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Karst systems represent the main drinking water source for 20-25% of world’s population, although covering only 7-10% of the global land surface (Ford & Williams, 2007). Karst aquifers are highly vulnerable to external pollution, so their protection and management is of critical importance to sustain water resources. In Greece, starting from the 1970’s, water demands for agricultural, domestic and industrial use increased significantly, mainly in coastal areas (Daskalaki & Voudouris, 2008). The main Hellenic aquifers are hosted in alluvial deposits, in Neogene deposits and in carbonate rocks. The latter cover about 35% of the country and are located in Western, Central and Southern Greece (Daskalaki & Voudouris, 2008). Karst aquifers are developed in limestones and dolomites (Triassic – Cretaceous), and in marbles (Paleozoic – Mesozoic). Their hydrogeological behaviour is controlled by tectonic deformation. About 45% of them is located inland, while the rest is in coastal areas (Voudouris & Kazakis, 2018). During several field sampling campaigns from 2016 to 2020, 126 karst water samples were collected in Greece. Physicochemical parameters (temperature, pH, electric conductivity and redox potential) were measured in situ. Analyses of major ions and trace elements were performed at the laboratories of INGVPalermo. Results were compared with the limits set by the Directive 98/83/EC that fixes quality standards for drinking water. Temperatures of sampled waters ranged from 8.7 to 31 °C, pH from 6.5 to 8.4, whilst Total Dissolved Solids from 206 to 25,617 mg L-1. Most of the samples showed a typical alkaline-earth bicarbonate composition, whilst those sited along the coastline presented elevated concentrations of Na+ (up to 7,680 mg L-1), Cl- (up to 14,200 mg L-1) and SO4 2- (up to 1,940 mg L-1), sometimes exceeding the EC limits, suggesting a seawater contamination. Furthermore, karst springs contaminated by seawater, displayed high concentrations of B (up to 3,870 μg L-1) and Sr (up to 7,080 μg L-1). Nitrate concentrations were always below the EC limit (50 mg L-1), indicating a low contamination from fertilizers. Few low chloride waters showed a metal enrichment, such as Tempi springs (Thessaly) that presented enrichments in Sr (up to 242 μg L-1), Mo (up to 2.27 μg L-1), Cs (up to 1.57 μg L-1) and As (up to 17 μg L-1). Such enrichments could be attributed to the local petrological environment. Generally, karst water samples can be considered suitable for human consumption. Water quality degradation of Hellenic karst springs is mainly due to seawater intrusion or to local geogenic contamination.

Karst aquifers are considered to be one of the most important aquifer types, as they constitute the main drinking water resource for the majority of the global population (Ford et al., 2007). They are generated from the dissolution of carbonate rocks (e.g. limestone, dolomite, marble etc.), a phenomenon commonly known as “karstification”. This process is mainly caused by the acidity of water enriched in dissolved CO2, with the concentration of the latter being dependent on both the temperature and the CO2 partial pressure of the atmosphere in contact with the water (Bakalowicz, 2005). Carbonate rocks cover about 35% of the land surface of Greece and are mainly located in the western, central and southern parts of the country (Daskalaki et al., 2008). The Hellenic karst aquifer resources are more abundant in the western part of Greece, which receives the highest amount of precipitation (1800 mm/a) (Mimikou, 2005). The karst system constitutes a strategic resource of water in the region and preserving its quantity and quality is of the utmost importance for the sustainability of the area. Seventy samples of natural water were collected from karst springs in the northern (Macedonia-Thrace) and in the central parts of Greece, during 3 campaigns from 2016 to 2018. Sampling sites were selected on the basis of the springs flow rates (> 50 L/s). Water temperature, pH, Eh and electric conductivity were measured in situ with portable instruments; major ions were determined by Ionic Chromatography (IC) on filtered (anions) or filtered and acidified (cations) samples, whereas trace elements were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) on filtered and acidified samples. All analyses were performed at the laboratories of INGV of Palermo. Chemical compositions were compared with the limits fixed by the Directive 98/83/EC, which is the most recent EU legislation that sets quality standards for drinking water. Regulations concerning the quality of drinking water as established by the Directive allow Member States to adapt the monitoring of water quality to local conditions (Karavoltsos et al., 2008). Temperatures of the sampled waters ranged from 8 to 25 °C, pH from 6.5 to 8.4, whilst Total Dissolved Solids (TDS) from 206 to 15,418 mg/L. The highest concentrations of sodium, potassium, chloride and sulfate were found in the karst springs of Central Greece (26.6-5610 mg/L; 1.56-204 mg/L; 81.06-9467 mg/L; 15-2420 mg/L, respectively), where values had sometimes exceeded the limits set by the Directive 98/83/EC, indicating a contamination due to sea water intrusion. Based on the chloride concentrations, samples were subdivided into low (Cl- < 100 mg/L) and high (Cl- > 100 mg/L) chloride karst waters. All water samples were plotted in a Langelier Ludwig diagram (Fig. 1) with the low chloride waters presenting a typical alkaline-earth bicarbonate composition. Exception is the samples of Kaliakuda, Sidirokastro and Koromilia that display enrichment in alkalis possibly due to hydrothermal activity. On the other hand, the most chloride-rich waters plot close to the sea water composition point while two samples (Rema, Mylos Kokkosi) are aligned along the seawater-groundwater mixing line (Fig. 1). Concentration ranges of major and trace elements for all waters are presented in Fig. 2, with low and high chloride samples being plotted with different symbols. High chloride group displays much higher values for Mg, SO4, Cl, Na, K, Sr, B, Li, Rb and Cs with respect to low chloride, with differences in the median values between two and three orders of magnitude. Species deriving from carbonate dissolution (Ca and HCO3) show the lowest range of concentrations both for low and high chloride waters (Fig. 2). Low chloride waters show a wide range of concentrations (three to four orders of magnitude) for trace elements such as Li, Fe, Rb, As, Mn, Cu and Cs. Trace elements were above the legislation limits (Directive 98/83/EC) mostly in the case of high chloride karst springs, showing elevated concentrations of Boron (up to 1861 μg/L), Strontium (up to 5026 μg/L) and Arsenic (up to 12.1 μg/L). In terms of Boron and Strontium, the exceeding values seem to be generally related to the intrusion of sea water. On the other hand, Arsenic, whose maximum admissible level is 10 μg/L, was above limit also in the low chloride water (17 μg/L) of Tempi, Thessalia. Few low chloride waters show a metal enrichment, such as Tempi (Sr = 242 μg/L, Mo = 2.27 μg/L, and Cs = 1.57 μg/L) and Kaliakuda (V = 3.89 μg/L, Mn = 3.65 μg/L, Fe = 71.26 μg/L, Cu = 11.55 μg/L, Zn = 22.61 μg/L, Rb = 54.7 μg/L), whilst nitrate concentrations that could indicate contamination from fertilizers or from septic tanks, are always below the maximum admissible value (50 mg/L). Most of the analyzed waters can be considered suitable for human consumption. Water quality deterioration of Hellenic karst springs is mainly due to sea water intrusion, whilst only few low chloride waters show significant enrichments in trace metals that rarely exceeds the drinking water standards. These higher contents are probably of natural origin due to local geological setting.

