The long-term performance of the permeable reactive barriers in remediating contaminated groundwater may diminish as a result of oxidation, precipitation on the particle surfaces, and pore space clogging. Evaluating its performance through monitoring could address this dilemma. We investigate the spectral induced polarization (SIP) response of zero valent iron (ZVI)-activated carbon (AC)-sand mixtures.The chargeablity exhibits a perfect linear relation to the volumetric concentration of ZVI (2.5-50%) and AC (2.5%-75%) with r = 0.99. However, the low-frequency electrical conductivity shows low sensitivity to the volumetric content of ZVI and AC. The relaxation time increases with the particle sizes. When these two particles are mixed, chargeablity is approximated as a superposition of their individual values. In terms of phase values and frequencies of the phase peaks, it also exhibits this superposition effect. Furthermore, we conducted 720-hour SIP measurements on ZVI-AC-sand columns flushed with NaCl or NaNO₃ solutions. It suggests that precipitation of 0.06 mm thick sedimentation onto the ZVI surface induced by changes in redox chemistry observed in micromorphology images, resulting an increase in the normalized chargeability by 44.05%, the scaled relaxation time and Cole–Cole model exponent by 1098.99% and 23.11%. Compared to flow-through by NaCl solution, changes in these parameters are more pronounced for columns saturated with NaNO₃ solution, indicating the corrosion of ZVI. Our findings illustrate that induced polarization parameters vary in response to the chemical alteration of ZVI-AC-sand mixed media, showing the potential for noninvasive long-term monitoring of the reactive barriers.

How to cite: Mao, D., Ma, X., Furman, A., Schwartz, N., Chao, C., Xia, T., Yang, K., and Guan, X.: Unveiling the characteristics of ZVI-AC-sand mixtures in remediating contaminated groundwater using spectral induced polarization, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2394, https://doi.org/10.5194/egusphere-egu24-2394, 2024.

Karst aquifers are particularly vulnerable to changes in environmental factors such as climate change or pollutants. In the critical zone, the role of the superficial layer, the soil and the so-called epikarst, is crucial as it can delay water infiltration and host temporary perched water reservoirs, due to high contrasts in hydraulic conductivity with deeper layers. In an effort to better characterise the effect of the plant activity on the water content in the shallow subsurface, we have designed a time-lapse ERT experiment at the Rochefort Cave Observatory (Belgium). We present results from (at least) 6 months of daily 3D ERT measurements on 64 electrodes installed in a 40x60 cm grid covering a 6.0 x 1.8 m surface area centred on a young beech tree. The ERT dataset is supported by data from a vertical profile of soil moisture probes and in-cave water percolation gauges. Our study also includes an artificial drying and sprinkling experiment which main purposes are to replicate extreme weather events and investigate their effect on the soil moisture condition.

This experiment also serves as a testbed for using OhmPi as a monitoring tool in the field. OhmPi is an open-source, open-hardware resistivity meter, which runs on a Raspberry Pi. It is designed for enabling flexible data acquisition, and is primarily dedicated to the research community. Relying on low-cost components and devices, and using a low-power injection module (0-50V), OhmPi is particularly suited for small-scale field and laboratory experiments.

How to cite: Watlet, A., Kaufmann, O., Mahieu, A., Imig, A.-F., Guyard, H., Goderniaux, P., Forquet, N., Fargier, Y., Dubois, V., Blanchy, G., and Clément, R.: Monitoring 3D soil moisture dynamics at a karst forest site with OhmPi, an open source resistivity meter, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19108, https://doi.org/10.5194/egusphere-egu24-19108, 2024.

The impacts of climate change, combined with population growth, necessitate practical and effective solutions for locating groundwater resources and ensuring drinking water quality. Our contribution explores recent advances in geoelectrical and electromagnetic imaging methods applied to investigate groundwater systems. Geoelectrical and electromagnetic imaging techniques are popular methods for characterising subsurface properties, such as electrical resistivity or dielectric permittivity. These electrical properties are strongly related to the hydrogeological characteristics of the subsurface. Therefore, geoelectrical and electromagnetic investigations can provide valuable insights into finding groundwater resources, assessing the water quality in terms of contaminations and conducting effective groundwater management.

Our study examines state-of-the-art approaches in modelling and instrumentation of induced polarisation and electrical resistivity tomography, as well as time- and frequency-domain electromagnetics and ground-penetrating radar methods. We review recent impactful and innovative groundwater case studies where the above-mentioned methods were applied and further developed. Emphasising the combination of geoelectrical and electromagnetic methods, the studies provide insights into the variation of electrical subsurface properties at different scales, contributing to an improved understanding of the hydrological dynamics in the studied areas. Furthermore, we provide an outlook on the potential for applying geoelectrical and electromagnetic imaging techniques for large-scale groundwater investigations in the exascale computing area.

