Water chemistry at Hontomín-Huermeces (Burgos, Spain): insights for a pre-, intra- and post-CO2 injection geochemical monitoring.

In this study, the very first geochemical and isotopic data related to surface and spring waters and dissolved gases in the area of Hontomín-Huermeces (Burgos, Spain) are presented and discussed. Hontomín-Huermeces was selected as a pilot site for the injection of pure (>99 %) CO2. Injection and monitoring wells are planned to be drilled close to 6 oil wells completed in the 1980’s. Stratigraphical logs indicate the presence of a confined saline aquifer at the depth of about 1,500 m into which less than 100,000 tons of liquid CO2 will be injected, possibly starting in 2013. The chemical and isotopic features of the spring waters suggest the occurrence of a shallow aquifer having a Ca2+(Mg2+)-HCO3- composition, relatively low salinity (Total Dissolved Solids _800 mg/L) and a meteoric isotopic signature. Some spring waters close to the oil wells are characterized by relatively high concentrations of NO3- (up to 123 mg/L), unequivocally indicating anthropogenic contamination that adds to the main water-rock interaction processes. The latter can be referred to Ca-Mg-carbonate and, at a minor extent, Al-silicate dissolution, being the outcropping sedimentary rocks characterized by Palaeozoic to Quaternary rocks. Anomalous concentrations of Cl-, SO42-, As, B and Ba were measured in two springs discharging a few hundreds meters from the oil wells and in the Rio Ubierna, possibly indicative of mixing processes, although at very low extent, between deep and shallow aquifers. Gases dissolved in spring waters show relatively high concentrations of atmospheric species, such as N2, O2 and Ar, and isotopically negative CO2 (<-17.7 h V-PDB), likely related to a biogenic source, possibly masking any contribution related to a deep source. The geochemical and isotopic data of this study are of particular importance when a monitoring program will be established to verify whether CO2 leakages, induced by the injection of this greenhouse gas, may affect the quality of the waters of the shallow Hontomín-Huermeces hydrological circuit. In this respect, carbonate chemistry, the isotopic carbon of dissolved CO2 and TDIC (Total Dissolved Inorganic Carbon) and selected trace elements can be considered as useful parameters to trace the migration of the injected CO2 into near-surface environments.

Effects on the shallow aquifers by CO2 leakages in a tertiary basin (province of Murcia, Spain)

Geological storage of CO2 is nowadays internationally considered as the most effective method for greenhouse gas emission mitigation, in order to minimize its effects on the global climatology. One of the main options is to store CO2 in deep saline aquifers at more than 800m depth, because it reaches its supercritical state. Study of the CO2 natural accumulations as natural analogues of an artificial CO2 storage is very useful in order to understand the CO2 long term behaviour and thus to predict its possible impact on the surficial environment and life. Therefore the main objective of this work is to detect the affection of the CO2 leakages from a deep saline aquifer on the shallow aquifers, all of them located in the Gañuelas-Mazarrón Tertiary basin (Province of Murcia, Spain). This CO2 storage and leakage natural system can be analogous to an artificial CO2 storage with leakage phenomena. In order to reach these objectives, groundwaters from different aquifers in the site have been sampled and analysed for major elements, free and dissolved gases and stable isotopes, particularly ∂ 13 C and 3 He/ 4 He. The results obtained allow to conclude that this natural system is an interesting example of natural analogue for an artificial CO2 storage affected by leakage processes because the shallow fresh aquifers in the site are polluted by CO2 from the deep saline aquifer as a consequence of an intensive over-exploitation of these freshwater aquifers

A study on the shallow water effect on a ship's pivot point

Information on the pivot point of a turning ship is collected, taking into account practical notes and manuals on ship maneuvering as well as experimental data and simulated results which all together reveal a consistent behavior when varying water depth or some ship particulars. Results from the studies already carried out on the Riverine Support Patrol Vessel (RSPV) of the Colombian Navy are included in this one, in order to estimate the pivot point’s position and to contrast those results with theory and available empirical observations. Linear manoeuvrability theory is tested and its results show poor approximation with respect to the kinematic equations. As to the depth variation effect, by means of fullscale experiments it is confirmed that the pivot point’s position, when going to shallow water, always varies in the same way, proving to be coherent with the available information on this phenomenon.

