Long-term Geochemical Monitoring Research Articles

Chemical and thermal changes in hot spring waters and fumarolic gases related to volcanic unrest at Meakandake volcano, Japan: Results of long-term geochemical monitoring from 1986 to 2022

Phreatic eruptions at Nakamachineshiri and Ponmachineshiri reflect the vigorous fumarolic and hydrothermal activities of Meakandake volcano. Thus, phreatic eruptions are likely to reoccur in the future, and hydrothermal systems should be understood to mitigate the risks of potential future hazards. Here, we reveal the results of long-term geochemical monitoring of hot spring waters and fumarolic gases since 1986 and discuss the chemical and temperature changes in hot spring waters, discharging at the foot, related to volcanic unrest. In Nakamachineshiri, related to the increase in seismic and fumarolic activity since 2019, the HCl concentration of the fumarolic gas has increased, and δD and δ18O values have also increased to approach those of the andesitic water. Simultaneously, with the increase in fumarolic and hydrothermal activity, the temperature and Cl concentration of the Meakan Onsen (MO) hot spring water discharged at the western foot of Nakamachineshiri have increased. The chemical characteristics of the fumarolic gas and MO water indicate that they were derived from a hydrothermal system in the shallow subsurface of Nakamachineshiri. Thus, we can consider that the temperature and chemical increases in the MO water represent activation of the system, and monitoring the MO water provides important information for understanding the present state of Nakamachineshiri. At Ponmachineshiri, phreatic eruptions occurred in 1988, 1996, 1998, 2006, and 2008, and seismic and fumarolic activity has sometimes increased since 1986 when we began our monitoring program. At the western foot of Ponmachineshiri, hot spring water is discharged at Yunotaki (YNT_U and YNT_L) and Lake Onneto (ONT). The results of our monitoring suggest that the YNT_U water is formed by mixing thermal water with a high HCO3 molar ratio (HCO3-type) and thermal water containing Cl and SO4 (Cl-SO4-type), which causes temperature and chemical changes related to the mixing ratio of these thermal waters related to volcanic unrest, especially increases in seismic activity. The Cl and SO4 concentrations in the ONT water began to increase approximately two years before the phreatic eruptions and increase in fumarolic activity. Such chemical increases can be explained by the increase in the supply of Cl-SO4-type thermal water and are likely to indicate precursor changes in volcanic unrest. Therefore, we expect to utilize these hot spring waters to understand the present state of Ponmachineshiri and detect precursor changes to future volcanic unrest.

Long-term geochemical monitoring of the soil cover in the impact zone of diamond mining enterprises: a case study in the Nakyn kimberlite field, Russia.

The most severe disturbance of the earth's surface occurs when the open-cut method of mineral deposits mining is used. The geoecological situation was assessed based on the nature of the soil cover based on the example of an industrial site of a diamond mining and processing plant located in the permafrost zone. During the period from 2007 to 2018, the soil cover of the industrial site is characterized by polyelement contamination. In the surface, soil horizons were an increase in the concentrations of mobile forms of Mn, Zn, Cd, Cr, Co, and Ni. It is identified that AO, ABcr, and CR are the accumulation horizons if the soil profile is preserved. Mobile forms Mn, Zn, Ni, Cr, Co, and As can migrate along with the soil profile to a depth of 40-50cm depending on the amount of soil organic matter, the degree of its decomposition, and the scale of the cryoturbation. Research in 2018 allowed us to localize and confirm the increase in the area of contamination of the industrial site. Areas with an extremely dangerous category of soil cover contamination increased by 3 times compared to 2014. The results obtained are the basis for a more detailed study of the horizons of geochemical accumulation and the creation of artificial geochemical barriers with the development of technologies for the subsequent extraction of useful components.

Long-Term Geochemical Monitoring of the Source of the Angara River (Runoff from Lake Baikal)

The long-term data (1950–2017) on the chemical composition of the water from the source of the Angara River, which is the only run-off from Lake Baikal, a large and clean fresh water reservoir, are presented. The absence of water contamination despite the anthropogenic influence is shown. The similarity and constancy of the current composition of the waters in the lake and the Angara River are recorded. It is recommended to continue geochemical studies of this unique world heritage site.

Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): Insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids

More than three decades of geochemical monitoring of hot springs and fumaroles of La Soufrière of Guadeloupe allows the construction of a working model of the shallow hydrothermal system. This system is delimited by the nested caldera structures inherited from the repeated flank collapse events and the present dome built during the last magmatic eruption (1530AD) and which has been highly fractured by the subsequent phreatic or phreatomagmatic eruptions. Because it is confined into the low volume, highly compartmented and partially sealed upper edifice structure, the hydrothermal system is highly reactive to perturbations in the volcanic activity (input of deep magmatic fluids), the edifice structure (sealing and fracturing) and meteorology (wet tropical regime).The current unrest, which began with a mild reactivation of fumarolic activity in 1990, increased markedly in 1992 with seismic swarms and an increase of degassing from the summit of the dome. In 1997 seismic activity increased further and was accompanied by a sudden high-flux HCl-rich gas from summit fumaroles. We focus on the interpretation of the time series of the chemistry and temperature of fumarolic gases and hot springs as well as the relative behaviours of halogens (F, Cl, Br and I). This extensive geochemical time series shows that the deep magmatic fluids have undergone large changes in composition due to condensation and chemical interaction with shallow groundwater (scrubbing). It is possible to trace back these processes and the potential contribution of a deep magmatic source using a limited set of geochemical time series: T, CO2 and total S content in fumaroles, T and Cl− in hot springs and the relative fractionations between F, Cl, Br and I in both fluids.Coupling 35years of geochemical data with meteorological rainfall data and models of ion transport in the hydrothermal aquifers has allowed us to identify a series of magmatic gas pulses into the hydrothermal system since the 1976–1977 crisis. The contrasting behaviours of S- and Cl-bearing species in fumarolic gas and in thermal springs suggest that the current activity is the result of a new magma intrusion which was progressively emplaced at shallow depth since ~1992. Although it might still be evolving, the characteristics of this new intrusion indicate that it has already reached a magnitude similar to the intrusion that was emplaced during the 1976–1977 eruptive crisis. The assessment of potential hazards associated with evolution of the current unrest must consider the implications of recurrent intrusion and further pressurisation of the hydrothermal system on the likelihood of renewed phreatic explosive activity. Moreover, the role of hydrothermal pressurisation on the basal friction along low-strength layers within the upper part of the edifice must be evaluated with regards to partial flank collapse. At this stage enhanced monitoring, research, and data analysis are required to quantify the uncertainties related to future scenarios of renewed eruptive activity and magmatic evolution.

Geochemistry of fluids discharged over the seismic area of the Southern Apennines (Calabria region, Southern Italy): Implications for Fluid-Fault relationships

The first comprehensive geochemical data-set of the fluids circulating over a 14,000 km 2 -wide seismic-prone area of the Southern Apennines, Calabria Region (Italy), is presented here. The geochemical investigations were carried out with the twofold aim of constraining the origin and interactions of the circulating fluids and to investigate possible relationships with local faults. Sixty samples of both thermal and cold waters were collected, from which the dissolved gases were extracted. The geochemical features of the water samples display different types and degrees of water–rock interactions, irrespective of the outlet temperature. The calculated equilibrium temperatures of the thermal waters (60–160 °C) and the low heat flow of the whole study area, are consistent with a heating process due to deep water circulation and rapid upflow through lithospheric structures. The composition of the dissolved gases reveals that crustal-originating gases (N 2 and CO 2 -dominated) feed all the groundwaters. The 3 He/ 4 He ratios of the dissolved He, in the range of 0.03–0.22Ra c for the thermal waters and 0.05–0.63Ra c for the cold waters (Ra c = He isotope ratio corrected for atmospheric contamination), are mainly the result of a two-component (radiogenic and atmospheric) mixing, although indications of mantle-derived He are found in some cold waters. As the study area had been hit by 18 of the most destructive earthquakes (magnitude ranging from 5.9 to 7.2) occurring over a 280-a time span (1626–1908) in the Southern Apennines, the reported results on the circulating fluids may represent the reference for a better inside knowledge of the fault-fluid relationships and for the development of long-term geochemical monitoring strategies for the area.

