HS8.1.6

Previous field self-potential (SP) surveys have suggested this passive, non-intrusive geophysical method to be a powerful tool for characterizing subsurface flow of water in shallow fractured systems. However, to accurately interpret the measured signal requires knowledge of electrochemical properties of these settings. Despite the interest, the controls on the electric surface charge and the zeta potential of gneiss at conditions relevant to naturally fractured systems remain unreported. There are no published zeta potential measurements conducted in such systems at equilibrium, hence, the effects of composition, concentration and pressure remain unknown. This study reports zeta potential values measured in a fractured gneiss sample, obtained from the Lewisian complex in NW Scotland, and saturated with NaCl solutions of various concentrations, artificial seawater and artificial groundwater solutions under equilibrium conditions at confining pressures of 4 MPa and 7 MPa. The constituent minerals of the sample were identified using X-ray diffraction and linked to the concentration and composition dependence of the zeta potential. The results reported in this study demonstrate that the zeta potential of gneiss was unique and dissimilar to pure minerals such as quartz, calcite, mica or feldspar. Moreover, the measured zeta potentials suggest that divalent ions (Ca²⁺, Mg²⁺ and SO₄²⁻) acted as potential determining ions. The zeta potential was also found to be independent of salinity in the NaCl experiments, which is unusual for most reported data. Moreover, the impact of fracture aperture on the electrokinetic response was investigated and likely implications for characterization of fractured systems using SP analyzed. Our novel results are an essential first step for interpreting field SP signals and facilitate a way forward for characterization of water flow through fractured basement aquifers.

How to cite: Vinogradov, J., Hidayat, M., Kumar, Y., Healy, D., and Comte, J.-C.: Zeta Potential of Fractured Gneiss Saturated with NaCl and Natural Aqueous Solutions – Impact of Composition, Concentration and Fracture Aperture on SP Signal in Response to Water Flow in Fractured Crystalline Bedrock, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13059, https://doi.org/10.5194/egusphere-egu22-13059, 2022.

The zeta potential is a measureable electrical potential of paramount importance to understand the electrochemical properties of colloids and grains in contact with brines. However, the zeta potential remains poorly understood because it takes place at the nanoscale of the electrical double layer on the mineral surface. Streaming potential measurements on quartz-rich Fontainebleau and Lochaline sandstones carried out at high salinity (above 0.1 M NaCl) yield surprisingly high zeta potential values. We found that placing the shear plane, where the zeta potential is defined, slightly closer to the mineral surface than the outer Helmholtz plane significantly improves the predictions of the zeta potential and surface charge density of quartz at high salinity as well as the values of the equilibrium constant describing sodium adsorption in the Stern layer. Our results have strong implications for the modelling of the electrochemical properties of minerals in contact with highly saline solutions.

How to cite: Leroy, P., Li, S., Maineult, A., and Vinogradov, J.: The zeta potential of quartz. Surface complexation modelling to elucidate high salinity measurements, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2636, https://doi.org/10.5194/egusphere-egu22-2636, 2022.

Characterizing and monitoring water flow in the critical zone is of uttermost importance to understand the water cycle. Water link several process within critical zone from aquifer recharge and solute transfer to eco-hydrology, many eco-systemic services and biogeochemical reactions. However, the in situ quantification of water flow is technically challenging using traditional hydrological methods and numerous gaps of knowledge remain. The self-potential (SP) method is a passive geophysical method that relies on the measurement of naturally occurring electrical field. One of the contributions to the SP signal is the streaming potential, which is of particular interest in hydrogeophysics as it is directly related to both the water flow and porous medium properties. Unlike tensiometers and other point sensors, which use the measurement of state (e.g., matric pressure) at different locations to infer the intervening processes, the SP method measures signals generated by dynamic processes (e.g. water movement). However, the amplitude of the SP signal depends on multiple soil properties which are dependent to soil type, moisture content, and water chemistry (composition and pH). During the last decades, many models have been proposed to relate the SP signal to the water flow. In this contribution, we will present a soil-specific petrophysical model to describe the electrokinetic coupling generated from different water fluxes in the critical zone: rain water infiltration and water uptake from tree-roots. We tested a fully coupled hydrogeophysical approach on a large SP dataset collected in a two-dimensional array at the base of a Douglas-fir tree (Psuedotsuga menziesii) in the H.J. Andrews Experimental Forest in central Oregon, USA. We collected SP measurements over five months to provide insight on the propagation of transpiration signals into the subsurface with depth and under variable soil moisture. The coupled model, which included a root-water uptake term linked to measured sap flux, reproduced both the long-term and diel variations in SP measurements, thus confirming that SP has potential to provide spatially and temporally dense measurements of transpiration-induced changes in water flow. Similar set-ups are being installed on several test-sites of the French Critical Zone observatory network, OZCAR: Larzac, LSBB, Strengbach. This will allow us to test the approach under different climatic conditions, different soil types and in different ecohydrological systems.

How to cite: Jougnot, D., Loiseau, B., Carrière, S., Champollion, C., Voytek, E., and Lesparre, N.: In situ monitoring of rain infiltration and evapotranspiration in the critical zone using self-potentials, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11720, https://doi.org/10.5194/egusphere-egu22-11720, 2022.

