Evaluation of Subsurface Moisture Dynamics and Leakage Pathways Through Geoelectrical Mapping: Insights from Nagi Lake, South Sikkim

Abstract

Sikkim, located in the northeastern Himalaya, is highly vulnerable to natural hazards and increasing depletion of surface and subsurface water resources, particularly springs and lakes. In South Sikkim, several lakes exhibit rapid drainage behavior, among which Nagi Lake shows near-complete water loss shortly after rainfall, indicating the presence of subsurface leakage pathways. This study investigates shallow subsurface moisture dynamics and identifies potential seepage-prone zones beneath the Nagi Lake basin using geoelectrical methods. Electrical resistivity profiling was conducted along seven survey lines during the non-rainy season (October–November 2025) to minimize the influence of transient rainfall-induced moisture variations. Profiling was carried out using the Wenner method, achieving investigation depths of approximately 6.5–9 m. Additionally, Vertical Electrical Sounding (VES) using the Schlumberger configuration was performed at selected locations to examine deeper subsurface conditions. Resistivity results indicate that profiles L1, L2, L3, L4, and L7 contain relatively higher moisture restricted to the upper ~5 m, whereas profiles L5 and L6 exhibit persistently low resistivity values from the surface to depths of ~9 m, suggesting sustained subsurface moisture accumulation. The downward extension of low-resistivity zones along L5 and L6 indicates possible preferential seepage pathways or localized subsurface water storage. VES results further reveal a higher density of subsurface anomalies below ~14 m in these areas. These low-resistivity anomalies are interpreted as potential subsurface flow pathways. Although confirmation of active seepage requires additional hydrological or time-lapse investigations, the findings provide important baseline geophysical insights for lake rejuvenation.

1. Introduction

The state of Sikkim, located in the northeastern region of India, occupies a unique tectonic and hydro-geomorphic setting. Bordered by Nepal, Bhutan and China and lying within the active Himalayan convergence zone, Sikkim is characterized by intense crustal deformation and is classified among India’s high-seismic hazard regions. For instance, a recent study using powerful hybrid approach Analytic Hierarchy Process (AHP)–Entropy-based multi-criteria decision-making (MCDM) models, which integrate subjective expert-based weighting (AHP) with objective data-driven weighting (Entropy), estimated that over 43% of the area and 54 % of the population are located in “high-to-very high” seismic hazard zones [1]. Now, as per the revised seismic zonation map of India by the Bureau of Indian Standards, Sikkim is in Zone VI [2]. In addition to the seismic hazard, Sikkim is facing mounting pressures on its water resources. The mountainous terrain means that many rural and urban populations depend heavily on natural springs, rivulets, seepages and lakes for drinking water, irrigation and livestock. Compounding these stresses, Sikkim’s lake systems, including both glacial and non-glacial lakes, have exhibited signs of instability and rapid drainage. A recent investigation of Chochen Lake in Pakyong District noted that following the 2011 earthquake, formerly perennial lakes in high seismic zones have increasingly become seasonal, attributed to the development of fracture networks beneath the lake basins that facilitate seepage [3].

Recent studies have highlighted the importance of integrating advanced hydro-geophysical, geotechnical, and modeling approaches to better understand subsurface hydrological processes and associated uncertainties. Process-based hydraulic modeling and uncertainty assessment have been widely applied to characterize soil–water interactions and subsurface flow dynamics in heterogeneous environments [4]. Similarly, coupled thermo-hydro-mechanical and damage models have been developed to evaluate fluid movement and stress responses in complex geological systems [5], while parametric geotechnical investigations provide valuable insights into subsurface structural variability and foundation stability [6]. Recent geophysical and hydro-mechanical studies further demonstrate the role of environmental forcing and seepage interactions in controlling subsurface mechanical properties and water movement [7,8,9,10]. In addition, isotopic techniques and advanced data-driven methods such as deep learning has been increasingly employed to identify groundwater sources, quantify ecohydrological dependencies, and evaluate key drivers of subsurface water quality and dynamics [11,12]. Moreover, integrated monitoring approaches have proven essential for understanding rainfall-induced hydrological hazards and subsurface instability in environmentally sensitive regions [13]. These studies collectively highlight the growing importance of multidisciplinary frameworks for investigating subsurface hydrological processes, providing a broader methodological context for the present geoelectrical investigation of moisture dynamics and seepage pathways beneath the Nagi Lake basin.

