Long-term Water-rock Interaction Research Articles

Strength and acoustic emission weakening mechanism of sandstone after prolonged water immersion: Insights from micro-fracture

In this study, a series of uniaxial compression and acoustic emission (AE) tests were conducted on sandstone specimens immersed in water for different durations (0–720 h). The mechanical properties and AE characteristics of dry, unsaturated, saturated, and long-term saturated sandstone specimens were comparatively analyzed. The experimental results show that with the passage of immersion time, the uniaxial compressive strength and elastic modulus of the specimens both decrease, while the Poisson’s ratio increase; the mechanical parameters change exponentially with the immersion time and are linearly related to the water content; the dominant failure mode evolves from tensile failure to shear failure gradually; and the AE intensity and frequency both weaken. Besides, the mechanism of long-term water–rock interaction was explored. It was proposed that the weakening of rock strength and AE signals is largely attributed to the transformation in the micro-fracture form (from transgranular fracture to intergranular fracture), which is induced by the deterioration of grain cementation. This view is supported by optical microscopic observations on micro-fracture behavior as well as AE response.

Hydrogeological controls on chromium enrichment along the groundwater flow path in the Baiyangdian Catchment, North China Plain

Although the natural occurrence of high chromium (Cr) groundwater has been intensively investigated in bedrock or sedimentary aquifers, the impacts of hydrogeological conditions on dissolved Cr distribution are poorly understood. In this study, groundwater samples from recharge mountain area (Zone I) through runoff area (Zone II) to discharge area (Zone III) were taken from bedrock and sedimentary aquifers approximately along the flow path in Baiyangdian (BYD) catchment, China, to reveal how hydrogeological conditions and hydrochemical evolution contributed to Cr enrichment in groundwater. Results showed that dissolved Cr was dominated by Cr(VI) species (>99 %). Around 20 % of studied samples had Cr(VI) exceeding 10 μg/L. Groundwater Cr(VI) was of natural origin, which generally increased along the flow path, and high concentrations (up to 80.0 μg/L) were observed in deep groundwater of Zone III. At the local scales, geochemical processes including silicate weathering, oxidation, and desorption under weakly alkaline pH, predominately contributed to Cr(VI) enrichment. Principal component analysis showed that oxic conditions were the principal control of Cr(VI) in Zone I, and geochemical processes (especially Cr(III) oxidation and Cr(VI) desorption) predominantly enhanced groundwater Cr(VI) enrichment in Zones II and III. However, at the regional scale, Cr(VI) enrichment was dominantly facilitated by the low flow rate and recharge of paleo-meteoric water due to the long-term water-rock interaction in the BYD catchment.

Comparison of water-caprock and CO2-water-caprock geochemical reactivity in the Triassic Heshanggou Formation of Ordos Basin

This study investigates the influence of geochemical reactions among CO2, water and key minerals on caprock sealing using laboratory experiments and numerical simulations, based on mudstone caprock of the Triassic Heshanggou Formation at the Shenhua CCS demonstration project in Ordos Basin. The results show that there is an obvious dissolution occurring in caprock after long-term water-rock interactions without CO2 injection, with a maximum increase of 11% in caprock permeability. The dissolution of kaolinite, chlorite, K-feldspar and quartz results in the increase of caprock permeability, and the precipitation of illite and hematite is the key factor effectively slowing down the change of permeability. However, the self-sealing capability of the caprock is continuously enhanced under the condition of CO2 injection, with permeability reduced by 40% of the original permeability at maximum. The precipitation of carbonate minerals directly induces the change of caprock permeability, and the precipitation of Ca-smectite and quartz further promotes the caprock sealing. Dolomite, siderite, magnesite and dawsonite are the key CO2 trapping minerals for the enhancement of caprock sealing, and the maximum amount of CO2 trapped in mineral phase is 35.44 kg/m3 medium. The dissolution of albite, chlorite and calcite provides the necessary Na+, Mg2+, Ca2+ and Fe2+ for the precipitation of carbonate minerals, which is conducive to the enhancement of caprock sealing, while chlorite and calcite are the key minerals affecting the caprock sealing. The dissolution of K-feldspar is not conducive to the enhancement of caprock sealing. This study could provide a theoretical basis for selection of reservoir-caprock and safety evaluation of CO2 geological storage.

