CO2-rich Springs Research Articles

СO2-rich thermal waters of the Neitrino Research Gallery (Baksan Neitrino Observatory, North Caucasus)

Isotope-geochemical (δ2H and δ18O in H2O, δ15N in N2, δ13C in CO2 and CH4,3He/4He) studies of СО2-rich thermal waters (up to 41.3oC) which located in 4 km long tonnel of the Baksan Neutrino Observatory were carried out. The δ13C(CO2) values (–8.0… –6.4‰) indicate the volcanogenic genesis of CO2. The presence of the volcanogenic component is also emphasized by high values of 3He/4He (346 × 10–8). At the same time, nitrogen present in the gas phase is characterized by values of δ15N = +1.3‰, indicating its crustal genesis. In the tunnel, an elevated concentration of CH4 (up to 0.5%) was noted in one of the gas outlets. This methane is characterized by high δ13C values (–33.5 and –26.0‰), which, in general, are typical for other СО2-rich springs of the Eastern Elbrus Mountains. In contrast with the other CO2-rich springs of the North Caucasus, which are practically indistinguishable from surface waters in terms of δ2H and δ18O values, the CO2-rich thermal springs of the tunnel are noticeably enriched in heavy oxygen isotope (18O). This is a consequence of oxygen exchange between the waters and the host rocks at elevated temperatures. In the δ18O-δ2H diagram, the figurative points of thermal waters form a trend that reflects the mixing of isotope-light infiltration waters with isotope-heavy waters.

CO<sub>2</sub> hypogene speleogenesis in an endogenic travertine system, Mesa del Oro, New Mexico, USA

The Mesa del Oro travertine, deposited by upwelling CO2-rich spring waters, is a large and thick mantle of calcite overlying soft Triassic Chinle Formation mudstones in western central New Mexico, USA. The travertine contains a system of small caves known as the Pronoun Cave Complex. Whut Cave is the largest known cave in the complex. While the travertine hydrologic system is no longer active, there is ample evidence that Whut Cave was formed by hypogene speleogenesis. The Whut Cave entrance is a small roughly circular shaft that descends ~four meters and intersects a linear fissure-controlled passage. Mn-oxide was once mined below the entrance, leaving a five-meter-deep pit that represents the path of upwelling waters that were likely charged with CO2 and supersaturated with CaCO3 once reaching the surface. The roomiest part of the cave is immediately adjacent to the entrance, defining where most of the upwelling of groundwater and gases took place. Gypsum and other sulfates were only noted microscopically. A sample of travertine at the surface near the entrance yielded a uranium-series age beyond the limit of the method and a δ234U age of 735 ± 109 ka BP (before present). A piece of breccia vein in the mine dump associated with the Mn-oxide ore, and a rill of a drape-like crust on the cave wall, both interpreted to be products of hypogene speleogenesis and representing the oldest speleogenetic material, yielded δ234U ages of 848 ± 110 and 723 ± 109 ka BP. Altogether, materials interpreted to be speleogenetic yielded ages of ~900 – 300 ka BP. Although the cave is dry and hydrologically inactive today, cave development by CO2 hypogene speleogenesis was coeval with endogenic travertine deposition and nearby volcanism. Our interpretation suggests that all these events are linked.

Controls on riverine calcium isotope ratios during basalt weathering in the Skagafjörður watershed, Iceland

