Hydrogeological and mixing process of waters in deep aquifers in arid regions: south east Tunisia

This study addresses the hydrogeochemistry of thermal and cold waters from south east Tunisia. Temperature intervals are 38.5–68 °C and 22–27.8 °C for thermal water and cold water, respectively. Three distinct hydrogeological systems supply water either for irrigation or for drinking; they are: (1) the Continental Intercalaire geothermal aquifer (CI), (2) the Turonian aquifer and (3) the Senonian aquifer. A synthetic study including hydrochemical, hydrogeological and geothermal approaches have been applied in order to evaluate the inter-aquifers water transfer in south east of Tunisia. By using silica geothermometers and saturation indices for different solid phases, estimated thermal reservoir temperature varies between 52 and 87 °C and between 75 and 110 °C, respectively. Based on chemical and thermal data, mixing, which occurs between the ascending deep geothermal water and shallow cold water, is about 57 % cold water.     

Hydrochemistry and geothermometry of thermal groundwater of southeastern Tunisia (Gabes region)

Given the vital importance of water and energy in desert regions, we undertook a study dealing with the deep reservoirs in Gabes area, which is located in the southeastern part of Tunisia. Geothermal resources are taken from the Intercalary Continental [or Continental Intercalaire (CI)], known as the largest deep aquifer in Tunisia and are used in a number of applications, mainly in agriculture. Previous investigations performed on the thermal waters of this area focused on the genesis of the deep waters with regard to the thermal features of geothermal reservoirs. A more detailed investigation has been carried out, considering both deep and shallow waters. In order to estimate the potential temperatures of deep reservoir in the Gabes area, we developed a synthetic study including chemical geothermometers, multiple mineral equilibrium approach, and other approaches. Chemical types of the thermal waters and effects of mixing between shallow cold waters with deep thermal waters were also discussed. In fact, the application of Na–K–Mg diagram relative to deep geothermal reservoir capitulate estimated temperatures (about 90°C). In addition, the multiple mineral equilibrium approach submits a similar estimated temperature ranging between 65 and 70°C, showing a disequilibrium status which indicates a possibly mixing with surface water. Indeed, wells exploiting the CI aquifer in the south part of the studied area showed the same characteristics, corroborating the reliability of the applied methods.     

Mixing processes in hydrothermal spring systems and implications for interpreting geochemical data: a case study in the Cappadocia region of Turkey

Mixing is a dominant hydrogeological process in the hydrothermal spring system in the Cappadocia region of Turkey. All springs emerge along faults, which have the potential to transmit waters rapidly from great depths. However, mixing with shallow meteoric waters within the flow system results in uncertainty in the interpretation of geochemical results. The chemical compositions of cold and warm springs and geothermal waters are varied, but overall there is a trend from Ca–HCO₃ dominated to Na–Cl dominated. There is little difference in the seasonal ionic compositions of the hot springs, suggesting the waters are sourced from a well-mixed reservoir. Based on δ¹⁸O and δ²H concentrations, all waters are of meteoric origin with evidence of temperature equilibration with carbonate rocks and evaporation. Seasonal isotopic variability indicates that only a small proportion of late spring and summer precipitation forms recharge and that fresh meteoric waters move rapidly into the flow system and mix with thermal waters at depth. ³H and percent modern carbon (pmC) values reflect progressively longer groundwater pathways from cold to geothermal waters; however, mixing processes and the very high dissolved inorganic carbon (DIC) of the water samples preclude the use of either isotope to gain any insight on actual groundwater ages.     

Geochemical and isotopic characteristics of fluids in the Niutuozhen geothermal field,North China

