Hydrothermal Aquifer Research Articles

Fluid geochemistry of the Cerro Galán geothermal system (Southern Puna, Argentina): Implications for the geothermal potential of one of the youngest giant calderas in the Andes

The exploration of novel geothermal systems, particularly those promising for electrical power generation, plays a fundamental role in incorporating new renewable sources into the energy matrix. Geothermal systems associated with volcanic calderas are considered ideal targets for exploration. This study focuses on the geochemical features of fluids from the Cerro Galán hydrothermal system, which is hosted within a major resurgent caldera with >3.5 Myr of magmatic evolution situated on the Southern Puna (Central Volcanic Zone of the Andes, NW Argentina). The main aim is constructing the first geochemical conceptual model and provide information on the geothermal potential of this interesting resource. The main hydrothermal reservoir consists of a NaCl aquifer with estimated temperatures up to 187 °C at depth. This reservoir is likely hosted within the fractured pre-caldera basement rocks, mainly including Miocene-Pliocene volcanic rocks and Proterozoic-Cambrian igneous and metamorphic rocks. The confinement of the deep reservoir is attributed to the deposits of the Toconquis Group and Cueva Negra Ignimbrite, along with the basal section of the Cerro Galán Ignimbrite, which exhibit low permeability due to hydrothermal alteration. The presence of a phreatic explosion crater near one of the hot spring-rich areas is likely indicating past over-pressurization of the hydrothermal aquifer, resulting from efficient sealing. Furthermore, the absence of anomalous soil CO2 flux values on the top of the reservoir, except where the thermal spring discharges are located, can be explained by an effective cap-rock layer. Deep circulation of meteoric water, enriched with atmospheric gases, receives inputs of magmatic fluids (∼11% of primordial helium), leading to the development of the hydrothermal NaCl aquifer. However, this deep fluid contribution might be underestimated due to significant crustal assimilation (up to 50%) involved in the magma genesis of the Cerro Galán Volcanic Complex, a process which modifies the He isotopic signature of the magmatic endmember. The hot springs, characterized by high flow rate (up to 459 m3/h) are positioned at the intersection between the caldera margins and the NNE-SSW oriented tectonic structures, suggesting favorable permeability conditions. The preliminary geothermal gradient for the Cerro Galán area is estimated at around 98–101 °C/km. Such a high gradient can be attributed to the considerable heat flux generated by the transcrustal plumbing system of the Cerro Galán caldera, which includes the shallow crystal mush reservoir (<4 km depth). The preliminary geothermal potential of this giant caldera was performed using the volumetric method along with Monte Carlo simulations. The results indicate a probable power production capacity of 2.09 MWe and 10.85 MWe at 90 and 50% confidence level, respectively. The results presented in this work constitute a foundational knowledge base to promote a more advanced exploration phase for the geothermal resource. Additionally to the local energy demand, lithium and other metal mining operations, which are operating independently from the National Interconnected System, could potentially be interested in power generation through binary cycles.

Natural processes leading to large, pre-historic hydrothermal eruptions in geothermal areas: Rotokawa geothermal field, New Zealand

Hydrothermal eruptions are the most violent and hazardous phenomena within geothermal fields. The largest of these may produce kilometer-sized craters and breccia deposits that are tens of meters thick. The geological and hydrothermal priming that leads to these types of eruptions is poorly understood. To understand large hydrothermal eruptions, we investigated a series of prehistoric events at the Rotokawa geothermal field in New Zealand. By revising the stratigraphy and distribution of hydrothermal breccia deposits and correlating these with componentry, crater morphology, and subsurface geological structure, we estimated the frequency, priming processes, triggers, and dynamics of multiple eruptions. Seventeen large hydrothermal eruptions occurred centuries to millennia apart in the period from ca. 22 cal ka B.P. to ca. 3.4 cal ka B.P. Of six hydrothermal eruptions since ca. 7 ka, four produced oval-shaped craters up to 2 km in diameter, creating a broad, shallow depression within the geothermal field. The two youngest eruptions occurred northeast of earlier eruption centers and have narrower and elongated vents. We infer that in the central depression, newly formed craters rimmed by breccia deposits and high-relief country rock hosted temporary lakes tens of meters deep. Crater-lake breakout(s) and/or seismic events caused sudden pressure reduction above the hydrothermal aquifer, triggering hydrothermal eruptions. Northeast of the basin, hydrothermal alteration produced caprocks above intensively fractured areas. In this case, earthquakes are the most likely trigger for cap-rupture and eruption. All eruptions excavated shallow and large craters mostly within partially altered Oruanui Formation and pre-fragmented breccias. The size and localization of the eruptions was likely due to a combination of (1) availability of undisturbed porous ignimbrite hosting large thermal aquifers, (2) efficient crater excavation within or alongside pre-fragmented breccia, and (3) the location of fracture and fault zones that channeled deep fluid upflow, favoring priming processes. This study highlights how an interplay of tectonic, magmatic, and hydrologic processes is responsible for the timing, dynamics, and ultimate size of hydrothermal eruptions in geothermal fields. Some events may be very large and destructive depending on the right priming and geological conditions.

