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We measure groundwater flux and thermal parameters around a borehole performing a heat dissipation test by heating the armor of a single fiber-optic cable and interpreting the resulting heating curves with a new analytical method. The procedure is similar to thermal response tests, but benefitting from the high spatial and temporal resolution of distributed temperature sensing and lasting longer, so as to measure advective dissipation. Field installation relies on an innovative method in hydrogeology, which is based on the installation of the FO cable in the annular space of the well, close to the aquifer matrix. The proposed new analytical method, expands the traditional Moving Infinitesimal Line Source Model to account for the effects of the field set up and cable materials. In fact, we show that the resulting temperature build-up goes through four periods easy to identify using the log derivative of temperature (dT/d (ln(t))): Initial response, skin (cable insulation), conduction dominated and advection dominated. We test the proposed method in an unconsolidated shallow aquifer with controlled pumping. Results are of the same order of magnitude of independent estimates of groundwater velocity., This work was funded by the projects CGL2016-77122-C2−1−R / 2-R and PID2019-110212RB-C22 / C21 of the Spanish Government. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site. We would like to acknowledge Sensornet (UK) for their support and collaboration. We would also like to thank Dr. Javier Benitez Buelga for his collaboration in designing and setting up the fiber-optic field installation. The author Albert Folch is a Serra Húnter Fellow., Peer Reviewed, Postprint (published version)

Surface electrical resistivity tomography (ERT) is a widely used tool to study seawater intrusion (SWI). It is noninvasive and offers a high spatial coverage at a low cost, but its imaging capabilities are strongly affected by decreasing resolution with depth. We conjecture that the use of CHERT (cross-hole ERT) can partly overcome these resolution limitations since the electrodes are placed at depth, which implies that the model resolution does not decrease at the depths of interest. The objective of this study is to test the CHERT for imaging the SWI and monitoring its dynamics at the Argentona site, a well-instrumented field site of a coastal alluvial aquifer located 40¿km NE of Barcelona. To do so, we installed permanent electrodes around boreholes attached to the PVC pipes to perform time-lapse monitoring of the SWI on a transect perpendicular to the coastline. After 2 years of monitoring, we observe variability of SWI at different timescales: (1) natural seasonal variations and aquifer salinization that we attribute to long-term drought and (2) short-term fluctuations due to sea storms or flooding in the nearby stream during heavy rain events. The spatial imaging of bulk electrical conductivity allows us to explain non-monotonic salinity profiles in open boreholes (step-wise profiles really reflect the presence of freshwater at depth). By comparing CHERT results with traditional in situ measurements such as electrical conductivity of water samples and bulk electrical conductivity from induction logs, we conclude that CHERT is a reliable and cost-effective imaging tool for monitoring SWI dynamics., This work was funded by the project CGL2016-77122-C2-1-R/2-R of the Spanish Government. This project also received funding from the European Commission, Horizon 2020 research and innovation programme (Marie Sklodowska-Curie (grant no. 722028)). The author Albert Folch is a Serra Húnter Fellow., Peer Reviewed, Postprint (published version)