This study examines the Ca isotope (δ44/40Ca) geochemistry of Icelandic rivers with the overall aims of improving the δ44/40Ca tracer, constraining solute sources, and addressing the hypothesis that basalt weathering disproportionately regulates Earth’s long-term climate. We report Ca isotope and elemental data for mainstem rivers, tributary streams, thermal and non-thermal springs, basalt, calcite, soil, vegetation, and sediment collected from the Skagafjörður region in North Iceland. Waters and calcite were further analyzed for their carbon isotope (δ13C) composition, and we conducted experiments to characterize δ44/40Ca values of riverine colloids, the clay-size fraction (<2 µm) of soil, and exchangeable leachates obtained from soil and bedload sediment. The dataset includes analyses of travertine and coexisting water samples collected from a CO2-rich spring (Hvanná river) in South Iceland.Samples show a ∼2‰ range, with groundwater, the ultrafiltered fraction of river water, and hydrothermal calcite producing among the highest δ44/40Ca values and travertine, soil exchangeable Ca, and vegetation the lowest. Riverine δ44/40Ca values are on average ∼0.20‰ higher than those for basalt, which shows minimal isotopic variability. Mainstem glacial-fed rivers entering the Skagafjörður valley from the soil- and vegetation-free highlands have higher δ44/40Ca values than direct-runoff tributaries draining catchments to the north where pedogenesis is more pervasive. Riverine δ44/40Ca values correlate with Sr/Ca and Na/Ca ratios, as well as the saturation index of calcite (SIcal). Riverine δ13C values increase from ∼−8‰ (atmospheric) to ∼−3‰ (calcite) as SIcal values increase from highly-undersaturated to near-equilibrium. One watershed (Svartá river) shows unique trends, where riverine δ44/40Ca values and Sr/Ca ratios do not correlate, and soil exchangeable leachates produced anomalously high amounts of Ca. Clay-size fractions isolated from three horizons composing a brown andosol show slightly lower δ44/40Ca values than bulk basalt, but the values overlap with those for primary minerals. Colloids appear to have lower δ44/40Ca values than truly dissolved Ca. The proxy could be developed as a tracer for colloidal contributions to riverine dissolved loads, especially in soil-mantled catchments where the inputs appear most pronounced. Groundwater δ44/40Ca values correlate with pH, temperature, distance from the coast, and Sr/Ca ratios, all consistent with fractionation control yielding varying degrees of geochemical evolution. The combined riverine patterns largely reflect three-component mixing between basalt weathering, calcite weathering, and hydrothermal water inputs. Subsurface Ca isotope fractionation indirectly impacts riverine δ44/40Ca values by elevating the δ44/40Ca values of groundwater and hydrothermal calcite. This study highlights the significance of hydrothermal water inputs of Ca and HCO3 to Icelandic rivers, which have been previously underappreciated. Mixing calculations suggest that a maximum of ∼20% of the Ca in mainstem rivers derives from surficial basalt weathering. Small tributaries draining the flanks of the valley show the clearest signals of basalt weathering by atmospheric CO2, but these waters have much lower solute concentrations than those employed in previous attempts to estimate basalt weathering rates and parameterize the climate sensitivity of the basalt weathering feedback.

Radiological investigation of natural carbonated spring waters from Eastern Carpathians, Romania

The current study presents a radiological water-quality assessment on 64 spring water samples from four Romanian counties. The study area is abundant in CO2-rich spring waters consumed by locals and tourists. Gross alpha activities ranged between 21 ± 2 and 7530 ± 658 mBq L−1, with 27% of the samples exceeding the WHO threshold. Gross beta values ranged from 40 ± 2 to 5520 ± 430 mBq L−1, with 29% exceeding the recommended values. Radionuclide activities fluctuated between 0.6 ± 0.08 and 81 ± 6 Bq L−1 for 222Rn, 15 ± 2 to 1154 ± 112 mBq L−1 for 226Ra, and from 18 ± 2 to 64 ± 5 mBq L−1 for 210Po. The annual effective doses attributed to radium varied between 0.002 and 0.23 mSv yr−1.

Mineralogical and hydrogeological study of "pouhons" in the lower Palaeozoic formations of the Stavelot-Venn Massif, Belgium

Numerous naturally CO2-rich mineral water springs, locally called pouhons, occur in the Stavelot-Venn Massif. These water springs show a particular composition with a high content of iron, manganese and lithium, and are characterised by a red-orange colour resulting from iron hydroxide precipitation near the land surface. Radon measurements have shown that these ferruginous deposits are weakly radioactive. The Upper Cambrian black shales of the La Gleize Formation are also known to display radioactive anomalies. These rocks show enrichment in HFSE (Pb, U, Y, Ce, Zr, Ti, Nb) and are depleted in transition metals (Co, Ni, Cu, Zn). Specific minerals such as florencite-(Ce), monazite-(Ce), xenotime-(Y) and zircon have been identified and are probably at the origin of the radioactive anomalies. Uranium was gradually leached from these minerals, transported in solution, and finally concentrated in ferruginous muds. These muds are mainly composed of goethite (most often amorphous), residual quartz and calcite in some samples. The most probable hypothesis is that uranium is adsorbed in small concentrations on the goethite surface. On the other hand, the Ottré Formation (Ordovician) appears to be the main source of lithium, iron and manganese. Pouhon waters have therefore probably leached rocks of various mineralogy and chemical composition during their sub-surface circulation.

