For the successful subsurface thermal energy storage, accurate characterization of a target aquifer is essential. The active-distributed temperature sensing (DTS) thermal test, which often utilizes a fiber-optic cable both to monitor temperature and to serve as a heat source, has emerged as a promising tool for high-resolution estimation of Darcy flux. For the interpretation, most studies have assumed thermal dispersion to be negligible, yet thermal dispersion is expected to become significant under high flow velocity and heterogeneous hydraulic conductivity field. Despite its importance, estimating in-situ dispersivity remains highly challenging. To evaluate the possibility and sensitivity of thermal dispersivity estimates obtained from active-DTS tests, we incorporate thermal dispersion into the moving infinite line source model and validate it through the numerical model. After that, a sensitivity analysis was performed with two-dimensional numerical simulations under various Darcy fluxes (q, 1 – 10 m/d) and thermal longitudinal dispersivity conditions (α, 0 – 0.01 m). Our results demonstrate that increasing thermal dispersivities systematically reduced the magnitude of temperature increase and delayed the time to reach the plateau, both effects intensified as q increased. Based on these results, we jointly estimate Darcy flux and thermal dispersivity from single or multiple active-DTS tests and evaluate their uncertainties quantitatively. We expect that these findings will extend the applicability of active-DTS thermal tests as a versatile tool for aquifer characterizations and provide a chance for in-situ thermal dispersivity estimation. Keywords: Thermal dispersion; Active-DTS; Aquifer characterization; Moving infinite line source

The assessment and management of groundwater often depend on large regional numerical models that predict hydrological stresses, such as those caused by climate change and resource exploitation. While regional and continental-scale models have been developed to evaluate these impacts, they typically use coarse grid cells that smooth land surface topography. This study investigates the impact of topography-controlled water table smoothing on simulated groundwater discharge to streams (baseflow) and associated groundwater age. A simplified 2D cross-sectional model of a topographically driven regional aquifer system was developed, under the assumption that the water table is a replica of the land surface topography. Scenarios with varying topography, derived from resampling digital elevation model (DEM) resolutions ranging from 30 to 10,000 m, were analyzed using a consistent, high-resolution numerical hydrogeological model mesh. Results show that baseflow rates decrease significantly as resolution declines, primarily due to reduced hydraulic gradients, with a flux difference of an order of magnitude simulated between resolutions of 30 and 1000 m. Although shallow groundwater flows in more permeable aquifer layers are significantly affected, deeper regional flow remains stable across all scenarios. Regional groundwater flow paths and associated residence times are less sensitive to changes in resolution, particularly at depths greater than 50 m in the model used. The study thus demonstrates that low-resolution models need to overestimate hydraulic conductivity during calibration to accurately match fluxes to streams. This study highlights the critical importance of carefully considering topographic resolution in regional models to ensure representative predictions of streamflow driven by subsurface–surface interactions.

Traditional groundwater monitoring techniques lack the capabilities to measure groundwater fluxes at high spatial resolution over large distances. Breakthrough work by Simon et al. (2021) enabled the quantification of groundwater fluxes at high spatiotemporal resolution using actively heated fibre optic Distributed Temperature Sensing (A-DTS), establishing this as a promising hydrogeophysical method. However, current A-DTS interpretation methods, such as the analytical solution used to process A-DTS data, assume that groundwater flux is perpendicular to the cable. Yet, many field-based applications of A-DTS violate this assumption due to the multi-dimensional nature of groundwater flow. This study aims to characterise the effect of fibre optic cable orientation on the interpretation of groundwater fluxes, and determine the minimum angle for which the method remains applicable. This study presents methodological advancements to A-DTS by characterising how cable orientation relative to flow direction affects groundwater flux estimates in a controlled environment. Estimating groundwater fluxes from A-DTS relies on the Moving Analytical Line Source (MILS) analytical model describing heat dissipation. A key assumption of the MILS model is that the flow is perpendicular to the cable angle, a condition frequently violated in field applications. To establish critical angle thresholds for reliable groundwater flux estimation, a large-scale experiment was conducted at the Site Contrôlé Expérimental de Recherche pour la réhabilitation des Eaux et des Sols (SCERES) platform in Strasbourg, France. This 25×12×3 m experimental tank represents an ~1000 m3 artificial porous aquifer designed to reduce potential boundary effects. This study allowed us to test some of the assumptions underlying the A-DTS method. A hybrid cable containing fibre optic strands and a steel armour for heating, was installed in a configuration with five sections at different orientations relative to the flow direction though the horizontal plane. The cable was buried within the saturated porous medium and different flux rates were imposed to establish the critical angle thresholds for reliable groundwater flux estimation. We will discuss the advantages and limitations of A-DTS for high-resolution groundwater flux monitoring under controlled yet field-representative conditions.