Geoelectrical resistivity methods have proven to be effective tools for delineating subsurface moisture distribution, fracture networks, and leakage or seepage pathways in a wide range of hydrological settings. In recent years, advances in instrumentation, inversion techniques, and interpretation frameworks have significantly enhanced the reliability of resistivity-based investigations for lake–groundwater interaction studies [14,15,16]. For example, investigations at Kailana Lake (Takht Sagar, Rajasthan) demonstrated that the combined application of self-potential and electrical resistivity methods can successfully identify fracture-controlled leakage beneath lake beds [17]. More broadly, geoelectrical techniques have been widely applied to investigate subsurface leakage and seepage in lakes, reservoirs, natural dams, tailings dams, and landfill sites. Recent studies have shown that spatial variations in apparent resistivity are effective indicators of moisture accumulation, preferential flow paths, and structurally controlled permeability in both natural and engineered systems [18,19,20,21]. In tectonically active and fractured terrains, such as the Himalayan region, resistivity imaging has been particularly valuable for characterizing heterogeneous subsurface conditions and identifying zones of enhanced groundwater movement or retention [3,22,23]. Earlier foundational studies established the physical basis for using resistivity methods to detect water movement and leakage beneath surface water bodies [24,25,26,27]. These pioneering works laid the groundwork for modern applications, which now incorporate 2D and 3D imaging, improved inversion algorithms, and integration with geological and hydrological data. Collectively, both classic and recent studies highlight the versatility of geoelectrical resistivity methods as non-invasive, cost-effective tools for investigating subsurface hydrological processes and supporting sustainable water-resource management, particularly in data-limited and environmentally sensitive regions.

In contrast to earlier investigations in Himalayan lakes mainly utilizing resistivity–tracer combined Himalayan groundwater data [28] and Isotopic/geophysical techniques [29] to understand groundwater–surface water interactions, the present study adopts a static geoelectrical approach combining resistivity profiling and VES to characterize shallow subsurface moisture dynamics and delineate potential seepage-prone zones beneath the Nagi Lake basin. Interestingly, rapid seepage of water from the lake basin was observed almost immediately after rainfall, preventing any significant accumulation of water in the lake that would allow reliable stage-level monitoring. These field observations further support the hypothesis of strong subsurface drainage pathways beneath the lake basin. This phenomenon points to an urgent need to characterize the subsurface hydrogeological conditions and potential structural controls (such as fractures or seepage zones) that may be driving this rapid depletion. Accordingly, this study undertakes geoelectrical resistivity profiling along seven reference lines (L1–L7) across the Nagi Lake basin during the non-rainy season (October and November 2025) to minimize rain-led moisture interference and to explore subsurface structure, moisture distribution and potential inflow or seepage zones. Nagi Lake does not retain standing water since 2010 onwards, even during extreme rainfall events. Field evidence consistently shows that the lake bed and surrounding formations exhibit very high permeability, causing immediate infiltration. The study employed the Wenner electrode configuration, which provides several practical and methodological advantages, including a high signal-to-noise ratio, stable measurements, and reduced sensitivity to contact resistance and moderate cultural noise. Through the analysis of spatial variations in apparent resistivity and inferred vertical moisture-movement trends (upward versus downward), this investigation aimed to delineate subsurface moisture distribution and identify low-resistivity zones that are consistent with, though not diagnostic of, potential preferential seepage pathways. The persistence of high moisture content during the non-rainy season could suggest the possible presence of sustained subsurface water sources rather than transient rainfall infiltration. These findings will establish essential baseline geophysical data to support lake rejuvenation efforts, groundwater recharge assessment, and sustainable water-resource management in this seismically active and hydrologically stressed Himalayan region.

2. Study Area

Nagi Lake lies in the northeastern Indian state of Sikkim, which shares its borders with Nepal to the west, Bhutan to the east, and Tibet along its northern and eastern boundaries. Administratively, Sikkim is divided into six districts, which are Gangtok and Pakyong in the east, Mangan in the north, Namchi in the south, and Gyalshing and Soreng in the west, with Gangtok serving as the state capital. The Nagi Lake area in Namthang falls within a high-hazard seismic zone, as identified by seismic microzonation studies conducted by Nath et al. [30]. Numerous earthquake epicenters have been recorded in and around the region in the past, highlighting its pronounced seismic vulnerability (Figure 1).

Figure 1. Map showing study area location and the epicenter of earthquakes (with moment magnitude) and structural discontinuities around Nagi Lake (Data Source: GSI, 2025 [31]). ALOS-P DEM shows the topography (Source: ASF Data Search [32]).