Niutuozhen geothermal field is located in the Jizhong graben, belonging to the northern part of Bohai Bay Basin in North China. Chemical and isotopic analyses were carried out on 14 samples of the geothermal fluids discharged from Neogene Minghuazhen (Nm), Guantao (Ng), and Jixianian Wumishan (Jxw) formations. The δ²H and δ¹⁸O in water, δ¹³C in CH₄, δ¹³C in CO₂, and ³He/⁴He ratio in the gases were analyzed in combination with chemical analyses on the fluids in the Niutuozhen geothermal field. The chemical and isotopic compositions indicate a meteoric origin of the thermal waters. The reservoir temperatures estimated by chemical geothermometry are in the range between 60 and 108 °C. The results show that the gases are made up mainly by N₂ (18.20–97.42 vol%), CH₄ (0.02–60.95 vol%), and CO₂ (0.17–25.14 vol%), with relatively high He composition (up to 0.52 vol%). The chemical and isotopic compositions of the gas samples suggest the meteoric origin of N₂, predominant crustal origins of CH₄, CO₂, and He. The mantle-derived He contributions are calculated to be from 5 to 8% based on a crust–mantle binary mixing model. The deep temperatures in the Jxw reservoir were evaluated based on gas isotope geothermometry to be in the range from 141 to 165 °C. The mantle-derived heat fraction in the surface heat flow is estimated to be in the range of 48–51% based on ³He/⁴He ratios.     

Hydrogeochemistry and geothermometry of thermal springs from the Guelma region,Algeria

This paper reports the results of our studies, the chemical analysis of thermal spring’s waters and their geological settings, the use of different statistical methods to evaluate the origin of the dissolved constituents of spring waters and the estimation of the reservoir temperature of the associated geothermal fields of the Guelma region, Algeria. A major component in 13 spring water samples was analyzed using various techniques. The waters of the thermal springs at Guelma basin vary in temperature between 20 and 94oC. Q-mode hierarchical cluster analysis suggests three groups. The water springs were classified as low, moderate and high salinity. Mineral saturation indices (SI) calculated from major ions indicate the spring waters are supersaturated with the most of the carbonate minerals, and all of the spring water samples are under-saturated with evaporite minerals. The thermal spring waters have a meteoric origin, and all samples are immature with strong mixing between warm and shallow waters, where the temperatures of reservoirs to which the thermal waters are related ranged between 64° and 124°C. The deep circulation of meteoric waters in the study area is supplied by the high geothermal gradient around 4.5°C per 100 m and reaches a high temperature before rising to the surface. The estimated circulation depths ranged from 1425 and 3542 m.     

Numerical modelling of Alpine deep flow systems: a management and prediction tool for an exploited geothermal reservoir (Lavey-les-Bains, Switzerland)

The geothermal site of Lavey-les-Bains, Switzerland is an Alpine deep flow system in fractured crystalline rocks. Groundwater analyses since 1973 reveal a mixing process between a deep warm component (68°C and TDS 1.4 g/L) and cold shallow water. The production rate of the new deep well P600, installed in 1997, has amplified this mixing process in well P201, for which a decline in temperature and TDS has been observed. Numerical hydrogeological two-dimensional and three-dimensional models of heat, flow and mass transport have been developed to reproduce the geothermal system and to forecast the long-term exploitation potential of the geothermal resource. The computed temperature of the deep inferred reservoir (100–130°C) is in agreement with the geothermometers, whereas the simulated thermal water flux (5,400–9,000 m³/day) is probably underestimated. Different fluid production scenarios can reproduce the decline and stabilization phases of temperatures in the geothermal field since 1997. For P201, the mixing ratio calculated before and during the exploitation of P600 is comparable with observed data; the modelled temperature tends towards stabilization in P201 at 56°C after 10–15 years of production at P600. Another proposed new well is likely to reduce the thermal output of the existing wells.     

Hydrochemical characteristics and mixing behavior of thermal springs along the Bijiang River in the Lanping basin of China

The Lanping–Simao basin characterized by the occurrence of hydrothermal activities in the northern part of the basin is the largest depression basin in western Yunnan Province. Hot springs outcrop in the semi-drought red layers (TDS, 0.644–8.17 g/L). Mixing with shallow cold water in the flow system is the dominant hydrogeological process leading to the decrease in thermal water temperature and complex hydrochemical compositions with water types ranging from HCO₃–Ca·Mg to Cl–Na type. According to the silicon concentration and enthalpy evaluation, the geothermal reservoir temperatures are estimated between 118 and 204 °C, and the mixing ratio between cold and thermal groundwater is 0.76 on the average. The high concentrations of Na and Cl indicate that there is some well-mineralized cold water in the flow system circulating to the upward migration path accompanied with intense water–rock reactions in the fault developed basin. Ca and SO₄ originate from progressively longer flow path owing to the poor solubility of gypsum, anhydrite and Ca–montmorillonite. Hierarchical clustering and multivariate statistical methods recognized three hot springs groups and four parameters groups which classified the reservoir environment into two situations, one is in the high-pressure environment with intense degassing, and the other is in the lower pressure with weak minerals migration.     

