Uso de técnicas geofísicas en la planificación y gestión de recursos hídricos subterráneos. Aplicación en acuíferos mediterráneos = Using geophysical techniques in planning and management of groundwater resources. Application in Mediterranean aquifers

[spa] La presente tesis tiene como objetivos el desarrollo, aplicación, calibración y validación del uso de información geofísica para la gestión, la caracterización y el estudio de la vulnerabilidad a la contaminación de los recursos hídricos subterráneos, planteando la eficiente integración de toda la información disponible. El proyecto de tesis ha desarrollado nuevas metodologías de aplicación de diferentes técnicas no destructivas (geofísicas) y ha validado la interpretación de los resultados obtenidos como apoyo para delimitar las zonas más favorables para la explotación de los recursos hídricos subterráneos, así como para evaluar la vulnerabilidad a la contaminación de los acuíferos, facilitando la protección y el control de las zonas más sensibles. Las principales fortalezas de las metodologías desarrolladas son su rapidez, resolución y la favorable relación coste/beneficio. Para ello se ha planteado el uso combinado de técnicas geofísicas ampliamente difundidas y contrastadas, como son la tomografía de resistividad eléctrica (ERT), los sondeos eléctricos verticales (SEV) y la gravimetría, con los generalmente más invasivos y costosos, pero siempre necesarios, métodos de caracterización directa clásicos (sondeos y piezómetros de investigación, ensayos de bombeo e infiltración y ensayos de laboratorio). Para validar las metodologías, éstas se han aplicado a distintos tipos de acuíferos, con diferentes problemáticas a tratar y distintas escalas y estrategias de trabajo. Los casos de aplicación se han seleccionado porque ocupan y preocupan en la actualidad a los gestores locales de aguas subterráneas. El primer caso de aplicación estudiado es la Cuenca de Valls. El comportamiento de las aguas subterráneas en esta zona se puede asimilar al de un sistema acuífero multicapa. En la cuenca de Valls se ha realizado un estudio cuantitativo y cualitativo de las aguas subterráneas en una zona declarada vulnerable a la contaminación de las aguas subterráneas, con una escala de trabajo regional (quilométrica), a partir de la reinterpretación de información ya existente. El tratamiento específico efectuado a datos de sondeos eléctricos verticales y su reinterpretación, apoyada en los datos de pozos disponibles, ha permitido también caracterizar a escala regional las características hidráulicas del sistema acuífero y la vulnerabilidad a la contaminación del mismo a través de la zona no saturada. Los métodos geofísicos han mostrado su utilidad en esta aplicación, proporcionado información muy valiosa de forma no destructiva, es decir sin necesidad de realizar nuevas perforaciones en el terreno que podrían actuar como vías preferentes para el paso de contaminantes hacia el acuífero. En el segundo y tercer caso de aplicación, Cubeta d’Abrera y Vall Baixa del Llobregat, el estudio también ha sido cuantitativo y cualitativo pero en este caso el comportamiento de las aguas subterráneas estudiadas se puede asimilar al de un acuífero libre. En estas zonas, el estudio se ha realizado a una escala intermedia (centenares de metros en el meandro de Ca n’Albareda) y local (métrica para las balsas de recarga artificial). En la Cubeta de Abrera y la Vall Baixa el trabajo se ha centrado en recopilar información preexistente (ya que se trata de una de las zonas más estudiadas de Europa) y en adquirir nueva información con un nivel de detalle inédito en la zona (caso de información geofísica). El uso combinado de la información proporcionada por fuentes diversas, como la cartografía hidrogeológica de la zona, la estructura del subsuelo definida por sondeos mecánicos, los resultados de los ensayos granulométricos de las muestras del sondeo más próximo, de campañas de prospección electromagnética y la interpretación de perfiles geoeléctricos realizados específicamente para este estudio, ha permitido establecer una metodología combinada para la mejor caracterización del medio acuífero implicado en la recarga artificial. La tomografía de resistividad eléctrica (ERT) en particular se ha mostrado como un método no destructivo, de bajo coste y elevada resolución, flexible y adaptable a distintas escalas de trabajo y tipos de adquisición que interfiere poco en el funcionamiento de las balsas de recarga artificial y permite caracterizar de un modo eficiente heterogeneidades hidrogeológicas del subsuelo y monitorizar procesos de infiltración y de colmatación en balsas de recarga artificial.[eng] The goal of the present PhD thesis has been the development, application, calibration and validation of the use of geophysical information for management, characterization and study of vulnerability to contamination of groundwater resources in an efficient manner. During the thesis project we have developed new methodologies for applying different nondestructive techniques (geophysics) to validate the interpretation of the results as a support tool for the evaluation of the most favorable areas for groundwater collection, as well as assessing vulnerability to contamination of aquifers, facilitating the protection and control of the most vulnerable areas to be polluted. The main strengths of the methodologies developed here are the speed of application, their resolution and effective cost / benefit ratio. To achieve our aim we combine the use of widespread and reliable geophysical techniques, such as electrical resistivity tomography (ERT), vertical electrical soundings (VES) and gravimetry, with generally more invasive and expensive, but always necessary, hydrogeological classical methods like research piezometers, pumping and infiltration tests and laboratory assays. The methodologies have been applied to different types of aquifers, with different problems to deal with, at different scales and with different approaches. The case studies have been chosen because they concern local managers of groundwater resources. The first case study is an aquifer system located in the Valls Basin. We realize that groundwater behavior could be associated to a multilayer aquifer system and, therefore, we have carried out a regional scale quantitative and qualitative study of the aquifer system from the reinterpretation of existing information in a groundwater system partially declared vulnerable to the surface pollution due to nitrates. In the second and third case studies, Cubeta d'Abrera and Llobregat artificial recharge ponds respectively, the study has also been quantitative and qualitative but in these cases the behavior of the groundwater can be associated to an unconfined aquifer. In this part, work has focused on gathering background information (as it is one of the most studied areas of Europe) and on acquiring and processing new geophysics data with an unprecedented level of detail in this area. The use of complementary direct and indirect methods such as boreholes, grain-size analysis, infiltration tests and electromagnetic and geoelectrical surveys has allowed to improve the diagnosis of the saturated and non-saturated zone for better characterization of the geological environment involved in the managed recharge system. In particular, the ERT technique, a low cost and a high resolution tool has shown to be flexible and adaptable for surveying at different scales and arrays without disturbing the recharge process and also has demonstrated the usefulness of geophysical imaging to unveil hydrogeological heterogeneities and monitoring infiltration and clogging processes in surface Managed Aquifer Recharge systems