Water scarcity remains a pressing concern in the districts of Namchi, Gyalshing, and Soreng, where communities rely heavily on lakes and natural springs to meet their domestic and agricultural water requirements. Situated in the Namthang region of Namchi district, Nagi Lake is one such water body; however, it has completely dried up. The lake falls within the coordinates 27°10′0.6″ N to 27°10′1.6″ N and 88°28′12.3″ E to 88°28′14.9″ E, covering an area of roughly 1900 m². A road borders the lake on its northern and eastern sides, while the Norbu Tsholing Monastery lies immediately to its south. Nagi Lake is located near Namthang town, approximately 60 km southwest of Gangtok and 42 km west of Jorethang, close to the village of Nagi Pamphok. The Sikkim state has an estimated 3380.03 km² of forest and tree cover. In addition, about 44% of Sikkim’s total geographical area consists of scrublands, alpine pastures, and zones that remain permanently under snow.

2.1. Topography of the Study Area

The study area surrounding Nagi Lake exhibits a rugged and steeply undulating Himalayan terrain. The Digital Elevation Model (DEM) shown in Figure 1 reveals that the regional elevation ranges from approximately 133 m to over 5600 m, although the local topography around Nagi Lake is confined within moderate elevation ranges of 1400–2000 m. The lake itself is situated at an elevation of around 1580 m above mean sea level, positioned on an east–west trending ridge. The slopes around the lake are moderately steep, draining towards the surrounding valley floors through a network of first-order and second-order streams, many of which eventually contribute to the Lhonak and Tista River systems visible in the map (Figure 1) [33].

The Main Central Thrust (MCT) and associated regional lineaments mapped across the broader area have strongly influenced the geomorphology, producing linear valleys and structurally controlled drainage patterns. These structural features, combined with the rugged terrain, contribute to the region’s susceptibility to slope instability and seismic amplification.

2.2. Hydrology of the Study Area

Based on the groundwater resource data provided in the CGWB 2023 technical report, the hydrology of Namthang Block in Namchi District, Sikkim, indicates that groundwater availability is largely controlled by rainfall recharge and the mountainous terrain of the region. The total geographical area of Namthang Block is 9869 ha, of which about 4387 ha consists of hilly terrain, while 5482 ha represents the recharge-worthy area where groundwater replenishment can occur. The major contribution to groundwater recharge comes from rainfall, with approximately 487.02 hectare-meters during the monsoon period and 153.97 hectare-meters during the non-monsoon period, whereas recharge from other sources is relatively minor, contributing only 12.01 ham during monsoon and 1.08 ham during the non-monsoon season. The total annual groundwater recharge of the block is estimated to be 654.08 ham, indicating that precipitation is the dominant hydrological driver in the region. Figure 2 shows the annual variations in rainfall in Sikkim state. Of this recharge, about 65.41 ham is lost through natural discharge, leaving 588.67 hectare-meters as the annual extractable groundwater resource. Current groundwater utilization is relatively low, with 27.12 hectare-meters used for irrigation, 19.31 ham for industrial purposes, and 8.87 ham for domestic use, resulting in a total extraction of about 55.30 hectare-meters. Considering projected domestic requirements up to 2025 (about 9.20 ham), the net groundwater availability for future use remains high at approximately 533.04 hectare-meters. The stage of groundwater extraction is only about 9.39%, which places the Namthang Block in the “Safe” category, indicating that groundwater resources are presently underutilized and remain sustainable under current extraction levels. Overall, the hydrological regime of the area is primarily rainfall-fed with moderate recharge potential and relatively low groundwater development [34]. Figure 3 shows the litholog of the Nagi Lake area based on apparent resistivity of subsurface strata up to 35 m.

Figure 2. The annual rainfall variation in Sikkim (Source: IMD, Pune) [35].

Figure 3. The Litholog section of Nagi Lake area based on resistivity.

2.3. Geology of the Study Area

The study area lies within Namchi (South Sikkim), part of the Eastern Himalayas in the northeastern region of India. The region is situated in a high seismic hazard zone (Zone VI) as per BIS 1893 (Part 1), 7th revision—2025, owing to its location within the tectonically active Himalayan belt [2].

The area forms part of the Proterozoic Lesser Himalayan Sequence (LHS), which primarily consists of low-to-medium-grade metamorphic rocks. The dominant geological formation in the vicinity of Nagi Lake belongs to the Daling Group, specifically the Gorubathan Formation. The Chlorite–Sericite Schist and Quartzite lithology characterize this formation, indicating extensive regional metamorphism and deformation [33].