The Extended Water Footprint and illegal groundwater use in the Upper Guadiana Basin (Spain)

Tablas de Daimiel National Park is located in the Upper Guadiana Basin and represents one of the largest and most important wetlands in Europe. The long term ecological integrity of this wetland is inherently associated with the maintenance of a shallow groundwater table, namely the Western Mancha aquifer (WMA) or Aquifer 23. The intensive use of groundwater, mainly for irrigation, has led over the last decades to deep socio‐economic changes. Such intensive use has also lowered the water table of Aquifer 23, drastically reducing the flooded area of the wetland and threatening its ecological integrity. A number of plans and measures have been developed and implemented since the declaration of overexploitation of Aquifer 23 in the year 1987. The most recent one is the Special Plan for the Upper Guadiana (SPUG), approved in 2008. This Plan is the main measure to comply with achieving the objective of good quantitative and qualitative status required under the Water Framework Directive (2000). This paper offers a new type of integrated analysis which allows assessing under a common lens the physical, economic and social dimensions of groundwater use in the area. The first objective is to calculate the groundwater footprint of agricultural production in the Upper Guadiana basin and its evolution during 2000‐2008. For this purpose, we have applied the Extended Water Footprint (EWF) methodology ‐a novel approach based on the classical Water Footprint (WF) approach‐ that includes an assessment of the water productivity from an economic and social perspective. Compared to the classical WF, the EWF allows for a more complete overview of the sector, providing new insights for policy decisions (e.g. to define options and possibilities on water re‐allocation in order to achieve both better ecosystem conservation and social equity). The second objective is to use the EWF to compare the existing authorized and non‐authorized or illegal use of water. This allows us to discuss current initiatives by public authorities in relation to the existing frame of water rights

Hydrogeochemistry of surface and spring waters in the surroundings of the CO2 injection site at Hontomín–Huermeces (Burgos, Spain)

In this paper the very first geochemical and isotopic data related to surface and spring waters and dissolved gases in the area of Hontomín–Huermeces (Burgos, Spain) are presented and discussed. Hontomín–Huermeces has been selected as a pilot site for the injection of pure (>99%) CO2. Injection and monitoring wells are planned to be drilled close to 6 oil wells completed in the 1980s for which detailed stratigraphical logs are available, indicating the presence of a confined saline aquifer at the depth of about 1500 m into which less than 100,000 tons of iquid CO2 will be injected, possibly starting in 2013. The chemical and features of the spring waters suggest that they are related to a shallow hydrogeological system as the concentration of the Total Dissolved Solids approaches 800 mg/L with a Ca2+(Mg2+)-HCO3− composition, similar to that of the surface waters. This is also supported by the oxygen and hydrogen isotopic ratios that have values lying between those of the Global and the Mediterranean Meteoric Water Lines. Some spring waters close to the oil wells are haracterized by relatively high concentrations of NO3− (up to 123 mg/L), unequivocally suggesting an anthropogenic source that adds to the main water–rock interaction processes. The latter can be referred to Ca-Mg-carbonate and, at a minor extent, Al-silicate dissolution, being the outcropping sedimentary rocks characterized by Palaeozoic to Quaternary rocks. Anomalous concentrations of Cl−, SO42−, As, B and Ba were measured in two springs discharging a few hundred meters from the oil wells and in the Rio Ubierna. These contents are significantly higher than those of the whole set of the studied waters and are possibly indicative of mixing processes, although at very low extent, between deep and shallow aquifers. No evidence of deep-seated gases interacting with the Hontomín–Huermeces waters was recognized in the chemistry of the disolved gases. This is likely due to the fact that they are mainly characterized by an atmospheric source as highlighted by the high contents of N2, O2 and Ar and by N2/Ar ratios that approach that of ASW (Air Saturated Water) and possibly masking any contribution related to a deep source. Nevertheless, significant concentrations (up to 63% by vol.) of isotopically negative CO2 (<−17.7‰ V-PDB) were found in some water samples, likely related to a biogenic source. The geochemical and isotopic data of this work are of particular importance when a monitoring program will be established to verify whether CO2 leakages, induced by the injection of this greenhouse gas, may be affecting the quality of the waters in the shallow hydrological circuits at Hontomín–Huermeces. In this respect, carbonate chemistry, the isotopic carbon of dissolved CO2 and TDIC (Total Dissolved Inorganic Carbon) and selected trace elements can be considered as useful parameters to trace the migration of the injected CO2 into near-surface environments.