Hyporheic fluxes are typically regarded as highly variable both in space and time at the stream‐groundwater interface. However, Active‐Distributed Temperature Sensing (DTS) experiments conducted in a losing river section demonstrated low spatial variability (one order of magnitude) and remarkable temporal stability. In this abstract, we investigate the potential reasons for the observed low variability and notable stability of hyporheic flows.Experiments were conducted by burying several hundred meters of heatable Fiber‐Optic cables within streambed sediments in a large meander, where permanent stream‐losing conditions are observed. The absence of correlation between water fluxes in the hyporheic zone and variations in streambed topography suggests that the low spatial variability (one order of magnitude) of fluxes serves as an indicator of the low variability in streambed hydraulic conductivities. Repeated measurements taken during several field campaigns over three years demonstrated a remarkable stability of hyporheic flows throughout this period. To explain our findings, we analyzed the temporal variability of river stage and groundwater levels. Despite the rapid and sudden fluctuations of water levels, caused by upstream dam hydropeaking and groundwater pumping in the alluvial aquifer, the hydraulic gradients between the river and the aquifer remained relatively stable over time. Moreover, the speed at which the levels rebalance suggests that flows at the interface are primarily controlled by the high permeability of the streambed sediments rather than by the boundary conditions. These results can be considered for calibrating models that assess hyporheic processes.

Geothermal energy could decrease remote regions dependence on diesel by offering an alternative baseload energy. However, the geothermal exploration risk is high in remote regions due to limited temperature and ground thermal conductivity data, and resultant heat flux evaluations. Thermal response tests are commonly used in the heat pump industry to evaluate the effective thermal conductivity, but these tests are typically performed in shallow wells (< 200 m), assume the effective thermal conductivity to be purely due to conduction and neglect the influence of groundwater flow. Herein, fibre-optic distributed temperature sensing was used during active thermal response tests to produce a high-resolution in-situ effective thermal conductivity profile. The high-resolution profiles allow conduction-dominated segments to be isolated based on the temperature and effective thermal conductivity profiles. This method was applied to two boreholes in southwestern Yukon on the traditional territory of Kluane First Nation (KFN-L: 387 m and DRGW: 220 m). The heat flux was evaluated based on conductive segments of the temperature and thermal conductivity profiles. The temperature profile was corrected for topography and paleoclimate effects, and the internal heat generation was also considered. This resulted in heat flux estimation of 89 mW m-2 and 99 mWm-2 at KFN-L and DRGW, respectively. These values decrease exploration uncertainty around Burwash Landing, Yukon, where data scarcity is a challenge to geothermal exploration. This method could be applied in diverse geological settings to confidently estimate local terrestrial heat flux in pre-existing boreholes.