Field exposures shows that the phyllitic and schistose rocks are moderately to highly weathered and altered. The schistosity planes generally strike northwest to southeast (NW–SE) with a moderate dip towards the east. To the southeast of Nagi Lake, amphibolite rocks of the Triassic–Jurassic age are observed, in addition to meta-greywacke of the Proterozoic age belonging to the same Gorubathan Formation. Towards the west, the Rangit Window is demarcated by the Ramgarh Thrust, exposing Permian-aged Gondwana rocks (Bhareli and Damuda formations) and Proterozoic–Cambrian Buxa Group rocks. The Gondwana Group formations (Lower Gondwana, Rangit Pebble Slate, and Bhareli–Damuda formations) occur as tectonic windows within the older Daling and Buxa Groups. These represent sedimentary successions deposited in continental to shallow-marine environments during the Carboniferous to Permian periods. The Kanchenjunga Gneiss and Chungthang Formation belonging to the Central Crystalline Gneissic Complex (Proterozoic) form the higher-grade metamorphic basement in the region [36]. Figure 4 shows the geological formations of the study area.

Figure 4. Geological map (lithostratigraphy) of Nagi Lake and its surroundings (Source: GSI, 2025) [31].

3. Methodology

A total of seven survey lines (L1–L7) as shown in Figure 5 were conducted across the surface of the Nagi Lake area (Figure 6), and the spacing strategy was designed considering site-specific constraints, including irregular lake geometry and non-uniform embankment thickness. A closer spacing of 2 m was adopted between L1–L2 and L6–L7 to extend lateral coverage while maintaining linear continuity of the survey grid. The remaining lines were spaced at 4 m to achieve an optimal balance between resolution and coverage. Geoelectrical resistivity profiling was conducted sequentially along each of these lines (L1 to L7), using the Wenner electrode array configuration, and the obtained resistance (R) data are summarized in Table S1, with C₁ and C₂ as current electrode positions, through which the current is injected into the ground, and P₁ and P₂ as potential electrode positions, which measure the potential difference. The Wenner configuration was chosen due to its strong signal-to-noise ratio, suitability for detecting lateral and vertical resistivity variations, and its effectiveness in delineating subsurface fractures in heterogeneous geological settings.

Figure 5. Image and sketch of the outline of Nagi Lake showing the reference lines (L1 to L7) of geoelectrical resistivity profiling.

Figure 6. Geoelectrical resistivity profiling in progress.

For the present study, electrical measurements were acquired using the Aqua-meter CRM-500 instrument (Anvic Systems, Pune, India) for line profiling, which displays the resistance value as output. The instrument was set on 4-cycle mode for repeated current injection to get good averaged data, improving the signal-to-noise ratio of each single measurement. The electrodes were hammered well into the ground at each point to get the good contact between electrodes and the ground and to reduce the contact resistance. The survey begins from one end of the line with 1 m separation between two adjacent electrodes shifted along the survey line, maintaining the configuration until the whole length is covered and then repeating with 2 m separation between adjacent electrodes, then 3 m separation and so on up to the separation permitted by the line’s length (Table S1). All the four electrodes were aligned along these straight lines within the same horizontal plane. The check dam at the eastern margin of the lake is used as a reference origin point for each line except L1. Each line represents a distinct transect across the Nagi Lake basin, and the measurements obtained along these profiles provide insights into the vertical variations in subsurface resistivity.

In electrical resistivity surveys, field measurements are acquired as resistance, whereas the pseudo-section is conventionally expressed in terms of apparent resistivity to account for electrode geometry and subsurface current flow. For each survey line, the field-recorded resistance values, along with their corresponding electrode configuration, were imported into ResIPy (Version: 3.6.6), a specialized software for geoelectrical data inversion and visualization [37]. Using the software’s algorithms these datasets were then processed and 2D apparent resistivity(ρ_a) pseudo-sections, along with inversion models, were generated for all seven lines. Alternatively, we can obtain apparent resistivity (ρ_a) values by multiplying resistance (R) with the geometric factor (G) (Equation (1)):

$${\rho }_a = \mathrm{R}\ast \mathrm{G}$$

(1)

where G can be calculated by the following formula [38,39]:

$$\mathrm{G}=\mathsf{\pi }{\displaystyle \frac{{(\mathrm{C}1\mathrm{C}2/2)}^2-{(\mathrm{P}1\mathrm{P}2/2)}^2}{2(\mathrm{P}1\mathrm{P}2/2)}}$$

(2)

where $\mathrm{C}1\mathrm{C}2$ is the separation between current electrodes and P1P2 is the separation between potential electrodes.