An Overview of Deep Geothermal Energy and Its Potential on the Island of Ireland

Summary This paper provides a short overview of geothermal energy, including a discussion on the key geological controls on heat distribution in the subsurface, and on the different types of geothermal resource and their potential uses. We then discuss the island of Ireland as an example of the role that geothermal energy can play in decarbonising the heat sector in a region characterised by relatively low-enthalpy (temperature) resources. Significant shallow geothermal potential exists across the island via the deployment of ground source heat pumps. The geology of onshore Ireland provides relatively limited potential for deep hydrothermal aquifers with primary porosity and permeability. Therefore, deep geothermal exploration on the island is likely to be focused on fractured carbonate reservoirs of Carboniferous age, with recorded groundwater temperatures reaching 38°C at 1 km depth, or on lower permeability petrothermal reservoirs developed as Enhanced or Advanced Geothermal Systems. The exception to this occurs within Mesozoic basins in Northern Ireland where porous and permeable Permo-Triassic sandstones are preserved beneath Paleogene basalts. Geothermal potential also exists in equivalent basins immediately offshore Ireland. For example, Triassic sandstones within the Kish Bank Basin, a few kilometres off the coast of Dublin, have estimated reservoir temperatures of 20–120°C across the basin.

Stress-induced changes in hydrothermal gas discharges along active faults near Mt. Etna volcano (Sicily, Italy)

Near-continuous monitoring both of gas emissions (CO2, CH4 and H2S) and of water temperature at Santa Venera al Pozzo thermal springs (SE foot of Mt. Etna volcano, Sicily, Italy) was conducted from December 2017 to April 2019, using a novel and cheaper Chromatography Monitoring System (CMS) coupled with a water temperature sensor. The results showed methane as predominant gas and temporal changes in gas concentrations that were in part due to daily fluctuations, which caused small amplitude variations, and in part due to non-environmental causes. These latter were correlated with the occurrence of strong earthquakes and slow tectonic events related to magmatic intrusions, but not with input of magmatic gases into the thermal aquifer, given the non-magmatic origin of all monitored gases. Methane spikes were observed during many volcano-tectonic events and call for a deep source of this gas. H2S was detected only during the strongest local tectonic events, including a Mw 4.9 earthquake, suggesting that this gas has a common origin as CH4 (i.e., mixing between microbial and thermogenic gas), but it is released only when tectonic stress is applied for sufficiently long periods as to cause H2S oversaturation in the hydrothermal aquifer. Water temperature decreases were also observed immediately after the two strongest earthquakes in the area, which helped us produce a comprehensive model to explain the observed geochemical variations. Our approach allowed revealing the great sensitivity of gases such as CH4 and especially H2S to tectonic stress, thus making them valuable indicators of impending strong tectonic or volcano-tectonic events.

Atmospheric Concentration of CO2 and PM2.5 at Salina, Stromboli, and Vulcano Islands (Italy): How Anthropogenic Sources, Ordinary Volcanic Activity and Unrests Affect Air Quality.

Geogenic and anthropogenic sources of atmospheric particulate and CO2 can lead to threats to human health in volcanic areas. Although the volcanic CO2 hazard is a topic frequently debated in the related scientific literature, space and time distribution of PM2.5 are poorly known. The results of combined CO2/PM2.5 surveys, carried out at Salina, Stromboli, and Vulcano islands (Aeolian archipelago, Italy) in the years 2020–2021, and integrated with investigations on bioaccumulation of metallic particulate matter by the mean of data on the magnetic properties of oleander leaves, are presented in this work. The retrieved results indicate that no significant anthropogenic sources for both CO2 and PM2.5 are active in these islands, at the net of a minor contribution due to vehicular traffic. Conversely, increments in volcanic activity, as the unrest experienced by Vulcano island since the second half of 2021, pose serious threats to human health, due to the near-ground accumulation of CO2, and the presence of suspended micro-droplets of condensed hydrothermal vapor, fostering the diffusion of atmophile viruses, such as the COVID-19. Gas hazard conditions can be generated, not only by volcanic vents or fumarolic fields, but also by unconventional sources, such as the outgassing from shallow hydrothermal aquifers through drilled or hand-carved wells.