Geochemical characterisation and health concerns of mineral bottled waters in Catalonia (North-eastern Spain)

Spain currently produces around 7000 million litres of mineral water a year, of which about 20% is produced in Catalonia, and there is a need for greater regulation and research into bottled waters and their impact on human health. A total of 29 samples were analysed from different brands of commercially bottled water, and 71 chemical elements were determined in each sample. The aim was to classify each brand based on composition, compare lithological origins, verify compliance with international standards for drinking water, and report benefits for human health. More than 60% of the samples were of the calcium bicarbonate type, had a low mineral content, and were associated with granitic aquifers, ranging from leucogranites to granodiorites. In contrast, 17% were of the sodium bicarbonate type, had harder waters, and were related to thermal springs. The thermal springs of the bottled waters from the Montseny-Guilleres massif (Vichy Catalán, Malavella, and San Narciso) emerge at a temperature of 60 °C with their own natural gas. Two samples exceeded European standards for As and Hg concentrations in water for human consumption, while one showed a concentration of U greater than that set out in international recommendations

Geophysical Characterization of Hydraulic Properties around a Managed Aquifer Recharge System over the Llobregat River Alluvial Aquifer (Barcelona Metropolitan Area)

Managed aquifer recharge using surface or regenerated water plays an important role in the Barcelona Metropolitan Area in increasing storage volume to help operators cope with the runoff variability and unexpected changes in surface water quality that are aggravated by climate change. The specific aim of the research was to develop a non-invasive methodology to improve the planning and design of surface-type artificial recharge infrastructures. To this end, we propose an approach combining direct and indirect exploration techniques such as electrical resistivity tomography (ERT), frequency domain electromagnetics and data from double-ring infiltration tests, trial pits, research boreholes and piezometers. The ERT method has provided much more complete and representative information in a zone where the recharge project works below design infiltration rates. The geometry of the hydrogeological units and the aquifer-aquiclude contact are accurately defined through the models derived from the interpretation of ERT cross-sections in the alluvial aquifer setting. Consequently, prior to the construction of recharge basins, it is highly recommended to conduct the proposed approach in order to identify the highest permeability areas, which are, therefore, the most suitable for aquifer artificial recharg