Chronological Analysis and Remote Sensing of Craters on the Surface of Mars

In the present research, we carried out detailed chronological and compositional analyses along with detailed spectral analysis of three unnamed craters on the surface of Mars. Knowledge on chronology/age analysis and compositional analysis of Mars’ surface is essential for future manned and unmanned missions. The study area is near the landing site of previous landed missions, which could be used for future landing. The area is interesting to be studied because of its high elevation in the northeastern side and low elevation in the southern side, consisting of three major geological boundaries, i.e., Hesperian, Noachian, and Amazonian, which are further subdivided into fourteen units. Chronological investigations were carried out using the active machine learning approach and Craterstats 2.0 software, which revealed the age plot of 3.09 ± 0.04 Ga for Amazonian, 3.63 ± 0.0 Ga for Hesperian, and 3.73 ± 0.0 Ga for Noachian geological units, stating that N(1) craters’ density corresponds to the early Amazonian, early Hesperian, and late Noachian/early Hesperian periods according to the established crater density boundaries, respectively. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM)-derived browse products are used for the compositional study of the surface characteristics of Mars. A spectral investigation was performed on an unnamed crater belonging to the Amazonian period, which showed to be majorly composed of oxides as the primary mineral, indicating the spectra of hematite, boehmite, and akaganeite. A Hesperian unit-unnamed crater shows the signature of monohydrated sulfates, melilite, illite, and kaolinite minerals in the region. For the unnamed crater 3, which belongs to the Noachian period, it has diagnostic absorptions of clay minerals in their extracted spectra, indicating the sign of long-term water–rock interactions in the period. Derived chronology results and compositional studies of craters help in better understanding the geological formation units of Mars’ surface.

Hydrogeochemical and geothermal controls on the formation of high fluoride groundwater

High fluoride (F–) concentrations in groundwater affected by geothermal activity have been reported worldwide, but the genesis mechanism is not adequately understood. In this study, 92 groundwater samples and 63 sediment samples were collected from the China’s Guide basin for chemical analyses and laboratory tests. Results indicate that groundwater F– concentrations continuously increase along a flow path, being consistent with trends of Na+ and HCO3– concentrations, Na/Ca molar ratio, and pH values, while Ca2+ concentrations show a decreasing trend. Water-soluble F– contents in sediments increase as the depth increases. Higher-F– groundwater samples generally have lower δD and δ18O values than those of lower F– samples. The major hydrogeochemical processes controlling the formation of high-F– groundwater include precipitation of Ca2+ or cation exchange between Ca2+ and Na+ or both, which enhance dissolutions of F–-containing minerals and facilitate the release of F– into groundwater. Slightly alkaline environments and increased HCO3– and CO32– concentrations favor desorption of F– from solid surfaces into groundwater. Closed hydrogeological conditions and long-term water–rock interactions in confined aquifers accumulate F– in groundwater. Exposed geothermal springs around the Guide basin also contribute to the formation of high-F– groundwaters directly by the input of geothermal water and indirectly by heat transfer into aquifers at a regional scale, which is a problem deserving further investigation.

Long-term analyses of water–rock interaction and its effect on Gölcük Lake water quality (Isparta-Turkey)

Water quality varies significantly depending on degree of leaching from aquifer rocks and pathway rocks. Waters’ chemical compositions are determined by degree of water–rock interaction, mineralogical and geochemical properties of lithological structures, and environmental factors. Golcuk Lake is used partly as the source of Isparta City’s drinking water. Volcanic rocks in this region are lava, volcanic breccia, agglomerate, and tuff and range from andesite to trachyandesite. Main hydrogeochemical facies determined in the area are (Na+-HCO3−). Concentrations of Mg2+, Na+, SO42−, and Cl− in the lake water samples increased in some years resulting from ion exchange of rock forming minerals by rainwater. The purpose of this research is to provide a better understanding of long-term water–rock interaction and its effect on water quality. Water quality indices were calculated using the Turkish Standard (TS) 266 of the World Health Organization (WHO) and the use of Golcuk Lake water as drinking water was assessed by Isparta. The fluoride content in Golcuk Lake was determined to be higher than the average.

