Application of different geothermometrical techniques to a low enthalpy thermal system

The reservoir temperature of the waters in the low temperature carbonate-evaporitic geothermal system of Arnedillo has been estimated by using two different techniques: 1) chemical geothermometers and 2) geothermometrical modelling. By combining the results of both techniques a reliable range of temperature of 90 ± 20 oC has been proposed for the waters in the reservoir. Despite being a carbonate-evaporitic system, the cationic geothermometers have provided good results, which, together with the geothermometrical modelling, indicate that the waters have reached equilibrium with anhydrite, quartz, calcite, dolomite, albite and K-feldspar in the reservoir

Geochemical evolution of thermal waters in carbonate – evaporitic systems: The triggering effect of halite dissolution in the dedolomitisation and albitisation processes

The Fitero and Arnedillo geothermal systems are located in the NW part of the Iberian Range (Northern Spain). The geothermal reservoir is hosted in the Lower Jurassic carbonates, in contact with the evaporitic Keuper Facies. Thermal waters are of chloride-sodium type with discharge temperature of about 45 °C and near neutral pH. The Arnedillo waters are more saline with higher Na, Cl and sulphate contents, but lower Ca and Mg than the Fitero waters. All waters have attained mineral equilibrium at depth with calcite, dolomite, anhydrite, quartz, albite, K-feldspar and other aluminosilicates, except for the Fitero waters, which have not reached the equilibrium with the aluminosilicates. The calculated reservoir temperature is 81 ± 11 °C in Fitero and 87 ± 13 °C in Arnedillo. In order to identify the reasons for the differences found between the two systems some inverse and forward geochemical calculations were performed and the main water-rock interaction processes responsible for the chemical evolution of these waters have been evaluated. Halite dissolution has been found to be the triggering factor for the two most important geochemical processes in the system: a) albitisation process, due to the common ion effect (Na); and b) dedolomitisation process, associated with the salinity increase, which enhance the dissolution of anhydrite and, in turn, produces the precipitation of calcite (common ion effect, Ca) and the concomitant dissolution of dolomite. Halite dissolution may be an important driving force in the geochemical evolution of groundwater systems in contact with carbonates and evaporites, where equilibrium with K-feldspar, albite and anhydrite has already been attained. The evolution of the processes at pH, temperature and salinity ranges wider than those in the Fitero-Arnedillo system has been theoretically examined with additional reaction-path simulations, in order to generalise the geochemical behaviour of these processes in other environments

Comparison of different thermodynamic databases used in a geothermometrical modelling calculation

Different thermodynamic databases usually have common thermodynamic data for some minerals or aqueous species, but in many cases they present important differences. Four different thermodynamic databases (WATEQ4F, LLNL, DATA0.YMP.R5 and SOLTHERM) have been used in a geothermometrical modelling problem and they are compared in this work. The main differences found in the thermodynamic data are related to the order, degree, crystallinity and composition of the considered aluminosilicate phases and the effects of these properties in the experimental, or theoretical, data used for the fitting of the equilibrium constant at different temperatures

Tufa sedimentation in changing hydrological conditions: the River Mesa (Spain)

The processes controlling tufa deposition along the River Mesa (NE Spain) were studied from April 2003 to September 2009, based on six-monthly monitoring of physical and chemical parameters of the river water and sedimentological characteristics, including deposition rates on tablets. With a mean annual discharge around 1.5m3/s, the sedimentation rate (mean 2mm/yr) recorded important spatial, seasonal and interannual variations. The river waters are of the calcium bicarbonate type. In this study, three distinct river stretches were distinguished based on the steady groundwater inputs, some of low-thermal nature. Groundwater discharges controlled the water chemical composition, and some sedimentation features too. At each stretch, an increase in pCO2 and conductivity was measured around the spring sites. Decreasing trends in conductivity or alkalinity with high enough saturation values with respect to calcite were only clearly observed in the intermediate stretch, which had higher tufa deposition rates than the other two. Tufa deposition rates were higher in cool (autumn+winter) than in warm (spring+summer) periods. In some low-rainfall warm periods, tufa deposition was inhibited or limited due to the low flow –mainly from groundwater inputs– and to the dryness of some river sites, which indeed favoured erosion during flooding. A decrease in yearly deposition rates from April 2006 onwards paralleled an important reduction in the river discharge. Groundwater inputs, drought periods and flood events should therefore be considered to understand fluvial tufa sedimentation in semi-arid conditions

Metodologías y técnicas instrumentales para el estudio de sistemas de aguas ácidas

Los sistemas de aguas ácidas presentan una gran complejidad geoquímica, ya que involucran una gran variedad de procesos de interacción entre aguas superficiales o subterráneas, gases (sobre todo el oxígeno atmosférico), minerales responsables de la generación de acidez y aquellos que intervienen en la atenuación natural de los contenidos de elementos disueltos y distintos tipos de actividad biológica. Por ello la calidad y fiabilidad de un estudio geoquímico en este tipo de sistemas dependen en gran medida de que las técnicas de muestreo, preservación y análisis de aguas, minerales, gases y muestras biológicas sean las más adecuadas para el tipo de interpretaciones que se desea realizar. En este trabajo se detallan las principales características de los métodos y técnicas más habituales empleados en los estudios geoquímicos desarrollados en sistemas de aguas ácidas en entornos ricos en sulfuros, teniendo en cuenta no sólo los distintos tipos de muestras habituales sino también las peculiaridades de los distintos tipos de sistemas (cortas mineras, balsas de lodos, arroyos ácidos, etc). Además, se indican las posibles aplicaciones y dificultades de cada metodología y técnica descrita y se proporcionan referencias a otros estudios científico-técnicos que permiten profundizar en los fundamentos de su us