Accurate mapping of the heterogeneity of hydraulic properties, including hydraulic conductivity (Kh) and its anisotropy (Kv/Kh), is crucial for predicting groundwater flow and solute transport in aquifers. This study investigates the use of hydraulic tomography with periodic signals to map Kh, Kv/Kh and specific storage (Ss) in an unconsolidated littoral aquifer. The periodic signals were generated by the movement of a rod numerically controlled by a winch, which allowed the signal amplitude and period to be imposed. Thirty periodic slug tests with periods of 150, 300 and 600 s were conducted between isolated intervals in a source well and an observation well, generating 120 head responses. Numerical inversion in the time domain used the rod-induced flow rates and associated heads to estimate the heterogeneous fields. Significant differences in head amplitude and phase shift between test intervals at different locations highlighted the heterogeneity of the aquifer. The inversion results for single and combined periods are consistent with the values of previous studies and the heterogeneous nature of the littoral aquifer. Comparison of models from different periods revealed slight spatial and statistical variations in hydraulic properties and different hydraulic behavior when tested with independent hydraulic tests. While the fundamentals of understanding the information in the different periodic signals need to be further clarified, this study advances the application of hydraulic tomography under real field conditions and highlights its effectiveness in characterizing aquifer heterogeneity and anisotropy.

The traditional territory of the Lù'àn Män Ku Dän (Kluane Lake People) is found along the Saint Elias Mountains in Yukon. It hosts the Burwash Landing community, home of the Kluane First Nation, which is one of eleven self-governing First Nations operating in tripartite with Yukon Government and Canada. Burwash Landing is primarily dependent on diesel for space heating and power generation. Cutting-edge technologies were deployed in the scope of geothermal resource assessment to evaluate the thermal state and properties of the subsurface. Active distributed temperature sensing was conducted with a composite heating and fiber-optic cable installed in the water column of two existing wells with the objective of quantifying the geothermal potential and groundwater flow along available wellbores. Heat injection tests were made in the 220 and 385 m deep wells located on the south and north side of the Denali fault, near a probable releasing bend that is favorable to permeability. Melting glacier water infiltrates in mountains and groundwater flows toward Kluane Lake, which is hypothesized to be a major groundwater discharge zone. The shallower well is at an altitude of 925 masl and intercepted 40 m of quaternary deposits before hitting fractured bedrock while the deeper well is at the valley bottom near the lake (altitude of 795 masl) and entirely drilled in quaternary deposits. Passive temperature monitoring was initially made and revealed a geothermal gradient of 34 ⁰C km-1 and 47 ⁰C km-1 in the shallow south side and deep north side wells. Heat was injected during active tests for 2 and 3 days and thermal recovery was monitored for 6 and 8 days, respectively. Temperature was measured every 25 cm at 4-minute intervals. The infinite line source equation and the superposition principle were used to analyze data and calculate a thermal conductivity profile. Nearly continuous ground thermal properties and temperature profiles were combined to assess the Earth natural heat flux, considering paleoclimate and topographic corrections. Analysis indicated a heat flux above 90 mW m‑2, thought to be favorable for geothermal resource development. Peclet number analysis was undertaken to infer horizontal groundwater flow in permeable horizons. Results are being used to develop a regional groundwater flow and heat transfer model to evaluate temperature at kilometer depth and assess the communities’ geothermal potential. This presentation will illustrate how active temperature sensing can be deployed to reduce geothermal exploration risks, acknowledging Kluane First Nation that allowed us to better understand groundwater flow in this magnificent territory.

Groundwater is widely exploited, leading to groundwater depletion, and a reduction of river flows. While the impact of climate change and hydrologic forcings on the hydrogeological cycle has long been demonstrated, a lack of observations of subsurface thermal changes on the scale of decades is hampering the accurate understanding of how human activities change subsurface thermal regimes (i.e. the spatiotemporal evolution of the subsurface temperature). In this paper, we analyze new temperature data illustrating the complex interplay of climate warming and anthropogenically enhanced groundwater flow on subsurface thermal regimes from two sites in different hydrogeological settings. A heuristic numerical model is used for the interpretation of the observed temperature anomalies. We demonstrate that pumping may have a significant impact on the thermal regime of an aquifer, and that depending on hydrogeological conditions and the natural geothermal gradient, this impact might be even more important than that of climate warming.