The weakening mechanisms of the rock mechanics of marlite bank slopes under water–rock interaction conditions

Karst-related issues have become serious in the Three Gorges reservoir area, where the bank slopes consist of marlite. In this paper, the mineralogical and petrological properties and internal micro-structure development mechanism of the marlite were analyzed to determine its dissolution and weathering characteristics. In addition, the application of high-power electron microscopy identified micro-cracks in the marlite and micro-dissolution vugs on both sides of the micro-cracks. Montmorillonite was shown to be the dominant clay mineral accumulated at the edges of the vugs. Based on the test results of the rock mechanics and the natural dissolution processes, the concept of dissolution coefficient was proposed to describe the strength change pattern of the marlite. Next, the change pattern between the dissolution and strength weakening of the marlite was characterized with the typical four stages of rock compression and deformations. It was also shown that long-term water–rock interactions resulted in unique deformation characteristics in the slopes, while, vertically, the slope bodies could be divided into a strong dissolution zone, medium dissolution zone, and weak dissolution zone.

Evaluation of Long-Term Impacts of CO2 Leakage on Groundwater Quality Using Hydrochemical Data from a Natural Analogue Site in South Korea

Three hydrochemical types of CO2-rich water (i.e., Na-HCO3, Ca-Na-HCO3 and Ca-HCO3) occur together in the silicate bedrock (granite and gneiss) of Gangwon Province in South Korea. As a natural analogue of geological carbon storage (GCS), this can provide implications for the environmental impacts of the leakage of CO2 from deep GCS sites. By using hydrochemical and isotopic datasets that were collected for previous and current studies, this study aimed to carefully scrutinize the hydrochemical differences in the three water types with an emphasis on providing a better understanding of the impacts of long-term CO2 leakage on groundwater quality (especially the enrichments of minor and trace metals). As a result, the Na-HCO3 type CO2-rich water contained higher Li, Rb and Cs than the Ca-HCO3 type, whereas Fe, Mn and Sr were higher in the Ca-HCO3 type than in the Na-HCO3 type despite the similar geological setting, which indicate that the hydrochemical differences were caused during different geochemical evolutionary processes. The δ18O and δD values and tritium concentrations indicated that the Na-HCO3 type was circulated through a deep and long pathway for a relatively long residence time in the subsurface, while the Ca-HCO3 type was strongly influenced by mixing with recently recharged water. These results were supported by the results of principal component analysis (PCA), whose second component showed that the Na-HCO3 type had a significant relation with alkali metals such as Li, Rb and Cs as well as Na and K and also had a strong relationship with Al, F and U, indicating an extensive water-rock interaction, while the Ca-HCO3 type was highly correlated with Ca, Mg, Sr, Fe and Mn, indicating mixing and reverse cation exchange during its ascent with hydrogeochemical evolution. In particular, the concentrations of Fe, Mn, U and Al in the CO2-rich water, the result of long-term water-rock interaction and cation exchange that was enhanced by CO2 leakage into silicate bedrock, exceeded drinking water standards. The study results show that the leakage of CO2 gas and CO2-rich fluid into aquifers and the subsequent hydrogeochemical processes can degrade groundwater quality by mobilizing trace elements in rocks and consequently may pose a health risk.

Enrichment of Geogenic Ammonium in Quaternary Alluvial-Lacustrine Aquifer Systems: Evidence from Carbon Isotopes and DOM Characteristics.