Estudio de viabilidad para almacenamiento de CO2 en la cuenca del Guadalquivir

El objetivo de este proyecto es investigar la viabilidad del almacenamiento de CO2 en un acuífero salino profundo ubicado en el margen suroccidental de la Cuenca del Guadalquivir. Este proyecto está destinado a una operación industrial con tasas de emisión de CO2 superiores a medio millón de toneladas anuales. Se ha construido un modelo geológico de la formación almacén en Petrel y se ha simulado la inyección utilizando la versión composicional de ECLIPSE. El objetivo es inyectar CO2 manteniendo una tasa de inyección constante durante 30 años, el máximo periodo permitido por la legislación española sobre almacenamiento geológico de CO2. La cantidad de CO2 inyectada en cada uno de los casos ha sido determinada. Los resultados parecen indicar que la inyección de CO2 a escala industrial podría ser viable, aunque la viabilidad del proyecto podría verse comprometida por la escasa profundidad a la que se encuentra el contacto entre la formación almacén y el sello lateral. Antes de seguir adelante con el desarrollo del proyecto sería conveniente determinar mejor la continuidad lateral de la formación almacén y sus condiciones de sello. ABSTRACT The aim of this project is to investigate the feasibility of CO2 geological storage in a deep saline aquifer located onshore in the southwestern margin of Guadalquivir Basin. The project is addressed for an industrial scale operation with CO2 emission rates higher than half a million tons per year. A geological model of the target reservoir was built in Petrel and injection simulations were performed with the compositional version of ECLIPSE. The purpose is to inject CO2 at constant rate during 30 years, the maximum period allowed by the Spanish law on carbon dioxide geological storage. The amount of CO2 injected in each studied scenario has been determined. Results suggest that CO2 injection at industrial scale could be viable, but the project feasibility could be endangered by the shallow depth of the contact between the target reservoir and the lateral seal. Prior to injection, further work should include ascertaining the reservoir’s lateral continuity and better determination of its sealing conditions

Interaction between karstic aquifers and allogenic rivers: the aquifer of the national park of the ephemeral river Lobos Canyon (Spain)

A methodology is described for understanding the interaction of karstic aquifers with allogenic rivers, where little information is available. This methodology includes conventional hydrogen- ology methods tracer tests and measurements of flow into, out of and circulating within the karstic system. The method is designed to un- derstand the hydrogeological behaviour of a river in sufficient detail, given a short study pe- riod. The methodology is applied to a karstic system in Spain, obtaining useful, quantitative results for a hydrological year, such as an esti- mate of the water balance, differentiation be- tween autogenic and allogenic natural recharge, relationship and connection between the river and the aquifer, and measurements of infiltration capacity in watercourses under different hydro- logical situations. The paper deals with a useful example that could be applied to other rivers and aquifers where few data are available. It can be applied to aquifers under a natural regime and Mediterranean climate.

Hidrogeología e hidrodinámica del acuífero de los manantiales de Gormaz y su importancia en el caudal base del río Duero, España = Hydrogeology and hydrodynamic of the Gormaz Springs Aquifer and its importance to the base flow of the Duero River, Spain

