Geochemistry, geothermometry and influence of the concentration of mobile elements in the chemical characteristics of carbonate-evaporitic thermal systems. The case of the Tiermas geothermal system (Spain) (original) (raw)
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Geologica Acta, Vol.15, Nº 2, 67-87, 2017
The Caldes de Boí geothermal waters show important differences in pH (6.5-9.6) and temperature (15.9ºC-52ºC) despite they have a common origin and a very simple circuit at depth (4km below the recharge area level). These differences are the result of secondary processes such as conductive cooling, mixing with colder shallower waters, and input of external CO 2 , which affect each spring to a different extent in the terminal part of the thermal circuit. In this paper, the secondary processes that control the geochemical evolution of this system have been addressed using a geochemical dataset spanning over 20 years and combining different approaches: classical geochemical calculations and geochemical modelling. Mixing between a cold and a thermal end-member, cooling and CO 2 exchange are the processes affecting the spring waters with different intensity over time. These differences in the intensity of the secondary processes could be controlled by the effect of climate and indirectly by the geomorphological and hydrogeological setting of the different springs. Infiltration recharging the shallow aquifer is dominant during the rainy seasons and the extent of the mixing process is greater, at least in some springs. Moreover, significant rainfall can produce a decrease in the ground temperature favouring the conductive cooling. Finally, the geomorphological settings of the springs determine the thickness and the hydraulic properties of the saturated layer below them and, therefore, they affect the extent of the mixing process between the deep thermal waters and the shallower cold waters. The understanding of the compositional changes in the thermal waters and the main factors that could affect them is a key issue to plan the future management of the geothermal resources of the Caldes de Boí system. Here, we propose to use a simple methodology to assess the effect of those factors, which could affect the quality of the thermal waters for balneotherapy at long-term scale. Furthermore, the methodology used in this study can be applied to other geothermal systems.
The geothermal area of Rio Valdez is located in the central portion of the Isla Grande de Tierra del Fuego (South Argentina), ten kilometers south of the southeastern sector of the Fagnano Lake. It consists of a series of thermal springs with low discharge rates (£1 L ⁄ s) and temperatures in the range of 20-33°C distributed in an area of <1 km 2 . The thermal springs are characterized by alkaline, Na-HCO 3 waters with low salinity (0.53‚0.58 g ⁄ L), but relatively high fluoride contents (up to 19.4 mg ⁄ L). Their composition is the result of a slow circulation at depth, possibly through deep tectonic discontinuities connected with the Magallanes-Fagnano Fault (MFF) system. According to geothermometric calculations, thermal waters reach temperatures in the range of 100-150°C and an almost complete chemical equilibrium with the alkali-feldspars in the metavolcanic country rocks. The relatively high fluorine contents can be explained by the slow ascent and cooling of deep groundwaters followed by a progressive re-equilibration with F-bearing, hydrated Mg-silicates, such as chlorite, which has been recognized as an abundant mineral in the metavolcanics of the Lemaire Formation and metapelites and metagraywackes of the Yahgá n Formation. Finally, the isotopic composition of the investigated samples is consistent with the infiltration from local snow melting at altitudes in the range of 610-770 m asl. The comparison of our data with those collected in 1991 seems to suggest a possible progressive decline of the bulk thermal output in the near future. This possibility should be seriously considered before planning a potentially onerous exploitation of the resource. Presently, the only ways to exploit this geothermal resource by the population scattered in the area are the direct use of thermal waters and ⁄ or spa structures.