Earthquakes and volcanic eruptions represent a hazard. However, the impact of gases released in geodynamically active areas should not be underestimated. It is commonly known that geogenic sources release great amounts of gases, which, apart from having an important influence on the global climate, can also have a strong impact on human health causing both acute and chronic effects. In particular, CO2 and sulphur gases (mainly H2S and SO2) are the main compounds responsible for acute mortality due to their asphyxiating and/or toxic properties. One of the most known and also worst episodes occurred, took place on the 21th of August 1986 at Lake Nyos, Cameroon, when about 1700 people were killed and 850 injured by a massive CO2 release (D’Alessandro, 2006). Like other geodynamically active areas, Greece is also affected by a large number of geogenic gas manifestations (Daskalopoulou et al., 2018a). These occur either in the form of point sources (fumaroles, mofettes, bubbling gases) or of diffuse soil gas emanations (Daskalopoulou et al., 2018b). D’Alessandro and Kyriakopoulos (2013) made a preliminary estimation of the risk related to geogenic gases in Greece for the time period of 1992-2011; the whole population of the country was considered. In that period, at least two fatal episodes with a total of three victims took place, likely caused to the exposure to geogenic gases (specifically CO2). This would give a risk of 1.310-8 fatality from geogenic gas manifestations per annum. This value, although probably underestimated, is much lower than many other natural or anthropogenic risks. Since deaths due to natural gases are often wrongly attributed, it cannot be excluded that some fatal episode has not been recognized and thus that the risk is somewhat higher than assessed. Although very low, this risk should not be neglected, not only because it is possibly underestimated, but also because simple countermeasures could be adopted for risk reduction. Dangerous areas could be easily identified and delimited by geochemical prospecting and their hazards properly highlighted. Apart from the sites where fatal episodes occurred, many other hazardous sites have been recognized in Greece. Here we present data collected at Loutra Ypatis (central Greece). Study area Sperchios Basin – Evoikos Gulf Graben is a 130 km long actively spreading graben in Central Greece (1 cm/a). The high geothermal gradient of the area is evident by the presence of many thermal springs with temperatures that vary from 24 to 82 °C. In the waters of these springs, discharging along the normal faults bordering the graben, an abundant gas phase is bubbling. Loutra Ypatis is one of the emerging springs and its waters (31 °C) are exploited by a spa. The water is currently drained by a gallery and therefore the water level is about 5 m below ground at the bottom of a funnel-like hole (Fig. 1 left). For safety reasons the hole was covered by a closed building (Fig. 1 left and center). The gas, which is vigorously bubbling in the spring, is mostly (> 96%) composed of CO2 (D’Alessandro et al., 2014). The walls of the hole are covered of sulfur that derives from the partial oxidation of the H2S (2500 ppm) contained in the released gas (D’Alessandro et al., 2014). Methods In October 2015 atmospheric concentrations of CO2 were measured with a Licor LI820 NDIR spectrometer (range 0 to 20,000 ppm, accuracy of 2%), whilst in April 2016, the atmospheric concentrations of CO2 and H2S were measured with a Multi-GAS analyser manufactured by INGV-Palermo equipped with Licor LI-840 NDIR spectrometer (CO2 0-20,000 ppm) and an EZ3H electrochemical sensor by City Technology Ltd. (H2S 0–100 ppm). Simultaneous CO2, CH4 (both 0- 100%), CO, H2S (both 0-500 ppm) and O2 (0 – 25%) concentrations within the building were measured with a portable gas analyser GA2000 (Geotechnical Instruments). Results and discussion Due to the fact that a building covers the thermal spring, the intense bubbling activity of its waters creates a strong gas accumulation inside. The main component of the released gases is CO2, which has a higher density with respect to atmospheric air, thus creating the conditions for gas accumulation. About 2 m above the water level, CO2 concentrations of >95% and non-detectable O2 concentrations were measured. At higher levels above the water, CO2 concentrations were decreased but never below 50%. Such concentrations within the building are lethal for both animals and human beings. Of course, access is forbidden, but as the building is not perfectly sealed, the gases permeate to the outside through fissures and cracks. Figure 2 shows the CO2 concentrations measured in the air on October 2015 at 1.5 m height while walking around the walls of the edifice at about 2 m distance. Leaking of CO2 from the edifice is made evident by concentrations reaching values of more than 6000 ppm. The highest values were measured close to the entrance of the edifice were fissures and cracks are concentrated. Due to the tendency of CO2 to accumulate at lower levels, in this place, close to the ground, CO2 levels lethal to small animal can be reached. This was made evident by a dead bird found in that occasion (Fig. 1). In April 2016, due to the much windier conditions, CO2 concentrations at the same places reached values never exceeding 1000 ppm while H2S was always below 1 ppm. These values sharply increased getting closer to the fissures around the main entrance of the building and reached saturation of the sensors (CO2 > 20,000 ppm and H2S > 100 ppm) at a distance of few centimeters. The intense CO2 degassing observed at Loutra Ypatis may be responsible for elevated levels that can have an impact on human beings. It is worth noting that values measured in the atmosphere close to the building exceed the Occupational Recommended Exposure Limit of 5000 ppm (NIOSH, 2005). In closed spaces lethal levels can be easily reached. An older inhabitant of the close by village told us that in his childhood a playmate died by going inside the gallery that drains the thermal water out of the spa due to the high CO2 levels. Such episode underscores the need not to disregard the gas hazard created by intense natural gas manifestations like the thermal spring of Loutra Ypatis.