Geogenic ammonium in groundwater owing to mineralization of natural organic matter (NOM) has been reported in different geologic settings, but detailed mechanisms responsible for high ammonium concentration levels are poorly understood. To this end, we chose Quaternary high ammonium aquifer systems in central Yangtze River basins and used carbon isotopes in both dissolved organic carbon and inorganic carbon together with characterization of dissolved organic matter (DOM) and groundwater chemistry to reveal mechanisms related to the genesis of ammonium. The results indicate that high levels of geogenic ammonium (up to 33.50 mg/L as N) occur due to long-term water-rock interactions in a relatively sluggish hydrogeological environment with abundant organic matter that is rich in both C and N. The stable carbon isotope data suggest that ammonium in the groundwater is released from intensive degradation of organic matter with higher contents of ammonium associated with methanogenesis. The optical signatures of DOM indicate ammonium in the groundwater is mostly associated with terrestrial humic-like components rather than protein-like components. Molecular characterization of DOM by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) shows that, compared to low ammonium groundwater, high ammonium groundwater has larger mass weights, greater abundance of CHO+N compounds, higher percentages of lignin- and condensed-hydrocarbon-like components, lower H/C ratios, higher nominal oxidation state of carbon (NOSC) values and more double bonds, rings, and aromatic structures. Strong degradation of NOM and preferential utilization of energetically more favorable, terrestrial humic-like components (lignin-like as the main class) with high NOSC values facilitates the formation of high ammonium groundwater. To the best of our knowledge, this is the first effort to use carbon isotopes and DOM characteristics to identify enrichment mechanisms for geogenic ammonium in alluvial-lacustrine aquifer systems.

Self-healing effect of water-conducting fractures due to water-rock interactions in undermined rock strata and its mechanisms

In underground coal mining regions with damaged overlying strata, under the combined effects of long-term water-rock interactions (WRIs) and mining-induced stress, the permeability of water-conducting fractures (WCFs) decreases significantly due to the self-healing of fractures. Such self-healing helps to reduce the range of fractured water-conducting zone in the overlying strata and prevents the loss of groundwater. Understanding the self-healing mechanism is therefore of great significance for the scientific evaluation of mining damage to aquifers and their self-restoration capacity. It was found, according to the WRI experiments, that the self-healing mechanism involves the following three physical and chemical processes: (1) the sealing of fractures due to the swelling of hydrophilic clay minerals and subsequent blockage of the fracture space due to the argillisation and slaking of clay minerals; (2) the dissolution and corrosion of fracture surfaces due to the flow of groundwater, which leads to reduced surface roughness in the fractures, thereby allowing the fracture walls to seal more closely under the action of mining-induced stress; and (3) ion exchange between the anions and cations present in groundwater and in the rock minerals, including aluminosilicates or carbonates, which leads to the production of secondary minerals or crystalline precipitates that eventually fill or block the fractures. The long-term cumulative effect of the compaction and blockage of WCFs is that their permeability gradually decreases, thereby demonstrating the self-healing mechanism. The results can provide a theoretical reference for water preservation and ecological management of the water-deficient mining areas in northwest China.

Origin of fluids discharged from mud volcanoes in SE Iran

Abstract Onshore and offshore mud volcanism in the Makran accretionary prism is related to convergence of the Arabian and Eurasian plates. This study describes the chemical and isotopic composition of hydrocarbon-rich fluids from four active on-shore mud volcanoes located along the Makran coast (southern Iran), namely Borborok, Ain, Napag and Sand Mirsuban (Makran coast, southern Iran), as well as Pirgel mud volcano (SE Iran) that is located between the Taftan and Bazman igneous volcanoes. The main aim was to provide insights into the source region(s) of gases and waters discharged from these systems and the secondary processes controlling their chemical features. The four on-shore mud volcanoes emitted CH4-dominated gases, with significant concentrations of C2+ alkanes suggesting a dominant thermogenic origin, as confirmed by their δ13C–CH4 values. Carbon dioxide was present at relatively low concentrations (0.78–2.33%) with an isotopic signature (δ13C-CO2 from −34.2 to −11.1‰ vs. V-PDB) in the range of that typical of thermogenic gases. Hence, the geochemical features of these mud volcanoes point to the occurrence of a deep gas source rich in hydrocarbons, although the occurrence of an exploitable gas reservoir has to be confirmed by geophysical measurements and detailed geostructural surveys. Gas chemistry from Pirgel mud volcano completely differs with respect to that of the previous ones, since the former emits gases dominated by CO2 and showing relatively high R/Ra values (≈1.6), suggesting a significant fluid contribution from the nearby volcanic systems. Moreover, waters from the on-shore mud volcanoes showed a Na-Cl composition, typically associated with mud volcanism, whereas those discharged from Pirgel were Na-HCO3-type and rich in chemical species typical of hydrothermal fluids such as As. Waters from the on-shore volcanoes were characterized by a strong δ18O-positive shift and high B and Li concentrations, likely indicating clay mineral dehydration and long-term water-rock interaction. Such geochemical features were also shown by the waters from Pirgel, where the high concentrations of B and Li were possibly related to volcanic source. Estimated temperatures for the on-shore mud volcanoes estimated using the Mg–Li and Mg-K geothermometers range from 84 to 165 °C, corresponding to 3–7 km depth.