Hydrological processes along mountain hillslopes involve complex interaction between soil storage and surface and subsurface flow, drainage and evapotranspiration. To capture this complexity, time-lapse extensive and intensive measurements are needed, potentially capable of providing spatially dense information in 3D and time frequent data. To this end, hydro-geophysical methods (ground penetration radar, GPR, electromagnetic induction, EMI and electrical resistivity tomography, ERT) based on electrical and electromagnetic laws are widely used as they naturally link to the electrical properties of soil moisture. ERT produces, especially in time lapse mode and using permanent installations, very detailed images of the water dynamics along hillslopes. While ERT requires galvanic contact with the ground, and thus relatively slow operations, EMI can be applied over large areas in a very short time. This method has been used for decades, mainly to produce apparent electrical conductivity (ECa) maps. Only recently, inversion of EMI data as a function of depth has become a viable practice.

In this work, we present two cases of hillslope monitoring using non-invasive methods, both performed as part of the WATZON project, funded by the Italian Ministry of University and Research (MIUR). The first case is the Mediterranean catchment of the Alento River, in southern Italy. The monitoring was carried out using 7 different EMI surveys, acquired in multifrequency mode (FDEM) between August 2020 and December 2021. The purpose of this survey was to characterize the structure of the basin’s subsoil within the first few meters, as well as to record the variation of electrical conductivity (EC) associated with seasonal variations. The second case is related to the Apennines catchment of the Re Della Pietra, located at the border between Tuscany and Emilia-Romagna in central Italy. The monitoring was carried out through 6 different EMI surveys, acquired in multifrequency mode (FDEM) between August 2020 and May 2021. The purpose of this survey was to characterize the structure of the basin’s subsoil within the first few meters, as well as to record the variation of electrical conductivity (EC) associated with seasonal variations. Furthermore, ERT measurements were carried out along a fixed line on the ground, according to the direction of the maximum slope. The combination of EMI and ERT proved particularly effective in delineating the hydrologic dynamics of the hillslope.

How to cite: Cassiani, G., Censini, M., Nasta, P., Allocca, C., Sica, B., Lazzaro, U., Mazzitelli, C., Verdone, M., Dani, A., Manca di Villahermosa, F., Penna, D., and Romano, N.: Time-lapse multi-frequency EMI mapping and ERT profiling for the characterization of soil water behavior in mountain catchments., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6956, https://doi.org/10.5194/egusphere-egu22-6956, 2022.

The hyporheic zone is a thin porous sedimentary interface that connects the river to the water table. It is a place where a large part of the groundwater transits and mixes with surface water. Recent studies point to the key role of this zone, a natural biological reactor at the groundwater-river interface, in altering the nitrogen and carbon cycles, capturing and releasing contaminants and buffering river temperatures. Past studies have suggested that, locally hyporheic fluxes can overtake groundwater-river exchanges, although the physical conditions (permeability, roughness, head, ...) under which such scenario occurs remains unclear. In this study, geophysical monitoring of electromagnetic conductivity along a river reach (Sélune, France) was used to identify longitudinal variations in bed permeability, and identify potential hotspot of hyporheic exchanges.
In practice, we will measure electrical conductivity at different depths into the river sediment using the electromagnetic instrument “CMD explorer”. The instrument is displaced over the river surface using a floating board. A two-layer model is then considered to separate the contribution of sediment bed and water column conductivities. Based on the law (Archie, 1942) in a saturated environment, we link measured variations in electrical conductivity to variations in sediment properties such as pore volume change. In parallel, manual permeability measurements of the riverbed were performed to compare and validate the permeability deduced by the electromagnetic method.
The recent removal of two dams over the Sélune river which was associated with an abrupt change of transported sediment load motivates us to perform time-lapse measurements of electromagnetic conductivity in several sectors upstream and downstream of the dams, before and after removal. Such repeated measure provides interesting data on the time scales of bed clogging processes and its impact on hyporheic exchanges.

How to cite: Mouhamadoul, M. B.: Use of electromagnetic tools to evaluate the risk of silting of the ‘Sélune’ river bed following the dismantling of the 'Vezins' and 'Roche Qui Boit' dams., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1234, https://doi.org/10.5194/egusphere-egu22-1234, 2022.

Irrigated agriculture plays a crucial role in the food supply in many countries where ecological conditions are characterized by warm and dry summers with high solar radiation and evapotranspiration rates. Evaluating spatio-temporal variability of soil water is critical for the delignating of management zones and optimal irrigation scheduling. However, the soil water content variability is normally obtained using simple water balance models and for representative areas, not taking into consideration the variability of soil properties. This is because for large-scale studies, the traditional sampling method is extremely difficult to implement and it remains critical to finding alternative methods of characterization of soil texture, which is required for soil hydraulic parameters assessment.