Volatiles are transported from the deep crust or mantle to the surface in geodynamically active areas where seismic, volcanic and geothermal activity is present; the circulation of hydrothermal fluids in the crust is enhanced. In such areas, faults may act as preferential pathways for advective gas-carrying fluid transport. Towards the surface, pressure decrease allows the gases to escape from the fluids into soil gas and eventually into the atmosphere (King, 1986). The migration of carbon-bearing crustal and mantle fluids contributes to Earth’s carbon cycle (Berner & Kothavala 2001). However, till now, the mechanisms, magnitudes and time variations of carbon transfer from depth to the surface remain the least understood parts of the global carbon budget. Carbon dioxide and methane are the main contributors of the total amount of C-degassing from geological (volcanic and non-volcanic) sources. From the beginning of the last century, high attention has been paid to the reservoirs of CO2 and CH4 in the atmosphere because they represent the most dangerous species in terms of global warning. The increased amount of carbon dioxide and methane in the atmosphere has important implications for the energy balance and the chemical composition of the atmosphere. Mörner and Etiope (2002) calculated that 102-103 Mt of CO2 are presumably involved in the carbon cycle every year. This estimation though, is affected by high uncertainty as a number of sources and C-degassing environments that account for this high leakage were not taken into consideration. Greece belongs to the most geodynamically active regions of the world and as such, it has to be considered an area of intense geogenic degassing. Regarding carbon, the territory is characterized by the high hydrothermal and volcanic activity of the South Aegean Active Volcanic Arc (SAAVA), and by widespread geological seeps of buried carbon dioxide and methane. In the present work, we present more than 700 literature data of free gases spread along the whole Hellenic territory to get insight on geographic distribution and composition of the released fluids. Moreover, we review all the published studies on CO2 and/or CH4 output of high degassing areas of Greece that are mainly concentrated along the SAAVA in a first attempt to estimate the total geologic output of the nation. Helium isotope data propose that the highest mantle contribution (50 to 90%) is found along the SAAVA, whereas the lowest in continental Greece (0-20%), with the atmospheric contribution being mostly negligible. Based on the geographical distribution of the gases, it is evident that the R/RA ratios and CO2 concentrations increase in areas characterized by: i) thin crust; ii) elevated heat flow values; iii) recent (Pleistocene-Quaternary) volcanic activity; and iv) deep routed extensional or transtensional regional faults. The highest values are therefore found along the SAAVA and the lowest in the western part of Greece where CH4 emission is prevailing. Furthermore, it was noticed that the majority of the samples present a prevailing limestone C component, whilst only few samples have a prevailing mantle C component (Sano and Marty, 1995). It seems barely possible though to distinguish CO2 deriving from crustal and slabrelated limestones. Additionally, due to the complex geodynamic history, the mantle C isotope composition could be affected by subduction-related metasomatism and, similarly to the nearby Italian area (Martelli et al., 2008), the C isotope composition could be more positive. In this case, the mantle contribution is probably underestimated. In terms of geogenic carbon degassing, the best studied and most exhaling area is the SAAVA, which releases 104,090 t/a of CO2 and 20.26 t/a of CH4. Continental Greece on the contrary, is much less studied but may release CO2 in the same order of magnitude in its eastern-central and northern part. The western and south-western parts of Greece are conversely the main area of methane and higher hydrocarbon degassing. Methane output of Greece is much less constrained but the presence on its territory of one of the biggest thermogenic gas seepages of Europe releasing about 200 t/a of CH4 to the atmosphere underscores its potentially high contribution. Approximately 114,310 t/a of CO2 and 221 t/a of CH4 are released from the whole Hellenic territory (Daskalopoulou et al., submitted). This estimation though, should be considered minimum as there are processes and sources that have not been taken into consideration yet. More specifically, in the submarine manifestations found at greater depths, gases cannot reach the sea surface due to the dissolution process that takes place along the water column; this is especially true for CO2 that is more soluble in water respect to other gases (eg. Milos - Dando et al., 1995; Kolumbo - Rizzo et al., 2016 etc). Moreover, the geological and geodynamic regime can contribute in the formation of CO2 reservoirs. This is the case of Florina Basin (Pearce et al., 2004) where more than one CO2 reservoirs were created, with one of them being exploited by the company Air Liquide Greece. It is worth noting that this reservoir, found at a depth of approximately 300 m, produces 30,000 t/a of CO2 (Pearce et al., 2004). Moreover, in the same area, water is also used for water supply and irrigation purposes. This water though contains a great amount of dissolved CO2 great part of which is released to the atmosphere when the water is pumped to the surface. Another source that should be underscored is the quantification of geogenic CO2 dissolved in big karstic aquifers. Chiodini et al. (1999, 2000) demonstrated that the relatively high solubility of CO2 in water plays an important role in the quantification of carbon. This approach was proved for central Italy and it might be the case for continental Greece due to the similar geodynamic history. Finally, in ophiolitic sequences where serpentinization takes place, if and when the conditions are adequate (i.e. presence of effective catalysts – Etiope and Ionescu, 2015) an abiogenic origin for CH4 seems to be favored even at low temperatures. Ophiolitic sequences crop out widely in Greece along two N-S trending belts, whilst more hyperalkaline springs or dry seeps may be present. However, their flux in generally is very low and therefore their contribution to the total natural CH4 output has probably to be considered negligible.

Samples of efflorescences and encrustations of hydrothermal origin have been collected at three fumarolic areas in Greece. The three sites are Sousaki, Thiafi (Methana) and Kokkino Nero (Kos) and all belong to the South Aegean Active Volcanic Arc. Samples were analysed for their mineralogical (XRD and SEM-EDS) and chemical composition. Solutions obtained from both mineralization with HNO3 and leaching with distilled water, were analysed for major (ICP-OES), minor and trace metals (ICP-MS) and for sulfate contents (IC). Results show that their composition is mainly controlled by the petrological composition of the substrate (ultramafic rocks at Sousaki, felsic volcanic rocks at Methana and low-grade metamorphic rocks at Kos). The microenvironmental conditions (humidity, oxidizing or anoxic, exposed or sheltered from meteoric agents) as well as the rainfall regime of the area play also an important role. The presence of highly soluble sulfate minerals with elevated contents of many metals further underscores the significant influence of hydrothermal activity on elements’ mobility. The sometimes very high concentrations in toxic elements like Al, As, Co, Cr, Ni evidence also a potential environmental impact.

In the period from 2004 to 2017, more than 350 samples of free and dissolved gases were collected along the whole Hellenic area. Some literature data have also been taken into consideration (Rizzo et al., 2016). Samples have been analysed for their chemical and isotope composition. The concentrations range from 0.10 to 3370 μmol/mol for He, 600 to 995,000 μmol/mol for N2, 0.60 to 915,000 μmol/mol for CH4 and 17 to 1,002,000 μmol/mol for CO2, whereas the isotope values range from 0.01 to 7.10 for R/RA and -29.91 to +6.00 for δ13C-CO2. Considering the R/RA and 4He/20Ne ratios the atmospheric, mantle and crustal contributions for He have been calculated (Sano and Wakita, 1985). The highest mantle contribution (50 to 90%) is found in the South Aegean Active Volcanic Arc (SAAVA), whereas the lowest in continental Greece (0-20%). Atmospheric contribution is mostly negligible. Taking into consideration the geographical distribution of the gases, it is evident that the R/RA increases in areas characterized by: i) thin crust; ii) elevated heat flow values; iii) recent (Pleistocene-Quaternary) volcanic activity; and iv) deep routed extensional or transtensional regional faults. The highest values are therefore found along the SAAVA and the lowest in the western part of Greece. Furthermore, based on the CO2/3He and δ13C-CO2 values (Sano and Marty, 1995), the contribution of Sediment, Mantle and Limestone end-members for CO2 was determined. The majority of the collected samples present a prevailing limestone C component and only few samples have a prevailing mantle C component. However, with the present data, it is not possible to distinguish CO2 deriving from crustal and slab-related limestones. Additionally, due to the complex geodynamic history, the mantle C isotopic composition could be affected by subduction-related metasomatism and, similarly to the nearby Italian area (Martelli et al., 2008), the C isotope composition could be more positive. In this case, the mantle contribution is probably underestimated.

Mount Etna, located in eastern Sicily, is the largest stratovolcano in Europe and one of the most intensely degassing volcanoes of the world (Allard et al., 1991; Gerlach, 1991). In particular, previous estimates highlighted that Mt Etna emits about 1.6 % of global H2O fluxes from arc volcanism (Aiuppa et al., 2008) and 10 % of global average volcanic emission of CO2 and SO2 (D’Alessandro et al., 1997; Caltabiano et al., 2004). Furthermore, Gauthier and Le Cloarec (1998) underscored that Mt. Etna is an important source of volcanic particles, having a mass flux of particle passively released from the volcano during non-eruptive period estimated between 7 to 23 tons/day (Martin et al., 2008; Calabrese et al., 2011). In general, Etna is considered to be still under evolution and rather ‘friendly’, which, along with the above, makes it a favorable natural laboratory to study volcanic geochemistry. During the previous years, 6 field campaigns and the first 2 editions of the “Etna International Training School of Geochemistry. Science meets practice” were carried out at Mt. Etna and hosted in the Volcanological Observatory of Pizzi Deneri, a picturesque building located at the base of the N-E Crater at an altitude of 2850 m a.s.l, on the rim of the Ellittico caldera. The basic idea was to share scientific knowledge and experiences in a multidisciplinary community, using local resouces with a low-cost organisation. The “Etna International Training School of Geochemistry, 2018. Science meets practice” is addressed to senior graduate students, postdoctoral researchers, fellows, and newly appointed assistant professors, aiming to bring together the next generation of researchers active in studies concerning the geochemistry and the budget of volcanic gases. Direct sampling of high-to-low temperature fumaroles, plume measurement techniques (using CO2/SO2 sensors such as Multi-GAS instruments, MAX-DOAS instruments and UV SO2 cameras, alkaline traps and particle filters) and measurement of diffuse soil gas fluxes of endogenous gases (CO2, Hg0, CH4 and light hydrocarbons) are introduced during brief theoretical sessions before being directly applied in the field. An active participation of the students to the fieldwork is always requested. Hence, the teaching approach includes frontal lesson, practical demonstrations and field applications. “Etna International Training School of Geochemistry, 2017. Science meets practice” was partially funded by EGU. Highlights of the 2016 and 2017 summer school will be presented and a new summer school for 2018 is announced. . .