Site-specific heterogeneity in geological materials plays a crucial role in groundwater (GW) and surface water (SW) interaction, especially in ecosystems sensitive to groundwater influx; for example, salmonid habitats are influenced by localized GW input to streams. While numerous methods have emerged to better understand mechanisms governing GW–SW interaction, few studies compare these methods directly. Therefore, the objective of this study is to evaluate the strengths and limitations of an innovative active heat tracing method to quantify the role of riverbed heterogeneity on GW–SW exchanges. This method was compared with several established techniques such as seepage meters, piezometers, and passive heat tracing at a field site on the Sainte-Marguerite River in Quebec, Canada. The measured spatial variation of the exchange rates due to the presence of a sandbar with coarse materials was shown to be statistically significant. Additionally, temporal analysis helped to identify variations of GW flux even during the cold season when GW flux was expected to be limited due to frozen ground and low infiltration from the snow-covered ground surface. Seepage meters and active heat tracing allowed for spatial analysis of GW–SW interaction, while piezometers with water level loggers and passive heat tracing with installed temperature sensors in the riverbed were convenient for identifying temporal variation of GW–SW exchange rates. The combination of temperature sensors and a heating cable was used for the first time as a tool for active heat tracing and showed good potential to evaluate riverbed thermal properties and GW seepage rates in the river.

Groundwater is widely exploited, leading to substantial drops in groundwater levels, which modify the recharge/discharge relationships between large-scale hydrogeological units. In addition, this anthropogenic hydraulic forcing is also responsible for changes in thermal regimes of groundwater systems. While the impact of climate and hydrologic forcings on the hydrogeological cycle has long been demonstrated, we still have insufficient knowledge on the influence of human activities on thermal regimes of groundwater systems and, as a consequence, on groundwater quality. The major scientific obstacle that prevents accurate understanding of the impact of these forcings on the critical zone’s thermal dynamics is a dire lack of field observations, i.e., repeated temperature-depth profiles collected over decades. In this paper, we analyse new temperature data illustrating the complex interplay of climate warming and enhanced groundwater flow on subsurface thermal regimes from two sites in different hydrogeological settings: (1) an aquifer in fractured crystalline bedrock (Ploemeur, France), and (2) a sedimentary aquifer (La Cabine, Netherlands). A simple numerical model is then used for the interpretation of the observed temperature anomalies. We first demonstrate that pumping may have a significant impact on the thermal regime of the critical zone, and that depending on hydrogeological conditions and the natural geothermal gradient, this impact might be even more important than that of the climate change.

Characterizing the spatiotemporal variability of water fluxes at the stream-groundwater interface is extremely challenging due to the lack of methods for estimating hyporheic flows at different scales. To address this, we demonstrate the potential of Active-Distributed Temperature Sensing (DTS) methods for measuring and mapping hyporheic flow in a lowland stream. Experiments were conducted by burying a few hundred meters of heatable Fiber-Optic cables within streambed sediments in a large meander, where permanent stream-losing conditions are observed along the stream reach. We propose a new methodology to filter ambient temperature variations along the heated section of the DTS cable and to extend the application of Active-DTS to losing streams. After data processing, the results show that, along lateral and longitudinal stream profiles, both thermal conductivity and water flux values follow normal distributions with relatively small standard deviations. Hyporheic fluxes vary by one order of magnitude. The absence of correlation between water fluxes within the hyporheic zone and streambed topography variations suggests that the variability is mainly controlled by local streambed heterogeneities. This means that the spatiotemporal variability of fluxes may be used as a marker of the variability of streambed hydraulic conductivities. The relatively low spatial variability (one order of magnitude) in hyporheic flow suggests a small variability of streambed properties. This is an important result for calibrating models assessing hyporheic processes, in which the hydraulic conductivity distribution is generally assumed. Additionally, measurements made over three years yield similar estimates showing the remarkable stability of hyporheic flows through time. Characterizing the interactions between groundwater and surface water is extremely challenging although such interactions control water quality and ecosystems resilience to climate changes. Here, we used an innovative approach based on heated fiber optic cables, called Active-Distributed Temperature Sensing, to image the spatial variability of hyporheic fluxes in a lowland stream. Our results show that the instrumental developments as well as the data processing methodology are very robust to accurately measure in-situ the thermal conductivity of stream sediments and hyporheic fluxes within the streambed. Interestingly, groundwater flux variability was found relatively limited and not correlated to the morphology of the riverbed. In addition, measurements made over three years yield similar estimates showing the excellent reproducibility of the measurements and the remarkable stability of hyporheic flows through time. These results shed new light about the spatial and temporal variability of hyporheic fluxes in a lowland river. Active-Distributed Temperature Sensing was used in a lowland stream to assess and map the spatiotemporal variability of stream infiltration An innovative field setup and a new methodology was developed to remove ambient temperature variations from the raw temperature signal Results suggest relatively homogeneous streambed properties and show remarkable stability of hyporheic flow during few years