Specifically, for the Wenner configuration, the geometric factor can be derived by using a as the constant spacing between all adjacent electrodes in Equation (2):

$$\mathrm{G}=2\pi a$$

(3)

About one month later (between 22 and 26 November), we repeated the profiling survey on the same survey lines and also performed a Vertical Electrical Sounding (VES) survey for further confirmation. We focused mainly on line 5 and line 6 as these two lines have shown more prominent low-resistivity anomalies during previous surveys.

For VES, the Schlumberger Electrode Configuration was used. From the center of each line, the current electrodes (A, B) were moved away on both sides symmetrically, starting with 1 m separation from the center, i.e., AB/2 = 1 m, till the ends of the line. And the potential electrodes (M, N) were positioned accordingly about the center of the spread (data shown in Table S2). The acquired resistance data were then processed and (ρ_a) vs. AB/2 were plotted via the software IPI2Win (Version: 3.0.1.a) (http://geophys01.geol.msu.ru/ipi2win.htm, accessed on 30 November 2025) [40]. The software provides the number, thickness, and resistivity of sub-layers. The apparent resistivity (ρ_a) was calculated by the same method as mentioned above, using the geometric factor (Equation (2)). Here, C1C2 is represented as AB and P1P2 as MN. The Nagi Lake area is not uniformly flat; therefore, VES and resistivity profiling could be carried out only over relatively flat and accessible portions of the lake bed to avoid distorted or biased measurements caused by uneven topography and unstable electrode contact. Further, due to the restricted dimensions of the lake basin, establishing a continuous hydraulic linkage between shallow low-resistivity zones (0–9 m) and deeper fracture systems (8–30 m) using resistivity profiling alone is beyond the scope of this study.

To minimize errors during measurements with the Aqua-meter CRM-500, stable electrode contact is ensured, cables and connections are routinely checked, and the instrument is calibrated to prevent drift. Repeat measurements were performed at selected stations to ensure consistency, and contact resistance was monitored during acquisition to maintain acceptable limits. Noisy or unstable readings were filtered prior to inversion.

4. Results

4.1. Profiling Survey

The generated pseudo-sections and their inversion illustrate how resistivity varies with pseudo-depth beneath each transect, enabling the identification of anomalous zones such as fractures, saturated layers, or weathered materials. The resulting pseudo-section plots for lines 1 to 7 are presented in Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13, respectively, along with their inverted modal section. The pseudo-section represents the raw measured data, while the inversion model section represents the interpreted subsurface resistivity structure derived from those data. Thus, the inversion model is generated from and constrained by the apparent resistivity pseudo-section.

4.1.1. Line 1 Profiling

Electrical resistivity profiling along line 1 (length: 56 m) produced an apparent resistivity pseudo-section extending to a pseudo-depth of about 7 m (Figure 7a). Both the pseudo-section and the inversion model show a low-resistivity zone (<450 Ω·m) from the surface to nearly 4 m depth, indicating relatively high subsurface moisture content. Such low resistivity values generally correspond to moist, porous, clay-rich, or weathered/fractured materials. The inversion results (Figure 7b) confirm that this conductive zone represents a genuine subsurface feature rather than a measurement artifact. The vertical continuity of the anomaly suggests downward infiltration of moisture, likely from rainfall, runoff, or residual lake water, indicating that the near-surface layers consist of permeable materials allowing vertical percolation and temporary moisture storage within the upper ~4–5 m.

Figure 7. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 1, depicting subsurface resistivity variations.

Figure 7. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 1, depicting subsurface resistivity variations.

4.1.2. Line 2 Profiling

Electrical resistivity profiling along line 2 (length: 73 m) produced a pseudo-section with an investigation depth of about 9 m (Figure 8a). Both the pseudo-section and inversion model show a distinct low-resistivity zone (<450 Ω·m) in the central part of the profile extending from the surface to nearly 5 m depth, indicating moisture-rich subsurface materials. Such low resistivity values commonly correspond to clay-bearing sediments, weathered formations, or fractured zones with high moisture content. The inversion model (Figure 8b) confirms that this conductive zone represents a real subsurface feature. The gradual increase in resistivity with depth suggests that moisture is mainly concentrated in the shallow layers, likely due to downward infiltration from rainfall, runoff, or residual lake water, while deeper layers appear relatively drier and more resistive.

Figure 8. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 2, depicting subsurface resistivity variations.

Figure 8. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 2, depicting subsurface resistivity variations.