Potential impact of leaking CO2 gas and CO2-rich fluids on shallow groundwater quality in the Chungcheong region (South Korea): A hydrogeochemical approach

This study evaluates the potential impact of leaking CO2 gas and CO2-rich waters on shallow groundwater quality by focusing on the mobilization of trace elements. For this study, Group I (acidic CO2-rich waters with low TDS), Group II (slightly acidic CO2-rich waters with high TDS), and Group III (CO2-poor waters with low TDS) were sampled in the study area occurring naturally CO2-rich waters. The carbon isotope data (δ13C and 14C) indicate that the dissolved CO2 in CO2-rich water originates from old and deep-seated CO2. Our results show that Group I is formed by the direct input of CO2 gas into recently recharged shallow groundwater, whereas Group II discharging along fractures or wells occurs by long-term CO2-water-rock interactions in the subsurface. Group I shows higher concentrations of trace elements (Al, Ba, Be, Cr, Cs, Fe, Mn, Ni, Rb, and U) than Group III, and its Al and Mn concentrations slightly exceed the US EPA drinking water standard. However, Group II has significantly higher concentrations of trace elements than Group I. In particular, harmful Be exceeds the US EPA drinking water standard by over 6.5-fold. In addition, Fe and Mn exceed the US EPA drinking water standard by 27.7-fold and 16.1-fold, respectively. The higher concentration of bicarbonate by pH buffering in Group II also increases the mobilization of U6+. Our results show that the mobilization of trace elements in the aquifer having little buffering capacity is mainly determined by the degree of CO2-water-rock interactions. This study indicates that upwelling of deep CO2-rich fluids with long-term water-rock interactions can be harmful to shallow potable groundwaters by supplying high levels of trace elements into shallow aquifers.

The Dynamic Response Law of Bank Slope under Water-Rock Interaction

During the reservoir operation process, the long-term security and stability of the bank slope is affected by dynamic response characteristics of its seismic action directly. Aimed at the typical bank slope existing in the actual reservoir environment, an experiment considering reservoir water level fluctuation and soaking-air-drying cyclic water-rock interaction has been designed and conducted while the cyclic loading test was performed in different water-rock cycles. Research results indicate the following: Firstly, in the process of water-rock interaction, the dynamic characteristics of sandstone show evident degradation trend, with the increase of the damping ratio and Poisson’s ratio and decrease of dynamic elastic modulus, and the former six water-rock cycle degradation effects are particularly obvious. Secondly, the numerical analog computation analysis of dynamic response in typical bank slope shows that as the water-rock interaction period is increased, the dynamic response of the slope hydro-fluctuation belt zone increases gradually, while the other parts weaken. Thirdly, under the long-term water-rock interaction process, the hydro-fluctuation belt zone gradually becomes a “soft layer” which is sensitive to the earthquake effect and dynamic response, resulting in a direct influence on long-term seismic performance of the bank slope. Therefore, it is necessary to make better protection for the bank slope hydro-fluctuation belt zone.

Geochemical Genesis of Arsenic in the Geothermal Waters from the Rehai Hydrothermal System, Southwestern China

The geothermal waters from the Rehai magmatic hydrothermal system, located in the Tengchong volcanic region, south-western China, are characterized by diversified hydrochemical types and a wide range of pH values. The neutral hot springs in Rehai generally have much higher arsenic concentrations than the acid hot springs. Further inspection shows that the acid, sulfate-rich springs are essentially locally-perched groundwaters heated by H2S-rich steam separated from deep geothermal fluid. Hence, the dissolution of near-surface rocks at moderate temperatures is the sole source of arsenic in the steam-heated acid springs. In comparison, the arsenic in the neutral hot springs, which represent the deep-circulating geothermal fluids, comes from long-term water-rock interactions at much higher reservoir temperatures. Moreover, the possibility that magmatic fluid is a potential arsenic source for the neutral springs in Rehai cannot be ruled out.