Geothermal Sustainability or Heat Mining?

Heat mining” is, in fact a complete deceptive misnomer. When a mineral deposit (e.g. copper) is mined and the ore has been taken out, it will be gone forever. Not so with geothermal resources: The heat and the fluid are coming back! Namely, the heat and fluid extraction create heat sinks and hydraulic minima; around these, strong temperature and pressure gradients develop. Along the gradients, natural inflow of heat and fluid arises to replenish the deficits. The inflow from the surroundings can be strong: around borehole heat exchangers, heat flow densities of several W/m2 result, whereas terrestrial heat flow amounts only to about 50 – 100 mW/m2.&#x0D; The regeneration of geothermal resources after production, in other words, extraction of fluid and/or heat) is a process that runs over different timescales, depending on the kind and size of the utilization system, the production rate, and the resource characteristics. The resource renewal depends directly on the heat/fluid backflow rate. Heat, respectively fluid production from geothermal resources can be accomplished with different withdrawal rates. Although forced production is more attractive financially (with quick payback), it can nevertheless degrade the resource permanently. The longevity of the resource (and thus the sustainability of production) can be ensured by moderate production rates. The sustainable geothermal production level depends on the utilization technology as well as on the local geologic conditions. The stipulation of the sustainable production level requires specific clarifications, especially by numerical modelling, based on long-term production strategies. In general, resource regeneration proceeds asymptotically: strong at the beginning and slowing down subsequently, reaching the original conditions only after infinite time. However, regeneration to 95 % can be achieved much earlier, e.g. within the lifetime of the extraction/production system. In other words, geothermal resources may under certain circumstances may be considered as having potential regrowth, like biomass. Concerning the requirements for such sustainable production, it is convenient to consider four resource types and utilization schemes. These may be treated by numerical model simulations that consider heat extraction by geothermal heat pumps, hydrothermal aquifer, used by a doublet system for space heating, high enthalpy two-phase reservoir, tapped to generate electricity, and enhanced Geothermal Systems (EGS).

First gas and thermal measurements at the frequently erupting Gamalama volcano (Indonesia) reveal a hydrothermally dominated magmatic system

The first gas and thermal measurements at the summit of the Gamalama volcano indicate that the system is dominated by hydrothermal processes. This is highlighted by the prevalence of H2S over SO2 (H2S/SO2 = 2–8), a high CO2/SO2 ratio (76–201), and a low heat transfer (3.0 MW) to the surface. A relative variation in gas composition is observed along the degassing fracture zone, possibly due to partial S scrubbing. Despite this surface hydrothermal signature, the system exhibits high gas equilibrium temperatures (425–480 °C), indicating that fluids are not exclusively derived from a boiling hydrothermal aquifer, but also sourced by cooling and crystallizing basaltic magma at deep that continues to inject magmatic fluids into the system. This hydrothermally dominated activity on Gamalama possibly persisted over the last two decades, during which a high number of eruptive events were witnessed. The period coincides with the opening of large fractures at the summit that subsequently shifted the volcanic activity from the crater center to the peripheral fractures zones. These fractures that possibly developed in response to the regional geodynamics, have weakened the hydrothermal seal, allowing the pressure developed by the hydrothermal-magmatic system and promoted by the high annual rainfall, to rapidly exceeds the tensile strength of the seal leading to the numerous phreatic eruptions.

