largescale semi-quantitative assessment. A reliable study on the dispersion dynamics of contaminants in air cannot exclude the acquisition of high-frequency data through active analyzers

Real-time measurements of Hg0 in volcanic, geothermal and anthropogenic systems: a multi-methodological approach using Lumex® instrumentation

Mercury represents a pollutant of global concern and strong environmental impact since is highly toxic. Hg is present in air in the oxidation states of 0 and +2, the former being the dominant species with a residence time of 1-2 years due to its high volatility, relatively low solubility and chemical inertness. Both volcanic/geothermal and anthropogenic systems are crucial contributor to the release of Hg0 in the atmosphere. In this work, a Lumex® (RA-915M) was used to evaluate the environmental impact in air of Hg0 from: i) the abandoned Hg mining site and geothermal areas from Mt. Amiata (Siena, Central Italy) and ii) selected Mediterranean volcanic and geothermal systems. The Lumex® instrumentation, based on atomic absorption spectrometric technique with Zeeman effect, allows to measure Hg0 at high frequency, in real-time and at a wide range of concentrations (from 2 to 50,000 ng/m3). Hg0 measurements were coupled with those of other pollutants, such as CO2 H2S, and SO2. Carbon dioxide was measured using a Multi-GAS instrument manufactured by INGV-Palermo, whereas H2S and SO2 using Thermo Scientific® Model 450i analyzer. GPS and meteorological parameters were continuously recorded, too. The data acquisition was carried out along transects at an approximately constant speed or at selected fixed points. Wherever possible, the analytical data were then converted into a spatial interpolation providing a qualitative model for the areal dispersion of the contaminants. The Lumex® device was also applied to measure Hg0 concentrations in interstitial soil gases collected from a probe inserted into the soil at 70 cm depth, in order to produce Hg0 maximum concentration maps in Hg-polluted areas (e.g. Abbadia San Salvatore Hg mining area, Mt. Amiata). Diffuse Hg0 soil fluxes were measured using a chamber positioned above the soil from which, at periodic time intervals, gases were extracted and injected into the Lumex® device. This instrument was also applied to measure Hg0 concentrations along vertical profiles in thermal wells at Santorini (Greece) and Vulcano (Italy) by using a Rilsan® tube lowered into the wells at pre-defined depths. With this approach, a significant stratification of the air masses in terms of Hg0, strictly dependent on water temperature, air pressure and well depth, was observed. The efficiency of Lumex® for these different approaches demonstrated the reliability of this instrument to produce Hg0 data that can be used to identify gaseous Hg-emitters in natural and anthropogenic environments, especially when coupled with other physical and chemical parameters

Hydrogeochemical processes controlling water and dissolved gas chemistry at the Accesa sinkhole (southern Tuscany, central Italy).

The 38.5 m deep Lake Accesa is a sinkhole located in southern Tuscany (Italy) that shows a peculiar water composition, being characterized by relatively high total dissolved solids (TDS) values (2 g L-1) and a Ca(Mg)-SO4 geochemical facies. The presence of significant amounts of extra-atmospheric gases (CO2 and CH4), which increase their concentrations with depth, is also recognized. These chemical features, mimicking those commonly shown by volcanic lakes fed by hydrothermal-magmatic reservoirs, are consistent with those of mineral springs emerging in the study area whose chemistry is produced by the interaction of meteoric-derived waters with Mesozoic carbonates and Triassic evaporites. Although the lake has a pronounced thermocline, water chemistry does not show significant changes along the vertical profile. Lake water balance calculations demonstrate that Lake Accesa has &gt;90% of its water supply from sublacustrine springs whose subterranean pathways are controlled by the local structural assessment that likely determined the sinking event, the resulting funnel-shape being then filled by the Accesa waters. Such a huge water inflow from the lake bottom (~9·106 m3 yr-1) feeds the lake effluent (Bruna River) and promotes the formation of water currents, which are able to prevent the establishment of a vertical density gradient. Consequently, a continuous mixing along the whole vertical water column is established. Changes of the drainage system by the deep-originated waters in the nearby former mining district have strongly affected the outflow rates of the local mineral springs; thus, future intervention associated with the ongoing remediation activities should carefully be evaluated to preserve the peculiar chemical features of Lake Accesa.</p

Measurements of Hg0 (and H2S) at the Solfatara Crater (Southern Italy): Estimating the atmospheric distribution with a real-time approach