The Sperchios Basin is an actively spreading rift area with deeply rooted extensional faults and also a site of quaternary volcanic activity. Such geologic conditions favoured the formation of many hydrothermal systems, whose surface expressions are among the biggest thermal springs of the whole Greece. Degassing of deeply derived geogenic CO2 is highlighted by strong bubbling within the main pools of the springs. Flux measurements were made in the Thermopyles spring with the floating chamber method and results showed that bubbling gases in the spring release about 1 ton/day of CO2. The outgoing stream has a flow of more than 250 l/s of water rich in CO2 (about 16 mmol/l). Although no bubbling is visible along the stream, after 300 m, the CO2 content decreases to 2 mmol/l, indicating an intense CO2 degassing. This was quantified in more than 10 tons/day, suggesting that most of the degassing is not visible. Output estimations at the close by thermal springs of Psoroneria and Ypatis have not been made yet, but considering that the bubbling in their pools and the water outflow rates are similar, the CO2 emitted will be of the same order of magnitude. Further contributions from the Sperchios area may come from the Kamena Vourla springs and from diffuse soil degassing. To sum up, the best estimate of the total CO2 output of the studied area is in the order of many tens of tons/day. Such output is comparable to that of the single active volcanic systems along the South Aegean Volcanic Arc (Sousaki, Methana, Milos, Santorini, Kos and Nisyros) and it highlights the importance of hidden degassing along CO2-oversaturated streams.

Several gas samples have been collected from natural gas manifestations in the island of Milos. Most of them are located underwater along its coasts, whereas three anomalous degassing fumarolic areas (Kalamos, Paleochori and Adamas) were identified on land. Almost all the gases have CO2 as the prevailing gas species, with concentrations ranging from 88 to 99% for the samples taken underwater, while the on-land manifestations present a wider range (15-98%), being sometimes heavily contaminated by air. Methane reaches up to 1.0%, H2 up to 3.2% and H2S up to 3.5% indicating a hydrothermal origin of the gases. The isotope composition of He shows values ranging from 2.55 to 3.39 R/RA, highlighting a significant mantle contribution with the highest values measured at Adamas. C-isotope composition of CO2 is in the range from -1.9 to +1.3‰ vs. V-PDB, with most of the values found around -0.5‰ indicating a prevailing limestones origin. Isotope composition of CH4, ranging from -18.4 to-5.0‰ vs. V-PDB for C and from -295 to +7‰ vs. V-SMOW for H, points to a geothermal origin with sometimes evident secondary oxidation processes. CO2-flux measurements showed values up to 1100, 1500 and 8000 g/m2/day in the areas of Kalamos, Paleochori and Adamas respectively and up to about 23,000 g/m2/day in the marine area of Kanava. The south-western part of the island was covered with a lower density prospection, revealing only few anomalous CO2 flux values (up to 650 g/m2/day). Preliminary CO2 output estimations gave values of 1.1, 0.6 and 5.5 tons/day for the three fumarolic areas and of 1.1 tons/day for Kanava. The total output of the island (about 10 tons/day) is comparable to the other volcanic/geothermal systems of the south Aegean active volcanic arc (Nisyros, Kos, Nea Kameni, Methana and Sousaki).

Nisyros is the easternmost island of the South Aegean Active Volcanic arc in Greece. It is an active stratovolcano known for its intense hydrothermal activity. In this study we present the results of the mineralogical analyses of 20 soil samples collected in the caldera area. Samples were analysed through X-ray diffraction and the results allow us to divide them in two groups: Lakki Plain and Stefanos Crater. In their majority the soils of Lakki Plain have a main mineralogical assemblage that consists of quartz, feldspar and gismondine (Ca2Al4Si4O16·9H2O). Gismondine as well as stellerite (CaAl2Si7O18·7H2O), which is appearing in some samples, derive from hydrothermal alteration, whilst wollastonite (CaSiO3) is also found as a product of thermally metamorphosed siliceous carbonates. In the Stefanos Crater soils, due to the acid environment and the relatively high temperatures of the fumaroles (about 100°C), the main assemblages mostly comprises hydrothermal alteration minerals like quartz, sulfur, wollastonite, gypsum and gismondine. The lower amount of feldspars with respect to the Lakki Plain soils can be justified by the high percentage of gismondine, their alteration products. Voltaite (K2Fe(II)5Fe(III)3Al(SO4)12·18H2O) was also found in some of the samples as a product of hydrothermal alteration of Si deposits. Close to the fumaroles, the occurrence of alunogen (Al2(SO4)3·17H2O), a sulphide alteration mineral found in fumarolic environments, is also noticeable.

The geologic emissions of greenhouse gases (CO2 and CH4) give an important natural contribution to the global carbon budget. However, the contribution of these emissions to the global carbon cycle and their possible role on the climate change remain still poorly quantified (Guliyev and Feizullayev, 1997; Milkov, 2000; Etiope et al., 2015 and references therein). Methane, the most abundant organic compound in Earth's atmosphere, may be created either from existing organic matter or synthesized from inorganic molecules. Accordingly, it can be differentiated in two main classes: a) biotic (either microbial or thermogenic) and b) abiotic. For this study, 115 gas samples of fumarolic, thermal and cold discharges from all over the Hellenic territory were collected and both chemical (CO2, H2S, CH4, N2, O2, Ar, H2 and light hydrocarbons) and isotopic (13C-CO2, 13C-CH4, D-CH4) analyses were performed, in order to investigate the genetic processes that produced CH4 in fluids related with the complex geodynamic setting of Greece. On the basis of the spatial distribution of the gas discharges and their type of emission, the whole dataset was subdivided into 3 main “domains”, as follows: 1) Volcanic Arc (VA) - 34 samples; 2) External Hellenides (EH) - 23 samples of cold emissions and of hyperalkaline aqueous solutions; 3) Internal Hellenides (IH) - 62 samples of cold and geothermal emissions. Almost each group is characterized, as long as subdivided in 3 groups based on the type of emission (on-land free or dissolved gases and subaqueous gases) and a 4th group includes literature data.