Geothermics, 2011
and Rb, whereas cold waters show low salinity, high concentrations of NO 3 , and significant As content when mixed with geothermal waters. Modeling of the geothermal fluids indicates that the fluid is supersaturated with aragonite and calcite, which matches the travertine precipitation close to the present discharge areas. Moreover, the barite and fluorite are also are near equilibrium levels, indicating possible control of Ba and F solubility by these mineral phases, which also precipitate in some discharge areas. Likewise, the fluid is supersaturated with respect to quartz, indicating the possibility of siliceous precipitation near the discharge areas of the present geothermal fluids. Taking into account the Na-K, Na-K-Ca, and SiO 2-temperature geothermometers, the temperature of the reservoir may be estimated to be about 135 • C. The chemistry of the geothermal fluids has changed from a recent high-enthalpy system, which precipitated siliceous deposits, to the present low-enthalpy system, which precipitates carbonated deposits (travertine). Multivariate analysis of the groundwater shows high correlations between K, Ca, As, Br, Ag, and Ba, suggesting that As is introduced to the environment via geothermal fluids. Moreover, As concentrations in hot groundwater are associated with high concentrations of Li and Si, as has been observed in other geothermal fields. Metal concentrations in the hydrothermal deposits show high values of Ag, As, Ba, Pb, Sb, and Zn, mainly in the siliceous deposits of the town of Caldes de Malavella, where the geothermal system deposited materials with high As concentrations (123-441 ppm). The similarities between the geochemical characteristics of the hydrothermal deposits and the groundwater suggest that the metals in these deposits and fluids have the same origin.
Journal of Volcanology and Geothermal Research, 2006
The Ribeira Grande geothermal field is a water-dominated geothermal system, located within Água de Pau/Fogo Volcano in the central part of the São Miguel Island. This geothermal system is exploited for energy production by wells sustaining two power plants. The wells produce from a formation of pillow lavas divided into different aquifers, with a fairly isothermal zone from 800 to 1300 m in depth, where reservoir temperature reaches 230 to 245°C. Below the depth of 1300 m there is a slight temperature reversal. The fluid produced has excess enthalpy and, separated at atmospheric pressure, is characterized by mineralization of sodiumchloride type up to 6-7 g/l, the concentration of dissolved silica varies between 450 and 650 mg/l and the pH ranges between 8 and 8.6. The gas phase is dominantly CO 2 , at a concentration of 98% of NCG. The composition of the deep geothermal fluid was obtained by computer simulation, using the WATCH program, and was compared with the composition of the bottom-hole samples. The approximations, in this simulation, were considered the singleand multi-step steam separation. The reference temperatures were based on: (i) the measured temperature in wells; (ii) the Na/K geothermometric temperature and (iii) the enthalpy-saturation temperature. According to both the measured and geothermometric temperatures, the deep fluid of the wells has two phases with a steam fraction up to 0.34, at higher well discharges. The measured enthalpy is always greater than the calculated enthalpy. The calcite equilibrium indicates scaling, since the fluid is flashing, around 2.28 mg/l CaCO 3 at the maximum discharge. The geothermal wells exploit three different aquifers, the lower of which is liquid and slightly colder than the upper ones. The intermediate is a two-phase aquifer with a steam fraction up to 0.081. The upper aquifer is probably of steam phase. The main differences between the aquifers are the temperature and boiling; both enthalpy and flashing increase as aquifer depth decreases.
