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.

The freshwater‐seawater mixing zone is a critical region for chemical activity. Yet little is known about the influence of ever present spatial heterogeneity on the dynamics of mixing and calcite dissolution, which play a key role in the understanding of karst development. We analyze the impact of different heterogeneity structures and strengths on the local and global response of mixing and dissolution rates across the saltwater freshwater mixing zone. We find that the initial heterogeneity structure significantly impacts observed dissolution and mixing patterns, which sheds some new light on karst propagation in coastal aquifers., Data associated with this manuscript are available in the DIGITAL.CSIC repository at https://digital.csic.es/ under the DOI (https://doi.org/10.20350/digitalCSIC/8955). This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska Curie Grant Agreement No. 722028 (ENIGMA ITN). Maria Pool acknowledges the support of the Spanish Ministry of Science and Innovation through the Torres‐Quevedo program (PTQ2018‐010081)., Peer reviewed

We use Approximate Bayesian Computation and the Kullback–Leibler divergence measure to quantify to what extent horizontal and vertical equivalent electrical conductivity time-series observed during tracer tests constrain the 2-D geostatistical parameters of multivariate Gaussian log-hydraulic conductivity fields. Considering a perfect and known relationship between salinity and electrical conductivity at the point scale, we find that the horizontal equivalent electrical conductivity time-series best constrain the geostatistical properties. The variance, controlling the spreading rate of the solute, is the best constrained geostatistical parameter, followed by the integral scales in the vertical direction. We find that horizontally layered models with moderate to high variance have the best resolved parameters. Since the salinity field at the averaging scale (e.g., the model resolution in tomograms) is typically non-ergodic, our results serve as a starting point for quantifying uncertainty due to small-scale heterogeneity in laboratory-experiments, tomographic results and hydrogeophysical inversions involving DC data., This work has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska - Curie grant agreement number 722028 (ENIGMA ITN). The authors would like to thank Jesús Carrera for enriching discussions motivating this work and Jürg Hunziker for sharing his code for implementing the circulant embedding technique., Peer reviewed

The characterization of saline water intrusion (SWI) and its hydrodynamics is a key issue to understand sub-marine groundwater discharge (SGD) and manage groundwater resources in coastal areas. To test and comparedifferent methods of characterization and monitoring, a new experimental site has been constructed in a coastalalluvial aquifer north of Barcelona city (Catalonia, Spain). The site is located between 30 and 90 m from theseashore and comprises 16 shallow piezometers organized in nests of three with depths ranging between 15 and25 m and 4 solitary piezometers. The objective of this paper is to combine different recently developed mon-itoring techniques to evaluate temporal variations in the aquifer hydrodynamics of the site at different spatialscales before and after the dry season of 2015. At the site scale,fibre optic distributed temperature sensing (FO-DTS), for thefirst time applied to study SWI, and cross-hole electrical resistivity tomography (CHERT) has beenapplied. At the meter/borehole scale, electrical conductivity of the formation has been applied not only in arepeated manner (¿time lapse¿), but also for thefirst time at relatively high frequency (1 sample every 10 min).CHERT has provided a better characterization of the seawater intrusion than electrical conductivity data ob-tained from piezometers. The combination of techniques has allowed improving the understanding of the systemby: 1) characterizing the extent and shape of SWI; 2) differentiating two different dynamics in the aquifer; and 3)identifying preferentialflow paths over different time and spatial intervals. Future challenges and the appli-cation 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.

Mixing along the salt-freshwater interface is critical for geochemical reactions, transport, and transformation of nutrients and contaminants in coastal ecosystems. However, the mechanisms and controls of mixing are not well understood. We develop an analytical model, based on the coupling between flow deformation and dispersion, which predicts the mixing dynamics along the interface for steady-state flow in coastal aquifers. The analytical predictions are compared with the results of detailed numerical simulations, which show that nonuniform flow fields, inherent to seawater intrusion in coastal aquifer, result in a non-monotonic evolution of mixing width and mixing rates along the interface. The analytical model accurately captures these dynamics over a range of freshwater flow rates and dispersivities. It predicts the evolution of the mixing width and mixing rates along the interface, offering a new framework for understanding and modeling mixing and reaction processes in coastal aquifers., This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 722028 (ENIGMA ITN). Maria Pool acknowledges the support of MCIN/AEI/10.13039/501100011033 through the Torres-Quevedo program (PTQ2018-010081). MD acknowledges the support of MCIN/AEI/10.13039/501100011033 through the project HydroPore (PID2019-106887GB-C31) and the Grant CEX2018-000794-S., Peer reviewed

Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute concentrations. This spatial and temporal variability generally leads to effective behaviors and emerging phenomena that cannot be predicted from conventional approaches based on homogeneous assumptions and models. However, it is not always clear when, why, how, and at what scale the 4D (3Dĝ€¯+ĝ€¯time) nature of the subsurface needs to be considered in hydrogeological monitoring, modeling, and applications. In this paper, we discuss the interest and potential for the monitoring and characterization of spatial and temporal variability, including 4D imaging, in a series of hydrogeological processes: (1) groundwater fluxes, (2) solute transport and reaction, (3) vadose zone dynamics, and (4) surface-subsurface water interactions. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. We then highlight recent innovations that have led to significant breakthroughs in high-resolution space-time imaging and modeling the characterization, monitoring, and modeling of these spatial and temporal fluctuations. We finally propose a classification of processes and applications at different scales according to their need and potential for high-resolution space-time imaging. We thus advocate a more systematic characterization of the dynamic and 3D nature of the subsurface for a series of critical processes and emerging applications. This calls for the validation of 4D imaging techniques at highly instrumented observatories and the harmonization of open databases to share hydrogeological data sets in their 4D components., This work has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement number 722028 (ENIGMA ITN)., Peer reviewed

The salt-freshwater interface (SFI) in coastal aquifers, where water bodies of differing chemical compositions converge, represents as a hotspot for chemical disequilibrium and biogeochemical reactions. The interplay between variable density flow, mixing and reaction in porous media leads to complex and highly localized reaction patterns across the SFI, which are difficult to visualize, understand and model. In this study, we present a novel experimental setup to quantify mixing-driven reactions at the SFI in a quasi two-dimensional laboratory-scale sand tank. We use a fast irreversible chemiluminescence reaction to obtain high resolution images of reaction intensity at the interface. We show that the reaction rate varies spatially along the interface, with up to a factor of four between minimum and maximum. We present a mechanistic model quantifying the evolution of the reaction rate along the interface by describing the coupled effects of solute dispersion, fluid compression and the change of the interface geometry under different flow rates. This modeling framework captures the spatial distribution of reaction along the SFI observed experimentally. It also explains and predicts the enhancement of the effective reaction rate across the interface when increasing freshwater flow. These findings provide a novel experimental technique and a new modeling framework to image and predict mixing-driven chemical reactions in coastal aquifers., This research has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant 722028 ITN ENIGMA., Peer reviewed

This dataset includes experimental images of a chemiluminescent reaction taking place across the salt-freshwater interface. Images are provided for several steady state saltwater wedges., European Commission
ENIGMA - European training Network for In situ imaGing of dynaMic processes in heterogeneous subsurfAce environments 722028, Peer reviewed

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.