Groundwater systems are components of the Critical Zone, in dynamic balance with the climate and human pressure. Water and heat fluxes control biogeochemical processes and regulate both resource potential and system vulnerability. Evaluating system responses to climate and land use changes requires an estimation of subsurface structure, flow dynamics, and the spatial distribution of recharge and discharge zones. Classical hydrological data, such as water levels and river discharge, are commonly coupled with unconventional methodologies, including heat tracing, electrical resistivity tomography, and surface wave analysis. Their coupling via time-lapse monitoring and inversion frameworks enables the characterization of thermal, electrical, and mechanical interactions, thus encouraging a transition from static structure description to transient representations of groundwater. We identify key challenges, including data limitations, model uncertainty propagation, and the integration of transient variables and parameters into inversion workflows. Petrophysical and geostatistical approaches help overcome these issues by coupling geophysical and groundwater models, quantifying spatio-temporal uncertainties, and addressing scale change. Finally, choices regarding experimental design, parameter reduction, dimensionality, model hypotheses, and inversion scale must be assessed to balance parsimony with model accuracy. These developments underscore the central role of hydrogeophysics in advancing Critical Zone science and sustainable groundwater management. Emphasizing transient processes is essential for capturing how subsurface systems evolve over time in response to environmental changes.

The first objective of this study is to present unique field data on a three-year pilot test during which air containing 8 mol% O2(g) was injected as a cushion gas into a natural gas reservoir, a carbonate-cemented sandstone aquifer located in the Paris Basin (France). 10-year system survey showed that: the oxygen was fully depleted several months after injection completion, meanwhile CO2(g) was detected around 2-6 mol%; the pH decreased from 8 to 6, while reducing conditions shifted to mildly oxidizing ones with increasing concentration of sulfates in equilibrium with gypsum. 3 years after injection completion, the pH gradually returned to its near initial state and sulfates were reduced by 2 to 3 times. The second objective is to develop a multiphase reactive transport model based on the field data. Simulations were constructed using the HYTEC reactive transport code, progressing from 0D-batch to 2D-reservoir configurations. The model reproduced the gas-water-rock reactive sequence: 1/ full depletion of the injected O2(g) due to pyrite oxidation, 2/ leading to acidity production and dissolved sulfates, 3/ acidity buffering by calcite dissolution, 4/ followed by gypsum precipitation and CO2(g) exsolution. The model demonstrated that pyrite kinetics was the most significant factor governing not only the amount of O2(g), CO2(g) and dissolved minerals, but also the spatial extent of these chemical reactions and, hence, the gas spread inside the reservoir. The formulated advective Damköhler number for oxygen consumption indicated advection- and reaction-dominant regimes explaining the gas composition and extension. The developed field-based model could be used as a workflow for other gas storage facilities, e.g. biomethane, compressed air, and CO2. For underground biomethane storage, the O2(g) contents recommended in Europe, i.e. the EASEE-gas specification 2005-001-02, should have a low impact on gas composition and reservoir geochemistry when the reservoir contains efficient pH-buffers such as calcite.

Given the current climate and anthropogenic evolution, water management becomes one of the greatest challenges of the 21st century. Number of studies have analyzed observed hydrologic trends and their connections with the changing climate. Impacts include changes in runoff, river discharge and groundwater recharge. Water quality is also impacted, through its many facets including the water temperature. Despite the important progress made in climate modelling, the impact of the predicted global warming on hydrological processes remains uncertain; particularly, in large hydrosystems. The Seine River basin has a surface of 78,650 km², it includes the Seine River and its 50 tributaries, it is populated by 30% of France inhabitants. The Seine River basin crosses 14 departments and 4 regions, including the Paris metropolitan area. Climate change poses a vulnerability due to its potential political, social, and economic consequences in the Seine basin. The agricultural activities and number industries depend on water resources or are located on the river sides. Our ability to adapt water resource management strategies to the climate change depends on our ability to understand and estimate the actual evolution of water resource. The terrestrial water budget is now considered as a single continuum. This integrated conceptualisation needs to simulate the spatial and temporal dynamics of water exchanges between the surface and groundwater. Here we propose to improve the representation of the surface water budget with the goal to decrease the uncertainty of the whole water budget of the Seine hydrosystem. We used the process-based physical land surface model ORCHIDEE (tag 2.2) to estimate surface water budget and heat balance for the period 1980-2018. This application takes advantage of high resolution land-use and albedo maps from ESA-CCI database, and various soil map databases. The model was satisfactorily able to reproduce the discharges of each sub-catchment, the actual evapotranspiration fluxes and LAI. With these results, we are able to estimate the the partitioning of the surface water balance of each catchment of the Seine basin. These results have wide ranging implications such as the estimation of energy balance in the basin, the estimation of spatialisation of the aquifer recharge, and the feedback between aquifers and the surface.