Heat transport in fractured aquifers is determined by the combined effects of flow velocity heterogeneity in the fracture system, and diffusive exchange between the fluid in the fractures and the rock matrix, which can be assumed as impervious. We analyze the impacts of this diffusive exchange on the response to heat transport, as opposite to the pure advective displacement, which governs solute transport. We focus on the post-peak behavior where we observe pre-asymptotic regimes with slopes that differ from the signature of matrix diffusion, which exhibits a decay rate of -3/2. This deviation is driven by the variability of both velocity field and fracture aperture field. We derive theoretical models that predict these pre-asymptotic tails under three extreme cases that can be related with specific network structures, that is, networks dominated by large or small fractures, networks with highly or poorly channelized flow. These theoretical predictions are compared with results from numerical simulations in different sets of three-dimensional discrete fracture networks. We determine that the combined observation of solute and heat transport responses allows classifying the network in terms of connectivity structure, and partially characterizing the fracture aperture variability in terms of upscaled parameters.

Considering the need of characterizing temporal dynamic of groundwater and the lack of available methods, we investigate the feasibility of active-Distributed Temperature Sensing (DTS) measurements to monitor and quantify groundwater fluxes variations over time. Active-DTS, which consists here of heating a Fiber Optic (FO) cable and in monitoring the temperature elevation, has proven to be very efficient to quantify the spatial distribution of groundwater fluxes in saturated porous media at high resolution with low uncertainties. However, the approach has never been tested to continuously monitor groundwater fluxes changes. To test this, we rely on both numerical simulations and sandbox experiments to assess the sensitivity of temperature elevation to variable flow conditions and our ability to interpret associated temperature variations. Results confirm that the temperature elevation and evolution over time is sensitive to flow conditions and that associated temperature variations can be used to characterize groundwater fluxes variations. First, experimental and numerical results show that when a flow change is followed by a long-enough steady-state flow period the temperature stabilizes independently of previous fluxes conditions. In such case, the stabilization temperature can easily be interpreted to estimate groundwater fluxes using the analytical model commonly used under steady flow conditions to interpret active-DTS measurements. Furthermore, we demonstrate here that, under certain flow conditions depending on the nature of flow variations, the approach offers the possibility of continuously monitoring fluxes variations. For instantaneous flow changes, it is even possible to go further by reproducing temperature signal variations over time by applying the superposition principle to the analytical model. In the end, these preliminary tests are particularly promising and open new perspectives for monitoring and/or quantifying the temporal dynamic of groundwater fluxes at different temporal scales including diurnal and short-term periodic fluxes variations.

Active distributed temperature sensing (ADTS) experiments are very useful to provide in-situ and distributed estimates of thermal conductivities of the subsurface and of groundwater flows. However, the data interpretation can be seen as difficult considering the large amount of data collected along a heated fiber-optic cable and the lack of associated tools for their automated analysis. In this context, we developed an automated routine program for the interpretation of ADTS measurements: the ADTS Toolbox. It contains several codes written in MATLAB that calculate, for each measurement point located along a heated section, both the thermal conductivity of the surrounding material and the groundwater flux. In addition, it provides uncertainties on the estimated thermal conductivities and fluxes according to the temperature resolution (noise) or to errors on temperature measurements. By offering the possibility of automatically interpreting ADTS measurements, the ADTS Toolbox facilitates the use and interpretation of ADTS experiments for characterizing at high resolution the groundwater flows distribution and for imaging the thermal conductivities variability.