Hydrogeochemistry and Hydrogeological Model of CO2-rich Springs on Buryatia Territory (Russia)

The work presents the results of isotopic-geochemical and hydrodynamic studies of CO2-rich springs of the Tunka Valley, Buryatia. The waters are remarkably diverse in terms of cationic composition, although the dominant anion is hydrocarbonate. In general, the springs studied are characterized by high concentrations of microcomponents. To form these springs, the presence of active deep-seated faults is necessary. Such fractured zones serve as channels bringing gas to the upper horizons of groundwater. The 2D numerical model presented convincingly proves that the Arshan field’s waters are meteoric waters of long-term water circulation (up to 220,000 years).

Hydrochemical, isotopic and microbiota characterization of telese mineral waters (Southern Italy)

The study deals with the analyses of springs and wells at the base of Montepugliano Hill that represents the SE edge of the wide carbonate Matese massif (Campania, southern Italy). At the base of the hill, from west to east and for almost one kilometre, cold springs HCO3-Ca type (Grassano springs, ~ 4.5 m3/s; TDS: about 0.45 g/L) pass to hypothermal, HCO3-Ca type, sulphurous and CO2-rich springs (~ 1 m3/s with TDS > 1 g/L). Some of the latter are widely used in Telese Spa and Centro Relax Spa. Chemical and isotopic analyses carried out for this study support the hypothesis that all these waters (mineral and non-mineral) have the same catchment area, which is located in the Matese massif. As regards the sulphurous springs, they receive both meteoric waters infiltration and uprising of deeper waters rich in endogenous CO2 and H2S gases through important faults systems. Far from these faults, the chemistry of groundwater is scarcely (or not at all) affected by these deep fluid enrichment processes. This scheme is very significant; in fact, when very important groundwater resources are present, it is possible to use both mineral waters in Spa and, in areas far from the faults, those not yet mineralized. Finally, at Montepugliano Hill, in the final stage of the flow path, groundwater is also affected by change in the microbiome: this could provide a basis for comparison between various mineral waters.

Hydrochemical Parameters to Assess the Evolutionary Process of CO2-Rich Spring Water: A Suggestion for Evaluating CO2 Leakage Stages in Silicate Rocks

Eighteen water samples collected from eight CO2-rich springs in the northern part of the Gyeongsang sedimentary basin (GSB), South Korea showed distinct hydrochemistry, in particular, pH, total dissolved solids (TDS), and Na contents, and they were classified into four groups: (1) Group I with low pH (average of 5.14) and TDS (269.8 mg/L), (2) Group II with high TDS (2681.0 mg/L) and Na-enriched (202.9 mg/L), (3) Group III with intermediate Na content (97.5 mg/L), and (4) Group IV with Na-depleted (42.3 mg/L). However, they showed the similar partial pressure of CO2 (0.47 to 2.19 atm) and stable carbon isotope ratios of dissolved inorganic carbon (−6.3 to −0.6‰), indicating the inflow of deep-seated CO2 into aquifers along faults. In order to elucidate the evolutionary process for each group of CO2-rich springs, a multidisciplinary approach was used combining stable hydrogen (δD), oxygen (δ18O) and carbon (δ13C), and radioactive carbon (14C) isotopic, geophysical, and hydrochemical data. The highest δD and δ18O ratios of water and the relatively young 14C ages in Group I and the lowest δD and δ18O in Group II indicated the short and long residence time in Group I and II, respectively. The electrical resistivity tomography (ERT) survey results also supported the fast rising through open fractures in Group I, while a relatively deep CO2-rich aquifer for Group III. Group II had high contents of Mg, K, F, Cl, SO4, HCO3, Li, and As, while Group I showed low contents for all elements analyzed in this study except for Al, which exceeded the World Health Organization (WHO) guideline for drinking-water quality probably due to the low pH. Meanwhile Group IV showed the highest Ca/Na as well as Ca, Fe, Mn, Sr, Zn, U, and Ba, probably due to the low-temperature dissolution of plagioclase based on the geology and the ERT result. The levels of Fe, Mn, and U exceeded the WHO guidelines in Group IV, while As in Group II. The different hydrochemistry suggests a distinct evolutionary process for each group. Group I seems to represent a fast discharge from the CO2-rich aquifer to a discharge point, experiencing a low degree of water-rock interaction, while Group II seems to represent a slow discharge with a high degree of water-rock interaction. GSB is a potential site for geological carbon storage (GCS), and injected CO2 may leak through various evolutionary processes given heterogenous geology as CO2-rich springs. The study result suggests that the combined use of pH, Na, K, Li, and Ca/Na are effective hydrochemical monitoring parameters to assess the leakage stage in silicate rocks in GCS projects. Besides, aluminum (Al) can be risky at the early stage of CO2 leakage, while Fe, Mn, U, and As at the later stage of CO2 leakage.