The Greek territory belongs to the geodynamically active Alpine-Himalayan orogenic belt. As such, it shows intense seismic activity, active volcanic systems and areas of enhanced geothermal fluxes. One of these areas is the Sperchios basin and the northern part of Evia island in central Greece, which present widespread thermal manifestations [1]. Five of them with temperatures from 33 to 80°C present bubbling gases whose dominating species are either CO2 or N2. All gases contain from 27 to 4000 ppm of CH4. The isotopic composition of CH4 in these gases covers a wide range with δ13C values ranging from -21.7 to +16.9‰ and δ2H values ranging from -124 to +301‰. The hottest manifestation displays the lowest isotopic values within the typical range of volcanic and geothermal systems. All the remaining samples fit a methane oxidation trend reaching very positive values. If we consider the lowest values as the deep hydrothermal marker the obtained ΔH/ΔC values range between 5 and 13 which are close to those typical of microbially driven oxidation [2]. Although the outlet temperature of the hottest manifestations is at the upper limit for methanotrophic microrganisms [3], we can hypothesize that environmental conditions are not favorable for their survival at this site while they can thrive in the other strongly consuming methane and producing very positive isotopic values. [1] D’Alessandro et al. (2014), Mar. Petrol. Geol. 55, 295-308; [2] Kinnaman et al. (2007), Geochim. Cosmochim. Acta 71, 271-283; [3] Sharp et al. (2014), Environ. Microbiol. 16, 1867-1878

The Aegean area, which is geodynamically very active, is characterised by intense seismic activity, presence of active volcanic systems and anomalous geothermal gradients. Like other regions of intense geodynamic activity it is also characterized by extensive geogenic degassing. Gas manifestations are not only widespread on land but are also very frequent underwater. Many of these, as for example those of Milos and Santorini, are known since long time and have been previously studied although the targets were generally the hot waters or the sediments affected by the emissions (Smith and Cronan 1978, Dando et al. 1995, Price et al. 2013, Megalovassilis 2014).The present study aims at producing the first catalogue of the shallow submarine gas manifestations of the Aegean Sea andto characterizegeochemically the emitted gases.

Greece belongs to the most geodynamically active regions of the world and as such, it has to be considered an area of intense geogenic degassing. In terms of carbon, the territory is characterized by the high hydrothermal and volcanic activity of the South Aegean Active Volcanic Arc (SAAVA), and by widespread geological seeps of buried carbon dioxide and methane. In the present work, we investigate the island of Gyali located in the volcanic system Kos-Gyali-Nisyros. Nine gas samples have been collected on the island of Gyali in areas found both on land, in a small lake (∼2000 m2) along its beach, and in the sea close to the shore at shallow depths (

Geothermal areas of Greece are located in regions affected by recent volcanism and in continental basins characterised by elevated heat flow. Many of them are found along the coast, and thus, water is often saline due to marine intrusion. In the current study, we present about 300 unpublished and literature data from thermal and cold mineral waters collected along Greece. Samples were analysed for major ions, Li, SiO2 and isotopes in water. Measured temperatures range from 6.5 to 98 °C, pH from 1.96 to 11.98, while Total Dissolved Solutes (TDS) from 0.22 to 51 g/L. Waters were subdivided into four main groups: (1) thermal; (2) cold; (3) acidic (pH 11). On statistical basis, thermal waters were subdivided into subgroups according to both their temperature [warm ( 75 °C)] and TDS [low salinity ( 30 g/L)]. Cold waters were subdivided based on their pCO2 [low ( 0.85 atm)]. δ18O–H2O ranges from − 12.7 to + 2.7‰ versus SMOW, while δ2H–H2O from − 91 to + 12‰ versus SMOW being generally comprised between the Global Meteoric Water Line and the East Mediterranean Meteoric Water Line. Positive δ18O shifts with respect to the former are mostly related to mixing with seawater, while only for a few samples these shifts point to high-temperature water–rock interaction processes. Only a few thermal waters gave reliable geothermometric estimates, suggesting reservoir temperatures between 80 and 260 °C.

Twenty gas samples have been collected from the natural gas manifestations of Milos Island, the majority of which is found underwater along its coast. Furthermore, three anomalous degassing fumarolic areas (Kalamos, Paleochori and Adamas) have been recognized on-land. Almost all the gases are CO2-dominated with CO2 ranging from 88 to 99% vol for the samples taken underwater, while the on-land manifestations show a wider range (15–98%) due to air contamination. Methane reaches up to 1.0% vol, H2 up to 3.2% vol and H2S up to 3.5% vol indicating a hydrothermal origin of the gases. The isotope composition of He points out to mantle contributions up to 45%, while the C-isotope composition of CO2 (from −1.9 to +1.3‰ vs. V-PDB with most of the values around −0.5‰) suggests a prevailing limestone origin. Isotope composition of CH4, ranging from −18.4 to −5.0‰ vs. V-PDB for C and from −295 to +7‰ vs. V-SMOW for H, points to a geothermal origin with sometimes evident secondary oxidation processes. Additionally, CO2-flux measurements showed high values in the three fumarolic areas (up to 1100, 1500 and 8000 g/m2/d at Kalamos, Paleochori and Adamas respectively) with the highest CO2-flux values (up to about 23,000 g/m2/d) being measured in the sea at Kanavas with a floating chamber. The south-western part of the island was covered with a lower density prospection revealing only few anomalous CO2 flux values (up to 650 g/m2/d). The total output of the island (30.5 t/d) is typical of quiescent closed-conduit volcanoes and comparable to the other volcanic/geothermal systems of the south Aegean active volcanic arc (Nisyros, Kos, Nea Kameni, Methana and Sousaki).

The complex geology of Greece includes two important parallel running ophiolitic belts. The Othrys Massif in central Greece belongs to the westernmost of them. In the current study, 33 water samples from cold hyperalkaline and hypothermal (T < 40°C) alkaline springs and 30 gas samples (either dissolved or free) were collected at 17 different sites in and around this wide ophiolite outcrop, aiming to determine the origin of fluids and evidence gas-water-rock interaction processes taking place in the area. Water samples were analysed for their chemical (major ions and trace elements) and isotope (d18O-H2O, d2H-H2O) composition. They can be subdivided into alkaline (pH 11 and Ca-OH composition). Trace elements generally showed very low concentrations and mostly inversely correlated with pH. Gases were analysed for their chemical (He, Ne, Ar, H2, O2, N2, CH4, C2H6, CO2 and H2S) and isotope (d13C-CH4, d2H-CH4, d13C-CO2) composition. Samples from alkaline waters were mainly dominated by CH4 (from 128,000 to 915,000 μmol/mol), while hyperalkaline waters showed a N2-rich composition (from 727,000 to 977,000 μmol/mol). Methane had a wide range of isotope compositions (d13C-CH4 from -74.5 to -14.5 and d2H-CH4 from -343 to -62 ). Alkaline waters present the most negative isotope values for CH4, evidencing a biogenic (both thermogenic and microbial) origin. Many of the hyperalkaline waters had CH4 isotope values compatible with an abiogenic origin through serpentinization processes but occasionaly very negative values were recorded, indicating sometimes a clear biogenic contribution. Finally, few samples both from alkaline and hyperalkaline waters showed some evidence of secondary oxidation processes.