4.1.3. Line 3 Profiling

Electrical resistivity profiling along line 3 extends for a total length of 74 m with an effective depth of investigation of approximately 9 m (Figure 9a). Both the apparent resistivity pseudo-section and the corresponding inversion model reveal a prominent low-resistivity zone (<450 Ω·m) concentrated in the central portion of the profile. Such low resistivity values commonly indicate the presence of moisture-rich sediments, clay-bearing materials, or highly weathered subsurface layers with elevated pore-water content. The inversion section (Figure 9b) shows that this low-resistivity anomaly extends from the near surface to a depth of approximately 5–6 m, suggesting that the central segment of the profile contains relatively saturated or weakly consolidated materials. In contrast, comparatively higher resistivity zones toward the flanks and at greater depths likely correspond to more compact sediments, less saturated materials, or partially weathered bedrock. The vertical variation in resistivity, characterized by lower resistivity in the upper-to-mid subsurface and relatively higher values with increasing depth, indicates gradual changes in lithology and moisture distribution within the subsurface. This pattern suggests that the central part of line 3 may act as a localized zone of enhanced infiltration or subsurface water accumulation, possibly controlled by variations in sediment composition, permeability, or minor structural discontinuities. Overall, the resistivity distribution along line 3 highlights heterogeneity in subsurface moisture conditions beneath the lakebed, with the central segment representing a relatively more conductive and potentially water-saturated zone compared to the surrounding areas.

Figure 9. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 3, depicting subsurface resistivity variations.

Figure 9. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 3, depicting subsurface resistivity variations.

4.1.4. Line 4 Profiling

Electrical resistivity profiling along line 4, with a total length of 74 m, provides subsurface imaging to an approximate depth of 9 m (Figure 10a). Both the apparent resistivity pseudo-section and the corresponding inversion model indicate a distinct low-resistivity zone (<400 Ω·m) located near the central portion of the profile. In geoelectrical investigations, such low resistivity values are generally associated with moisture-rich sediments, clay-bearing materials, or highly weathered subsurface layers with elevated pore-water content. The inversion section (Figure 10b) further reveals that this conductive zone extends from near the surface to a depth of approximately 5–6 m, forming a vertically continuous anomaly beneath the central part of the profile. This feature suggests the presence of relatively saturated or weakly consolidated sediments in comparison with the surrounding subsurface materials. In contrast, higher resistivity values observed toward the margins of the profile and at greater depths likely correspond to more compact sediments, less saturated layers, or partially weathered bedrock. The gradual increase in resistivity with depth indicates a transition from relatively moisture-rich near-surface materials to comparatively drier or more consolidated subsurface layers. Such resistivity variations reflect heterogeneity in lithology and moisture distribution beneath the lakebed. Overall, the central segment of line 4 represents a localized conductive zone, which may act as a preferential pathway for moisture accumulation and limited vertical infiltration within the subsurface.

Figure 10. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 4, depicting subsurface resistivity variations.

Figure 10. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 4, depicting subsurface resistivity variations.

4.1.5. Line 5 Profiling

Electrical resistivity profiling along line 5, extending over a length of 72 m, provides subsurface imaging to an approximate depth of 9 m (Figure 11a). The apparent resistivity pseudo-section and the corresponding inversion model reveal a distinct low-resistivity zone (~300 Ω·m) occurring primarily in the lower part of the profile. In geoelectrical investigations, such low resistivity values commonly indicate moisture-rich sediments, clay-bearing layers, or highly weathered materials containing elevated pore-water content. The inversion section (Figure 11b) shows that this conductive anomaly is more pronounced at depth and gradually narrows upward, terminating at approximately 1 m below the ground surface. This geometry suggests that the conductive zone represents a localized accumulation of moisture within the subsurface, possibly controlled by lithological variations, reduced permeability layers, or zones of weathered sediments that retain water.

In contrast, moderately higher resistivity values observed toward the near-surface and along the flanks of the profile may correspond to relatively drier or more compact materials. The upward tapering of the conductive zone therefore reflects a localized subsurface moisture concentration rather than evidence of upward groundwater discharge. Instead, the resistivity distribution likely indicates moisture retention within a confined subsurface layer or a perched saturation zone beneath the survey line. Overall, the resistivity pattern along line 5 highlights heterogeneity in subsurface moisture distribution, with the deeper conductive zone representing a relatively saturated horizon beneath comparatively less conductive near-surface materials. However, in the absence of detailed hydrogeological data, the interpretation derived from the geoelectrical results should be considered indicative, and additional geological or hydrological investigations would be required to further constrain the subsurface processes influencing the observed resistivity variations.