Monitoring of temporal and spatial variations in fumarole helium and carbon dioxide characteristics at Po^|^aacute;s and Turrialba volcanoes, Costa Rica (2001-2009)

We report results of a 9-year monitoring program that took place from 2001 to 2009 of the helium and carbon isotope (3He/4He, δ13C) and relative abundances ratios (CO2/3He) of fumarole sites at Poás and Turrialba volcanoes, Costa Rica. Over the monitoring period, helium isotopes (3He/4He), δ13C(CO2) and CO2/3He values varied between 6.7-7.6 RA (where RA = 1.4 × 10-6), -5.5 to -1.3‰ (vs. PDB) and 8.2-59.5 (×109), respectively, at Poás Volcano. Corresponding values for Turrialba Volcano were 6.9-8.0 RA, -4.4 to -2.7‰ and 9.4-19.6 (×109), respectively. Notably, fumarole sites at both volcanoes underwent changes in temperature, intensity and areal extent during the 9-year period, and Poás Volcano experienced hydrophreatic eruptions and structural damage induced by a nearby earthquake. At both volcanoes, there were significant and sympathetic temporal changes involving all three geochemical parameters—notably in 2001 and 2006 at Poás and in 2001 and between 2005-2007 at Turrialba. We dismiss increased hydrothermal interaction, magma degassing and calcite precipitation as likely causes of the observed variations. Instead, by ascribing endmember compositions to the three principal contributors to the CO2 inventory—mantle wedge as well as limestone and organic (sedimentary) carbon (both slab and crust derived)—we show that changes in observed He-CO2 relationships mainly reflect enhanced crustal contributions of CO2 and increased inputs from magma degassing. Such changes in the relative roles of crust and magma as suppliers of CO2 are readily apparent in the He-CO2 temporal record. This work supports calls for long-term geochemical monitoring to be included within hazard assessment and mitigation studies at active volcanoes.

The Porretta thermal springs (Northern Apennines): seismogenic structures and long-term geochemical monitoring

The thermal springs of Porretta are located on a seismically active area of the Northern Apennines. In 19th Century a chemist identified anomalous behaviour of the thermal waters in concomitance with local seismic events. Recent studies assessed the geochemical features of the circulating fluids (e.g., waters carry a dissolved CH4-dominated gas phase with a radiogenic signature of the helium isotopic ratio) and observed anomalous hydrologic and geochemi- cal signals possibly related to crustal strain phenomena due to local seismic events. Long-term geochemical moni- toring was carried out from 2001 to 2006 with the aim of detecting the behaviour of the circulating fluids possibly coinciding with seismic activity. The collected data reveal a sensitivity of the thermal waters to the activity of the main fault crossing the village of Porretta and identify a «seismogenic» structure crossing the village.

Long-term geochemical monitoring and extensive/compressive phenomena: case study of the Umbria Region (Central Apennines, Italy)

Long-term geochemical monitoring performed in the seismic area of the Umbria-Marche region of Italy (i.e. Central Apennines) has allowed us to create a model of the circulation of fluids and interpret the temporal chemical and isotopic variations of both the thermal springs as well as the gas vents. Coincident with the last seismic crisis, which struck the region in 1997-1998, an enhanced CO2 degassing on a regional scale caused a pH-drop in all the thermal waters as a consequence of CO2 dissolution. Furthermore, much higher 3He/4He isotope ratios pointed to a slight mantle-derived contribution. Radon activity increased to well above the ±2 sinterval of the earlier seismic period, after which it abruptly decreased to very low levels a few days before the occurrence of the single deep-located shock (March 26, 1998, 51 km deep). The anomalous CO2 discharge was closely related to the extensional movement of the normal faults responsible for the Mw 5.7, 6.0 and 5.6 main shocks that characterized the earlier seismic phase. In contrast, a clear compressive sign is recognizable in the transient disappearance of the deep-originating components related to the Mw 5.3, 51 km-deep event that occurred on March 26, 1998. Anomalies were detected concomitantly with the seismicity, although they also occurred after the seismic crisis had terminated. We argue that the observed geochemical anomalies were driven by rock permeability changes induced by crustal deformations, and we describe how, in the absence of any release of elastic energy, the detection of anomalies reveals that a seismogenic process is developing. Indeed, comprehensive, long-term geochemical monitoring can provide new tools allowing us to better understand the development of seismogenesis.