Nisyros Island (Greece) is an active volcano hosting a high-enthalpy geothermal system. During June 2013, an extensive survey on Hg concentrations in different matrices (fumarolic fluids, atmosphere, soils, and plants) was carried out at the Lakki Plain, an intracaldera area affected by widespread soil and fumarolic degassing. Concentrations of gaseous elemental mercury (GEM), together with H2S and CO2, were simultaneously measured in both the fumarolic emissions and the atmosphere around them. At the same time, 130 samples of top soils and 31 samples of plants (Cistus creticus and salvifolius and Erica arborea and manipuliflora) were collected for Hg analysis. Mercury concentrations in fumarolic gases ranged from 10,500 to 46,300 ng/m3, while Hg concentrations in the air ranged from high background values in the Lakki Plain caldera (10-36 ng/m3) up to 7100 ng/m3 in the fumarolic areas. Outside the caldera, the concentrations were relatively low (2-5 ng/m3). The positive correlation with both CO2 and H2S in air highlighted the importance of hydrothermal gases as carrier for GEM. On the other hand, soil Hg concentrations (0.023-13.7 μg/g) showed no significant correlations with CO2 and H2S in the soil gases, whereas it showed a positive correlation with total S content and an inverse one with the soil pH, evidencing the complexity of the processes involving Hg carried by hydrothermal gases while passing through the soil. Total Hg concentrations in plant leaves (0.010-0.112 μg/g) had no direct correlation with soil Hg, with Cistus leaves containing higher values of Hg with respect to Erica. Even though GEM concentrations in the air within the caldera are sometimes orders of magnitude above the global background, they should not be considered dangerous to human health. Values exceeding the WHO guideline value of 1000 ng/m3 are very rare (

Florina Basin is located in northern Greece, close to Mount Voras where the volcanic activity of Late Messinian age began. In the area, many CO2-rich gas emissions are present as a bubbling free-phase in groundwater (both springs and wells) and soil gases. Volcanism along with the geological and geodynamic regime of the basin, created the ideal conditions for CO2 accumulation in vertically stacked reservoirs. One of these, industrially exploited by the company Air Liquide Greece, produces 30,000 t/a of CO2. Results show that CO2 concentrations in the gases of Florina can arrive up to 99.8% and are mostly above 90%. Moreover, C-isotope composition (-2.1 to + 0.3 h vs. VPDB) indicates a mixed mantle-limestone origin for CO2, while He isotope composition (R/RA from 0.21 to 1.20) shows a prevailing crustal origin with an up to 15% mantle contribution. Helium and methane, with concentrations spanning over three orders of magnitude, show a positive correlation and a consequent high variability of He/CO2 and CH4/CO2 ratios. This variability can be attributed to the interaction of the uprising gases with groundwater that chemically fractionates them due to their different solubility. Based on the CO2, CH4 and He concentrations, gas samples collected in the basin can be divided in 3 groups: a) deep reservoir gases, b) enriched in less soluble gases and c) depleted in less soluble gases. The first group consists of gas samples collected at the Air Liquide extraction wells, which tap a 300m deep reservoir. This group can be considered as the least affected by fractionation processes due to interaction with groundwater. The gases of the second group due to their interaction with shallower unsaturated aquifers, become progressively enriched in less soluble gases (He and CH4). Finally, the third group represents residual gas phases after extensive degassing of the groundwater during its hydrological pathway.

Κατά την περίοδο 2004 με 2017, συλλέχθηκαν περισσότερα από 350 δείγματα ελεύθερων και διαλελυμένων αερίων στην Ελλάδα. Μαζί με αυτά συνυπολογίστηκαν και βιβλιογραφικά δεδομένα. Για καλύτερη κατανόηση της μελέτης, η χώρα διαιρέθηκε σε τέσσερις γεωλογικές ενότητες (Εξωτερικές [EH] και Εσωτερικές [IH] Ελληνίδες, Ελληνική Ενδοχώρα [HH] και ενεργό ηφαιστειακό τόξο [VA]) και με βάση τον συγκεκριμένο διαχωρισμό ερευνήθηκε η πιθανή σχέση της κύριας γεωχημικής σύστασης των αερίων με τα διάφορα γεωλογικά και γεωδυναμικά καθεστώτα των περιοχών δειγματοληψίας. Τα δείγματα αναλύθηκαν για την χημική (O2, N2, CH4, CO2, He, Ne, Ar, H2, H2S και C2-C6 υδρογονάνθρακες) και ισοτοπική τους (R/RA, δ13C-CO2, δ13C-CH4 and δ2H-CH4) σύσταση. Οι συγκεντρώσεις κυμαίνονταν από 0.10 έως 3370 μmol/mol για το He, 600 έως 995,000 μmol/mol για το N2, 0.60 έως 915,000 μmol/mol για το CH4 και 17 έως 1,000,000 μmol/mol για το CO2, ενώ οι ισοτοπικές τους τιμές από 0.01 έως 7.10 R/RA, -29.91 έως +6.00 vs. V-PDB για το δ13C-CO2, -79.8 έως +45.0‰ vs. V-PDB για το δ13C-CH4 και -311 έως +301‰ vs. V-SMOW για το δ2H-CH4. Λαμβάνοντας υπ’όψιν τις τιμές των R/RA και 4He/20Ne, υπολογίστηκε η συνεισφορά της ατμόσφαιρας, του μανδύα και του φλοιού για το He. Η μεγαλύτερη μανδυακή συνεισφορά (έως και 90%) βρέθηκε στο VA, ενώ η χαμηλότερη (0-20%) στο EH. Η συνεισφορά της ατμόσφαιρας ήταν σχετικά αμελητέα. Σύμφωνα με την γεωγραφική κατανομή των αερίων, είναι εμφανές πως το R/RA αυξάνεται σε περιοχές που χαρακτηρίζονται από: i) λεπτό φλοιό; ii) αυξημένες τιμές ροής θερμότητας; iii) πρόσφατη (Πλειστόκενο-Τεταρτογενής) ηφαιστειακή δραστηριότητα; και iv) τοπικά εκτατικά ή διασταλτικά ρήγματα. Οι υψηλότερες τιμές βρέθηκαν κατά μήκος του VA και οι χαμηλότερες στις EH. Επιπλέον, με βάση τις τιμές των CO2/3He και δ13C-CO2, υπολογίστηκε η συνεισφορά των Ιζηματογενών, Μανδυακών και Ασβεστολιθικών end-members του CO2. Οι πλειοψηφία των δειγμάτων παρουσίασε μία κυρίως ασβεστολιθική σύσταση για τον C, ενώ μόνο λίγα δείγματα έδειξαν μανδυακή σύσταση. Παρ’όλα αυτά, με τα υπάρχοντα δεδομένα, είναι αδύνατος ο διαχωρισμός του CO2 που προέρχεται από τους ασβεστολίθους του φλοιού και από εκείνους της υπό βύθισης πλάκας. Επί προσθέτως, λόγω της σύνθετης γεωδυναμικής ιστορίας, η ισοτοπική σύσταση του μανδυακού C θα μπορούσε να είναι επηρεασμένη από μετασωμάτωση συσχετιζόμενη με την βύθιση της πλάκας, όπως συμβαίνει και στην γειτνιακή περιοχή της Ιταλίας, κάνοντας την ισοτοπική του σύσταση πιο θετική. Σε τέτοια περίπτωση, η συνεισφορά του μανδύα θεωρείται ότι έχει υποτιμηθεί. Κάποια δείγματα παραθέτουν πολύ χαμηλες τιμές CO2/3He και δ13CO2 λόγω απώλειας του CO2, η οποία έχει προκληθεί είτε από διάλυση του CO2 σε υπόγεια νερά μικρού βάθους είτε από την καθίζηση του ασβεστίτη που λαμβάνει μέρος στε ορισμένες θερμές πηγές. Από την άλλη μεριά, οι τιμές του CH4/(C2H6+C3H8) (από 1.5 έως 93,200) σε συνδιασμό με τα ισοτοπικά χαρακτηριστικά του CH4, συνιστούν πως οι ελαφριές αλκάνες προέρχονται από διαφορετική αρχική πηγή και αρκετές φορές επηρεάζονται από δευτερογενή διαδικασίες. Στα αέρια των EH παρατηρείται μια σχετικά αποκλειστική βιοτική, κυρίως μικροβιακή, προέλευση για το CH4. Τα αέρια των ψυχρών πηγών των IH έχουν κυρίως θερμογενή προέλευση, αν και κάποια από αυτά συνδέονται με ηπειρωτικές σερπεντινιώσεις και μοιάζει να έχουν αβιοτική προέλευση. Το CH4 στις θερμές πηγές των IH, HH and VA και στα φουμαρολικά αέρια του VA φαίνεται να είναι κυρίως αβιοτικό, αν και τα χημικά και ισοτοπικά χαρακτηριστικά του μοιάζει να έχουν επηρεαστεί από δευτερογενή οξείδωση του CH4 θερμογενούς προέλευσης. Τέλος, κάποια από τα δείγματα παρουσιάζουν αρκετά θετικές ισοτοπικές τιμές (δ13C-CH4 έως +45.0‰ vs. V-PDB και δ2H-CH4 έως +301‰ vs. V-SMOW) πιθανότατα λόγω οξείδωσής τους από μικρόβια. In the period from 2004 to 2017, more than 350 samples of free and dissolved gases were collected along the whole Hellenic area. Some literature data have also been taken into consideration. For a better comprehension of this study, Greece was subdivided in four geologic units (External [EH], Internal [IH] Hellenides, Hellenic Hinterland [HH] and active Volcanic Arc [VA]) and based on that division, I investigate the possible relationship of the main geochemical composition of the gases with the different geological and geodynamical settings of the sampling sites. Samples have been analysed for their chemical (O2, N2, CH4, CO2, He, Ne, Ar, H2, H2S and C2-C6 hydrocarbons) and isotope (R/RA, δ13C-CO2, δ13C-CH4 and δ2H-CH4) composition. The concentrations range from 0.10 to 3370 μmol/mol for He, 600 to 995,000 μmol/mol for N2, 0.60 to 915,000 μmol/mol for CH4 and 17 to 1,000,000 μmol/mol for CO2, whereas the isotope values range from 0.01 to 7.10 for R/RA, -29.91 to +6.00 vs. V-PDB for δ13C-CO2, -79.8 to +45.0‰ vs. V-PDB for δ13C-CH4 and -311 to +301‰ vs. V-SMOW for δ2H-CH4. Considering the R/RA and 4 He/20Ne ratios the atmospheric, mantle and crustal contributions for He have been calculated. The highest mantle contribution (up to 90%) is found in the VA, whereas the lowest in continental Greece (0-20%). Atmospheric contribution is mostly negligible. Taking into consideration the geographical distribution of the gases, it is evident that the R/RA increases in areas characterised by: i) thin crust; ii) elevated heat flow values; iii) recent (PleistoceneQuaternary) volcanic activity; and iv) deep routed extensional or transtensional regional faults. The highest values are therefore found along VA and the lowest in EH. Furthermore, based on the CO2/3He and δ13C-CO2 values, the contribution of Sediment, Mantle and Limestone endmembers for CO2 was determined. The majority of the collected samples present a prevailing limestone C component and only few samples have a prevailing mantle C component. However, with the present data, it is not possible to distinguish CO2 deriving from crustal and slab-related limestones. Additionally, due to the complex geodynamic history, the mantle C isotope composition could be affected by subduction-related metasomatism and, similarly to the nearby Italian area, the C isotope composition could be more positive. In this case, the mantle contribution is probably underestimated. Some samples display very low CO2/3He and δ13CO2 values due to the CO2 loss caused either by dissolution of CO2 in shallow groundwater or by the calcite precipitation that is taking place in most of the thermal springs. On the other hand, the CH4/(C2H6+C3H8) ratios (from 1.5 to 93,200) coupled with CH4 isotopic features, suggest the light alkanes derive from a different primary source and are sometimes affected by secondary processes. An almost exclusive biotic, mainly microbial, origin of CH4 can be attributed to EH gases. Cold gases at IH have mainly a thermogenic origin, although some gases connected to continental serpentinization may have an abiogenic origin. Methane in gases bubbling in thermal waters of IH, HH and VA and fumarolic gases of the VA seem to have a prevailing abiogenic origin, although their chemical and isotopic characteristics may have been produced by secondary oxidation of thermogenic CH4. Finally, in some of the sampled gases, the isotopic values are extremely positive (δ13C-CH4 up to +45.0‰ vs. V-PDB and δ2H-CH4 up to +301‰ vs. V-SMOW) most probably caused by microbial oxidation.