Geochemistry of thermal springs, Alhama de Granada (southern Spain)
Applied Geochemistry, 2001
The waters of the thermal springs at Alhama de Granada vary in temperature between 27 and 45 C. Temporal changes in the composition of the principal spring (BanÄ os Viejos) indicate that a small degree of mixing may occur between deep thermal waters and shallow groundwater. Slight compositional variations also occur between the various thermal springs in the study area. These spatial variations are due to the dierent local hydrodynamic conditions in the springs. Towards the north in less hydraulically transmissive rocks, cooling of the rising water is more noticeable, as are ion exchange and processes of SO 4 reduction. The chemical composition of the water is related to the dissolution of evaporites (SO 4 and Cl salts), carbonates and silicates, and to the possible existence of sources of S within the rock. Estimates of the mean residence times have been obtained based on 14 C DIC and T. The state of thermodynamic equilibrium at the spring discharge was calculated using the SOLMINEQ.88 program. The results indicate that all the samples are supersaturated with respect to quartz, chalcedony, cristobalite, calcite, aragonite and dolomite, and undersaturated with respect to gypsum, anhydrite and halite. The use of dierent geothermometers and modelling of saturation indices for quartz, albite and anhydrite indicate temperatures of about 110 C. #
Applied Geochemistry, 1995
h order to derive T-P c0J geoindicators suitable for thermal waters coming from the hydrothermal systems of medium-low temperature which arc hosted in carbonate-evaporite rocks, the theoretical concentration of Ca, Mg, HCO,. SO,, F and their complexes in aqueous solutions in equilibrium with a mineral assemblage made up of calcite, dolomite, anhydrite and fluorite has been calculated at temperatures between 0 and 150°C. at P cn_ in the 0.1-100 bar interval and for total molality of mobile species.Na and Cl ranging between 0.0001 a&l 0.3. The results of such calculations indicated that: (1) the Ca. Me. SO, and F contents and the SO, I(F)' and Ca/Mg ratios are potential geothermometers; ?2\ the (HcQyp/S& ratio is a Pco? indicator; (j) 'th'e HCO,/F ratio and the HC03 content arc indicators of both PcO, and T. A first geothermometric and geobarometric evaluation of the thermal springs of the Etruscan Swell generally shows equilibrium temperatures in the interval 50-100°C and Pco? from about 1 bar to tens of bars. These estimations are consistent with the general knowledge of the hydrothermal systems of medium-low temperature present in the region.
Review: Thermal water resources in carbonate rock aquifers
Hydrogeology Journal, 2010
The current knowledge on thermal water resources in carbonate rock aquifers is presented in this review, which also discusses geochemical processes that create reservoir porosity and different types of utilisations of these resources such as thermal baths, geothermal energy and carbon dioxide (CO 2 ) sequestration. Carbonate aquifers probably constitute the most important thermal water resources outside of volcanic areas. Several processes contribute to the creation of porosity, summarised under the term hypogenic (or hypogene) speleogenesis, including retrograde calcite solubility, mixing corrosion induced by cross-formational flow, and dissolution by geogenic acids from deep sources. Thermal and mineral waters from karst aquifers supply spas all over the world such as the famous bath in Budapest, Hungary. Geothermal installations use these resources for electricity production, district heating or other purposes, with low CO 2 emissions and land consumption, e.g. Germany's largest geothermal power plant at Unterhaching near Munich. Regional fault and fracture zones are often the most productive zones, but are some-times difficult to locate, resulting in a relatively high exploration uncertainty. Geothermal installations in deep carbonate rocks could also be used for CO 2 sequestration (carbonate dissolution would partly neutralise this gas and increase reservoir porosity). The use of geothermal installations to this end should be further investigated.
Journal of Volcanology and Geothermal Research, 2017
This study focused on the geochemical and isotopic features of thermal fluids discharged from five zones located in the high altitude Puna plateau (Jujuy Province between S 22°20′-23°20′ and W 66°-67°), i.e. Granada, Vilama, Pairique, Coranzulí and Olaroz. Partially mature waters with a Na +-Cl − composition were recognized in all the investigated zones, suggesting that a deep hydrothermal reservoir hosted within the Paleozoic crystalline basement represents the main hydrothermal fluid source. The hydrothermal reservoirs are mainly recharged by meteoric water, although based on the δ 18 O-H 2 O and δD-H 2 O values, some contribution of andesitic water cannot be completely ruled out. Regional S-oriented faulting systems, which generated a horst and graben tectonics, and NE-, NW-and WE-oriented transverse structures, likely act as preferentially uprising pathways for the deep-originated fluids, as also supported by the Rc/Ra values (up to 1.39) indicating the occurrence of significant amounts of mantle He (up to 16%). Carbon dioxide, the most abundant compound in the gas phase associated with the thermal waters, mostly originated from a crustal source, although the occurrence of CO 2 from a mantle source, contaminated by organic-rich material due to the subduction process, is also possible. Relatively small and cold Na +-HCO 3 −-type aquifers were produced by the interaction between meteoric water and Cretaceous, Palaeogene to Miocene sediments. Dissolution of evaporitic surficial deposits strongly affected the chemistry of the thermal springs in the peripheral zones of the study area. Geothermometry in the Na-K-Ca-Mg system suggested equilibrium temperatures up to 200°C for the deep aquifer, whereas lower temperatures (from 105 to 155°C) were inferred by applying the H 2 geothermometer, likely due to re-equilibrium processes during the thermal fluid uprising within relatively shallow Na-HCO 3 aquifers. The great depth of the geothermal resource (possibly N 5000 m b.g.l.) is likely preventing further studies aimed to evaluate possible exploitation, although the occurrence of Liand Ba-rich deposits associated may attract financial investments, giving a pulse for the development of this remote region.