Geochemistry of gas and water discharge from the magmatic-hydrothermal system of Guallatiri volcano, northern Chile

This work presents the first chemical and isotopic (δ13C-CO2, δ13C-CH4, 3He, 4He, 20Ne, 40Ar, 36Ar, δ18O, and δD) data for fluid discharges from Guallatiri volcano, a remote and massive stratovolcano, which is considered as the second most active volcano of the Central Volcanic Zone (CVZ) in northern Chile. Fumarolic gases had outlet temperatures of between 80.2 and 265 °C, and showed a significant magmatic fluid contribution marked by the occurrence of SO2, HCl, and HF that are partially scrubbed by a hydrothermal aquifer. The helium isotope ratios (< 3.2) were relatively low compared to those of other active volcanoes in CVZ, possibly due to contamination of the magmatic source by 4He-rich crust and/or crustal fluid addition to the hydrothermal reservoir. Geothermometry in the H2O-CO2-CO-H2-CH4 system suggests equilibrium temperatures of up to 320 °C attained in a vapor phase at redox conditions intermediate between those typical of hydrothermal and magmatic environments. Thermal springs located 12 km northwest of the volcano’s summit had outlet temperatures of up to 50.1 °C, neutral to slightly basic pH, and a sodium bicarbonate composition, typical of distal fluid discharges in volcanic systems. Cold springs at the base of the volcanic edifice, showing a calcium sulfate composition, were likely produced by interaction of shallow meteoric water with CO2- and H2S-rich gases. A geochemical conceptual model was constructed to graphically represent these results, which can be used as an indication for future geochemical monitoring and volcanic hazard assessment.

Geochemical features of hydrothermal systems in Jujuy Province, Argentina: Hints for geothermal fluid exploration

Fluid primary source(s) and chemical-physical processes controlling water and gas chemistry of thermal springs from Eastern Cordillera, sub-Andean Ranges and Santa Bárbara (Jujuy Province, northern Argentina) were investigated to provide information for a preliminary evaluation of the geothermal potential in these areas. Thermal manifestations in Eastern Cordillera (Reyes) and part of those in the western sector of sub-Andean Range (Aguas Calientes) are fed by shallow aquifers, interacting with Quaternary- Neogene rocks and the upper portion of Pliocene-Miocene formations (Orán Group), whereas the meteoric water recharge area is located at >2500 m a.s.l., corresponding to Chañi hill. Differently, El Jordán thermal spring in the sub-Andean Range is fed by a hydrothermal aquifer hosted within highly porous and fractured formations of the Salta Group (Yacoraite Formation) and recharged by meteoric water from Sierra de Calilegua (∼1500 m a.s.l.). The latter is the recharge area of the La Quinta geothermal waters as well, but these have been fed at higher altitudes (>2500 m a.s.l.) in the range. The hydrothermal reservoir feeding the other thermal springs from the Santa Barbara system (Caimancito, El Palmar, and Siete Aguas) is recharged by meteoric water from Zapla Ranges and Santa Barbara Hill at <2500 m a.s.l. The high-TDS (>16,000 mg/L) Na+-Cl- La Quinta thermal springs are produced by interaction with the evaporite deposits of Salta Group, including halite, whereas the chemistry of El Palmar, El Jordán and Caimancito thermal springs, showing a Na+-SO42-(Cl−) composition, depends on mixing with shallower SO42--rich waters interacting with gypsum deposits of Anta Formation. Dissolved and bubbling gases from all the investigated provinces are related to CO2- and CH4-rich crustal fluids produced by both thermogenic processes occurring within the hydrothermal systems and microbial activity at relatively low depth, with low to negligible mantle contribution, as indicated by the 3He/4He values ≤ 0.21 Ra. The fluid reservoir feeding the Quinta thermal springs shows the highest estimated temperatures (>200 °C), which, considering the depth of Salta Group in the Santa Barbara system (~2000 m), support the idea, suggested by previous authors, of an anomalous geothermal gradient for this area, a promising pre-requisite for future exploitation of the geothermal resource.

Preliminary conceptual model of the Cerro Blanco caldera-hosted geothermal system (Southern Puna, Argentina): Inferences from geochemical investigations