Figure 11. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 5, depicting subsurface resistivity variations (October 2025).

Figure 11. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 5, depicting subsurface resistivity variations (October 2025).

To verify the stability of the observed resistivity pattern, the survey along line 5 was repeated after approximately one month. The resulting pseudo-section and inversion model (Figure 12a,b) show a resistivity distribution very similar to that obtained during the initial survey, with only minor variations in apparent resistivity values. Although light rainfall occurred a few days prior to the second survey, it did not significantly influence the subsurface moisture conditions. The consistency between the two datasets suggests that the identified conductive zone represents a relatively stable subsurface feature rather than a short-term response to recent precipitation.

Figure 12. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 5, depicting subsurface resistivity variations (November 2025).

Figure 12. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 5, depicting subsurface resistivity variations (November 2025).

4.1.6. Line 6 Profiling

Electrical resistivity profiling along line 6, approximately 65 m in length, provides subsurface imaging to a depth of about 8 m (Figure 13). The profile is located roughly 4 m north of line 5, and the overall resistivity pattern shows similarities to that observed along the adjacent line. Both the apparent resistivity pseudo-section and the corresponding inversion model indicate the presence of a distinct low-resistivity zone (~300 Ω·m) extending from deeper levels and gradually narrowing upward, terminating at approximately 1 m below the ground surface. Such low resistivity values are generally associated with moisture-rich sediments, clay-bearing layers, or highly weathered materials containing elevated pore-water content. The geometry of the conductive anomaly suggests a localized zone of enhanced moisture accumulation within the shallow subsurface, possibly controlled by lithological heterogeneity, variations in permeability, or reduced vertical drainage through less permeable layers. In contrast, relatively higher resistivity values observed toward the near-surface and along the margins of the profile likely correspond to comparatively drier or more compact subsurface materials. The similarity of the resistivity structure to that observed along line 5 indicates that the conductive zone may represent a laterally continuous moisture-enriched horizon beneath this part of the study area rather than an isolated feature.

Figure 13. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 6, depicting subsurface resistivity variations (October 2025).

Figure 13. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 6, depicting subsurface resistivity variations (October 2025).

To assess the temporal stability of the resistivity pattern, the survey along line 6 was repeated after approximately one month (Figure 14a,b). The repeated pseudo-section and inversion model exhibit a resistivity distribution broadly similar to that obtained during the initial survey, with only minor variations in apparent resistivity values. The overall geometry and depth extent of the conductive anomaly remain largely unchanged, indicating that the observed resistivity structure represents a relatively stable subsurface feature. These results suggest that short-term environmental factors, such as light rainfall occurring a few days prior to the second survey, did not significantly influence the subsurface moisture conditions in this area.

Figure 14. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 6, depicting subsurface resistivity variations (November 2025).

4.1.7. Line 7 Profiling

Electrical resistivity profiling along line 7, the shortest survey line with a length of approximately 52 m, provides subsurface imaging to a depth of about 6.5 m (Figure 15a). The profile is located roughly 2 m north of line 6 and about 6 m from line 5. Both the apparent resistivity pseudo-section and the corresponding inversion model (Figure 15b) show a relatively low-resistivity zone confined mainly to the shallow subsurface, with resistivity values gradually increasing with depth. In geoelectrical investigations, such low resistivity values in the near-surface layers are typically associated with moisture-rich soils, clay-bearing sediments, or weathered materials containing elevated pore-water content. The gradual increase in resistivity with depth suggests a transition to comparatively drier or more compact subsurface materials, reflecting changes in lithology and moisture distribution beneath the survey line.

Figure 15. (a) Apparent resistivity pseudo-section and (b) inversion modal section of electrical resistivity profiling along line 7, depicting subsurface resistivity variations.

Unlike lines 5 and 6, which exhibit deeper and vertically converging conductive anomalies, line 7 does not show a pronounced low-resistivity zone extending into deeper layers. The conductive zone is largely restricted to the upper part of the profile, indicating the absence of significant subsurface moisture accumulation at depth along this line. This contrast suggests that the moisture-enriched zone identified in lines 5 and 6 is spatially limited and does not extend into the area covered by line 7. Overall, the resistivity distribution along line 7 reflects typical near-surface moisture conditions and background subsurface characteristics within the study area, providing a useful reference for distinguishing localized conductive anomalies observed in adjacent profiles from the general hydrogeophysical setting.