A review of the geology and origin of CO2 in mineral water springs in east Belgium

Naturally CO2-rich mineral water springs (pouhons) in east Belgium occur in the context of the Rhenohercynian domain of the Variscan fold-and-thrust belt, mostly within the Cambro-Ordovician Stavelot-Venn Massif. The origin of the CO2 is still unclear, although different hypotheses exist. In this review study, we show pouhon waters are of the calcium bicarbonate type (~310 mg/l HCO3- on average), with notable Fe (~15 mg/l) and some Ca (~43 mg/l). Pouhon waters are primarily meteoric waters, as evidenced by H and O isotopic signature. The δ13C of CO2 varies from -7.8 to +0.8‰ and contains up to ~15% He from magmatic origin, reflecting a combination of carbonate rocks and mantle as CO2 sources at depth. Dinantian and Middle Devonian carbonates at 2–6 km depth could be potential sources, with CO2 generated by dissolution. However, carbonates below the Stavelot-Venn Massif are only predicted by structural models that assume in-sequence thrusting, not by the more generally accepted out-of-sequence thrust models. The mantle CO2 might originate from degassing of the Eifel magmatic plume or an unknown shallower magmatic reservoir. Deep rooted faults are thought to act as preferential pathways. Overall low temperatures of pouhons (~10 °C) and short estimated residence times (up to 60 years) suggest magmatic CO2 is transported upwards to meet infiltrating groundwater at shallower depths, with partial to full isotopic exchange with carbonate rocks along its path, resulting in mixed magmatic-carbonate signature. Although the precise role and interaction of the involved subsurface processes remains debatable, this review study provides a baseline for future investigations.

Quantitative assessment of deep-seated CO2 leakage around CO2-rich springs with low soil CO2 efflux using end-member mixing analysis and carbon isotopes

This study examined a mountainous area with two hydrochemically distinct CO2-rich springs to understand the origin, flow, and leakage of CO2, which may provide implications for precise monitoring of CO2 leakage in geological carbon storage (GCS) sites. The carbon isotopic compositions of dissolved inorganic carbon (DIC) in CO2-rich water (δ13CDIC) and those of soil CO2 (δ13CCO2) indicated a deep-seated CO2 supply to the near-surface environment in the study area. The hydrochemical difference (e.g. pH, total dissolved solids) for the two CO2-rich springs separated by 7 m, despite similar δ13CDIC and partial pressure of CO2, was considered as the result of different evolution of shallow groundwater affected by deep-seated CO2 preferentially rising along fracture zones. Electrical resistivity tomography also suggested flow through fracture zones beneath the CO2-rich springs, showing low resistivity compared to other surveyed zones. However, soil CO2 efflux was low compared to that in other natural CO2 emission sites, and in particular it was noticeably low near the CO2-rich springs, whereas δ13CCO2 was high close the CO2-rich springs. The dissolution of CO2 in the near-surface water body seemed to decrease the deep-seated CO2 leakage through the soil layer, while δ13CCO2 imprinted the source. End-member mixing analysis was performed to assess the contribution of deep-seated CO2 to the low soil CO2 efflux by assuming that atmospheric CO2 and soil CO2 (by respiration) as well as deep-seated CO2 contribute to the soil CO2 efflux. For each end-member, characteristic δ13CCO2 and CO2 concentrations were defined, and then their apportionment to soil CO2 efflux was estimated. The resultant proportion of deep-seated CO2 was up to 8.8%. Unlike the spatial distribution of high soil CO2 efflux, high proportions exceeding 3% were found around the CO2-rich springs along the east-west valley. The study results indicate that soil CO2 efflux measurement should be combined with carbon isotopic analysis in GCS sites for CO2 leakage detection because CO2 dissolution in the underground water body may blur leakage detection on the surface. The implication of this study is the need to quantitatively assess the contribution of deep-seated CO2 using the soil CO2 concentration, soil CO2 efflux, and δ13CCO2 at each measurement site.