Hydrogeochemistry and geothermometry of the Jowshan thermal springs, Central Iran

Jowshan geothermal system comprises 6 thermal springs with outlet temperatures ranging from 39.3 to 46.6°C. The thermal water of these springs is presently used for swimming and as a treatment for rheumatism, sinusitis and skin diseases. The pH value of these springs is slightly acidic to neutral and the electrical conductivities about 1500 μS/Cm. The presence of many faults in the area, the alignment of all springs along the Sirch Fault and the similar chemical and isotopic composition of all springs in combination with the hydrogeological setting and geochemistry of water samples indicate that these springs are associated with deep circulation of meteoric water. According to this heating mechanism, meteoric waters infiltrate through fault openings to depth and after heating by geothermal gradient rise to the ground surface due to the hydraulic and buoyancy forces, a mechanism which is common in the southern parts of Iran. The use of various chemical geothermometers and mineral equilibrium states suggests a range of temperature about 50–90°C for the reservoir of Jowshan geothermal system.     

Geochemical characterization and origins of the thermal springs in southern Gaoligong Mountains,China

Hydrogeochemistry and environmental isotope data were utilized to understand origins and characteristics of the thermal springs in southern Gaoligong Mountains, China. The groundwater at the thermal springs has low values of total dissolved solids, and its main water types are Na-HCO₃. The thermal springs are mainly recharged from meteoric precipitations. The recharge areas are located near the springs at an approximate elevation of 1,800 m. The groundwater of the thermal springs is immature and partially equilibrated with a strong mixture of the shallow cold waters during the flow process. The shallow cold water accounts for more than 90 %. The temperatures of thermal reservoir that feed the springs are between 146 and 260 °C, and the calculated groundwater circulation depths range from 2,000 to 5,700 m below ground surface.     

Characteristics of the Reservoir of the Rehai Geother-mal Field in Tengchong,Yunnan Province,China1

Abstract The Rehai geothermal field in Tengchong County, Yunnan Province is a significant high- temperature hydrothermal convective system. The geothermal reservoir is composed of granite. Various geothermometers are used to evaluate the reservoir temperature. The most likely temperature of the reservoir as represented by T_Na-K-Ca is about 230 °C. The chemical and isotopic compositions of fluids before boiling within the reservoir are estimated. The mixing and dilution of cold and warm waters are discussed. The Rehai geothermal field is a high- temperature (hot) water system with the subsurface boiling zone close to the surface. The reservoir pressure at different depths is calculated. And finally the water-rock equilibration is inferred.     

Properties of geothermal resources in Kebilli region, Southwestern Tunisia

The Kebilli region is located in the Southwestern part of Tunisia, and is characterized by the presence of deep and shallow geothermal systems (continental intercalary and complex terminal). Chemical and isotopic contents are used to classify the type and determine the origin of thermal water. An evaluation of reservoir temperature and a possible geothermal fluid mixing are also carried out. Both continental intercalary-deep aquifer and complex terminal-shallow aquifer are of Na–(Ca)–Cl–(SO₄) mixed water type. The use of different geothermometers and the computation of saturation indexes for different solid phases suggests that the thermal reservoir temperature of the continental intercalary is between 92 and 105 °C, while the fluid temperature from the shallow complex terminal aquifer ranges from 50 to 75 °C. Also, the isotopic data indicates the old origin of all groundwater of Southwestern Tunisia. Mixing effects characterizing the continental intercalary and the complex terminal aquifers were identified using δ²H and δ¹⁸O relationship. It appears that the upward movement of thermal water from the deep aquifer to shallow ones is probably due to the abundant fractures in the research area.     