New chemical and original isotopic data on waters from El Tatio geothermal field, northern Chile
GEOCHEMICAL JOURNAL, 2005
The El Tatio geothermal field is located at an height of 4200-4300 m on the Cordillera de los Andes (Altiplano). Geysers, hot pools and mudpots in the geothermal field and local meteoric waters were sampled in April 2002 and analyzed for major and trace elements, δ 2 H, δ 18 O and 3 H of water, δ 34 S and δ 18 O of dissolved sulfate, δ 13 C of dissolved total carbonate, and 87 Sr/ 86 Sr ratio of aqueous strontium. There are two different types of thermal springs throughout the field, that are chloride-rich water and sulfate-rich water. The chemical composition of chloride springs is controlled by magma degassing and by water-rock interaction processes. Sulfate springs are fed by shallow meteoric water heated by ascending gases. In keeping with the geodynamic setting and nature of the reservoir rocks, chloride water is rich in As, B, Cs, Li; on the other hand, sulfate water is enriched only in B relative to local meteoric water. Alternatively to a merely meteoric model, chloride waters can be interpreted as admixtures of meteoric and magmatic (circa andesitic) water, which moderately exchanges oxygen isotopes with rocks at a chemical Na/K temperature of about 270°C in the main reservoir, and then undergoes loss of vapor (and eventually mixing with shallow water) and related isotopic effects during ascent to the surface. These chloride waters do not present tritium and can be classified as submodern (pre-1952). A chloride content of 5,400 mg/l is estimated in the main reservoir, for which δ 2 H and δ 18 O values, respectively of-78‰ and-6.9‰, are calculated applying the multistage-steam separation isotopic effects between liquid and vapor. From these data, the meteoric recharge (Cl ≈ 0 mg/l) of the main reservoir should approach a composition of-107‰ in δ 2 H and-14.6‰ in δ 18 O, when a magmatic water of δ 2 H =-20‰, δ 18 O = +10‰ and Cl = 17,500 mg/l is assumed. The 87 Sr/ 86 Sr ratios of the hot springs are quite uniform (0.70876 to 0.70896), with values within the range observed for dacites of the Andean central volcanic zone. A water δ 18 O-87 Sr/ 86 Sr model was developed for the main geothermal reservoir, by which a meteoric-magmatic composition of the fluids is not excluded. The uniform δ 34 S(SO 4 2-) values of +1.4 to +2.6‰ in the chloride waters agree with a major deep-seated source for sulfur, possibly via hydrolysis in the geothermal reservoir of sulfur dioxide provided by magma degassing, followed by isotopic exchange between sulfate and sulfide in the main reservoir. This interpretation is supported by the largely negative δ 34 S(SO 4 2-) value in steam-heated water sulfate (-9.8‰) and mass-balance calculation, which exclude leaching at depth of igneous iron-sulfides with δ 34 S near zero per mill. All the δ 13 C values of total carbonate in the chloride waters are negative, with variable values from-9.2 to-20.1‰, pointing to an important proportion of biogenic carbon in the fluids. The interpretation of these data is problematic, and a number of alternative explanations are reported in the text.