Search Results (66)

The hyporheic zone (HZ) mediates biogeochemical exchanges between rivers and aquifers, yet its spatial and temporal dynamics in large, regulated rivers remain poorly characterized due to limitations of point-based measurements. Here, we combined three time-lapse electrical resistivity tomography (T-ERT) surveys with continuous hydrological and hydrochemical monitoring along a meandering reach of the lower Yellow River, generating a two-dimensional, profile-integrated view of HZ geometry under three hydrodynamic states: low flow (1 December 2020), natural rising stage (1 March 2021), and peak stage during the Xiaolangdi (XLD) water-and-sediment regulation (1 July 2021). Absolute tomograms identified two hydrostratigraphic units: an upper sandy-silt cap (35–170 Ω·m) and an underlying sand aquifer (12–35 Ω·m). Percent-difference tomograms, relative to the low-flow baseline, revealed lateral HZ expansion from ~15 m and vertical growth of 2.5 m at the rising stage to ~36 m and 4.5 m at peak stage, with local resistivity decreases exceeding 38%. In contrast, the deeper mixing zone varied by <10% across surveys. Temperature, rainfall infiltration, and groundwater freshening could not explain the observed patterns. These results were corroborated by three independent lines of evidence: lateral conductivity excursions and in-well temperature records at floodplain well W2, and analytical Darcy–Archie calculations, all consistent with the predicted lateral extent and mixing fraction. River stage, amplified by the XLD release, emerged as the dominant control on two-dimensional HZ geometry. This study provides direct empirical evidence of hyporheic dynamics in a large regulated river and demonstrates that T-ERT, supported by sparse hydrological data, offers a minimally invasive and effective tool for characterizing hyporheic zones. Full article

The characterisation of subsurface electrical resistivity is a fundamental requirement for geoscientific and engineering applications, including groundwater exploration and structural assessments. This study examines the sequential cooperative inversion of direct current resistivity and frequency-domain electromagnetic data and compares the results to the inverse models obtained from separate (individual) inversions of the datasets. The proposed cooperative framework is applied to both synthetic datasets generated through forward modelling and field data acquired at the Morgenzon Farm site, South Africa, to delineate a dolerite dyke of hydrogeological significance. Individual inversions identified distinct features but exhibit limitations: direct current resistivity highlights a two-layered medium with minor anomalies, while frequency-domain electromagnetic data identify a resistive anomaly. In contrast, the sequential cooperative inversion approach, which uses the output of one dataset to constrain the other, provides improved subsurface imaging results, reduces ambiguity, and enables the integration of complementary information from both methods. The results indicate that resistivity models constrained by inverse frequency-domain electromagnetic data provide improved representation of subsurface geometry and amplitude compared to individual approaches. These findings support the use of a non-destructive testing approach for improved subsurface imaging, facilitating better-informed decision-making in infrastructure projects and resource management. Full article

Time-Domain Electromagnetic (TDEM) soundings are widely recognized as an effective method for subsurface characterization at intermediate depths. This study applies TDEM surveying to the municipality of Alfaro (La Rioja, Spain), where ten stations with 200 × 200 m loops were acquired and processed using Occam 1D inversion. The resulting models were integrated into a 3D environment in geomatic software (Seequent, Oasis montaj 2025.1) to generate a continuous geoelectrical volume of the subsurface. Three major resistivity domains were identified: a shallow resistive unit (40–80 ohm·m), an intermediate unit (20–40 ohm·m), and a deep conductive domain (<20 ohm·m). The pseudo-3D model revealed a thickening of the intermediate–resistive domain toward the central and western sectors, interpreted as the most favorable zone for groundwater extraction. This workflow demonstrates that integrating 1D TDEM inversion results into a pseudo-3D geoelectrical framework improves the spatial interpretation of resistivity distributions and provides a practical decision-support framework for identifying optimal drilling sites. Full article

Saline water intrusion poses a major threat to groundwater security in arid and semi-arid regions, reducing freshwater availability and challenging sustainable water resource management. Accurate delineation of the fresh-saline water interface is therefore essential; however, conventional hydrochemical and laboratory-based assessments remain costly, invasive, and spatially limited. Resistivity methods have long been used to infer subsurface salinity, as low resistivity typically reflects clay-rich saline water and higher resistivity reflects freshwater-bearing sand or gravel. Yet, resistivity values for similar lithologies frequently overlap, causing ambiguity in distinguishing fresh and saline aquifers. To overcome this limitation, Dar–Zarrouk (D–Z) parameters are often applied to enhance hydrogeophysical discrimination, but previous studies have relied exclusively on one-dimensional (1D) D–Z derivations using vertical electrical sounding (VES), which cannot resolve the lateral complexity of alluvial aquifers. This study presents the first application of electrical resistivity tomography (ERT) to derive two- and three-dimensional D–Z parameters for detailed mapping of the fresh-saline water interface in the alluvial aquifers of Punjab, Pakistan. ERT provides non-invasive, continuous, and high-resolution subsurface imaging, enabling volumetric assessment of aquifer electrical properties and salinity structure. The resulting 2D/3D models reveal the geometry, depth, and spatial continuity of salinity transitions with far greater clarity than VES-based or purely hydrochemical methods. Physicochemical analyses from boreholes along the ERT profiles independently verify the geophysical interpretations. The findings demonstrate that ERT-derived 2D/3D D–Z modeling offers a cost-effective, scalable, and significantly more accurate framework for assessing fresh-saline water boundaries. This approach provides a transformative pathway for sustainable groundwater monitoring, improved well siting, and long-term aquifer protection in salinity-stressed alluvial regions. Full article