The Cerro Blanco Caldera (CBC) is the youngest collapse caldera system in the Southern Central Andes (Southern Puna, Argentina). The CBC is subsiding with at an average velocity of 0.87 cm/year and hosts an active geothermal system. A geochemical characterization of emitted fluids was carried out based on the chemical and isotopic compositions of fumaroles, and thermal and cold springs discharged in this volcanic area with the aim of constructing the first hydrogeochemical conceptual model and preliminary estimate the geothermal potential. The main hydrothermal reservoir, likely hosted within the pre-caldera basement rocks, has a Na+-Clˉ(HCO3)ˉ composition with estimated temperatures ≥135 °C. The unconsolidated, fine-grained Cerro Blanco ignimbrite likely acts as the cap-rock of the hydrothermal system. The presence of phreatic eruption breccias in the surrounding area of the geothermal fumaroles supports the effectiveness of the pyroclastic deposit as sealing rocks. The isotopic data of water (δ18O and δD) indicate a meteoric recharge of the hydrothermal reservoir, suggesting as recharge areas the sectors surrounding the CBC, mainly towards the W and NW where large outcrops of the pre-caldera basement exist. A fault-controlled hydraulic connection between the hot springs and the hydrothermal reservoir is proposed for the Los Hornitos area. The fumaroles show the typical compositional features of hydrothermal fluids, being dominated by water vapor with significant concentrations of H2S, CH4 and H2. Considering the high geothermal gradient of this area (∼104 °C/km) and the relatively high fraction of mantle He (∼39%) calculated on the basis of the measured R/Ra values, the hydrothermal aquifer likely receives inputs of magmatic fluids from the degassing magma chamber. The preliminary geothermal potential at CBC was evaluated with the Volume Method, calculating up to E = 11.4*1018 J. Both the scarce presence of superficial thermal manifestations and the occurrence of an efficient cap-rock likely contribute to minimize the loss of thermal energy from the reservoir. The results here presented constitute the necessary base of knowledge for further accurate assessment of the geothermal potential and ultimately the implementation of the geothermal resource as a viable energy alternative for small localities or mining facilities isolated from the National Interconnected System due to their remote localization.

Update on the fluid geochemistry monitoring time series for geothermal systems in Dominica, Lesser Antilles island arc: 2009–2017

With updated geochemical and isotopic compositions obtained over the period 2009 to 2017, this study presents revised fluid characterizations for the active volcanic-hydrothermal systems on the island of Dominica in the Lesser Antilles, which were first reported in 2011. Hydrothermal waters of Dominica cover a wide spectrum of pH, temperatures and chemical composition. The pH of the thermal waters ranges from acidic to neutral (pH values of 1–7.8) and the waters are predominantly Na-SO4 in character (Na = 14–2127 mg/L; SO4 = 1–1725 mg/L), likely formed as a result of dilution of acidic H2S-rich gases in near surface oxygenated groundwater, and have experienced limited water-rock interaction. The geochemical composition of the waters for most of the hydrothermal systems studied indicate no significant changes, with the exception of the Boiling Lake, which experienced a short (~6 week) episode of instability in November 2016 which appeared to be associated with a small mud-rich explosion. Unlike the last such event in December 2004, which was reported to be earthquake-triggered, this event is possibly the consequence of a moderate-sized landslide into the lake. The lake draining episodes have been accompanied by changes in composition between Na-SO4 and Na-Cl, which is attributed to hydrothermal fluid contributions from two different aquifers: a shallower acid-sulphate hydrothermal aquifer and a deeply-sourced brine aquifer.Reservoir temperatures determined by quartz geothermometers have not changed significantly over the monitoring period, suggesting steady-state degassing of the magma chambers. In two areas, temperatures have increased: Watten Waven (from 83–90 °C to 89–139 °C) and Sulphur Springs (from 145–152 °C to 93–243 °C). The elevated reservoir temperatures have affected the isotopic composition of the waters (δ18O = −5.7 to 9.1‰ and δD = −8 to 20.5‰), that reflect a dominantly meteoric source, with boiling/degassing and evaporation also playing an important role. The time series data suggests that some hydrothermal areas are experiencing increased steam evaporation over time whereas other waters are becoming more meteoric. The δ13CDIC is decoupled from the deuterium and oxygen-18 isotopes and shows very little variation over time, but a broad range in values from −11 to +5‰. The dominant process contributing to δ13CDIC is degassing of primarily magmatic CO2, as exhibited by the bubbling pools. Hydrothermal streams have experienced mixing with biogenic CO2 sources, including plant respiration and methanogenesis. The slight variations observed from site to site are likely a consequence of fractional degassing of the magma chamber during exsolution of CO2. Over the sampling period 2014–2017, the temperatures and δ13C values do not change, which suggests a current steady-state of degassing.