Enhanced Groundwater Protection and Management Using Gravity and Geoelectrical Data (Valls Basin, Spain)

The basis for the protection and prevention of groundwater pollution lies in the accurate assessment of vulnerability in terms of the exposure of groundwater bodies to contaminants before they are potentially discharged into the environment. The vulnerability assessment consists of calculating the ease with which pollutants can reach the aquifer from the surface through the vadose zone, which effectively reduces the pollutant load when the transit time is long. Index methods are mostly used, as they are based on input data that are readily available, easy to implement and interpret, and which are simple and practical. However, there are also limitations, as some methods are somewhat subjective and provide only a qualitative approximation. This case study aims to develop a methodology that can quantitively estimate the hydrogeological parameters of the aquifer formations of the Valls basin using geophysical methods and the Dar Zarrouk parameters. The specific treatment carried out on data from gravity stations and vertical electric soundings, supported by the available well data, allows for the delineation of the most favourable areas for the exploitation of groundwater resources (higher hydraulic transmissivity) and the areas most susceptible to pollution (with a shorter transit time) on a regional scale. Geophysical methods have proved useful, sustainably providing valuable information without the need to drill new boreholes that could act as preferential pathways for pollutants into the aquifer

ERT monitoring of two Managed Aquifer Recharge ponds in the alluvial aquifer of the Llobregat River (Barcelona, Spain)

Over the past twenty years, there has been growing interest in the use of the subsurface for water storage using shallow ponds, where water is infiltrated to the subsurface and later groundwater is recovered from pumping wells. This scheme is designed as surface Managed Aquifer Recharge (MAR). Llobregat artificial recharge ponds are MAR systems located in alluvial aquifers near Barcelona with strong significance for water supply to the city. The recharge ponds showed low infiltration rates since the beginning (Ca n'Albareda) and a significant decrease after some months (Sant Vicenç). Consequently, different methodologies were designed for monitoring the systems and evaluating the effectiveness of the selected areas and maintenance procedures. For this purpose, we combined the use of Electrical Resistivity Tomography (ERT) with standard hydrogeological methodologies, including water table monitoring from piezometers and infiltration tests. The combination of direct and indirect methods have allowed us to improve the diagnosis of the subsurface involved in the managed recharge system. The ERT technique has shown to be a cost‐effective and high‐resolution tool, flexible and well adaptable for surveying at different scales without disturbing the recharge process. As a consequence, we demonstrate the usefulness of ERT imaging to unveil hydrogeological heterogeneities and monitoring infiltration, clogging effect and clean‐up processes in surface MAR projects

Estudio comparativo de diferentes técnicas geofísicas para el control de la intrusión salina en acuíferos costeros: Aplicación al acuífero de Oued Laou (Marruecos)

Groundwater is a fundamental and strategic resource for water supply of coastal areas, especially in the Mediterranean basin where the irregularity of precipitation, both seasonally and year-onyear, put at risk the availability of surface water in the rivers, which go from being dry for long periods to causing torrential discharges. In particular, if the exploitation of coastal aquifers is not properly managed and the quality of groundwater is not monitored regularly, important sectors of the aquifer may be affected by saltwater intrusion, and cause the abandonment of the wells. Geophysical methods are a very effective complement to provide indirect information on groundwater salinity in areas of the aquifer where wells or piezometers are not available for sampling. This study presents the results obtained from the application of different geophysical methods in the coastal aquifer of Oued Laou (Morocco) as a complement to hydrochemical data. Four geophysical methods, two electrical: vertical electrical sounding and electrical resistivity tomography, and two electromagnetic: frequency domain EM and time domain EM sounding. The comparative analysis of the results obtained by each method has made it possible to assess the coherence and complementarity of each method, as well as their corresponding advantages and limitations in terms of resolution and cost-effectiveness

Coupled Geophysical and Hydrogeochemical Characterization of a Coastal Aquifer as Tool for a More Efficient Management (Torredembarra, Spain)