In coastal regions, the interaction between freshwater and seawater creates a dynamic system in which the spatial distribution of salinity critically constrains the use of freshwater for human consumption. Although saline intrusion is a globally widespread phenomenon, its inland extent varies significantly with hydrological conditions, posing a persistent threat to groundwater quality and sustainability. This study aimed to characterize salinity distribution using an integrated karst-watershed approach, thereby enabling the identification of both lateral and vertical salinity gradients. The study area is in the northwestern Yucatan Peninsula. Available hydrogeological data were analyzed to determine aquifer type, soil texture, evidence of saline intrusion, seawater fraction, vadose zone thickness, and field measurements. These included sampling from 42 groundwater sites (open sinkholes and dug wells), which indicated a fringe zone approximately 5 km in size influenced by seawater interaction, in mangrove areas and in three key zones of salinity patterns: west of Mérida (Celestun and Chunchumil), and northern Yucatan (Sierra Papacal, Motul, San Felipe). Vertical Electrical Sounding (VES) and conductivity profiling in two piezometers indicated an apparent seawater influence. The interface was detected at a depth of 28 m in Celestun and 18 m in Chunchumil. These depths may serve as hydrogeological thresholds for freshwater abstraction. Results indicate that saltwater can extend several kilometers inland, a factor to consider when evaluating freshwater availability. This issue is particularly critical within the first 20 km from the coastline, where increasing tourism exerts substantial pressure on groundwater reserves. A coastal-to-inland salinity was identified, and an empirical equation was proposed to estimate the seawater fraction (fsea%) as a function of distance from the shoreline in the Cenote Ring trajectory. Vertically, a four-layer model was identified in this study through VES in the western watershed: an unsaturated zone approximately 2.6 m thick, a confined layer in the coastal Celestun profile about 9 m thick, a freshwater lens floating above a brackish layer between 8 and 25 m, and a saline interface at 37 m depth. The novelty of this study, in analyzing all karstic water surfaces together as a system, including the vadose zone and the aquifer, and considering the interactions with the surface, is highlighted by the strength of this approach. This analysis provides a better understanding and more precise insight into the integrated system than analyzing each component separately. These findings have significant implications for water resource management in karst regions such as Yucatan, underscoring the urgent need for sustainable groundwater management practices to address seawater intrusion. Full article

In this study, we develop a methodology to quantitatively integrate well resistivity logs and audio-magnetotelluric (AMT) sounding in the context of groundwater investigations. The experiments were conducted in a complex Quaternary depositional environment within a region of intensive groundwater use. A synthetic resistivity model was constructed from well resistivity log data and used for forward modelling to generate synthetic AMT responses, which were then inverted using Occam’s algorithm. The AMT-derived resistivity model was subsequently compared with the model obtained from the inversion of the field AMT sounding using the Pearson correlation coefficient (r) and the root mean square error (RMSE). A scalar shift factor (k) was introduced to optimize the match between both models. The comparison between the AMT-derived resistivity model and the well resistivity logs yielded a Pearson correlation coefficient of 0.669 and an RMSE of 0.1911. The optimal scalar shift factor was k = 1.1854, with a 95% confidence interval of [1.1470, 1.2246], indicating only a minor discrepancy. These results demonstrate that AMT can successfully recover a resistivity structure consistent with well resistivity logs. The proposed quantitative integration and validation workflow provides a robust framework to reduce the inherent ambiguity in AMT interpretation and highlights its potential as a direct hydrogeophysical tool for groundwater studies. Full article