Hydrogeochemical characterization of an evaporite karst area affected by sinkholes (Ebro Valley, NE Spain)

The main processes controlling the hydrochemistry of an alluvium-covered evaporite karst area with high sinkhole risk (Ebro Valley, NE Spain) are examined by means of multivariate analyses (Principal Component Analysis and Hierarchical Cluster Analysis), ion correlations and geochemical speciation-solubility calculations. The hydrogeochemistry of the studied system seems to be governed by the interaction between the groundwater from the salt-bearing evaporitic karst aquifer and from the overlying Ebro River alluvial aquifer. The observed hydrochemical features in the alluvial-karst aquifer system are mainly determined by the relative contribution of gypsum/anhydrite and halite dissolution, showing a wide spectrum from relatively fresh recharge waters (mainly irrigation waters) to highly evolved groundwater from the evaporitic aquifer. The variability of these contributions is especially evident at sinkhole ponds which, in some cases, seem to be associated with discharge areas of the karst aquifer in the valley bottom alluvium. Calculated saturation indexes suggest that, in contrast to gypsum, the amounts of halite in the sampled portions of evaporitic aquifer are not large enough to attain equilibrium, which is consistent with the predominance of gypsum/anhydrite reported for these materials. Furthermore, the observed Na:Cl and Ca:SO4 correlations and stoichiometries suggest that other possible processes, such as glauberite dissolution or Na/Ca-exchange, generally play a minor role (compared to halite and gypsum dissolution) in this system. Another important process in the system is the dissolution of carbonate minerals (dolomite and, possibly, calcite) fostered by the input of CO2(g), which is probably produced by pedogenic processes. Dolomite dissolution seems to be particularly relevant in the evaporitic materials probably due to dedolomitisation triggered by gypsum/anhydrite dissolution

Caracterización geoquímica del proceso de mezcla de aguas termales y no termales en los manantiales de Jaraba (Aragón, España).

En este trabajo se presenta el estudio realizado sobre el proceso de mezcla de aguas que se produce en el sistema termal de Jaraba (Aragón, España). Los diferentes manantiales de este sistema muestran una variación espacial en su composición y temperatura de surgencia. El uso de gráficos de correlación binaria entre cloruros, temperatura y elementos de comportamiento conservativo permite justificar esta variabilidad por la actuación de un proceso de mezcla de las aguas termales con otras más frías y superficiales. Se ha identificado un conjunto de surgencias especialmente afectado por este proceso (con proporciones de mezcla en torno a un 60% de polo termal) y la comparación con estudios previos sugiere que esta situación se mantiene de forma relativamente constante en el tiempo. Componentes como alcalinidad, sílice, magnesio y calcio muestran un comportamiento reactivo durante la mezcla y están afectados por procesos adicionales de reequilibrio que, actualmente, están siendo investigados mediante el empleo de técnicas de modelización geoquímica

Aplicación de geotermómetros químicos a un sistema termal de baja temperatura

The Tiermas geothermal system is one of the areas with the greatest geothermal potential in Arago´n, however, its hydrogeological and geochemical features are still poorly known. In this study, the main hydrochemical features of these waters are presented and the reservoir temperature is approached by applying chemical geothermometers. These waters have a sulphate chloride sodium affinity, with nearly 40 oC of spring temperature and an approximate flow rate of 200 l/s. The most likely aquifer seems to be located in the Paleocene and Eocene carbonates. However, due to the structural complexity of the area, the waters would also interact with the evaporitic facies present in the EoceneâOligocene boundary. Two different hydrochemical groups have been distinguished based on their salinity, Na/Cl ratios, SO4 concentrations and TDS values. Despite the compositional variations detected in the springs, the geothermometric calculations allow to infer a reservoir temperature of 85 ± 17 °C. Good consistency has been obtained with the cationic geothermometers, which is an unusual situation for a geothermal system in carbonateâevaporitic materials.El sistema geotermal de Tiermas representa una de las zonas con mayor potencial geotérmico de Aragón y, sin embargo, se sabe todavía poco acerca de sus características hidrológicas y geoquímicas. En este trabajo se presentan los principales rasgos hidroquímicos de estas aguas y se determina su temperatura en profundidad mediante la aplicación de geotermómetros químicos. Estas aguas tienen una afinidad clorurada sulfatada sódica, una temperatura de surgencia de casi 40 °C y un caudal de unos 200 l/s. El acuífero más probable se localizaría en los carbonatos del Paleoceno y Eoceno, pero debido a la complejidad estruc- tural de la zona, las aguas entrarían en contacto con la facies evaporítica del tránsito EocenoâOligoceno, adquiriendo así dicha afinidad. Se han distinguido dos grupos hidroquímicos con una diferente salinidad, relación Na/Cl y concen- traciones de SO4 y TSD. A pesar de las variaciones composicionales detectadas en las surgencias, los cálculos geotermométricos realizados permiten establecer un rango de temperaturas en el reservorio de 85 ± 17 °C, habiéndose obtenido bue- nos resultados con los geotermómetros catiónicos, situación poco habitual en sistemas termales instalados en materiales carbonatados â evaporíticos

Geoquimica de las lagunas saladas de los monegros (Zaragoza). 1 - Determinacion experimental de los efectos del reequilibrio mirabilita-solucion con la temperatura en un sistema natural

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