Fiber-Optic Active Distributed Temperature Sensing (FO-ADTS) experiments were performed on an Aquifer Thermal Energy Storage system (ATES) site located on the university campus of Bordeaux, France. The experiments consisted in heating the steel core of the FO cable while monitoring the rate of temperature increase during the heating periods. The changes in temperature, that were monitored through time at every depth under various hydraulic conditions and in different boreholes, were used to evaluate both aquifer properties and wells conditions. A first ADTS experiment was conducted under cross borehole configuration using a pumping well and a monitoring well separated by a distance of 8.5 meters. Then, to check the reciprocity of the results, a second experiment was conducted by switching the monitoring and the pumping well. The results obtained through the use of analytical solutions for reproducing and interpreting the data lead to the following conclusions: (i) ADTS can be used to estimate both thermal conductivity and Darcy velocity distribution along boreholes, crucial properties for ATES performance. (ii) The proposed method is a promising tool to detect clogging locations in the boreholes when it occurs. This can be of great practical interest to maintain systems performance, since, once FO cables deployed, experiments could be easily repeated without opening boreholes and stop the system operation.

&lt;p&gt;Transport of solutes in aquifers is controlled by the heterogeneous spatial distribution of hydraulic properties, but the characterization of aquifer heterogeneity is quite challenging with conventional methods. Hydraulic tomography (HT) was developed to define the heterogeneous distribution of hydraulic conductivity (&lt;em&gt;K&lt;/em&gt;) and specific storage (S&lt;sub&gt;s&lt;/sub&gt;). HT involves the emission of a series of hydraulic head perturbations in a stressed well and the recording of this signal at several levels in the stressed and observation wells. All recorded hydraulic head responses are simultaneously analyzed through numerical inversion, which provides the spatial distribution of hydraulic properties at a scale relevant for local site investigations.&lt;/p&gt; &lt;p&gt;This communication reports on a tomographic experiment carried out in a heterogeneous and highly anisotropic granular aquifer at the Saint-Lambert research site near Quebec City, Canada. This site has already been the object of detailed characterizations with multiple hydraulic methods: pumping tests, packer slug tests, flowmeter profiles, vertical interference tests, and slug test tomography. A relatively new approach named oscillatory hydraulic tomography (OHT) was tested, in which multi-frequency oscillatory head perturbations are induced in an interval isolated by packers of the stressed well by a submerged rod that is electronically controlled by a winch system. Hydraulic responses are measured in the stressed intervals and in multiple intervals of an observation well.&lt;/p&gt; &lt;p&gt;This study was primarily aimed at testing, first on an operational level, if the OHT signal could be generated in the stressed well and propagated to the observation well in a highly anisotropic granular aquifer. Second, the study developed a rigorous workflow for the treatment of the measured hydraulic heads. Third, in terms of characterization efficacy, the study aimed to determine if multiple controlled frequencies would allow the assessment of &lt;em&gt;K&lt;/em&gt; spatial distribution.&lt;/p&gt; &lt;p&gt;Results show that the field experiment provided clear measured hydraulic responses that could be used to obtain the 2D distribution of hydraulic properties from the inversion of OHT measurements. Comparison was made of inversion results using a single oscillatory frequency and multiple frequencies. Under conditions of realistic field measurement noise and uncertainty, it will be valuable in future work to compare the imaging capabilities of oscillatory hydraulic tomography against other tomographic methods. Further investigation is also needed to examine the information content of oscillatory hydraulic tomographic data for characterizing &lt;em&gt;K&lt;/em&gt; and &lt;em&gt;Ss&lt;/em&gt; heterogeneities through a sensitivity and resolution analysis. This study demonstrates the practical potential for the implementation of OHT experiments in relatively low permeability and highly anisotropic granular aquifers.&lt;/p&gt;