Low-lying coastal plains are increasingly threatened by saltwater intrusion, yet the extent of the phenomenon and the role of coastal dune systems remain unevenly assessed. In the northern Adriatic Sea (NE Italy), salinisation has been documented, but systematic, spatially resolved studies are lacking. This work investigates the Belvedere–San Marco relict dune system to assess its hydrogeological function and vulnerability to seawater intrusion. An integrated methodology combining borehole and core stratigraphy, in situ water electrical conductivity (EC) measurements, and multi-method geophysical surveys (FDEM, ERT, GPR, active seismics) was tested. Results reveal a consistent stratigraphy of permeable aeolian sands overlying clay-rich units, with groundwater EC values in the dune sector always remaining well below thresholds for brackish or saline conditions. Geophysical imaging reveals that the dunes are low-conductive bodies contrasting sharply with the conductive surrounding lowlands, thus indicating the persistence of a freshwater lens sustained by local recharge within the dunes. The Belvedere–San Marco dunes therefore act as both freshwater reservoirs and natural hydraulic barriers, buffering shallow aquifers against salinisation. This study demonstrated the applicability of integrated geophysical methods to extensively investigate shallow phreatic aquifers lying a few metres below the surface, and establishes a baseline for monitoring future changes under rising sea levels, subsidence, and increased groundwater exploitation. Full article

Groundwater management in volcanic islands represents a complex challenge due to the scarcity of surface resources, the strong heterogeneity of volcanic terrains, and the constant threat of marine intrusion. In Tenerife (Canary Islands, Spain), current regulations establish that only saline or brackish waters are permitted for exploitation, to be subsequently desalinated through reverse osmosis for urban and touristic supply. In this context, it is essential to develop geophysical methodologies capable of accurately characterizing subsurface salinity and optimizing the location of new boreholes. The present study applies Electrical Resistivity Tomography (ERT) profiles in the Los Cristianos area (Arona, Tenerife), later integrated into a three-dimensional model using Oasis Montaj software Version 2025.1. The results allow for the differentiation of four geoelectrical domains. The 3D modeling enabled a detailed characterization of the conductive domain, delineating the geometry of the marine intrusion. The findings confirm that the combination of ERT and 3D modeling constitutes an effective, replicable, and economically efficient methodology for precisely locating saline horizons and selecting the most suitable drilling sites, thereby providing an objective basis for the sustainable management of water resources in volcanic islands. Full article

This study presents a methodology for estimating hydraulic conductivity (K) from well geophysical logs, with the aim of improving the parameterization of hydrogeological models in data-scarce regions. The lack of data poses a challenge for aquifer characterization, especially in contexts requiring integrated groundwater management. In such contexts, indirect methods can support estimation of key hydraulic parameters. The proposed methodology was applied to wells which penetrate Neogene–Quaternary hydrogeological units of the sedimentary aquifer system in the Middle Magdalena Valley, Colombia. Effective porosity was estimated from sonic and gamma ray logs, while temperature profiles were derived from the regional geothermal gradient and calibrated with field measurements. Hydraulic conductivity was estimated using an approach based on the Csókás method and validated through comparison with 131 pumping tests and alignment with the parameterization of a previously calibrated regional groundwater flow model. Pumping tests yielded geometric mean K values of 1.56 m/day in floodplain deposits (QFal), 1.36 m/day in U4, and 0.96 m/day in U3. K values from well logs ranged from 1.65 to 2.95 m/day, within the same order of magnitude. These findings support the proposed methodology as a viable alternative for the parameterization of numerical hydrogeological models in data-scarce environments. Full article

Assessment of contaminant dispersal in sandstones requires hydraulic characterization with a combination of datasets that span from the core plugs to wellbores and up to the field scale as the matrix and fractures are both hydraulically conductive. Characterizing the hydraulic properties of the matrix is fundamental because contaminants diffuse into the fractured porous blocks. Fractures are highly conductive, and the determination of the number of hydraulically active rock discontinuities makes discrete fracture network models of solute transport reliable. Recent advances (e.g., active line source temperature logs) in hydro-geophysics have allowed the detection of 40% of hydraulically active fractures in a lithified sandstone. Tracer testing has revealed high (~10⁻⁴–10⁻² ms⁻¹) flow velocities and low (~10⁻²–10⁻⁴) effective porosities. Contaminants can therefore move rapidly in the subsurface. The petrophysical characterization of the plugs extracted from the cores, in combination with borehole hydro-geophysics, allows the characterization of either matrix or fracture porosity, but the volume of sandstone characterized is low. Tracer tests cannot quantify matrix or fracture porosity, but the observation scale is larger and covers the minimum representative volume. Hence, the combination of petrophysics, borehole hydro-geophysics, and tracer testing is encouraged for the sustainable management of solute transport in dual porosity sandstones. Full article

To further comprehend kinetic processes in the unsaturated zone, a series of soil column experiments was conducted to simulate downward and upward water movement under variable saturation conditions. High-accuracy spatial and temporal measurements were carried out using the time domain reflectometry—TDR—and Ground-Penetrating Radar—GPR—geophysical methods. Several custom spatial TDR sensors were constructed and used alongside point-measuring TDR sensors, which served as reference points for the calibration of the custom spatial waveguides. The experimental results validated the ability of the custom-made spatial sensors, and the TDR technique in general, to capture water movement and soil moisture changes with high precision during varying wetting processes and demonstrated the complementarity, the limitations, and the potential of the GPR method under the same conditions. The study proved that the combination of the aforementioned measuring technologies provides a better understanding of the kinetic processes that occur in variably saturated conditions. Full article