Long-term Water-rock Interaction in the Bohemian Cretaceous Basin (Central Europe): Field Evidence and Modelling

Two models of reversible and irreversible water – mineral interaction are developed to simulate geochemical evolution of old groundwater in the Cenomanian and Turonian sandstone aquifers in the Bohemian Cretaceous Basin in Central Europe. The kinetic constants of dissolution and equilibrium constants of reversible reactions are results of laboratory experiments published in literature. Reactive surface areas of minerals are calculated. The model simulates the evolution of the chemical composition of groundwater whose age was derived from isotopic data. The results are compared to the chemical composition of groundwater along the trajectory of the prevailing flow. The model represents the interaction between groundwater and sandstones containing quartz, amorphous silica, a mixture of clay minerals, calcite and traces of residual feldspars. The three processes of water rock interaction are the irreversible dissolution of feldspars and carbonate, aluminium and silica equilibrium. The composition of groundwater in the Cenomanian aquifer with a confined water table is a result of geochemical evolution during the last 26,000 years, while the groundwater in the Turonian aquifer with a free water table is a mixture of old groundwater and recent water infiltrated from the surface. The proposed models are applicable to confined aquifers with residual rock matrix derived from granitic rocks decomposed by weathering without mixing of waters of different ages.

How does the water–rock interaction of marly rocks affect its mechanical properties in the Three Gorges reservoir area, China?

Marly rock is a sensitive rock group of landslides in the Three Gorges reservoir area, China. It is composed predominantly of carbonate and clay minerals, water–rock interaction (WRI) of which could activate landslides in the reservoir area. To study the mechanism by which WRI affects the mechanical properties of marly rock, samples were collected from two boreholes (depth 301.78 and 307.14 m) and slope surface. Then, laboratory tests were designed to study the quantitative relationship between mineral contents and mechanical properties of intact rock, to analyze both change process of mineral composition and microstructure under short-term and long-term WRI. Finally, the change in mechanical properties and its effect on slope stability are suggested. This study indicates that the uniaxial compressive strength and Poisson ratio can be estimated by linear regression equations: (1) σ c = 8.959 × (C/Q) − 0.744 × CM + 58.516; (2) μ = 0.014 × (C/Q) − 0.001 × CM + 0.234. The chemical reactions of WRI mainly included dissolution and ion exchange. On the slope surface, dissolution mainly acts on calcite, illite, dolomite, feldspar and other minerals dissolved in water. Underground, both chemical reactions of dissolution and ion exchange easily approach equilibrium with long-term seepage. Small-size minerals and micropores damage the stable microstructure of marly rock. These changes of minerals and microstructure can trigger shallow slope failure and develop deep creep deformation along some crash zones in the reservoir shoreline.

Modeling of Long-Term Water Rock Interaction in the Bohemian Cretaceous Basin

This work focuses on a model of water-rock geochemical interaction and its testing on data from hydrogeological and isotopic research of rates of percolation of ground water in Cenomanian and Turonian aquifers of the Bohemian Cretaceous Basin. A simple geochemical model of percolation in Cenomanian and Turonian aquifers is presented. Using calibration with measured data, the dissolution rates of minerals or reaction surfaces of minerals are obtained. The results are discussed.

Hydrogeochemistry of high fluoride groundwater in shallow aquifers, Hangjinhouqi, Hetao Plain

Hydrochemical characteristics of groundwater, the lithological features of the aquifer sediments and fluoride in rocks were studied to determine the distribution and genesis of high fluoride groundwater (>1.5mg/L) in Hangjinhouqi, which is an endemic fluorosis areas. Groundwater with high fluoride (>1.5mg/L) is mainly of HCO3–Na or HCO3–Cl–Na type for both fresh and brackish water. Different from other halide ions of Cl−, Br− and I−, F− in groundwater decreased with the TDS. Analysis of fluorine in rocks showed that the average of total fluorine in rocks in Hangjinhouqi (1278.84mg/kg) is much higher than the background value of Inner Mongolia (316.2mg/kg). During long-term water–rock interaction, fluoride in these rocks from Yin Mountains will be released out and then accumulated in the groundwater of sand or clay (silty clay) aquifers in the Hangjinhouqi area. All the water samples fall below the Ksp curve of CaF2. Due to CaF2 solubility control, Na-predominant water is favorable for F enrichment, that's why groundwater with high fluoride displayed low TDS in the study area.