Reliable groundwater recharge quantification at the regional scale (e.g., watershed or subwatershed) is fundamental to sustainable water resource management. Although modeling at the watershed scale is gaining wide support, the long‐term monitoring needed for model calibration is often not readily available, as many watersheds worldwide remain ungauged. In response to this situation, we propose a new approach to estimate groundwater recharge at the watershed scale. This approach is fast and accurate and takes into account the existing variability without requiring long‐term monitoring. Only a single field campaign to acquire soil water content and pore water isotopic composition depth profiles is needed. The principle is to extend a physically based, one‐dimensional unsaturated zone flow model from the local (i.e., profile) to the watershed scale, using an index method for distributed recharge based on a GIS. The methodology was validated in a gauged watershed, where previous studies have estimated recharge using a spatialized water balance model calibrated using long‐term discharge monitoring data. Scaling was investigated by comparing recharge values obtained using the local‐scale approach at 10 study sites within the watershed with coinciding values obtained at the watershed scale. Recharge values were similar in terms of both dynamics and quantity. Using the pore water isotopic fingerprint of ungauged watersheds is therefore confirmed to be a suitable approach for understanding spatiotemporal recharge variability.

Energy transition requires the use of low-carbon energies such as geothermal energy for the production of electricity or heat. Geothermal exploitation has a number of preferential targets, including fault zones in the context of graben. High temperatures can, indeed, be observed where fluids rise through fault zones, but geothermal processes are complex to understand and to model in such a 3D tectonic context. For instance, seismic observations and then observations at well-scale show structures on different spatial scales that can overlap and interconnect. These structures then present strong heterogeneities in physical properties (e.g. fault core or damage zones). In addition, this knowledge evolves over time, from the exploration to drilling and exploitation phases. One of the challenges of numerical modelling is to represent this complexity while being readily upgradeable in the light of exploration. We are developing an adaptive approach using the CIMLIB/EXALIB library. Geometrical complexity and physical properties are defined by distance functions (level set functions) to geologic objects that are inherited from a geologic modelling software. Coupled fluid flow and heat transport processes are then modelled in 3D with adaptive meshing. The mesh can, indeed, be adapted according to static criteria such as geometry or dynamic criteria such as physical processes. This approach will be illustrated by examples derived from an ongoing GIS-Geodenergies project in the Upper Rhine Graben.

The evolution of water temperature at the stream–aquifer interface and the associated heat fluxes have a major influence on the ecological state. Here, a synthetic case is developed to characterize the behavior of the stream–aquifer interface submitted to meteorological and hydrological forcings as well as to various hydraulic and thermal properties, representative of a wide range of lithofacies from clay to gravel/sand. The thermal regime of the stream–aquifer system is driven by two pseudo-periodic cycles: the annual cycle and the diurnal cycle. A thermo-hydrogeological model coupled with a parameter sampling script is used. The results highlight the drivers that have the greatest influence on heat fluxes: (1) the meteorological conditions through the seasonal thermal gradients established between the stream and the aquifer during winter and summer periods, and (2) the hydraulic conductivity at the stream–aquifer interface. Depending on the hydraulic conductivity values at the stream–aquifer interface, two thermal regimes exist: for high hydraulic conductivities, advective fluxes clearly prevail, while for lower hydraulic conductivities, conductive fluxes predominate in most cases. This depth is reduced in upwelling cases and modulated by the thermal regime. The predominance of either the streambed- or aquifer-property effects depends on the equivalent hydraulic conductivity. In downwelling conditions under a given meteorological setup, stream–aquifer disconnection leads to increased advective fluxes compared with its connected counterpart. Results from this study provide better insight into heat fluxes at the stream–aquifer interface, which will ultimately result in a better stream heat-balance model.