Pathways and fate of REE in the shallow hydrothermal aquifer of Vulcano island (Italy)

We investigated the geochemical behaviour of major and Rare Earth Elements (REE), together with oxygen and deuterium isotopic composition in the aquifer of Vulcano, the southernmost island of the Aeolian archipelago (Italy).Studied wells, located at different distances from the crater, are characterised by different contributions of the rising volcanic fluids. In particular, those located in the proximity of La Fossa crater are affected by a strong interaction with volcanic-hydrothermal fluids and show REE behaviour similar to that of fresh rocks, suggesting a congruent dissolution of the solid matrix. Samples from the other wells, located in an area where the volcanic deposits are hydrothermally altered as an “advanced argillic facies”, are enriched in HREE and mirror the corresponding depletion observed in the altered rocks. Moreover, the different grade of interaction with hydrothermal fluids determines the main ligand that complexes the REE. The main ligand is CO32– in the wells that are more directly affected by hydrothermal circulation, whereas SO42− dominates in those located at greater distances from La Fossa crater.This information provides further clues to the complex groundwater circulation model of Vulcano Island, which is regulated by the variable mixing and interacting of rising volcano-hydrothermal fluids, meteoric infiltration and seawater, differently interacting with fresh and altered rocks.

The suitability of the Pieve Fosciana hydrothermal system (Italy) as a detection site for geochemical seismic precursors

The Serchio valley is one of the regions of highest seismic risk in Tuscany, Italy. As a part of a seismic prevention/prediction pilot project funded by Regional Government of Tuscany, the geochemical and hydrologic processes that control the composition of the thermal waters emerging near the Pieve Fosciana village (Prà di Lama site), have been investigated to assess the suitability of the site for the monitoring of geochemical precursors of earthquakes. The investigation protocol considered the coupled use of chemical analyses on water aliquots sampled during discrete surveys, and of continuous signals for selected physical-chemical parameters acquired via an automatic station equipped with a suite of parameter-specific sensors. Concentrations of major inorganic dissolved aqueous components have been processed with geochemical modeling techniques to obtain a quantitative description of water–rock interactions occurring at depth in local Mesozoic aquifers. The geochemical background of the system was defined by combining these data, and an integrated hydrogeological and geochemical model of the hydrothermal aquifer feeding the springs was elaborated. Based on the very low variability of physical and geochemical parameters over a long observation time, the relatively long and deep underground circulation paths of discharged waters under confined conditions, the site-specific sensitivity of monitored parameters to local crustal strain, and the proximity to seismically active neotectonic structures, the Pieve Fosciana system ranks as a favorable site for recognizing fluids possibly connected to energy release and/or permeability variations induced by seismogenic processes at depth. The integrated geological, hydrogeological, geochemical and modeling approach presented here could be profitably applied for the design of similar projects elsewhere.

Assessing hydrothermal groundwater flow path using Kohonen's SOM, geochemical data, and groundwater temperature cooling trend.

Assessing groundwater flow path in a thermal aquifer, such as El Hamma aquifer, southeastern Tunisia, and its lateral communication with the adjacent Jeffara-Gabes aquifers, is a very complex operation which requires the integration of several approaches to understand and explain the reality of phenomenon. In this study, geochemical and isotopic data, Kohonen self-organizing map, temperature cooling trend, and kriging techniques were used to assess groundwater flow path in hydrothermal aquifer of El Hamma-Gabes, Tunisia. For this objective, 32 sampled wells are analyzed for major ions, electric conductivity, pH, total dissolved solids, and stables isotopes (δ2H and δ18O). Geochemical diagrams reveal that groundwater chemistry was controlled by evaporation, and rock-water interaction with a dominant water facies was Cl·SO4-Na·Ca-Mg. Kriging techniques were used to highlight groundwater flow path. Kohonen self-organizing map shows that the waters are clustered into three classes according to chemical and isotopic composition. These clusters represent a hydrothermal groundwater class from the Continental Intercalaire aquifer, a shallow groundwater class corresponding to Jeffara-Gabes aquifer and mixed water class. Groundwater cooling trend and stable isotopes indicate that groundwater flow is toward west to east part of study area, indicating a recharge of Jeffara aquifer from El Hamma thermal aquifer.

Mass Addition at Mount St. Helens, Washington, Inferred From Repeated Gravity Surveys