Aplicat embargament des de la data de defensa fins els dia 6 de juny de 2021, Detailed characterization of coastal aquifers is essential for proper management of coastal groundwater resources, and coastal areas in general. Still, there is a general lack of systematic monitoring and detailed characterization of the seawater intrusion (SWI). Part of this lack is related to the limitations of traditional tools and methodologies. In this thesis we aim to provide new approaches and methodologies for the characterization of coastal aquifers, and in particular for the SWI dynamics. At a first stage, three relatively new monitoring systems were tested to characterise the SWI: Cross-Hole Electric Resistivity Tomography (CHERT), Time Laps Induction Logging (TLIL) and Fiber Optics Distribute Temperature Sensing (FO-DTS). We did that by installing the fiber optic cable and CHERT electrodes outside the borehole casing, and allowing to deploy additional tools through the borehole. This multipurpose approach allowed to deploy multiple tools in parallel, decreasing installation costs and improving characterization by combining complementary information. In fact the combination of the three methods allowed to discern different levels of SWI and multiple dynamics. From these techniques we were interested in further exploring the use of FO-DTS. However, before that we had to study the use of temperature as a tracer of the SWI. Although the fundamentals of heat transport in groundwater are well known, the use of temperature as a natural tracer for SWI is limited. Numerical modelling was used to explore the potential of temperature for studying SWI dynamics at field scale. The results were used to propose a framework for the interpretation of thermal data at the SWI. The framework gives guidelines to identify preferential paths of the SWI, flow direction and even roughly estimate groundwater flux rates, in multilayered coastal aquifers. We concluded that temperature has a potential for SWI motorisation largely unexplored. Based on the promising results from the m, Una caracterización detallada de los acuíferos costeros es esencial para la gestión de los recursos hídricos costeros, y en general de las zonas costeras. A pesar de su importancia, en general no existe un monitoreo sistemático de la zona de intrusión salina (SWI). Parte de esta carencia se debe a limitaciones en las herramientas y metodologías tradicionalmente empleadas. El objetivo de esta tesis es proporcionar nuevos enfoques y herramientas para la caracterización de los acuíferos costeros, y en particular de la SWI. Inicialmente, tres sistemas de monitoreo fueron testados en campo para caracterizar la SWI: Cross-Hole Electric Resistivity Tomography (CHERT), Time Laps Induction Logging (TLIL) and Fiber Optics Distribute Temperature Sensing (FO-DTS). Para poder operar las tres herramientas en paralelo, el cable de fibra óptica y los electrodos de la CHERT se instalaron entre el entubado y el sedimento. De este modo redujimos costes de instalación y mejoramos el proceso de caracterización con la combinación de información complementarias. De hecho, la combinación de las tres permitió´ detectar diferentes niveles y dinámicas de la SWI. De estas tres técnicas, nos hemos querido concentrar en explorar el uso de la FO-DTS para el monitoreo de acuíferos costeros. Sin embargo, antes de todo tuvimos que estudiar el uso de la temperatura como trazador de la SWI. A pesar de que los fundamentos del transporte de calor en agua subterránea están establecidos, el uso de la temperatura como trazador natural de la SWI es limitado. Para estudiar la SWI se han hecho dos modelos numéricos. De los resultados obtenidos surge un posible marco teórico para la interpretación de datos térmicos de la SWI. Usando la distribución vertical de temperaturas podríamos diferenciar niveles de SWI, la dirección de flujo, e incluso aproximar la velocidad del mismo. Con estos resultados llegamos a la conclusión de que el uso de la temperatura para el seguimiento de la SWI tiene todavía un gran potenc, Postprint (published version)

The characterization of saline water intrusion (SWI) and its hydrodynamics is a key issue to understand submarine groundwater discharge (SGD) and manage groundwater resources in coastal areas. To test and compare different methods of characterization and monitoring, a new experimental site has been constructed in a coastal alluvial aquifer north of Barcelona city (Catalonia, Spain). The site is located between 30 and 90 m from the seashore and comprises 16 shallow piezometers organized in nests of three with depths ranging between 15 and 25 m and 4 solitary piezometers. The objective of this paper is to combine different recently developed monitoring techniques to evaluate temporal variations in the aquifer hydrodynamics of the site at different spatial scales before and after the dry season of 2015. At the site scale, fibre optic distributed temperature sensing (FO-DTS), for the first time applied to study SWI, and cross-hole electrical resistivity tomography (CHERT) has been applied. At the meter/borehole scale, electrical conductivity of the formation has been applied not only in a repeated manner (“time lapse”), but also for the first time at relatively high frequency (1 sample every 10 min). CHERT has provided a better characterization of the seawater intrusion than electrical conductivity data obtained from piezometers. The combination of techniques has allowed improving the understanding of the system by: 1) characterizing the extent and shape of SWI; 2) differentiating two different dynamics in the aquifer; and 3) identifying preferential flow paths over different time and spatial intervals. Future challenges and the application of these techniques in other areas are also discussed., This work was funded by the projects CGL2013-48869-C2-1-R/2-R and CGL2016-77122-C2-1-R/2-R of the Spanish Government. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site. The authors want to thank the support of the Generalitat de Catalunya to MERS (2018 SGR-1588). This work is contributing to the ICTA ‘Unit of Excellence’ (MinECo, MDM2015- 0552). Part of the funding was provided by the French network of hydrogeological observatories H+ (hplus/ore/fr/en) and the ANR project EQUIPEX CRITEX (grant ANR-11-EQPX-0011). V Rodellas acknowledges financial support from the Beatriu de Pinós postdoctoral program of the Generalitat de Catalunya (2017-BP-00334). M. Diego‐Feliu acknowledges the economic support from the FI‐2017 fellowships of the Generalitat de Catalunya autonomous government (2017FI_B_00365). This project also received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No 722028., Peer Reviewed, Postprint (author's final draft)