Isotope–geochemical characterization and geothermometrical modeling of Uttarakhand geothermal field,India

Uttarakhand geothermal area, located in the central belt of the Himalayan geothermal province, is one of the important high temperature geothermal fields in India. In this study, the chemical characteristics of the thermal waters are investigated to identify the main geochemical processes affecting the composition of thermal waters during its ascent toward the surface as well as to determine the subsurface temperature of the feeding reservoir. The thermal waters are mainly Ca–Mg–HCO₃ type with moderate silica and TDS concentrations. Mineral saturation states calculated from PHREEQC geochemical code indicate that thermal waters are supersaturated with respect to calcite, dolomite, aragonite, chalcedony, quartz (SI > 0), and undersaturated with respect to gypsum, anhydrite, and amorphous silica (SI < 0). XRD study of the spring deposit samples fairly corroborates the predicted mineral saturation state of the thermal waters. Stable isotopes (δ¹⁸O, δ²H) data confirm the meteoric origin of the thermal waters with no oxygen-18 shift. The mixing phenomenon between thermal water with shallow ground water is substantiated using tritium (³H) and chemical data. The extent of dilution is quantified using tritium content of thermal springs and non-thermal waters. Classical geothermometers, mixing model, and multicomponent fluid geothermometry modeling (GeoT) have been applied to estimate the subsurface reservoir temperature. Among different classical geothermometers, only quartz geothermometer provide somewhat reliable estimation (96–140 °C) of the reservoir temperature. GeoT modeling results suggest that thermal waters have attained simultaneous equilibrium with respect to minerals like calcite, quartz, chalcedony, brucite, tridymite, cristobalite, talc, at the temperature 130 ± 5 °C which is in good agreement with the result obtained from the mixing model.     

Petrology of mafic enclaves in the 2006–2012 eruptive products of Bezymianny Volcano,Kamchatka

This paper reports the results of the first comprehensive petrological study of mafic enclaves widespread in the products of recent (2006–2012) eruptions of Bezymianny Volcano, Kamchatka. Four types of mafic enclaves were distinguished on the basis of the composition and morphology of minerals, P–T conditions of formation of mineral assemblages, and structural and textural characteristics of the rocks. Disequilibrium assemblages of mafic enclaves indicate a complex structure of the magmatic plumbing system of the volcano, including a shallow chamber with andesite–basaltic andesite magmas and a deep reservoir filled in part with plagioclase–hornblende cumulates and fed by basic magmas with mantle harzburgite xenoliths. The mafic enclaves were formed at different levels of the magmatic plumbing system of the volcano and correspond to different degrees of mixing of interacting magmas. The most abundant enclaves were formed during magma ascent from the deep reservoir (960–1040°C, 5–9 kbar) into the shallow andesitic chamber (940–980°C). Enclaves of plagioclase–hornblende cumulates from the basic magmas feeding the deep reservoir (T > 1090°C and P > 9 kbar) are much less common.     

Hydrogeochemical and isotopic characteristics of the Ilica geothermal system (Erzurum,Turkey)

In this paper, the hydrochemical isotopic characteristics of samples collected from geothermal springs in the Ilica geothermal field, Eastern Anatolia of Turkey, are examined and described. Low-temperature geothermal system of Ilica (Erzurum, Turkey) located along the Eastern Anatolian fault zone was investigated for hydrogeochemical and isotopic characteristics. The study of ionic and isotopic contents shows that the thermal water of Ilica is mainly, locally fed by groundwater, which changes chemically and isotopically during its circulation within the major fault zone reaching depths. The thermal spring has a temperature of 29–39 °C, with electrical conductivity ranging from 4,000 to 7,510 µS/cm and the thermal water is of Na–HCO₃–Cl water type. The chemical geothermometers applied in the Ilica geothermal waters yielded a maximum reservoir temperature of 142 °C according to the silica geothermometers. The thermal waters are undersaturated with respect to gypsum, anhydrite and halite, and oversaturated with respect to dolomite. The dolomite mineral possibly caused scaling when obtaining the thermal waters in the study area. According to the enthalpy chloride-mixing model, cold water to the thermal water-mixing ratio is changing between 69.8 and 75 %. The δ¹⁸O–δ²H compositions obviously indicate meteoric origin of the waters. Thermal water springs derived from continental precipitation falling on to higher elevations in the study area. The δ¹³C ratio for dissolved inorganic carbonate in the waters lies between 4.63 and 6.48 ‰. In low-temperature waters carbon is considered as originating from volcanic (mantle) CO₂.     