Se cuantifican las descargas subterráneas de un acuífero a un río que lo atraviesa utilizando correlaciones estadísticas. El río Duero, España, incrementa su caudal base en varios m3/s, al atravesar unos afloramientos carbonatados mesozoicos en un pequeño tramo de su cabecera; esto es de especial importancia en época de estiaje, cuando la mayor parte del caudal base del río procede de manantiales que allí se sitúan. Dichos afloramientos corresponden a uno de los dos acuíferos calcáreos confinados, que se desarrollan en paralelo y están hidráulicamente desconectados por una capa impermeable, que forman el sistema acuífero de los manantiales de Gormaz. Este sistema se encuentra en estado de régimen natural y está apenas explotado. Se define el modelo conceptual de funcionamiento hidrogeológico, considerando el papel hidrogeológico de la falla de Gormaz, situada en la zona de descarga del sistema. Analizando información geológica antecedente y la geofísica exploratoria realizada, se obtuvo un mejor conocimiento de la geometría y los límites de los acuíferos, definiéndose un sistema acuífero con una zona de recarga en el sur, correspondiente a los afloramientos calcáreos, los cuales se confinan hacia el norte bajo el Terciario, hasta intersecar con la falla normal de Gormaz. El salto de falla genera una barrera para las formaciones permeables situadas al extremo norte (margen derecha del río Duero); a su vez, el plano de falla facilita el ascenso del agua subterránea del sistema acuífero en estudio y pone en conexión hidráulica los dos acuíferos. Se estimaron, además, los parámetros hidráulicos de los acuíferos en los alrededores de la falla. La buena correlación entre los niveles piezométricos y las descargas subterráneas al río Duero han permitido la reconstrucción del hidrograma de los manantiales de Gormaz en el periodo 1992-2006. Se calcula así que la contribución subterránea al río Duero es de 135.9 hm3/año, que supone el 18.9% de la aportación total del río. In a short stretch of its headwaters, the base flow of the River Duero increases by several m3/s as it traverses some Mesozoic carbonate outcrops. This is of special importance during the dry season, when the majority of the base flow of the river proceeds from springs in this reach. The outcrops correspond to one of two confined calcareous aquifers that developed in parallel but which are not hydraulically connected because of an impermeable layer. Together, they constitute the aquifer system of the Gormaz Springs. The system is still in its natural regime and is hardly exploited. This study defines the conceptual model of hydrogeological functioning, taking into consideration the role of the Gormaz Fault, which is situated in the discharge zone of the system. Analysis of both antecedent geological information and geophysical explorations has led to a better understanding of the geometry and boundaries of the aquifers, defining an aquifer system with a recharge zone in the south corresponding to in the calcareous outcrops. These calcareous outcrops are confined to the north below Tertiary formations, as far as their intersection with the normal fault of Gormaz. The throw of the fault forms the barrier of the permeable formations situated in the extreme north (right bank of the River Duero). In turn, the fault plane facilitates the upflow of groundwater from the aquifer system and creates hydraulic connection between the two aquifers. In addition, the study estimated the hydraulic parameters of the aquifer around the fault. The close correlation between piezometric levels and the groundwater discharges to the River Duero has enabled the reconstruction of the hydrogram of Gormaz springs over the period 1992-2006. By this means, it is calculated that the groundwater contribution to the River Duero is 135.9 hm3/year, or 18.9% of the total river inflow.

El análogo natural de almacenamiento y escape de Co2 de la cuenca de Gañuelas-Mazarrón : implicaciones para el comportamiento y la seguridad de un almacenamiento de Co2 en estaso supercrítico