Greece is located on a convergent plate boundary comprising the subduction of the African Plate beneath the Eurasian, while the Arabian plate approaches the Eurasian in a northwestward motion. It is considered to be one of the most tectonically active regions of Earth with a complex geodynamic setting, deriving from a long and complicated geological history. Due to this specific geological background, conditions for the formation of many thermal springs are favoured. In the past years, almost all the already known sites of degassing (fumaroles, soil gases, mofettes, gas bubbling in cold and thermal waters) located in the Hellenic area were sampled at least one time. Collected samples were analysed for their chemical (He, Ne, Ar, O2, N2, H2, H2S, CO, CH4 and CO2) and isotopic composition (He, C and N). Some of these sites have been selected for systematic sampling. Four of them have records longer than 10 years with tens of samplings also considering some literature data. Two of the sites are located in active volcanic areas (Santorini and Nisyros) while the other two are close to actively spreading graben structures with intense seismic activity (Gulf of Korinth and Sperchios basin). Results allowed to define long term background values and also some interesting variation related to seismic or volcanic activity.

Volcanic emissions represent one of the most relevant natural sources of trace elements to the troposphere. Due to their potential toxicity, they may have important environmental impacts from local to global scale. They can also severely affect the atmospheric and terrestrial environment at timescales ranging from a few to millions of years. Mt. Etna volcano is known as one of the largest global contributors of magmatic gases (CO2, SO2 and halogens) and particulate matter, including some toxic trace elements. The aim of this study is to characterize the chemical composition and the mineralogical features of the volcanogenic aerosol passively emitted from Mt. Etna. Twenty-five samples were collected by filtration technique from different sites between 2008 and 2014. Chemical and mineralogical analyses allowed to discriminate two main constituents: the first is mainly referable to the silicate component in the volcanic plume, like lithic, juvenile fragments or glass shards and crystals (e.g. plagioclases, pyroxenes, oxides); the second constituent consists of soluble compounds like sulphosalts or halide minerals (sulphates, chlorides and fluorides). Fluxes of major and trace metals emitted in the atmosphere have been estimated. By comparing the Etnean trace elements with those from European anthropic emissions, we conclude that Mt. Etna is the main persistent point source of major and trace metals in the Mediterranean region. Results gathered from this investigation is of fundamental importance due to the exposure and potential impact of harmful chemical compounds for hundred thousand tourist visits each year to the summit of Mt. Etna.