Helium and carbon isotopes in the dissolved gases of Friuli Region (NE Italy): Geochemical evidence of CO 2 production and degassing over a seismically active area

The first geochemical data showing the existence of an active degassing activity over a large seismically active sector of the Southern Alps (Friuli Region, NE Italy) are presented. The dissolved gases, helium and carbon isotopic systematics of 46 water samples taken from 13 sites running along E–W and NE–SW faults besides the natural degassing of a 5000 km 2 wide area are investigated. The chemical composition of the dissolved gases revealed that a CO 2-rich gas phase feeds the local groundwaters. 3He/ 4He ratios ( R) normalized to the atmospheric 3He/ 4He ratio (Ra = 1.39 × 10 − 6 ) and corrected for the atmospheric contamination ( R/Ra c), range from 0.29 to 1 as a result of a two component (radiogenic and atmospheric) mixing. The δ 13C values of total dissolved inorganic carbon (TDIC) ranging from − 15.28 to − 0.75‰ vs. PDB, show the occurrence of multiple gas–water interactions. The mixing between the atmospheric air and a crustal source and the gas–water interactions occurring at various extents appears to be the main control on the observed He–C systematics. The natural CO 2 degassing was evaluated by a soil gas survey carried out by a grid of about 100 measuring sites located over the area that generated destructive seismic sequences (e.g. the Gemona sequence of 1976; main shocks M6.4 of 6th May and M6.1 of 11th and 15th September). The results obtained show that a significant amount of crustal-originated gases are released over the continental area of Eastern Southern Alps. The evidence that carbon dioxide is associated with radiogenic-type helium denotes the lack of the mantle as primary energy and degassing source, highlighting the possibility that CO 2 is produced by thermo-mechanical processes occurring at seismogenic depth. The information provided here may be used to start up a long-term geochemical monitoring of this seismically active area and could be able to detect the modifications occurring to the circulating fluids to gain a better insight on the relationships between the fluids' geochemistry and the activity of the local seismogenic faults.

Long-term (1997-2007) geochemical monitoring of gases from the Umbria-Marche region

The paper presents the results of ten-year long geochemical monitoring carried out in the area hit by the 1997–98 Umbria–Marche seismic sequence (Northern Apennines, Italy) showing the advancement in our understanding of the reciprocal relationships between fluids and tectonic structures in this area. Long-term geochemical monitoring has allowed us to define the geochemical features of the gas phase and to model its origins and interactions. All the released gases, including dissolved and venting gases, have predominantly crustal origin and have suffered from substantial modifications in their geochemical features. The whole seismogenic process is considered to be responsible for the modifications recorded over a wide time interval, including the production of CO 2 from mechanical stress on carbonate rocks. We interpret the modifications occurred to the gas phase during the 1997–98 period as related to faulting activity, while those recorded after the end of the seismic crisis are interpreted as being a consequence of the crustal relaxation. The data show that it is almost impossible to interpret the temporal variation without coupling long-term geochemical monitoring and knowledge of the genetic features of the fluids. Therefore the information coming from the circulating fluids is a powerful tool to better understand the physical processes governing the faulting activity.

Tectonic pumping: earthquake-induced chemical flux detected in situ by a submarine cable experiment in Sagami Bay, Japan

We successfully deployed an in situ automatic chemical analyzer sensitive to manganese (Mn) in seawater for a period of 81 days for the first time on the deep seafloor of Sagami Bay along a convergent plate boundary south of Japan. The in situ Mn analyzer (GAMOS-IV) was connected to a submarine cable as a means to supply power and to relay real time data. During the observation period from April 5 till June 25, 2006, the amount of measured Mn was seen to increase abruptly up to 10 times that of the background level only on April 21, probably triggered by a M5.8 earthquake which occurred ∼7 km south-southwest of the observation site. This study demonstrates the suitability of submarine cables for the long-term geochemical monitoring of deep sea environments.

Tectonic pumping: earthquake-induced chemical flux detected in situ by a submarine cable experiment in Sagami Bay, Japan

We successfully deployed an in situ automatic chemical analyzer sensitive to manganese (Mn) in seawater for a period of 81 days for the first time on the deep seafloor of Sagami Bay along a convergent plate boundary south of Japan. The in situ Mn analyzer (GAMOS-IV) was connected to a submarine cable as a means to supply power and to relay real time data. During the observation period from April 5 till June 25, 2006, the amount of measured Mn was seen to increase abruptly up to 10 times that of the background level only on April 21, probably triggered by a M5.8 earthquake which occurred ∼7 km south-southwest of the observation site. This study demonstrates the suitability of submarine cables for the long-term geochemical monitoring of deep sea environments.