How to cite: Rulff, P., Castillo-Reyes, O., Koyan, P., Martin, T., Deleersnyder, W., and Carrizo Mascarell, M.: Geoelectrical and electromagnetic imaging methods applied to groundwater systems: recent advances and future potentials, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-654, https://doi.org/10.5194/egusphere-egu24-654, 2024.

It is well-recognized that soil moisture plays an important role in the management of water resources, as well as soil and crop production. This research proposed the use of time-lapse Electrical Resistivity Tomography (ERT) to overcome the limitations of point-based soil moisture measurement, which often fails to capture detailed spatiotemporal data. ERT is a widely used geophysical technique for the non-destructive exploration of subsurface media’s resistivity. Since the electrical resistivity is sensitive to the water content in soil, the variation of the soil’s resistivity in time and space can be obtained by using this technique that can be correlated to water transport in soil. Thus, using time-lapse ERT for the exploration of water transport in the soil was launched in this study.

This research conducted a time-lapse ERT survey executed in a farm during a sprinkling rainfall. A 50 meters time-lapse ERT survey was employed for 29 days with a hybrid-array configuration at a fallow land. The electrode spacing was 1 meter and measurement were conducted every 2 hours, thus a resistivity section of the land with 50 meters in length and 4 meters in depth was estimated with a period of 2 hours. In addition, five moisture meters were set up in the middle of the ERT survey line and at depths of 10, 20, 30, 50, and 100 cm, respectively. Then, the variation of the resistivity was compared with the precipitation data and the soil moisture readings from the meters. The results showed that the decrease of soil resistivity was consistent with the increase of the precipitation and soil moisture. The water transport rates in soils estimated by this technique and moisture meters were similar, they were 20 mm/hour and 16 mm/hour, respectively.

This study demonstrates that time-lapse ERT is an effective tool for dynamically monitoring water transport in soils. By employing this technique, near real-time 2D soil moisture monitoring becomes feasible, which could significantly enhance the optimization of water resources and crop production, when integrated with an automatic irrigation system.

How to cite: Liu, J.-W., Chang, Y., and Liu, T.-S.: Water transport in agricultural soils estimated by time-lapse electrical resistivity tomography technique, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4487, https://doi.org/10.5194/egusphere-egu24-4487, 2024.

Near-surface geo-electrical methods monitoring the electrical conductivity have gained particular interest for environmental studies. Their sensitivity to key properties of storage and transport in porous media and their non-destructive nature make these methods a significant asset for studying the subsurface. Nevertheless, their quantitative interpretation depends on the efficiency of the used petrophysical relationship to link the physical properties, obtained from the electrical measurements, with the hydrological properties and state variables of interest. Therefore, the electrical conductivity of a porous medium is related to several geological parameters such as mineral matrix, porosity, permeability, and degree of water saturation. All of these parameters are controlled by the pore structure which plays a key role in the distribution of the conductive fluid. During reactive processes, the pore structure is significantly affected which translates into surface and volume variations. This evolution of the pore space leads to changes in the macroscopic hydraulic properties and, therefore, the electrical conductivity. In this study, we present an analytical fractal model to describe the electrical conductivity evolution during reactive processes. Under the assumption that the pore system is represented by a bundle of tortuous capillaries with constrictivity, we account for the reactive processes in the model by considering the geometrical variations in the pore structure (i.e., the increase and decrease of the pores aperture). The derivation of the electrical conductivity is based on upscaling procedures and a fractal law which describes the size distribution of pores. Considering the electrical charges dragged by the water in one capillary, we upscale the electrical property and obtain closed-mathematical expressions to calculate the electrical conductivity of the medium. This can be achieved thanks to the independence from scales of fractal media. For partially saturated conditions of the medium, the model’s expressions can estimate the electrical conductivity as a function of hydraulic properties. The performance of the model has been tested with published data from different soil and rock textures, under reactive fluid flow or partial saturation conditions. The comparison shows that the model can satisfactorily reproduce the behavior of the data. The fractal distribution is consistent with mico-CT results and the dissolution rate is within the same order of magnitude of the value obtained from experimental results. From a geometrical approach and within a fractal framework, we included the effect of reactive processes in the estimates of the medium electrical conductivity which opens up new possibilities to characterize media from geoelectrical techniques.