Το σύνθετο γεωδυναμικό καθεστώς της Ελλάδας πηγάζει από την πολύπλοκη γεωλογική της ιστορία, η οποία χαρακτηρίζεται από έντονη σεισμική δραστηριότητα και ενισχυμένη γεωθερμική βαθμίδα. Αυτή η δραστηριότητα σε συνδυασμό με το ενεργό ηφαιστειακό τόξο, ευνοεί την ύπαρξη πολλών εκπομπών αερίων, με αποτέλεσμα την κατηγοριοποίηση σε CO2-, N2- και CH4-αέρια, ανάλογα με το επικρατών είδος. Η παρούσα εργασία επικεντρώνεται στο μεθάνιο και τους ελαφρούς υδρογονάνθρακες (C2-C6) με σκοπό τον προσδιορισμό της προέλευσή τους. Οι συγκεντρώσεις του CH4 (, Greece has a very complex geodynamic setting deriving from a long and complicated geological history being characterized by intense seismic activity and enhanced geothermal gradient. This activity, with the contribution of an active volcanic arc, favours the existence of many gas manifestations. Depending on the prevailing gas species, the latter can be subdivided in three main groups: CO2-, N2- and CH4- dominated. In the present work, we focus on methane and light hydrocarbons (C2-C6) to define their origin. CH4 concentrations (

Like other geodynamically active areas, Greece is affected by a large number of geogenic gas manifestations. These occur either in form of point sources (fumaroles, mofettes, bubbling gases) or as diffuse emanations. We produced a catalogue of the geogenic gas manifestations of Greece also considering few literature data. Collected samples were analysed for their chemical (He, Ne, Ar, O2, N2, H2, H2S, CO, CH4 and CO2) and isotopic composition (He, C and N). Geogenic gases, apart from having important influences on the global climate, could have strong impact on human health. Gas hazard is often disregarded because fatal episodes are often not correctly attributed. Geodynamic active areas release geogenic gases for million years over wide areas and the potential risks should not be disregarded. A preliminary estimation of the gas hazard has been made for the last 20 years considering the whole population of Greece. In this period at least 2 fatal episodes with a total of 3 victims could be certainly attributed to CO2. This would give a risk of 1.310-8 fatality per annum. Such value, probably underestimated, is much lower than most other natural or anthropogenic risks. Nevertheless this risk, being unevenly distributed along the whole territory, should not be overlooked and better constrained in areas with high density of gas manifestations and high soil gas fluxes.

Quantification of gaseous emissions in geological systems is an important branch because it is a major source of greenhouse gas to the atmospheric budget. Of geological environments, there are two different categories: the first category includes emissions of the predominant carbon dioxide (CO2), while the second includes emissions of the predominant methane (CH4). The Hellenic territory has a very complex geodynamic setting deriving from a long and complicated geological history. It is strongly characterized by intense seismic activity and enhanced geothermal gradient. This activity, with the contribution of an active volcanic arc, favours the existence of many cold and thermal gas manifestations. Geogenic sources release huge amounts of gases, which, apart from having important influences on the global climate, could also have a strong impact on human health. Geochemical studies based on the isotopic composition of carbon and hydrogen, along with helium isotopic ratios have become a good indicator of the origin of the gas. The isotopic ratio 13C/12C of CO2 expressed in _ 13C (h, provides important information about the amount of CO2 released from the Earth’s crust or mantle. For methane, carbon and hydrogen isotopic compositions and C1/(C2+C3) hydrocarbon ratios can characterize the origin of methane: biogenic (thermogenic or microbial) or abiogenic. Helium isotopic ratios provide additional information about crustal or mantle origin of the gas. In the present work, a large set of chemical and isotopic data is presented aiming at the identification of areas with geogenic gas emissions and their characterization in terms of different gas composition and origin. The present catalogue should be the base for the estimation total nationwide geogenic CO2 and CH4 fluxes.

In the winter 2013, mercury concentrations in air from the mine-polluted area of Mt. Amiata (1738 m a.s.l.), in southern Tuscany (Central Italy), were measured by active moss biomonitoring. This area is part of the geologic anomaly of Hg in the Mediterranean basin, which contains about 65 % of the world’s cinnabar (HgS). Mt. Amiata covers some 400 km2 and is drained by several rivers. Esploitation activity at Abbadia S. Salvatore, in the SE sector of the mountain, sprang up during the 19th century as one of the largest mercury mining and smelting plants in Europe, after those of Almaden Spain. In this area, Sphagnum moss bags were exposed for about two months, from October to December 2013. At each site (10 sites), one covered and one uncovered moss bag were deployed. Concentrations of mercury in air were also investigated in the same sites with a portable spectrophotometer (Lumex RA-915M). After exposure, mosses were oven-dried, grinded and each sample was divided in two aliquots: one was analyzed for mercury by using a Hydra C cold vapor atomic absorption analyzer (INGV-Palermo), following 7473 US EPA method; the second was microwave digested in acid solution (HNO3 + H2O2). Extraction solutions were analyzed by ICP-MS for total concentrations of a large suite of trace elements, including potentially toxic elements e.g. As, Cd, Cr, Cu, Mo, Sb, Se, V. Mercury air concentrations measured with the Lumex showed extremely high values in the mine district of Abbadia, with median values ranging from 2,000 to 4,000 ng/m3 and maximum values up to 20,000 ng/m3, in contrast with the lower values (median values from 20 to 200 ng/m3) measured in the distal sites few kilometres from the mine-district area. In agreement with these results, in the vicinity of the district uncovered bags were in the range of 10,000 – 100,000 ng/g of Hg, whereas in the distal sites they were in the range of 1,000 – 10,000 ng/g. The moss-blank (unexposed moss) was ~100 ng/g. Covered moss bags were not significantly enriched in Hg with respect to the concentrations recovered from the moss-blank, suggesting that the mercury trapped in the mosses was mainly in particulate form. The particles carried from the winds were probably associated with soils re-mobilization, as also confirmed by the associated enrichments of some lithophile elements (Li and lanthanides) and anthropogenic element (As, Cr, Cd, Fe, Se, V). These preliminary results confirm the intense contamination of the study area not only for mercury but also for other potentially toxic elements.

Nisyros Island, Greece, is a stratovolcano known for its intense hydrothermal activity. On June 2013, during a multidisciplinary field campaign, soil samples were collected in the caldera area to determinate the main mineralogical paragenesis and to investigate the distribution of trace elements concentrations and the possible relationship to the contribution of deep originated fluids. Soil samples were analysed with XRD and for the chemical composition of their leachable (deionized water) and near total (microwave digestion) fraction both for major and trace elements. All data were compared to a local background soil. The results allow to divide the samples in 2 groups: Lakki Plain and Stefanos Crater. The latter, where a fumarolic area is located, shows a mineralogical paragenesis dominated by phases typical of hydrothermal alteration. Their very low pH values (1.9 – 3.4) evidence the strong impact of fumarolic gases which are probably also the cause of strong enrichments in these soils of highly volatile elements like S, As, Se, Bi, Sb, Tl and Te. The soils of the Lakki Plain, being less exposed to the fumarolic activity, show minearalogical and chemical characteristics intermediate between the background soil and those of Stefanos Crater.

The Aegean area, like other regions of intense geodynamic activity, is characterised by extensive geogenic degassing. Gas manifestations widespread on land are also very frequent underwater. The present study aims at producing a first catalogue of these manifestations together with the geochemical characterization of the emitted gases. Sixty-one samples at 21 different sites have been collected by diving up to 15 m depth, mainly along the south Aegean active volcanic arc (SAAVA). The sites displayed very different gas fluxes. Most of them showed a very sluggish gas bubbling while a few had spatially large and strong bubbling. The most intense manifestation has been found at Kos Island along Paradise beach. Here a total output of about 2.7 tons/day from an area of about 250 m2 has been measured in September 2016. Samples have been analysed for their chemical (He, Ne, Ar, O2, N2, H2, H2S, CH4 and CO2) and isotopic (He, Ar, CO2-C, CH4-C, CH4-H) composition by gas chromatography and mass-spectrometry methodologies.