Constraints on the hydrogeochemistry and origin of the CO2-rich mineral waters from the Eastern Carpathians – Transylvanian Basin boundary (Romania)

The Eastern Carpathians, along the Neogene to Quaternary volcanic chain, host important natural resources of mineral waters. Most of these are in the form of CO2-rich natural springs of different types, associated with CO2 gas emissions. In order to understand the origin of these waters, the natural processes responsible for the observed physical–chemical features and to check the exposure and vulnerability of the hydrogeological system to potential contamination and environmental changes, 121 mineral water springs and wells were investigated. The investigations performed, offered an insight on the processes affecting the waters after their infiltration that led to a wide range of chemical and isotopic compositions. The diversity in the geological structure of the area, alteration processes and interaction with CO2, all explain the variety of the hydrogeochemical facies of the investigated waters. Three major hydrogeochemical types that are the result of the geological variety of the study area and water–rock interaction were identified: Ca–Na–Mg–HCO3, Ca–Na–Mg/Na–Ca ± Mg–HCO3–Cl ± SO4, and Na–Cl waters. Further changes in the geochemical features are attributable to mixing between different water types during their ascent, enhanced by the tectonic setting of the area. Considering the isotopic composition (δ18O, δD) of the waters, most of them proved to be of meteoric origin. Extreme isotopic shifts towards enriched δ18O and δD values of +8.59 and −12.2‰ (V-SMOW) respectively were observed in the case of 11 springs. These shifts were attributed to the interaction of water with clay minerals and diagenesis. As most of these waters are of meteoric origin, their discharge may be influenced by meteorological factors. Moreover, the mixing of aquifers with very shallow waters most likely facilitates their exposure to external contamination.

CO2 release to the atmosphere from thermal springs of Sperchios Basin and northern Euboea (Greece): The contribution of “hidden” degassing

Greece is a region characterised by intense geodynamic activity that favours the circulation of hydrothermal fluids in the crust transporting volatiles from either the deep crust or the mantle to the surface. Elevated heat flow values are detectable at Sperchios Basin and North Euboea (central Greece), two areas defined by a system of deeply rooted extensional faults and Quaternary volcanic activity. This setting contributed to the formation of numerous hydrothermal systems, which are mostly expressed as CO2-rich thermal springs with intense bubbling.The CO2 output from six bubbling pools has been determined by flux measurements with the use of the floating chamber method. The highest bubbling CO2 output has been found at Thermopyles and Psoroneria (0.6 and 0.84 t/d, respectively), where the outgoing channels have an elevated flow (>250 l/s) of gas-charged water (>15 mmol/l of CO2). Although no bubbling is noticed along the stream, the CO2 content decreases by an order of magnitude after few hundreds of metres, indicating an intense degassing from the water. Taking into account the water flow and the amount of CO2 lost to the atmosphere, the CO2 output of the outgoing channels is quantified in 12.5 t/d for Thermopyles and 9.23 t/d for Psoroneria. Output estimation has also been made for the other springs of the area. The diffuse CO2 release from the outgoing channels has always been higher respect to that of the visible bubbling, suggesting that most of the degassing is “hidden”. Furthermore, the loss of CO2 from the water has determined a shift in dissolved carbonate species as demonstrated by the pH increase along the channel that led eventually to an oversaturation in carbonate minerals and therefore travertine deposition.The total CO2 output to the atmosphere of the study area is estimated at ~27 t/d, with the major contribution deriving from the degassing along the outflow channels of the thermal springs. Such output is comparable to that of the single active volcanic systems along the South Aegean Active Volcanic Arc and highlights the importance of “hidden” degassing along CO2-oversaturated streams.