The aquifers of the Spanish Mediterranean coast are generally subjected to intense exploitationto meet the growing water supply demands. The result of the exploitation is salinizationdue to the marine saltwater intrusion, causing a deterioration in the quality of the water pumped,limiting its use for community needs, and not always being well delimited. To prevent deterioration,a groundwater control network usually allows precise knowledge of the areas affected by saltwaterintrusion but not the extent of the saline plumes. Moreover, the characterization of aquifer systemsrequires a model that defines the geometry of aquifer formations. For this objective, we integratedhydrogeological, hydrogeochemical, and electrical resistivity subsoil data to establish a hydrogeologicalmodel of the coastal aquifer of Torredembarra (Tarragona, NE Spain). In this research, wehave carried out a regional and local-scale study of the aquifer system to define the areas prone tobeing affected by saline intrusion (electrical resistivity values below 10 Wm). The obtained resultscould be used as a support tool for the assessment of the most favorable areas for groundwaterwithdrawal, as well as enabling the control and protection of the most susceptible areas to be affectedby saltwater intrusion.</p

Characterization of a Shallow Coastal Aquifer in the Framework of a Subsurface Storage and Soil Aquifer Treatment Project using Electrical Resistivity Tomography (Port de la Selva, Spain)

Water percolation through infiltration ponds is creating significant synergies for the broad adoption of water reuse as an additional non-conventional water supply. Despite the apparent simplicity of the soil aquifer treatment (SAT) approaches, the complexity of site-specific hydro-geological conditions and the processes occurring at various scales require an exhaustive under-standing of the system's response. The non-saturated zone and underlying aquifers cannot be considered as a black box, nor accept its characterization from few boreholes not well distribut-ed over the area to be investigated. The electrical resistivity tomography (ERT) is a non-invasive technology, highly responsive to geological heterogeneities that has demonstrated useful to provide the detailed subsurface information required for groundwater modeling. The relation-ships between the electrical resistivity of the alluvial sediments and the bedrock and the differ-ence in salinity of groundwater, highlight the potential of geophysical methods over other more costly subsurface exploration techniques. The results of our research show that ERT coupled with implicit modeling tools provides information that can significantly help to identify aquifer geometry and characterize the saltwater intrusion of shallow alluvial aquifers. The proposed approaches could improve the reliability of groundwater models and the commitment of stakeholders to the benefits of SAT procedures

Hydrogeophysical Assessment of the Critical Zone below a Golf Course Irrigated with Reclaimed Water Close to Volcanic Caldera

The geometry and the hydraulic properties of the unsaturated zone is often difficult to evaluate from traditional soil sampling techniques. Soil samples typically provide only data of the upper layers and boreholes are expensive and only provide spotted information. Non-destructive geophysical methods and among them, electrical resistivity tomography can be applied in complex geological environments such as volcanic areas, where lavas and unconsolidated pyroclastic deposits dominate. They have a wide variability of hydraulic properties due to textural characteristics and modification processes suh as compaction, fracturation and weathering. To characterize the subsurface geology below the golf course of Bandama (Gran Canaria) a detailed electrical resistivity tomography survey has been conducted. This technique allowed us to define the geometry of the geological formations because of their high electrical resistivity contrasts. Subsequently, undisturbed soil and pyroclastic deposits samples were taken in representative outcrops for quantifying the hydraulic conductivity in the laboratory where the parametric electrical resistivity was measured in the field. A statistical correlation between the two variables has been obtained and a 3D model transit time of water infiltration through the vadose zone has been built to assess the vulnerability of the aquifers located below the golf course irrigated with reclaimed water

Application of Resistivity and Seismic Refraction Tomography for Landslide Stability Assessment in Vallcebre, Spanish Pyrenees

Geophysical surveys are a noninvasive reliable tool to improve geological models without requiring extensive in situ borehole campaigns. The usage of seismic refraction tomography (SRT), electrical resistivity tomography (ERT) and borehole data for calibrating is very appropriate to define landslide body geometries; however, it is still only used occasionally. We present here the case of a Spanish Pyrenees slow-moving landslide, where ERT, SRT and lithological log data were integrated to obtain a geological three-dimensional model. The high contrasts of P-wave velocity and electrical resistivity values of the upper materials (colluvial debris and clayey siltstone) provided accurate information on the geometry of the materials involved in the landslide body, as well as the sliding surface. Geophysical prospecting allowed us to identify the critical sliding surface over a large area and at a reduced cost and, therefore, gives the geophysical method an advantage over borehole data. The three-dimensional model was used to carry out stability analyses of a landslide in 2D and 3D, which, coherently with previous studies, reveal that the lower part is more unstable than the upper units