Modeling water and solute transport in the vadose zone for groundwater resource management requires an accurate determination of soil hydraulic parameters. Estimating these parameters by inverse modeling using in situ observations is very common. However, little attention has been given to the potential of pore water isotope information to parameterize soil water transport models. By conducting a Morris and Sobol sensitivity analysis, we highlight the interest of combining water content and pore water isotope composition data in a multi-objective calibration approach to constrain soil hydraulic property parameterization. We then investigate the effect of the sampling frequency of the observed data used for model calibration on a synthetic case. When modeling is employed in order to estimate the annual groundwater recharge of a sandy aquifer, it is possible to calibrate the model without continuous monitoring data, using only water content and pore water isotopic composition profiles from a single sampling time. However, even if not continuous, multi-temporal data improve model calibration, especially pore water isotope data. The proposed calibration method was validated with field data. For groundwater recharge estimate studies, these results imply a significant reduction in the time and effort required, by avoiding long-term monitoring, since only one sampling campaign is needed to extract soil samples.

RationaleA method to measure the δ2H and δ18O composition of pore water in soil samples using direct vapour equilibration and laser spectrometry was first described in 2008, and was rapidly adopted. Here, we describe an improved setup to measure pore water δ2H and δ18O values through direct vapour equilibration with a laser spectrometer, combining a liquid and a vapour mode for water isotope analyses, and resulting in improved accuracy.MethodsWe first tested new gas sampling bags as part of the equilibration protocol. Then, to assess measurement accuracy, vapour samples from equilibrated liquid waters of known isotope composition were measured in the liquid mode of the analyser using the new setup as well as the manufacturer's vapour mode. Various modes of preparing liquid water standards, namely equilibration, nebulisation, and vapourisation, were tested to determine the best calibration in terms of accuracy. Finally, the proposed modified liquid setup was validated by analysing water vapour equilibrated from soil pore water of a known composition.ResultsThe δ2H and δ18O measurements were found to be more accurate by the modified liquid mode than by the factory‐setup vapour mode. The strong and non‐linear dependence of measured δ2H and δ18O values on H2O concentration in vapour mode, especially at concentrations equal to the vapour pressure saturation typically found in laboratories, is problematic for corrections. Regarding calibration and standards, the use of two equilibrated liquid water standards was found to best calibrate measurements in the modified liquid setup. Finally, the modified liquid mode setup and its calibration, as described here, were shown to be appropriate for soil pore water analysis.ConclusionsThe proposed modified setup results in more precise δ2H and δ18O soil pore water values than the usual protocols. An average standard deviation of 0.04‰ for δ18O values and 0.3‰ for δ2H values, based on 228 soil sample analyses, was obtained.

A water table mapping method that accounts for surface-water-groundwater (SW-GW) connectivity and human pressure, such as pumping and underground structures occurrence, has been elaborated and tested in the heavily urbanized Parisian area. The method developed here consists of two steps. First, hard data (hydraulic head) and soft data (dry wells) are used as conditioning points for the estimation of the SW-GW connection status. A disconnection criteria of 0.75 m is adjusted on observed unsaturated zone depth (UZD). It is a default value in areas where such data are missing. The second step consists of the final mapping of the water table. Given the knowledge of the disconnection criteria, the final map is achieved with an ordinary kriging of the UZD that integrates the surface water elevation without unsaturated zone where it is relevant. The methodology is demonstrated on two datasets of UZD observations that were collected under low- and high-flow conditions.

The Tsagaan Els basin is a ~16,300 km 2 endorheic basin located in the Gobi Desert in Mongolia. This Cretaceous sedimentary basin is crossed by the North Zuunbayan fault, which determines two partially connected hydrogeological sub-basins: Zuunbayan and Unegt. Two roll-front uranium ore deposits have been discovered in these sub-basins by Orano Mining and its subsidiary COGEGOBI LLC, and the regional aquifers have been surveyed for several years. Based on that knowledge, this study presents the regional hydrogeological functioning of this multi-layered aquifer system supported by a regional groundwater model. The latter was implemented with MODFLOW and calibrated through PEST using piezometric levels. Recharge rates were adjusted to between 0.6 and 3.1 mm/year. The resulting water budget indicates an important transfer of water from Unegt sub-basin toward Zuunbayan sub-basin, occurring at a few main springs located along the North Zuunbayan fault. Evaporation loss is potentially high during these transfers but the remaining water seeps through and later evaporates in the depression located at the center of Zuunbayan sub-basin, which is the terminal discharge playa of the entire basin. These results can be helpful for future water management plans.