The application of hydrogeophysical techniques to delineating aquifers was conducted in De Aar, the eastern part of the Karoo region, Northern Cape, South Africa. Previously, recharge estimations in this region assumed a uniform aquifer type, overlooking the presence of diverse aquifer systems. This study identified both unconfined and confined aquifers to improve recharge potential assessments. Vertical electrical resistivity sounding (VES) and ground telluric methods were applied. Six VES stations and eleven profiles were measured using a 1D Wenner array configuration. The VES data, processed with IPI2win software, generated a 2D subsurface model. In contrast, the telluric data were analyzed using an automated algorithm to create a 2D profile. The electric potential difference curve was interpreted in comparison with lithological cross-sections. The VES results revealed three to four distinct layers of low-resistivity (0.9–8.1 Ωm), moderate-resistivity (22.4–125 Ωm), and high-resistivity (68–177 Ωm) values, indicating three lithological formations. The telluric data suggested that shallow groundwater boreholes were located in areas with groundwater levels above 50 m. These findings, which matched the lithological data, pointed to a double-layer aquifer system, suggesting that recharge estimates should be carried out to different aquifer layers. The study demonstrated how hydrogeophysical methods can effectively delineate aquifer systems and enhance the identification of recharge areas. Full article

Continuous measurements of soil moisture in the deeper parts of the unsaturated zone remain an important challenge. This study examines the development of an integrated system for the continuous and 3-D monitoring of the vadose zone processes in a cost- and energy-efficient way. This system comprises TDR, ERT and GPR geophysical techniques. Their capacities to adequately image subsurface moisture changes with continuous and time-lapse measurements are assessed during an artificial infiltration experiment conducted in a characteristic urban site with anthropogenic fills and much compaction. A 3-D array was designed for each method to expand the information of a single TDR probe and obtain a broader image of the subsurface. Custom spatial TDR probes installed in boreholes made with a percussion drilling instrument were used for soil moisture measurements. Moisture profiles along the probes were estimated with a numerical one-dimensional inversion model and a standard calibration equation. High conductivity water used during all infiltration tests led to the detection of the flow by all techniques. Preferential flow was present throughout the experiment and imaged sufficiently by all methods. Overall, the integrated approach conceals each method’s weaknesses and provides a reliable 3-D view of the subsurface. The results suggest that this approach can be used to monitor the unsaturated zone at even greater depths. Full article

In this work, electrical resistivity tomography was carried out together with physical hydrogeology techniques to evaluate the karst aquifer in the northwest region of the Yucatán Peninsula in a study area near the western edge of the Ring of Cenotes of the Chicxulub Crater. In addition, based on a systematic compilation of open-access data of water levels reported for the peninsular aquifer, maps of groundwater isolines and groundwater flows were generated using IDW interpolation, Empirical Bayesian Kriging, and the Flow Net method. From these results, a shallow aquifer is observed, with the presence of heterogeneities such as possible dissolution conduits and/or flooded caverns, approximately 20 m below ground level, formed by the dissolution processes of limestone rocks. On a regional scale, the geomorphological influence of the Ring of Cenotes on groundwater flows was observed. In general, the flow directions observed from these maps coincide with those conceptualized for this region of the peninsular aquifer. Nevertheless, some differences were observed depending on the interpolation method used. Our results contribute to hydrogeological studies carried out in the periphery of this ring, where the vulnerability of the aquifer to anthropogenic contamination has been highlighted due to the intrinsic features of the karst environment. Full article

Low-frequency electromagnetic induction (EMI) is a non-invasive geophysical method that is based on the induction of electromagnetic (EM) waves into the subsurface to quantify changes in electrical conductivity. In this study, we present an open (design details and software are accessible) and modular system for the collection of EMI data. The instrument proposed allows for the separations between the transmitter to be adjusted and up to four receiving antennas as well as the acquisition frequency (in the range between 3 and 50 kHz) to permit measurements with variable depth of investigation. The sensor provides access to raw data and the software described in this study allows control of the signal processing chain. The design specifications permit apparent conductivity measurements in the range of between 1 mS/m and 1000 mS/m, with a resolution of 1.0 mS/m and with a sampling rate of up to 10 samples per second. The sensor allows for a synchronous acquisition of a time stamp and a location stamp for each data sample. The sensor has a mass of less than 5 kg, is portable and suitable for one-person operation, provides 4 h of operation time on one battery charge, and provides sufficient rigidity for practical field operations. Full article