AbstractRelative gravity measurements were made at 12 sites on Mount St. Helens and 4 sites far afield during the summers of 2010, 2012, 2014, and 2016. Positive residual gravity changes of 0.05–0.06 ± 0.01 mGal from 2010 to 2016—a sign of mass addition—remain at proximal sites after accounting for the effects of changes in Crater Glacier shape and mass. Modeling of the 2010–2016 monitoring data indicates mass addition in the volcano magma reservoir, the volcano conduit, and/or the shallow hydrothermal aquifer. Magma intrusion in the volcano's known reservoir is suggested by the joint inversion of GPS and gravity data (d = 5800 ± 710 m below sea level,ΔVm = 49.8 ± 8.6 × 106m3,ρ = 1930 ± 300 kg/m3); the modeled depth and location are consistent with that of the reservoir that fed the 2004–2008 eruption, and its mass change can explain up to 19% of the residual gravity. The other two potential sources—the conduit and shallow aquifer—are not well constrained. Magma addition along the volcano conduit can explain up to 62% of the residual gravity (ΔVm ≅ 31 × 106m3, ρm ≅ 2, 300 kg/m3). However, such an intrusion should have produced a measurable surface deformation, which is not observed in the GPS time series. Changes in the level of the volcano's shallow hydrothermal system (ρw = 1, 000 kg/m3) can explain 17% (ΔVrecharge ≅ 9 × 106m3) to 61% (ΔVrecharge ≅ 30 × 106m3) of the residual gravity. It would therefore seem that the bulk of the mass change measured at Mount St. Helens during 2010–2016 was caused by shallow accumulation of water beneath the floor of the crater.

Isotopes and geochemistry to assess shallow/thermal groundwater interaction in a karst/fissured-porous environment (Portugal): a review and reinterpretation

As a distinctive fingerprint of the groundwater sources and water–rock interaction within a karst/fissured-porous environment, isotopic (e.g. δ13C, δ18O, δ34S, 3H, and 14C) and geochemical data have been used to establish flow paths (diffuse flow) of thermal waters used in Caldas da Rainha Spa (Portuguese mainland). These thermal waters (T ≈ 32 °C) discharge from springs and boreholes located close to an N–S-oriented oblique fault (60°E). This hydrothermal system is dominated by deep karst/fissured-porous Lower Jurassic carbonate formations containing slow flowing groundwater. 14C determinations in the total dissolved inorganic carbon (TDIC) indicate a mean “age” of about 1600 years BP for the thermal waters. The HCO3−, Ca2+, and Mg2+ signatures are related to water/calcite–dolomite interaction, whereas Na+, Cl−, and SO42− concentrations are mainly associated with halite and gypsum dissolution. δ18O values indicate that the hydrothermal aquifer system is depleted in heavy isotopes comparing to the shallow aquifer systems, signifying that the main recharge must be related to the Lower Jurassic carbonate formations of the Candeeiros Mountain. The δ13C values measured in the TDIC are typical of carbonate dissolution enhanced by CO2 soil air dissolution. The δ34Ssulphate and δ18Osulphate values of the thermal waters indicate that the sulphate is clearly the result of water–rock interaction with evaporitic layers at depth. Considering a mean geothermal gradient in the region of about 30 °C/km, the silica and K2/Mg geothermometry seems to indicate more reliable circulation depths (1–2 km) for the thermal waters than the SO42−–H2O isotope geothermometer (3–5 km depth). The lack of mixing evidences between the thermal and the local shallow cold groundwaters indicates that both water sources are distinct. Furthermore, increasing knowledge on the local/regional hydrogeology is extremely important to achieve the sustainable use of such “invisible” georesources, since most thermal and mineral waters from karst aquifers worldwide are used both as a source of bottled water and a recreational resource (spa facilities, tourism, etc.).

Water-rock Interactions in a Site Investigated for Geochemical Precursors of Earthquakes: The Pieve Fosciana Spring (Italy)

The Serchio Valley is one of the regions of highest seismic risk in Tuscany, Italy. As a part of a seismic prevention/prediction pilot project funded by Regional Government of Tuscany, the Pieve Fosciana spring, a perennial thermo-mineral spring emerging in correspondence with some of the most important neo-tectonic structures of the Serchio Valley, was monitored for possible geochemical precursors of earthquakes. The investigation protocol considered the coupled use of chemical analyses on water aliquots sampled during discrete surveys, and of continuous signals for selected physical-chemical parameters acquired via an automatic station equipped with a suite of parameter-specific sensors. By combining these data, the geochemical background of the system was defined, and an integrated hydrogeological and geochemical model of the hydrothermal aquifer feeding the spring was elaborated.