Microbial diversity of two natural CO2-rich springs with contrasting hydrochemical features

Microbial communities in CO2-rich springs have adapted to environmental pressure including low pH resulting from high concentrations of CO2. Microbial community composition and hydrochemical characteristics in the CO2-rich springs are important features affected by high levels of CO2 in groundwater ecosystems, and they may be useful indicators of CO2 leakage from carbon capture and storage (CCS) sites where CO2 is injected into deep geological reservoirs for the mitigation of climate change. To determine how long-term CO2 leakage affects the groundwater ecosystems, we analyzed the bacterial and archaeal community composition and hydrochemical features of two different natural CO2-rich springs, Hansil (HS) and Chosukol (CS), located in south-eastern Korea which are natural analog sites for groundwater ecosystems affected by CO2 leakage from geological CO2 storage sites. Analyses of hydrochemical characteristics and 16S rRNA gene amplicon sequencing of bacteria and archaea revealed a marked distinction between the two springs. The pH, the concentrations of bicarbonate ion (HCO3-), major cations including Ca2+ and Mg2+, and specific trace elements including Fe and Mn were higher in HS as compared to CS while no distinction in pCO2 between the two springs was observed. Microbial community composition was distinct between the two springs at the genus level. Specifically, the relative abundance of Gallionella, an iron-oxidizer, was significantly higher in HS whereas that of Albidiferax known as dissimilatory iron-reducing bacteria was higher in CS than HS, reflecting the difference of iron concentration in the two springs. In our study, microbial community composition appeared to be more strongly affected by the concentration of iron than that of CO2. Our results enhance the understanding of hydrochemical features and microbial communities in natural CO2-rich springs and suggest that microbial communities may be useful indicators of CO2 leakage from CCS sites.

Tracing the migration of mantle CO2 in gas fields and mineral water springs in south-east Australia using noble gas and stable isotopes

Abstract Geochemical monitoring of CO2 storage requires understanding of both innate and introduced fluids in the crust as well as the subsurface processes that can change the geochemical fingerprint of CO2 during injection, storage and any subsequent migration. Here, we analyse a natural analogue of CO2 storage, migration and leakage to the atmosphere, using noble gas and stable isotopes to constrain the effect of these processes on the geochemical fingerprint of the CO2. We present the most comprehensive evidence to date for mantle-sourced CO2 in south-east Australia, including well gas and CO2-rich mineral spring samples from the Otway Basin and Central Victorian Highlands (CVH). 3He/4He ratios in well gases and CO2 springs range from 1.21 to 3.07 RA and 1.23–3.65 RC/RA, respectively. We present chemical fractionation models to explain the observed range of 3He/4He ratios, He, Ne, Ar, Kr, Xe concentrations and δ13C(CO2) values in the springs and the well gases. The variability of 3He/4He in the well gases is controlled by the gas residence time in the reservoir and associated radiogenic 4He accumulation. 3He/4He in CO2 springs decrease away from the main mantle fluid supply conduit. We identify one main pathway for CO2 supply to the surface in the CVH, located near a major fault zone. Solubility fractionation during phase separation is proposed to explain the range in noble gas concentrations and δ13C(CO2) values measured in the mineral spring samples. This process is also responsible for low 3He concentrations and associated high CO2/3He, which are commonly interpreted as evidence for mixing with crustal CO2. The elevated CO2/3He can be explained solely by solubility fractionation without the need to invoke other CO2 sources. The noble gases in the springs and well gases can be traced back to a single end-member which has suffered varying degrees of radiogenic helium accumulation and late stage degassing. This work shows that combined stable and noble gas isotopes in natural gases provide a robust tool for identifying the migration of injected CO2 to the shallow subsurface.

Multimodal geothermal exploration in the Lesser Antilles Arc at the Lamentin lowland (Martinique)

The aim of the last geothermal exploration campaign at the Lamentin lowland (Martinique) was to specify the characteristics and the extent of the local geothermal system. In this purpose, previous data were re-processed and new field data were acquired through geological, gravimetric, electromagnetic (CSEM and TEM) and fluids geochemical surveys. All the collected data were gathered in a 3D geological model leading to a new conceptual model. The Lamentin lowland is considered as an active graben controlled by major NW-SE faults and crossed by a NE-SW accommodating fault system. Recent, restricted and monogenic hydro-volcanism was highlighted, associated to large hydrothermal deposits from a first high temperature (HT) geothermal episode. Associated with thermal and soil gas anomalies and CO2-rich springs, they mark out the main faults. A shallow, thin and very conductive layer (<10 Qm) was mapped by CSEM and TEM, as the caprock of the previous HT geothermal system. Current geothermal fluid results from the mixing of 25-30% of seawater with fresh water flowing likely from the northern relief where annual precipitation exceeds 4 m. It is then heated to 115 ± 25°C and flows in a convective ascent through the major NW-SE fault. Northern Lamentin lowland appears to be a fault-limited and shallow medium enthalpy geothermal system.