El aumento de la temperatura media de la Tierra durante el pasado siglo en casi 1 ºC; la subida del nivel medio del mar; la disminución del volumen de hielo y nieve terrestres; la fuerte variabilidad del clima y los episodios climáticos extremos que se vienen sucediendo durante las ultimas décadas; y el aumento de las epidemias y enfermedades infecciosas son solo algunas de las evidencias del cambio climático actual, causado, principalmente, por la acumulación de gases de efecto invernadero en la atmósfera por actividades antropogénicas. La problemática y preocupación creciente surgida a raíz de estos fenómenos, motivo que, en 1997, se adoptara el denominado “Protocolo de Kyoto” (Japón), por el que los países firmantes adoptaron diferentes medidas destinadas a controlar y reducir las emisiones de los citados gases. Entre estas medidas cabe destacar las tecnologías CAC, enfocadas a la captura, transporte y almacenamiento de CO2. En este contexto se aprobó, en octubre de 2008, el Proyecto Singular Estratégico “Tecnologías avanzadas de generación, captura y almacenamiento de CO2” (PSE-120000-2008-6), cofinanciado por el Ministerio de Ciencia e Innovación y el FEDER, el cual abordaba, en su Subproyecto “Almacenamiento Geológico de CO2” (PSS-120000-2008-31), el estudio detallado, entre otros, del Análogo Natural de Almacenamiento y Escape de CO2 de la cuenca de Ganuelas-Mazarrón (Murcia). Es precisamente en el marco de dicho Proyecto en el que se ha realizado este trabajo, cuyo objetivo final ha sido el de predecir el comportamiento y evaluar la seguridad, a corto, medio y largo plazo, de un Almacenamiento Geológico Profundo de CO2 (AGP-CO2), mediante el estudio integral del citado análogo natural. Este estudio ha comprendido: i) la contextualización geológica e hidrogeológica de la cuenca, así como la investigación geofísica de la misma; ii) la toma de muestras de aguas de algunos acuíferos seleccionados con el fin de realizar su estudio hidrogeoquímico e isotópico; iii) la caracterización mineralógica, petrográfica, geoquímica e isotópica de los travertinos precipitados a partir de las aguas de algunos de los sondeos de la cuenca; y iv) la medida y caracterización química e isotópica de los gases libres y disueltos detectados en la cuenca, con especial atención al CO2 y 222Rn. Esta información, desarrollada en capítulos independientes, ha permitido realizar un modelo conceptual de funcionamiento del sistema natural que constituye la cuenca de Ganuelas-Mazarrón, así como establecer las analogías entre este y un AGP-CO2, con posibles escapes naturales y/o antropogénicos. La aplicación de toda esta información ha servido, por un lado, para predecir el comportamiento y evaluar la seguridad, a corto, medio y largo plazo, de un AGP-CO2 y, por otro, proponer una metodología general aplicable al estudio de posibles emplazamientos de AGP-CO2 desde la perspectiva de los reservorios naturales de CO2. Los resultados más importantes indican que la cuenca de Ganuelas-Mazarrón se trata de una cubeta o fosa tectónica delimitada por fallas normales, con importantes saltos verticales, que hunden al substrato rocoso (Complejo Nevado-Filabride), y rellenas, generalmente, por materiales volcánicos-subvolcánicos ácidos. Además, esta cuenca se encuentra rellena por formaciones menos resistivas que son, de muro a techo, las margas miocenas, predominantes y casi exclusivas de la cuenca, y los conglomerados y gravas pliocuaternarias. El acuífero salino profundo y enriquecido en CO2, puesto de manifiesto por la xx exploración geotérmica realizada en dicha cuenca durante la década de los 80 y objeto principal de este estudio, se encuentra a techo de los materiales del Complejo Nevado-Filabride, a una profundidad que podría superar los 800 m, según los datos de la investigación mediante sondeos y geofísica. Por ello, no se descarta la posibilidad de que el CO2 se encuentre en estado supe critico, por lo que la citada cuenca reuniría las características principales de un almacenamiento geológico natural y profundo de CO2, o análogo natural de un AGP-CO2 en un acuífero salino profundo. La sobreexplotación de los acuíferos mas someros de la cuenca, con fines agrícolas, origino, por el descenso de sus niveles piezométricos y de la presión hidrostática, el ascenso de las aguas profundas, salinas y enriquecidas en CO2, las cuales son las responsables de la contaminación de dichos acuíferos. El estudio hidrogeoquímico de las aguas de los acuíferos investigados muestra una gran variedad de hidrofacies, incluso en aquellos de litología similar. La alta salinidad de estas aguas las hace inservibles tanto para el consumo humano como para fines agrícolas. Además, el carácter ligeramente ácido de la mayoría de estas aguas determina que tengan gran capacidad para disolver y transportar, hacia la superficie, elementos pesados y/o tóxicos, entre los que destaca el U, elemento abundante en las rocas