The hydrothermal system of the Domuyo volcanic complex (Argentina): A conceptual model based on new geochemical and isotopic evidences

The Domuyo volcanic complex (Neuquén Province, Argentina) hosts one of the most promising geothermal systems of Patagonia, giving rise to thermal manifestations discharging hot and Cl−-rich fluids. This study reports a complete geochemical dataset of gas and water samples collected in three years (2013, 2014 and 2015) from the main fluid discharges of this area. The chemical and isotopic composition (δD-H2O and δ18O-H2O) of waters indicates that rainwater and snow melting are the primary recharge of a hydrothermal reservoir located at relative shallow depth (400–600m) possibly connected to a second deeper (2–3km) reservoir. Reactive magmatic gases are completely scrubbed by the hydrothermal aquifer(s), whereas interaction of meteoric waters at the surface causes a significant air contamination and dilution of the fluid discharges located along the creeks at the foothill of the Cerro Domuyo edifice. Thermal discharges located at relatively high altitude (~3150ma.s.l.), namely Bramadora, are less affected by this process, as also shown by their relatively high R/Ra values (up to 6.91) pointing to the occurrence of an actively degassing magma batch located at an unknown depth. Gas and solute geothermometry suggests equilibrium temperatures up to 220–240°C likely referred to the shallower hydrothermal reservoir. These results, confirming the promising indications of the preliminary surveys carried out in the 1980′s, provide useful information for a reliable estimation of the geothermal potential of this extinct volcanic system, although a detailed geophysical measurements is required for the correct estimation of depth and dimensions of the fluid reservoir(s).

Mechanisms of Formation of Natural Hydrogen Reservoirs in Thermal Aquifers - Impact of Bacteria and Gas Bubble Dynamics

Underground hydrogen reservoirs, whose existence has been confirmed recently by geologists all over the world, represent a new source of renewable energy. Hydrogen is formed as the product of reaction of serpentinization between ferromagnesian minerals of the Mantle and water in the offshore zones of subduction or in continental zones at considerable depth beneath hydrothermal aquifers. Initially dissolved in water, hydrogen can create free gas bubbles when the local concentration of H2 dissolved in water exceeds the equilibrium value. The bubbles raise up and create the gas cap of free hydrogen. This process is significantly influenced by methanogen bacteria inhibiting in aquifers and consuming hydrogen for their metabolism. The reactions initiated by bacteria leads to the appearance of other volatile components as methane. The dimension, the form of the gas cap and the concentration of hydrogen and methane determine the efficiency of gas production and the energy potential of the reservoir. In the present paper we develop the conceptual mathematical model of gas cap formation in hydrogen reservoir. The bacterial activity is described by the equation of population dynamics with specific kinetic functions obtained by the authors by treating experimental data. The formation of free gas is modelled in terms of the formation of oversaturated nuclei, their growth and simultaneous motion of isolated bubbles. As far as the bubbles grow, they stick together and create continuous free gas. Thus, the hydrogen motion consists of two stages: (i) the motion of isolated bubbles, controlled by the equations of ganglion dynamics; and (ii) the motion of continuous gas through water, governed by Darcy’s law. The bubble growth is ensured by the mass exchange with hydrogen dissolved in water, which involves a non-equilibrium in the average hydrogen concentrations. Such a problem of hydrogen raising is solved analytically in simplified situation without bacteria. The problem can be reduced to a system of nonlinear hyperbolic equations, which has either continuous or discontinuous solutions (shock waves) depending on the degree of the non-equilibrium. Multiples deformations of the shocks under gas raising are described. The impact of bacteria is studied numerically by using the open source Basilisk (developed by D’Alembert Institute). We have revealed non trivial scenarios of the appearance of piece-wise constant traveling waves, or auto-oscillations caused by the nonlinear population dynamics. The result obtained enables us to give estimation to the characteristic time of gas cap formation and the evolution of its composition in time.

Identification of Hydrothermal Aquifer Zone using Geo-Electrical Method in Kaloling Sinjai District

&lt;p&gt;Geothermal energy is one of the natural resources which emerged on the subsurface ofthe earth can be in a gaseous form (the vapor heat) or in the form of hydrothermal. The geothermal Panggo that located at watersheds of Kalamisu river, was one of the three sources of hydrothermal system existing in district Sinjai East. The existence of the geothermal system in this area will much give many advantagest if managed optimally. This research aims to map the spread of the hydrothermal aquifer zone at the subsurface and it potentials in Panggo village base on electrical properties. Methods used in this research were geo-electrical using Wenner and Schlumberger configurations.At all these research area, it is found the presence of zones which has low resistivity (&amp;lt; 20 Ωm), and it is interpreted as the spread of hydrothermal zones. The hydrothermal system appears at subsurface allegedly caused by the geological fault of Kalamisu across this region.&lt;/p&gt;