How to cite: Soldi, M., Rembert, F., Guarracino, L., and Jougnot, D.: Electrical conductivity estimation from a new fractal model for porous media under reactive processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9432, https://doi.org/10.5194/egusphere-egu24-9432, 2024.

Accurately determining soil hydraulic properties is a complex task due to significant variations in spatial information, posing ongoing challenges in managing subsurface and agricultural practices effectively. Geophysical methods, alongside traditional techniques, play a crucial role in monitoring subsurface state variables and inferring soil properties. Electrical Resistivity Tomography (ERT) is an appealing geophysical method due to its non-invasive, easy-to-apply and cost-effective nature. In ERT, electrical resistivity tomograms, obtained with surface measurements, are used to monitor the hydraulic state of the subsurface by translating the electrical tomograms to water content or pore-water salinity maps using calibrated petrophysical relations. However, obtaining 2D (or 3D) electrical tomograms from raw measurements requires the inversion of an ill-posed problem, which causes smoothing of the actual structure. Furthermore, the spatial resolution of the electrical tomograms is determined from the distances in the electrode placement, thus inherently upscaling the obtained structure. In this study, we explored the applicability of Physics-Informed Neural Networks (PINNs) for simultaneously upscaling soil properties, specifically the permeability and the petrophysical relations, and monitoring water dynamics at heterogeneous soils, using time-lapse geoelectrical measurements as the training data. High-resolution numerical simulations mimicking water infiltration to the subsurface were used as benchmarks to test the provided approach. Synthetic time-lapse ERT surveys with electrode spacing ten times larger than the numerical model resolution were conducted to provide upscaled 2D electrical resistivity tomograms. The electrical tomograms were fed to a PINNs system to obtain the permeability, petrophysical relations, and water content spatiotemporal maps simultaneously. To examine the system sensitivity to the measured data, an additional PINNs system that also incorporates water content measurements at 20 random locations was trained separately. Results have shown that the PINNs system could produce reliable results regarding the upscaled (heterogeneous) permeability and petrophysical relations fields. Water dynamics at the subsurface was accurately predicted by the PINNs system with an average error of ∼3% in the upscaled water saturation maps. The two separately trained PINNs systems have provided similar results in the obtained fields, indicating that the PINNs system can produce unique solutions for highly ill-posed problems. The addition of water content measurements at 20 random locations to the PINNs system training slightly improved the system outcomes, where a reduction of ∼0.25% in the upscaled water saturation average misfit was observed. Improvements were primarily located at the ERT low sensitivity zones, i.e., at the array's outskirts and large depths, thus implying the cost over benefits for obtaining additional hard data for training the system.

How to cite: Sakar, C., Schwartz, N., and Moreno, Z.: Upscaling Permeability, Petrophysical Relations and Water Saturation Maps of Heterogeneous Soils Using Physics-Informed Neural Networks Trained with Time-lapse Geo-electrical Tomograms, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6719, https://doi.org/10.5194/egusphere-egu24-6719, 2024.

Increasing anthropogenic and climate pressures on water resources and thermal energy call for a better understanding of the transient water storage and the water fluxes within the Critical Zone (CZ). Recharge, as the main water inflow feeding groundwater (GW), is critical for the proper management of GW systems. GW recharge is defined as the water percolating from the last unsaturated horizon down to the water table and is therefore broadly inaccessible to direct observations. Recharge is spatially heterogeneous and controlled by multiple factors such as porous media properties and hydrogeological conditions. Hydrogeophysics provide valuable approaches to determining hydraulic parameters in unconsolidated and unsaturated soils. In this domain, electromagnetic and electrical methods predominate due to their obvious dependence on water content. While crucial for water-related assessments, the transition to mechanical properties emphasizes the complementary role of seismic techniques. Specifically, seismic refraction tomography and surface-wave dispersion analysis stand out in delimiting boundaries between saturated and unsaturated zones. Recent studies underscore the synergy of employing both 2D electrical and seismic methods, showcasing their collective efficacy in identifying hydrofacieses and delineating the water table. However, these techniques fall short of providing a detailed saturation profile in the unsaturated zone. Recent studies suggest to employ the Van Genuchten model, coupled with a rock physics model that incorporates capillary suction effects, to determine the mechanical properties of the soil, accounting for both depth and saturation dependencies. This method enables the analytical 1D modeling of both P- and S-wave velocities in various hydrofacieses with various water table depths (in static conditions). Then by utilizing these velocity models, it is possible to calculate synthetic P-wave travel times (P-TT) and surface-wave dispersion (SWD) from an artificial seismic setup. This constitute a forward problem from saturation versus depth models towards seismic data. In this study, we propose to do the inverse problem, e.g. estimating the VG parameters (VG) from P-TT and SWD. We use the database provided by Carsell and Parrish to compute synthetic observations in wide a priori ranges. We propose the employment of a straightforward grid search and formulate the results in a Bayesian framework. Our results indicate that both SWD and P-TT are responsive to changes in water saturation, allowing for the retrieval of the VG parameters from observed data. Moreover, our study highlights that the sensitivity of geophysical data varies with soil composition, particularly underscoring the complexities of estimating VG parameters in soils with a high sand content.