Thermoplasmatales and sulfur-oxidizing bacteria dominate the microbial community at the surface water of a CO2-rich hydrothermal spring located in Tenorio Volcano National Park, Costa Rica.

Here we report the chemical and microbial characterization of the surface water of a CO2-rich hydrothermal vent known in Costa Rica as Borbollones, located at Tenorio Volcano National Park. The Borbollones showed a temperature surrounding 60°C, a pH of 2.4 and the gas released has a composition of ~ 97% CO2, ~ 0.07% H2S, ~ 2.3% N2 and ~ 0.12% CH4. Other chemical species such as sulfate and iron were found at high levels with respect to typical fresh water bodies. Analysis by 16S rRNA gene metabarcoding revealed that in Borbollones predominates an archaeon from the order Thermoplasmatales and one bacterium from the genus Sulfurimonas. Other sulfur- (genera Thiomonas, Acidithiobacillus, Sulfuriferula, and Sulfuricurvum) and iron-oxidizing bacteria (genera Sideroxydans, Gallionella, and Ferrovum) were identified. Our results show that CO2-influenced surface water of Borbollones contains microorganisms that are usually found in acid rock drainage environments or sulfur-rich hydrothermal vents. To our knowledge, thisis the first microbiological characterization of a CO2-dominated hydrothermal spring from Central America and expands our understanding of those extreme ecosystems.

Compositional data analysis and geochemical modeling of CO2-water-rock interactions in three provinces of Korea.

The CO2-rich spring water (CSW) occurring naturally in three provinces, Kangwon (KW), Chungbuk (CB), and Gyeongbuk (GB) of South Korea was classified based on its hydrochemical properties using compositional data analysis. Additionally, the geochemical evolution pathways of various CSW were simulated via equilibrium phase modeling (EPM) incorporated in the PHREEQC code. Most of the CSW in the study areas grouped into the Ca-HCO3 water type, but some samples from the KW area were classified as Na-HCO3 water. Interaction with anorthite is likely to be more important than interaction with carbonate minerals for the hydrochemical properties of the CSW in the three areas, indicating that the CSW originated from interactions among magmatic CO2, deep groundwater, and bedrock-forming minerals. Based on the simulation results of PHREEQC EPM, the formation temperatures of the CSW within each area were estimated as 77.8 and 150°C for the Ca-HCO3 and Na-HCO3 types of CSW, respectively, in the KW area; 138.9°C for the CB CSW; and 93.0°C for the GB CSW. Additionally, the mixing ratios between simulated carbonate water and shallow groundwater were adjusted to 1:9-9:1 for the CSW of the GB area and the Ca-HCO3-type CSW of the KW area, indicating that these CSWs were more affected by carbonate water than by shallow groundwater. On the other hand, mixing ratios of 1:9-5:5 and 1:9-3:7 were found for the Na-HCO3-type CSW of the KW area and for the CSW of the CB area, respectively, suggesting a relatively small contribution of carbonate water to these CSWs. This study proposes a systematic, but relatively simple, methodology to simulate the formation of carbonate water in deep environments and the geochemical evolution of CSW. Moreover, the proposed methodology could be applied to predict the behavior of CO2 after its geological storage and to estimate the stability and security of geologically stored CO2.

Microbiological and Geochemical Survey of CO2-Dominated Mofette and Mineral Waters of the Cheb Basin, Czech Republic.