The monitoring of temporal variabilities of groundwater flows is a critical point in many hydrogeological contexts, especially for the characterization of coastal aquifers, sub-surface heterogeneities or else groundwater/stream interactions. Considering the lack of available methods, we investigate the possibility of monitoring and quantifying groundwater fluxes variations over time through active-Distributed Temperature Sensing (DTS) measurements. Active-DTS, consisting in heating a fiber optic cable, performs very well for investigating the spatial distribution of groundwater fluxes but the method has never been tested to continuously monitor groundwater fluxes changes. In this context, both numerical simulations and sandbox experiments were performed in order to assess the sensitivity of temperature elevation to variable flow conditions. Results first demonstrate that when a flow change is followed by a long-enough steady-state flow stage the temperature elevation stabilizes independently of previous fluxes conditions. Thus, the stabilization temperature can easily be interpreted to estimate groundwater fluxes using the analytical model commonly used under steady flow conditions to interpret active-DTS measurements. Under certain flow conditions, depending on the nature of flow variations, the approach also allows the continuous monitoring of fluxes variations over time. If instantaneous flow changes occur, the superposition principle can even be used to reproduce the temperature signal over time. In summary, we demonstrated through these preliminary results the possibility of for monitoring and/or quantifying the temporal dynamic of groundwater fluxes at different temporal scales including diurnal and periodic fluxes variations, which open very interesting perspectives for the quantification of subsurface processes.

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 (3D + 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.

Being the world’s largest freshwater resource, groundwater is at a continuous risk of overabstraction for human water use. Beside substantial drops in groundwater levels that are the consequence of unsustainable groundwater abstraction, which modify the recharge/discharge relationships between large-scale hydrogeological units, this anthropogenic hydraulic forcing is also responsible for changes in thermal regimes within the critical zone. While the impact of global groundwater pumping on the hydrogeological cycle has long been demonstrated, we still have insufficient knowledge on the influence of human activities on groundwater temperatures and, as a consequence, on stream thermal regimes and groundwater quality.In this contribution we discuss temperature anomalies that develop in the shallow subsurface as a result of localized groundwater extraction. We study different hydrogeological settings, i.e., porous and fractured aquifers, that we explore via numerical modelling and comparison with field observations. In the field, we use repeated temperature-depth borehole profiles separated by decades, the advantage of which is that differencing the temperature logs for individual boreholes yields real temperature change and eliminates steady-state sources of curvature. Thus, it enables us to detect changes in subsurface thermal regimes, resulting from transient conditions, i.e., climate change and changes in groundwater hydrodynamics.

Groundwater recharge is difficult to estimate, especially in fractured aquifers, because of the spatial variability of the soil properties and because of the lack of data at basin scale. A relevant method, known as the water table fluctuation (WTF) method, consists in inferring recharge directly from the WTFs observed in boreholes. However, the WTF method neglects the impact of lateral groundwater redistribution in the aquifer; i.e., it assumes that all the WTFs are attributable to recharge. In this study, we developed the WTF approach in the frequency domain to better consider groundwater lateral flow, which quickly redistributes the impulse of recharge and mitigates the link between WTFs and recharge. First, we calibrated a 1D analytical groundwater model to estimate hydrodynamic parameters at each borehole. These parameters were defined from the WTFs recorded for several years, independently of prescribed potential recharge. Second, calibrated models are reversed analytically in the frequency domain to estimate recharge fluctuations (RFs) at weekly to monthly scales from the observed WTFs. Models were tested on two twin sites with a similar climate, fractured aquifer and land use but different hydrogeologic settings: one has been operated as a pumping site for the last 25 years (Ploemeur, France), while the second has not been perturbed by pumping (Guidel). Results confirm the important role of rainfall temporal distribution in generating recharge. While all rainfall contributes to recharge, the ratio of recharge to rainfall minus potential evapotranspiration is frequency-dependent, varying between 20 %–30 % at periods <10 d and 30 %–50 % at monthly scale and reaching 75 % at seasonal timescales. We further show that the unsaturated zone thickness controls the intensity and timing of RFs. Overall, this approach contributes to a better assessment of recharge and helps to improve the representation of groundwater systems within hydrological models. In spite of the heterogeneous nature of aquifers, parameters controlling WTFs can be inferred from WTF time series, providing confidence that the method can be deployed in different geological contexts where long-term water table records are available.