How to cite: Sanchez Gonzalez, R., Bodet, L., Gesret, A., and Rivière, A.: Bayesian sensitivity analysis of seismic data to van Genuchten parameters in unsaturated and unconsolidated soils, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12001, https://doi.org/10.5194/egusphere-egu24-12001, 2024.

Facing the societal issues related to water resources management, the development of ambient noise-based seismology for monitoring fluids in the subsurface is promising but still challenging ⁽¹⁾. In this study, we propose a seismological monitoring of shallow groundwater with high spatial resolution, by applying ambient noise interferometry techniques. On a glacio-alluvial plain containing a shallow aquifer near Grenoble (France), we installed a dense array of 50 seismic nodes settled during five days. A pumping test was performed in a borehole during the experiment, inducing a fast and heterogeneous response of the aquifer. We estimated relative changes in surface wave velocity (dV/V) from autocorrelations of ambient noise recorded by the 50 sensors. During the pumping phase, dV/V increases by more than 10% near the borehole, indicating a significant decrease in pore pressure. Mapping the seismological response to pumping suggests a high channelization of the hydrogeological paths. Poroelastic modeling combined with active seismic campaigns improves the interpretation of observations ⁽²⁾, paving the way to a high-resolution time-lapse 3D mapping of the water dome and potential fluxes.

References

1 - Gaubert‐Bastide, T., Garambois, S., Bordes, C., Voisin, C., Oxarango, L., Brito, D., & Roux, P. (2022). High‐resolution monitoring of controlled water table variations from dense seismic‐noise acquisitions. Water Resources Research, 58(8), e2021WR030680.

2 - Voisin, C., Garambois, S., Massey, C., & Brossier, R. (2016). Seismic noise monitoring of the water table in a deep-seated, slow-moving landslide. Interpretation, 4(3), SJ67-SJ76.

How to cite: Baudement, C., Guillemot, A., Larose, E., Garambois, S., Royer, A., Rey, E., and Cappoen, V.: Tracking shallow groundwater response to a pumping test using a dense passive seismic array , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17325, https://doi.org/10.5194/egusphere-egu24-17325, 2024.

Lake Sibaya is the largest groundwater-fed freshwater lake in South Africa. In the past several decades the lake levels have declined substantially, largely due to drought, human water demands, and the expansion of eucalyptus plantations. These falling lake levels have resulted in the formation of isolated basins, most notably a northern main basin and a southern secondary basin, where water levels behave independently. The southern basin plays an important water resource and ecological role in the area, consequently, there is a need to better understand the groundwater-surface water connectivity and hydrogeological structure.

The area is characterized by a complex depositional system comprising dune and fluvial-deltaic sediments which makes understanding the groundwater and surface water connectivity non-trivial. To better understand the subsurface structure land and waterborne transient electromagnetic (TEM) surveys were conducted using a towed TEM system. The TEM method utilizes a transmitter and a receiver coil to estimate the subsurface resistivity distribution to depths of 50 – 70 m. Firstly, a primary electromagnetic field is generated by passing an electric current around the transmitter coil. The primary electromagnetic field induces currents in the subsurface which then generate a secondary electromagnetic field. The receiver coil then detects the secondary electromagnetic field. The rate of decay of this secondary electromagnetic field can be used to model the subsurface resistivity distribution. The resistivity models can be used with local borehole data to constrain geological boundaries in the survey area.

The resistivity models derived from the surveys, combined with borehole data, revealed distinct geological layers comprising organic sediments, sands, silts, and calcareous sandstones. Furthermore, whereas the northern basin is connected to the deeper aquifer, the southern basin is not. This work highlights the ability of high-productivity TEM methods to gain a better understanding of complex hydrogeological systems and provide context for their management.

How to cite: McLachlan, P.: Assessing groundwater-surface water connectivity using land and waterborne transient electromagnetic surveys, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20461, https://doi.org/10.5194/egusphere-egu24-20461, 2024.