The Cheb Basin (NW Bohemia, Czech Republic) is a shallow, neogene intracontinental basin. It is a non-volcanic region which features frequent earthquake swarms and large-scale diffuse degassing of mantle-derived CO2 at the surface that occurs in the form of CO2-rich mineral springs and wet and dry mofettes. So far, the influence of CO2 degassing onto the microbial communities has been studied for soil environments, but not for aquatic systems. We hypothesized, that deep-trenching CO2 conduits interconnect the subsurface with the surface. This admixture of deep thermal fluids should be reflected in geochemical parameters and in the microbial community compositions. In the present study four mineral water springs and two wet mofettes were investigated through an interdisciplinary survey. The waters were acidic and differed in terms of organic carbon and anion/cation concentrations. Element geochemical and isotope analyses of fluid components were used to verify the origin of the fluids. Prokaryotic communities were characterized through quantitative PCR and Illumina 16S rRNA gene sequencing. Putative chemolithotrophic, anaerobic and microaerophilic organisms connected to sulfur (e.g., Sulfuricurvum, Sulfurimonas) and iron (e.g., Gallionella, Sideroxydans) cycling shaped the core community. Additionally, CO2-influenced waters form an ecosystem containing many taxa that are usually found in marine or terrestrial subsurface ecosystems. Multivariate statistics highlighted the influence of environmental parameters such as pH, Fe2+ concentration and conductivity on species distribution. The hydrochemical and microbiological survey introduces a new perspective on mofettes. Our results support that mofettes are either analogs or rather windows into the deep biosphere and furthermore enable access to deeply buried paleo-sediments.

The CO2 flux from hydrothermal systems of the Karymsky volcanic Centre, Kamchatka

The CO2 flux provided by the hydrothermal activity within the Karymsky Volcanic Centre, Kamchatka, was measured, and the CO2 balance of the Karymsky caldera lake was estimated in the framework of a Deep Carbon Observatory (DCO) project. The Karymsky Volcanic Centre located in the SE of the Kamchatka Peninsula, in the middle of the modern volcanic front, consists of two calderas, hosts a caldera lake and is characterized by hydrothermal activity that is manifested at several thermal fields. Within the Akademii Nauk (AN) caldera which is filled by a caldera lake, the Akademii Nauk springs discharge boiling water into the lake. The lake is drained by the Karymsky River that then crosses the caldera of the Karymsky volcano (Karymsky caldera) and drains the thermal field of CO2-rich Karymsky springs. The lake after the 1996 sublimnic eruption is in a steady-state condition with the total dynamic CO2 budget of about 4t/day, and has a total amount of CO2 stored inside of the lake of around 8000t. The thermal springs of the Karymsky caldera drained by the Karymsky River enrich the river in dissolved carbon species. A total CO2 output of 320t/day from both Karymsky Centre calderas was estimated, carrying around 130t/day carbon species (expressed as CO2) as dissolved species (HCO3 and CO2(aq)), and emitting to the atmosphere around 190t/day of CO2 as the diffusion flux from the soil and bubbling emanations from the springs.

Hydrochemical and Isotopic Characteristics of CO2-rich Groundwater in the Gyeongsang Sedimentary Basin, South Korea: A Natural Analogue Study on the Potential Leakage of Geologically-stored CO2

The potential of geologic carbon storage has been considered in the Gyeongsang sedimentary basin (GSB) at the southeastern part of South Korea. In order to understand the long-term behaviors of geologically-stored CO2, hydrochemical and multi-isotopic data (δ18O-δD of water and δ13C of dissolved inorganic carbon) of groundwater samples were collected from naturally seeping CO2-rich springs and ambient shallow groundwater wells in the GSB. The collected samples were classified into three hydrochemical groups: 1) dilute and acidic, CO2-rich springs, 2) high TDS, CO2-rich springs, and 3) CO2-poor groundwater. CO2-rich springs (i.e., Groups 1 and 2) were differentiated from Group 3 by their high carbon isotope values and partial pressure of CO2, suggesting that the CO2 in Groups 1 and 2 originates from a deep-seated (magmatic) source. Compared to the CO2-rich springs of Group 1, major cations and HCO3-, together with Fe, Mn and Sr, are enriched in CO2-rich springs of Group 2 and these constituents are highly correlated with TDS. This indicates that, during the ascent of CO2-rich fluids via fractures, CO2-rich springs of Group 2 experienced substantial CO2-water-rock interactions, possibly through longer or restricted pathways, whereas CO2-rich springs of Group 1 acquired the chemistry through lower degrees of water-rock interaction via the rapid ascent through more permeable pathways. The results of principal component analysis (PCA) with compositional data demonstrated that pH can be used as a prior monitoring parameter to detect the direct leakage of CO2 as represented by Group 1. In addition, the changes of ionic composition can be used as a supplementary tool to monitor the indirect leakage of CO2 accompanying a substantial CO2-water-rock interaction.