O,H, and Sr isotope evidences of mixing processes in two geothermal fluid reservoirs at Yangbajing,Tibet, China

The Yangbajing geothermal field with the highest reservoir temperature among Chinese hydrothermal systems is located about 90 km northwest to Lhasa City, capital of Tibet, where high temperature geothermal fluids occur in two reservoirs: a shallow one at a depth of 180–280 m and a deep one at 950–1,850 m. In this study, Oxygen-18 and deuterium isotope compositions as well as ⁸⁷Sr/⁸⁶Sr ratios of water samples collected from geothermal wells, cold springs and surface water bodies were characterized to understand the genesis of geothermal fluids at Yangbajing. The results show that the deep geothermal fluid is the mixing product of both magmatic and infiltrating snow-melt water, whereas the shallow geothermal fluid is formed by the mixing of deep geothermal fluid with cold groundwater. Using a binary mixing model with deep geothermal fluid and cold groundwater as two endmembers, the mixing ratios of the latter in most shallow geothermal water samples were calculated to be between 40 and 50%. The combined use of O, H, and Sr isotopes proves to be an effective approach to depict the major sources of geothermal fluids and the mixing processes occurring in two reservoirs at Yangbajing.     

A hydrochemical study of the Hammam Righa geothermal waters in north-central Algeria

This study focuses on the hydrochemical characteristics of 47 water samples collected from thermal and cold springs that emerge from the Hammam Righa geothermal field, located in north-central Algeria. The aquifer that feeds these springs is mainly situated in the deeply fractured Jurassic limestone and dolomite of the Zaccar Mount. Measured discharge temperatures of the cold waters range from 16.0 to 26.5 °C and the hot waters from 32.1 to 68.2 °C. All waters exhibited a near-neutral pH of 6.0–7.6. The thermal waters had a high total dissolved solids (TDS) content of up to 2527 mg/l, while the TDS for cold waters was 659.0–852.0 mg/l. Chemical analyses suggest that two main types of water exist: hot waters in the upflow area of the Ca–Na–SO₄ type (Hammam Righa) and cold waters in the recharge zone of the Ca–Na–HCO₃ type (Zaccar Mount). Reservoir temperatures were estimated using silica geothermometers and fluid/mineral equilibria at 78, 92, and 95 °C for HR4, HR2, and HR1, respectively. Stable isotopic analyses of the δ¹⁸O and δD composition of the waters suggest that the thermal waters of Hammam Righa are of meteoric origin. We conclude that meteoric recharge infiltrates through the fractured dolomitic limestones of the Zaccar Mount and is conductively heated at a depth of 2.1–2.2 km. The hot waters then interact at depth with Triassic evaporites located in the hydrothermal conduit (fault), giving rise to the Ca–Na–SO₄ water type. As they ascend to the surface, the thermal waters mix with shallower Mg-rich groundwater, resulting in waters that plot in the immature water field in the Na–K–Mg diagram. The mixing trend between cold groundwaters from the recharge zone area (Zaccar Mount) and hot waters in the upflow area (Hammam Righa) is apparent via a chloride-enthalpy diagram that shows a mixing ratio of 22.6 < R < 29.2 %. We summarize these results with a geothermal conceptual model of the Hammam Righa geothermal field.     

Hydrogeochemical and isotopic composition of a low-temperature geothermal source in northwest Turkey: case study of Kirkgecit geothermal area

Chemical and isotopic compositions of three hot springs and one cold spring in the Kirkgecit geothermal field, located 15 km southwest of Canakkale-Biga in the northwest of Turkey, were monitored five times during 2005 and 2007. The physico-chemical characteristics of the hot springs are average discharge 3–3.5 L/s, surface temperature 45–52°C, pH 8.9–9.3, and electrical conductivity (EC) 620–698 μS/cm. The cold spring has a temperature of 12–13°C, pH 7.5–8.3, and EC 653–675 μS/cm. The hot waters are Na-SO₄ type, whereas the cold water is Ca-HCO₃ type. Chemical geothermometers suggest that the reservoir temperature is around 80–100°C. The isotopic data (oxygen-18, deuterium and tritium) indicate that the thermal waters are formed by local recharge and deep circulation of meteoric waters.     