volcánicas ácidas de la cuenca, con contenidos de hasta 14 ppm, y en forma de uraninita submicroscópica. El estudio isotópico ha permitido discernir el origen, entre otros, del C del DIC de las aguas (δ13C-DIC), explicándose como una mezcla de dos componentes principales: uno, procedente de la descomposición térmica de las calizas y mármoles del substrato y, otro, de origen edáfico, sin descartar una aportación menor de C de origen mantélico. El estudio de los travertinos que se están formando a la salida de las aguas de algunos sondeos, por la desgasificación rápida de CO2 y el consiguiente aumento de pH, ha permitido destacar este fenómeno, por analogía, como alerta de escapes de CO2 desde un AGP-CO2. El análisis de los gases disueltos y libres, con especial atención al CO2 y al 222Rn asociado, indican que el C del CO2, tanto disuelto como en fase libre, tiene un origen similar al del DIC, confirmándose la menor contribución de CO2 de origen mantélico, dada la relación R/Ra del He existente en estos gases. El 222Rn sería el generado por el decaimiento radiactivo del U, particularmente abundante en las rocas volcánicas de la cuenca, y/o por el 226Ra procedente del U o del existente en los yesos mesinienses de la cuenca. Además, el CO2 actúa como carrier del 222Rn, hecho evidenciado en las anomalías positivas de ambos gases a ~ 1 m de profundidad y relacionadas principalmente con perturbaciones naturales (fallas y contactos) y antropogénicas (sondeos). La signatura isotópica del C a partir del DIC, de los carbonatos (travertinos), y del CO2 disuelto y libre, sugiere que esta señal puede usarse como un excelente trazador de los escapes de CO2 desde un AGPCO2, en el cual se inyectara un CO2 procedente, generalmente, de la combustión de combustibles fósiles, con un δ13C(V-PDB) de ~ -30 ‰. Estos resultados han permitido construir un modelo conceptual de funcionamiento del sistema natural de la cuenca de Ganuelas-Mazarrón como análogo natural de un AGP-CO2, y establecer las relaciones entre ambos. Así, las analogías mas importantes, en cuanto a los elementos del sistema, serian la existencia de: i) un acuífero salino profundo enriquecido en CO2, que seria análoga a la formación almacén de un AGPxxi CO2; ii) una formación sedimentaria margosa que, con una potencia superior a 500 m, se correspondería con la formación sello de un AGP-CO2; y iii) acuíferos mas someros con aguas dulces y aptas para el consumo humano, rocas volcánicas ricas en U y fallas que se encuentran selladas por yesos y/o margas; elementos que también podrían concurrir en un emplazamiento de un AGP-CO2. Por otro lado, los procesos análogos mas importantes identificados serian: i) la inyección ascendente del CO2, que seria análoga a la inyección de CO2 de origen antropogénico, pero este con una signatura isotópica δ13C(V-PDB) de ~ -30 ‰; ii) la disolución de CO2 y 222Rn en las aguas del acuífero profundo, lo que seria análogo a la disolución de dichos gases en la formación almacén de un AGP-CO2; iii) la contaminación de los acuíferos mas someros por el ascenso de las aguas sobresaturadas en CO2, proceso que seria análogo a la contaminación que se produciría en los acuíferos existentes por encima de un AGP-CO2, siempre que este se perturbara natural (reactivación de fallas) o artificialmente (sondeos); iv) la desgasificación (CO2 y gases asociados, entre los que destaca el 222Rn) del acuífero salino profundo a través de sondeos, proceso análogo al que pudiera ocurrir en un AGP-CO2 perturbado; y v) la formación rápida de travertinos, proceso análogo indicativo de que el AGP-CO2 ha perdido su estanqueidad. La identificación de las analogías más importantes ha permitido, además, analizar y evaluar, de manera aproximada, el comportamiento y la seguridad, a corto, medio y largo plazo, de un AGP-CO2 emplazado en un contexto geológico similar al sistema natural estudiado. Para ello se ha seguido la metodología basada en el análisis e identificación de los FEPs (Features, Events and Processes), los cuales se han combinado entre sí para generar y analizar diferentes escenarios de evolución del sistema (scenario analysis). Estos escenarios de evolución identificados en el sistema natural perturbado, relacionados con la perforación de sondeos, sobreexplotación de acuíferos, precipitación rápida de travertinos, etc., serian análogos a los que podrían ocurrir en un AGP-CO2 que también fuera perturbado antropogénicamente, por lo que resulta totalmente necesario evitar la perturbación artificial de la formación sello del AGPCO2. Por último, con toda la información obtenida se ha propuesto una metodología de estudio que pueda aplicarse al estudio de posibles emplazamientos de un AGP-CO2 desde la perspectiva de los reservorios naturales de CO2, sean estancos o no. Esta metodología comprende varias fases de estudio, que comprendería la caracterización geológico-estructural del sitio y de sus componentes (agua, roca y gases), la identificación de las analogías entre un sistema natural de almacenamiento de CO2 y un modelo conceptual de un AGP-CO2, y el establecimiento de las implicaciones para el comportamiento y la seguridad de un AGP-CO2. ABSTRACT The accumulation of the anthropogenic greenhouse gases in the atmosphere is the main responsible for: i) the increase in the average temperature of the Earth over the past century by almost 1 °C; ii) the rise in the mean sea level; iii) the drop of the ice volume and terrestrial snow; iv) the strong climate variability and extreme weather events that have been happening over the last decades; and v) the spread of epidemics and infectious diseases. All of these events are just some of the evidence of current climate change. The problems and growing concern related to these phenomena, prompted the adoption of the so-called "Kyoto Protocol" (Japan) in 1997, in which the signatory countries established different measurements to control and reduce the emissions of the greenhouse gases. These measurements include the CCS technologies, focused on the capture, transport and storage of CO2. Within this context, it was approved, in October 2008, the Strategic Singular Project "Tecnologías avanzadas de generación, captura y almacenamiento de CO2" (PSE-120000-2008-6), supported by the Ministry of Science and Innovation and the FEDER funds. This Project, by means of the Subproject "Geological Storage of CO2" (PSS- 120000-2008-31), was focused on the detailed study of the Natural Analogue of CO2 Storage and Leakage located in the Ganuelas-Mazarron Tertiary basin (Murcia), among other Spanish Natural Analogues. This research work has been performed in the framework of this Subproject, being its final objective to predict the behaviour and evaluate the safety, at short, medium and long-term, of a CO2 Deep Geological Storage (CO2-DGS) by means of a comprehensive study of the abovementioned Natural Analogue. This study comprises: i) the geological and hydrogeological context of the basin and its geophysical research; ii) the water sampling of the selected aquifers to establish their hydrogeochemical and isotopic features; iii) the mineralogical, petrographic, geochemical and isotopic characterisation of the travertines formed from upwelling groundwater of several hydrogeological and geothermal wells; and iv) the measurement of the free and dissolved gases detected in the basin, as well as their chemical and isotopic characterisation, mainly regarding CO2 and 222Rn. This information, summarised in separate chapters in the text, has enabled to build a conceptual model of the studied natural system and to establish the analogies between both the studied natural system and a CO2-DGS, with possible natural and/or anthropogenic escapes. All this information has served, firstly, to predict the behaviour and to evaluate the safety, at short, medium and long-term, of a CO2-DGS and, secondly, to propose a general methodology to study suitable sites for a CO2-DGS, taking into account the lessons learned from this CO2 natural reservoir. The main results indicate that the Ganuelas-Mazarron basin is a graben bounded by normal faults with significant vertical movements, which move down the metamorphic substrate (Nevado-Filabride Complex), and filled with acid volcanic-subvolcanic rocks. Furthermore, this basin is filled with two sedimentary formations: i) the Miocene marls, which are predominant and almost exclusive in the basin; xxiv and ii) the Plio-Quaternary conglomerates and gravels. A deep saline CO2-rich aquifer was evidenced in this basin as a result of the geothermal exploration wells performed during the 80s, located just at the top of the Nevado-Filabride Complex and at a depth that could exceed 800 m, according to the geophysical exploration performed. This saline CO2-rich aquifer is precisely the main object of this study. Therefore, it is not discarded the possibility that the CO2 in this aquifer be in supercritical state. Consequently, the aforementioned basin gathers the main characteristics of a natural and deep CO2 geological storage, or natural analogue of a CO2-DGS in a deep saline aquifer. The overexploitation of the shallow aquifers in this basin for agriculture purposes caused the drop of the groundwater levels and hydrostatic pressures, and, as a result, the ascent of the deep saline and CO2-rich groundwater, which is the responsible for the contamination of the shallow and fresh aquifers. The hydrogeochemical features of groundwater from the investigated aquifers show the presence of very different hydrofacies, even in those with similar lithology. The high salinity of this groundwater prevents the human and agricultural uses. In addition, the slightly acidic character of most of these waters determines their capacity to dissolve and transport towards the surface heavy and/or toxic elements, among which U is highlighted. This