Volcanic and geothermal areas are important emitters of natural gas compounds into the atmosphere, which can be of concern when discharging close to densely, populated sites. Mercury has a strong environmental impact, its organic and inorganic complexes being toxic. The dominant form of Hg in the atmosphere is gaseous elemental mercury (GEM), which has high volatility and residence time of 1-2 years. Volcanic degassing accounts for a significant part of the natural mercury emissions. No mercury limits or target values in ambient air are present in the EU legislations, whereas US-EPA and ATSDR impose 300 and 200 ng/m3, respectively, as a limit for chronic exposure. WHO has proposed the annual average value of 1,000 ng/m3 as a guideline for Hg0 in ambient air. The determination of Hg0 concentrations is often performed via passive/diffusive samplers, which provide time-integrated gas concentrations, but not able to assess the highly variable distributions of GEM. Different weather factors and photochemical reactions indeed affect the Hg0 dispersion. In volcanic/geothermal sites, GEM measurements can be associated with H2S, an irritating and suffocating substance and detectable at very low concentrations (7 \u3bcg/m3, ~5 ppb) due to its typical rotten eggs odor. WHO recommends a guideline value of 150 \u3bcg/m3 (~107 ppb) with a 24h averaging time. In April 2014 real-time Hg0 and H2S measurements in air were conducted at the Solfatara Crater, which is nested in the town of Pozzuoli (Southern Italy). The main aims were to (1) test this new methodological approach and (2) investigate the Hg0 the H2S concentrations and their spatial distribution. GEM and H2S continuous measurements were determined with a portable Zeeman atomic absorption spectrometer with high frequency modulation of light polarization (Lumex RA-915M, DL: 2ng/m3) and a pulsed fluorescence gas analyzer (Thermo 450i, DL: 1 ppb), respectively. The GEM and H2S and meteorological data were acquired along previously planned pathways at an average speed <5 km/h. The Hg0 and H2S concentrations were between 12 and 77 ng/m3 and 0.2 and 2400 ppb, respectively. The highest measured concentrations corresponded to the main gas discharging areas, whereas the lowest values were measured outside the crater and in the vegetated areas. The results of this study indicate that this technique approach is highly efficient and effective and provides reliable and reproducible Hg0 and H2S concentrations, which can be used to define the exposure that tourists and inhabitants, living close to volcanic and geothermal areas, may suffer

Geochemistry of hydrothermal fluids from the eastern sector of the Sabatini Volcanic District (central Italy).

This study reports a complete geochemical dataset of 215 water and 9 gas samples collected in 2015 from thermal and cold discharges located in the eastern sector of the Sabatini Volcanic District (SVD), Italy. Based on these data, two main aquifers were recognized, as follows: 1) a cold Ca-HCO3 to Ca(Na)-HCO3 aquifer related to a shallow circuit within Pliocene-Pleistocene volcanic and sedimentary formations and 2) a deep CO2-pressurized aquifer hosted in Mesozoic carbonate-evaporitic rocks characterized by a Ca- HCO3(SO4) to Na(Ca)-HCO3(Cl) composition. A thick sequence of low-permeability formations represents a physical barrier between the two reservoirs. Interaction of the CO2-rich gas phase with the shallow aquifer, locally producing high-TDS and low-pH cold waters, is controlled by fractures and faults related to buried horst-graben structures. The d18O-H2O and dD-H2O values indicate meteoric water as the main source for both the shallow and deep reservoirs. Carbon dioxide, which is characterized by d13C-CO2 values ranging from 4.7 to þ1.0‰ V-PDB, is mostly produced by thermo-metamorphic decarbonation involving Mesozoic rock formations, masking possible CO2 contribution from mantle degassing. The relatively low R/Ra values (0.07e1.04) indicate dominant crustal He, with a minor mantle He contribution. The CO2/3He ratios, up to 6 1012, support a dominant crustal source for these two gases. The d34SH2S values (from þ9.3 to þ11.3‰ V-CDT) suggests that H2S is mainly related to thermogenic reduction of Triassic anhydrites. The d13C-CH4 and dD-CH4 values (from 33.4 to 24.9‰ V-PDB and from 168 to 140‰ V-SMOW, respectively) and the relatively low C1/C2þ ratios (<100) are indicative of a prevailing CH4 production through thermogenic degradation of organic matter. The low N2/Ar and high N2/ He ratios, as well as the 40Ar/36Ar ratios (<305) close to atmospheric ratio, suggest that both N2 and Ar mostly derive from air. Notwithstanding, the positive d15N-N2 values (from þ0.91 to þ3.7‰ NBS air) point to a significant extra-atmospheric N2 contribution. Gas geothermometry in the CH4-CO2-H2 and H2S-CO2-H2 systems indicate equilibrium temperatures <200 C, i.e. lower than those measured in deep geothermal wells (~300 C), due to either an incomplete attainment of the chemical equilibria or secondary processes (dilution and/or scrubbing) affecting the chemistry of the uprising fluids. Although the highly saline Na-Cl fluids discharged from the explorative geothermal wells in the study area support the occurrence of a well-developed hydrothermal reservoir suitable for direct exploitation, the chemistry of the fluid discharges highlights that the uprising hydrothermal fluids are efficiently cooled and diluted by the meteoric water recharge from the nearby Apennine sedimentary belt. This explains the different chemical and isotopic features shown by the fluids from the eastern and western sectors of SVD, respectively, the latter being influenced by this process at a lesser extent. Direct uses may be considered a valid alternative for the exploitation of this resource.Published187-2016A. Geochimica per l'ambiente2IT. Laboratori sperimentali e analitici1VV. AltroJCR Journa