The origin of reservoir fluids in the geothermal field of Los Azufres,Mexico — isotopical and hydrological indications

The calculation of hydrological balance resulted in a potential, average annual infiltration rate of 446±206 mm/m² for the Los Azufres geothermal area, which corresponds to a total of 82×10⁶ m³ per a. Due to the highly fractured and faulted structure of the volcanic formations, a considerable potential for the infiltration of recent meteoric water into deeper sections of the volcanic formations can be assumed. Isotopic data indicate the minor importance of recent meteoric water for the recharge of the geothermal reservoir. Very negative δ¹³C values can be explained by the input of organic C from the surface, but the lack of ¹⁴C in the deep fluids reflects a pre-historic age for the infiltration event of fossil meteoric water. The dilution of the meteoric water by ¹⁴C-free CO₂ gas from a shallow magma chamber complicates the exact age determination of the infiltration event, which probably occurred during the Late Pleistocene or Early Holocene glacial period. Strong water–rock interaction processes, such as sericitization/chloritization, caused the primary brine composition to be camouflaged. A preliminary hydrological model of the reservoir can be postulated as follows: the fossil hydrodynamic system was characterized by the infiltration of meteoric water and mixing with andesitic and/or magmatic water. Strong water–rock interaction processes in the main part of the production zone prove the existence of former active fluid circulation systems. Due to changes in pressure and temperature, the rising fluids get separated into liquid and vapour phases at a depth of 1500 m. After cooling, the main portions of both phases remain within the convective reservoir cycle. Isotope analyses of hot spring waters indicate the direct communication of the reservoir with the surface at some local outcrops. A recent reactivation of the hydrodynamic system is caused by the geothermal production, as indicated by the detection of lateral communication between some production and reinjection wells.     

Hydrogeochemical and hydrogeological investigations of thermal waters in the Usak Area (Turkey)

Thermal waters of the Usak area have temperatures ranging from 33 to 63°C and different chemical compositions. These waters hosted by the Menderes Metamorphic rocks emerge along fault lineaments from two geothermal reservoirs in the area. The first reservoir consists of gneiss, schists, and marbles of the Menderes Metamorphic rocks. The recorded reservoir is Pliocene lacustrine limestone. Hydrogeochemical studies indicate that thermal waters were mixed with surface waters before and/or after heating at depth. The results of mineral equilibrium modeling indicate that all the thermal waters are undersaturated at discharge temperatures for gypsum, anhydrite, and magnesite minerals. Calcite, dolomite, aragonite, quartz, and chalcedony minerals are oversaturated in all of the thermal waters. Water from the reservoir temperatures of the Usak area can reach upto120°C. According to δ¹⁸O and δ²H values, all thermal and cold groundwater are of meteoric origin.     

Hydrochemistry of geothermal water in Tianshui and adjacent area, Gansu province, China

A geochemical study on thermal water has been carried out in Tianshui and its adjacent area, Gansu province, China. Chemical and isotopic contents were employed in the investigation on the origin and evolution of thermal water and the evaluation of reservoir temperature in the geothermal systems. Thermal waters in Wushan and Tianshui are characterized by outlet temperatures from 15 to 38°C and low TDS (226?C255?mg/L), defined as bicarbonate water. Its origin may be attributed to the interaction between meteoric rain, biotite plagioclase gneiss and carbonate reservoir rocks. In contrast, thermal waters in Tongwei and Qingshui have higher outlet temperatures of 25?C54.2°C and a moderate TDS of 915?C1,793?mg/L, regarded as sulfate waters. These sulfate waters may arise from the interaction between meteoric water, granite and amphogneiss. Isotopic data presented here suggest that thermal waters in the study area have a meteoric origin without being significantly effected by water?Crock isotope exchange. Chemical geothermometry indicates the existence of a deep geothermal reservoir of low-to-medium enthalpy (70?C111°C) in the Tianshui study area.