Geophysical techniques such as electromagnetic induction (EMI) provide enormous advantages compared to soil sampling because they allow for in-depth and non-invasive analysis, covering large areas in less time and at a lower cost. we carried out EMI surveys in a 23ha almond field, located in Alentejo, Portugal to evaluate the potential use of this methodology in mapping spatial distribution of soil texture in this water-scarce region. We firstly inverted field apparent conductivity data (σ_a) using a Quasi-3D inversion algorithm in order to obtain 3D electromagnetic conductivity images (EMCI) of the real soil electrical conductivity (σ) with depth. Afterward, we evaluated the possibility of establishing a linear regression (LR) relationship between σ and soil texture collected from 13 soil sample locations to a depth of 0.60 m. We concluded that it is possible to establish a relatively good LR between σ and clay and sand, allowing us to convert EMCI to clay and sand content maps and generate these maps for different depts. With this information at hand, pedotransfer functions were applied to define the soil hydraulic parameters necessary to run the distributed model and map the within soil variability at the field scale.

we used the MOHID-Land distributed process-based model to compute the variability of the soil water balance components in this field, at a resolution of 5m. Irrigation data was monitored on-site, at two locations, while weather data was extracted from a local meteorological station. The distributed modeling approach included the definition of potential evapotranspiration fluxes computed from the product of the reference evapotranspiration obtained according to the FAO56 Penman-Monteith equation and a crop coefficient for each stage of almond’s growing season, the variable-saturated flow using the Richards equation, and root zone water stress following a macroscopic approach. Modeling results are then used to present the maps of the variability of the seasonal actual crop transpiration and soil evaporation, the mean soil moisture, seasonal runoff, and seasonal percolation, which are then used to propose management zones for improving irrigation water use in the studied almond field.

Acknowledgments

This work was developed in the scope of SOIL4EVER “Sustainable use of soil and water for improving crops productivity in irrigated areas” project supported by FCT, grant no. PTDC/ASP-SOL/28796/2017.

How to cite: Farzamian, M., Ramos, T. B., Oliveira, A. R., Darouich, H., Castanheira, N., Paz, A. M., and Gonçalves, M.: Application of Electromagnetic Induction Method and Distributed Process-Based Modeling for Optimized Soil Water Variability Assessment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5419, https://doi.org/10.5194/egusphere-egu22-5419, 2022.

The monitoring of alpine rock glaciers has both scientific and economic relevance. The degradation of mountain permafrost is a relevant proxy of climate change and global warming, but also a possible source of hazards for mountain communities since it can trigger natural processes such as rockfalls, debris flows, and floods. Geophysical techniques have been used to study these periglacial forms, particularly electrical and seismic refraction methods. Nevertheless, the independent data processing applied to these measurements does not lead to quantitative estimation of the physical components (air, water, ice, and rock) in the frozen subsoil. Moreover, the structural interpretation of the ground with independent resistivity and seismic sections can introduce ambiguities. To quantify the composition of the mountain permafrost, Wagner et al. (2019) developed a petrophysical joint inversion approach of electrical resistivity and seismic refraction datasets. We applied this method to several datasets collected in the rock glaciers of Schafberg (Engadin, Switzerland) and Ritigraben (Canton of Valais, Switzerland). To estimate the parameters in Archie’s and Timur’s laws, we performed the petrophysical joint inversion with a range of plausible values, selecting the ones that guaranteed the lowest final root-mean-square (RMS) error between the model response and the observed data. Our approach can be applied wherever information from boreholes is unavailable. This is a common situation in rock glacier studies since drilling in high mountain environments is very complicated and expensive. Finally, to improve the quality of individual resistivity and seismic velocity sections, we applied the structurally-coupled cooperative joint inversion method to our datasets, developed by Günther and Rücker (2008). This approach is based on the exchange of structural information between the independent geophysical inversions of electrical and seismic datasets. The process is driven by 3 different coupling parameters and the choice of their values has been done again by running the inversion with a range of values, choosing those that guaranteed the lowest final RMS. This method can be useful to better define the active layer thickness and the lower boundary of frozen ground. From the obtained results, it is clear that combined use of petrophysically-coupled and structurally-coupled joint inversion can represent a significant improvement for the characterization of mountain permafrost, in comparison to the traditional independent geophysical inversions, even in the absence of prior information from boreholes. In future studies, both structural and petrophysical coupling could be incorporated into a single inversion framework to adaptively allow structural agreement if quantitative petrophysical agreement cannot be satisfied.

References
-    Wagner, F.M., Mollaret, C., Günther, T., Kemna, A., & Hauck, C. (2019). Quantitative imaging of water, ice, and air in permafrost systems through petrophysical joint inversion of seismic refraction and electrical resistivity data. Geophysical Journal International, 219(3), 1866–1875. doi:10.1093/gji/ggz402.
-    Günther, T., & Rücker, C. (2008). A new joint inversion approach applied to the combined tomography of DC resistivity and seismic refraction data. Symposium on the application of geophysics to engineering and environmental problems 2006 (pp. 1196–1202). doi:10.4133/1.2923578.

How to cite: Pavoni, M., Boaga, J., Wagner, F., Bast, A., and Phillips, M.: Combined use of structurally-coupled and petrophysically-coupled joint inversion for the characterization of rock glaciers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5861, https://doi.org/10.5194/egusphere-egu22-5861, 2022.