element is abundant in the acidic volcanic rocks of the basin, with concentrations up to 14 ppm, mainly as sub-microscopic uraninite crystals. The isotopic study of this groundwater, particularly the isotopic signature of C from DIC (δ13C-DIC), suggests that dissolved C can be explained considering a mixture of C from two main different sources: i) from the thermal decomposition of limestones and marbles forming the substrate; and ii) from edaphic origin. However, a minor contribution of C from mantle degassing cannot be discarded. The study of travertines being formed from upwelling groundwater of several hydrogeological and geothermal wells, as a result of the fast CO2 degassing and the pH increase, has allowed highlighting this phenomenon, by analogy, as an alert for the CO2 leakages from a CO2-DGS. The analysis of the dissolved and free gases, with special attention to CO2 and 222Rn, indicates that the C from the dissolved and free CO2 has a similar origin to that of the DIC. The R/Ra ratio of He corroborates the minor contribution of CO2 from the mantle degassing. Furthermore, 222Rn is generated by the radioactive decay of U, particularly abundant in the volcanic rocks of the basin, and/or by 226Ra from the U or from the Messinian gypsum in the basin. Moreover, CO2 acts as a carrier of the 222Rn, a fact evidenced by the positive anomalies of both gases at ~ 1 m depth and mainly related to natural (faults and contacts) and anthropogenic (wells) perturbations. The isotopic signature of C from DIC, carbonates (travertines), and dissolved and free CO2, suggests that this parameter can be used as an excellent tracer of CO2 escapes from a CO2-DGS, in which CO2 usually from the combustion of fossil fuels, with δ13C(V-PDB) of ~ -30 ‰, will be injected. All of these results have allowed to build a conceptual model of the behaviour of the natural system studied as a natural analogue of a CO2-DGS, as well as to establish the relationships between both natural xxv and artificial systems. Thus, the most important analogies, regarding the elements of the system, would be the presence of: i) a deep saline CO2-rich aquifer, which would be analogous to the storage formation of a CO2-DGS; ii) a marly sedimentary formation with a thickness greater than 500 m, which would correspond to the sealing formation of a CO2-DGS; and iii) shallow aquifers with fresh waters suitable for human consumption, U-rich volcanic rocks, and faults that are sealed by gypsums and/or marls; geological elements that could also be present in a CO2-DGS. On the other hand, the most important analogous processes identified are: i) the upward injection of CO2, which would be analogous to the downward injection of the anthropogenic CO2, this last with a δ13C(V-PDB) of ~ -30 ‰; ii) the dissolution of CO2 and 222Rn in groundwater of the deep aquifer, which would be analogous to the dissolution of these gases in the storage formation of a CO2-DGS; iii) the contamination of the shallow aquifers by the uprising of CO2-oversaturated groundwater, an analogous process to the contamination that would occur in shallow aquifers located above a CO2-DGS, whenever it was naturally (reactivation of faults) or artificially (wells) perturbed; iv) the degassing (CO2 and associated gases, among which 222Rn is remarkable) of the deep saline aquifer through wells, process which could be similar in a perturbed CO2- DGS; v) the rapid formation of travertines, indicating that the CO2-DGS has lost its seal capacity. The identification of the most important analogies has also allowed analysing and evaluating, approximately, the behaviour and safety in the short, medium and long term, of a CO2-DGS hosted in a similar geological context of the natural system studied. For that, it has been followed the methodology based on the analysis and identification of FEPs (Features, Events and Processes) that have been combined together in order to generate and analyse different scenarios of the system evolution (scenario analysis). These identified scenarios in the perturbed natural system, related to boreholes, overexploitation of aquifers, rapid precipitation of travertines, etc., would be similar to those that might occur in a CO2-DGS anthropogenically perturbed, so that it is absolutely necessary to avoid the artificial perturbation of the seal formation of a CO2-DGS. Finally, a useful methodology for the study of possible sites for a CO2-DGS is suggested based on the information obtained from this investigation, taking into account the lessons learned from this CO2 natural reservoir. This methodology comprises several phases of study, including the geological and structural characterisation of the site and its components (water, rock and gases), the identification of the analogies between a CO2 storage natural system and a conceptual model of a CO2-DGS, and the implications regarding the behaviour and safety of a CO2-DGS.