Characterisation and isotopic evolution of saline waters of the Outokumpu Deep Drill Hole, Finland – Implications for water origin and deep terrestrial biosphere

The isotopic composition of water and dissolved Sr as well as other geochemical parameters at the 2516m deep Outokumpu Deep Drill Hole, Finland were determined. The drill hole is hosted by Palaeoproterozoic turbiditic metasediments, ophiolite-derived altered ultramafic rocks and pegmatitic granitoids. Sodium–Ca–Cl and Ca–Na–Cl-rich waters (total dissolved solids up to ca. 70gL−1) containing significant amounts of gas, mainly CH4 (up to 32mmolL−1), N2 (up to 10mmolL−1), H2 (up to 3.1mmolL−1) and He (up to 1.1mmolL−1) discharge from fracture zones into the drill hole. This water is distinct from the shallow fresh groundwater of the area, and has an isotopic composition typical of shield brines that have been modified during long-term water–rock interaction. Based on water stable isotopes and geochemistry, the drill hole water profile can be divided into five water types, each discharging from separate fracture systems and affected by the surrounding rocks. The δ2H varies from −90‰ to −56‰ (VSMOW) and δ18O from −13.5‰ to −10.4‰ (VSMOW), plotting clearly above the Global and Local Meteoric Water Lines on a δ2H vs. δ18O diagram. The 87Sr/86Sr ratios range between 0.72423 and 0.73668. Simple two-component mixing between 2H and 18O rich end-member brine and meteoric water cannot explain the water stable isotopic composition and trends observed. Instead, hydration of silicates by ancient groundwaters recharged under different climatic conditions, warmer than at present, is the most likely mechanism to have caused the variation of the δ2H and δ18O values. Water types correlate with changes in microbial communities implying that different ecosystems occur at different depths. The different water types and microbial populations have remained isolated from each other and from the surface for long periods of time, probably tens of millions of years.

Strontium isotope geochemistry and its geochemical implication from hot spring waters in South Korea

Despite being a non-volcanic area, the southern Korean Peninsula has a number of hot water springs with temperatures more than 40 °C, most of which are commercially used for spa. The hot springs are located at the fringes of the granite body rather than the center of the Mesozoic granite. Here we report on the geochemical characteristics of the hot water springs. The hot springs in the Jurassic granite area are mainly of Na( Ca) HCO 3 type, whereas those in the Cretaceous granite area are of Na Cl ( SO 4) type. The stable isotope composition of O and H suggests a meteoric origin of the hot spring water. Further, the chemical components of the hot spring waters indicate that they were all derived from hot spring water–rock interaction rather than through anthropogenic input. The 87Sr/ 86Sr ratio of the hot spring waters in the Jurassic granite area ranges from 0.711088 to 0.729034 and that in the Cretaceous granite area ranges from 0.705652 to 0.707912. Such 87Sr/ 86Sr ratio distribution of hot spring waters corresponds with the distribution of the initial 87Sr/ 86Sr ratios for the emplacement age of the Jurassic and Cretaceous granites in the hot spring occurrence area. The 87Sr/ 86Sr ratios in the hot spring waters were also divided into three groups according to their geographical location: eastern, western, and southern groups. The constant 87Sr/ 86Sr ratio of hot spring water from one hot spring area during the last 6 years strongly suggests that the hot-spring-bearing aquifer system in South Korea should be separated from the upper cold shallow groundwater system. This indicates that the 87Sr/ 86Sr ratio of the hot spring waters in the granite area of South Korea originates from an equilibrium because of long-term water–rock interaction rather than short-term water–rock interaction due to the input of recent meteoric water. Hence, it may be concluded that hot spring water in the granite area may be a limited deep groundwater resource that might deplete with overuse.