4.2. Vertical Electrical Sounding (VES)

Vertical Electrical Sounding (VES) surveys were conducted along all profile lines to identify subsurface anomalies, particularly fracture zones occurring below a depth of 8 m. The VES measurements provided reliable resistivity information down to approximately 30 m depth; deeper investigation was not feasible due to the limited lateral extent of the lake area. Detailed interpretation was focused on lines 5 and 6, where prominent subsurface features were observed.

The log–log plots of apparent resistivity (ρ_a) versus half-current electrode spacing (AB/2) were used to interpret subsurface conditions, with fracture zones identified by deviations from the expected smooth resistivity curve [23]. The interpreted VES curves for lines 5 and 6 (Figure 16 and Figure 17) exhibit nearly identical trends, suggesting similar subsurface resistivity structures beneath both profiles.

Figure 16. Apparent resistivity vs. AB/2 plot of line 5. ρ is resistivity in Ω·m, h is layer thickness in meters, d is depth from surface in meters, and Alt is altitude in meters. Green circle is showing the fracture signature.

Figure 17. Apparent resistivity vs. AB/2 plot of line 6. ρ is resistivity in Ω·m, h is layer thickness in meters, d is depth from surface in meters, and Alt is altitude in meters. Green circle is showing the fracture signature.

Both VES curves indicate the presence of significant anomalies below approximately 14 m depth characterized by apparent resistivity values ranging from 1504 to 2234 Ω·m. These anomalies are interpreted as fracture networks within the subsurface. Such fractured zones likely act as preferential pathways for moisture movement and groundwater movement, and may represent potential seepage zones or groundwater sources. The presence of these fracture-controlled pathways could explain the observed similarity in moisture retention across the investigated sites.

4.3. Azimuthal Survey

An azimuthal resistivity survey was obtained in a previous study [23] at a significant seepage site, indicated by low-resistivity anomalies, aiming to ascertain the orientation of the fracture. Apparent resistivity was measured 2 m and 6 m away from the center along each marked direction line. The results indicate that the apparent resistivity is significantly lower along the seepage pathway than in other directions. This observation is supported by the polar plot of apparent resistivity versus azimuth (Figure 18), which clearly shows maximum resistivity in the southeast direction and minimum values in the northeast direction. Based on this analysis, the dominant fracture orientation is interpreted to be along N50° E. Since groundwater movement in hard rock terrains is largely controlled by fracture systems, the inferred seepage direction corresponds closely with the identified fracture trend, as illustrated by the red double arrow in Figure 18. Furthermore, geological outcrops in the vicinity of the lake display a dip direction of N40° E, indicating that variations in hydraulic head may cause the seepage direction to range between N50° E and N40° E.

Figure 18. Apparent resistivity vs. azimuth plot showing direction of the identified fracture (red arrow) (Source: [23]).

5. Discussion

Electrical resistivity surveying is based on the principle that subsurface materials exhibit characteristic resistivity values governed by factors such as moisture content, porosity, lithology, degree of weathering, and the presence of structural discontinuities, including fractures and joints. Accordingly, spatial variations in resistivity provide indirect constraints on subsurface hydrological and geological conditions [4,41]. In the present study, the geoelectrical data provide indicative insights into subsurface conditions that may be associated with the observed drying of Nagi Lake. However, it is important to emphasize that resistivity data alone do not uniquely constrain subsurface processes, and interpretations should therefore be regarded as non-unique and conditional. Low-resistivity zones identified in the electrical resistivity profiles are commonly associated with moisture-rich sediments, clay-bearing layers, weathered materials, or zones of enhanced porosity. Delineation of such conductive regions helps identify areas that may act as zones of moisture retention or preferential subsurface pathways, although their specific hydrogeological role cannot be established without independent validation [21].

The resistivity profiles along lines 1–4 and line 7 are characterized by relatively low resistivity values (<450 Ω·m) primarily confined to the shallow subsurface (~5 m depth). This distribution is consistent with near-surface moisture retention and shallow infiltration processes. The absence of laterally extensive or vertically continuous conductive features at depth suggests limited evidence for significant deep subsurface moisture accumulation along these profiles. In contrast, the resistivity profiles along lines 5 and 6 exhibit lower resistivity values (~300 Ω·m) extending across a greater depth range. The conductive zones display a gradual upward tapering geometry. Such patterns may indicate localized subsurface moisture accumulation; however, alternative explanations, such as lithological heterogeneity, clay-rich horizons, or variations in porosity and permeability, are equally plausible. In this context, the observed anomalies are interpreted cautiously and are not taken as definitive evidence of specific flow mechanisms or fracture-controlled seepage. The occurrence of similar conductive features along lines 5 and 6, and their relative absence along adjacent line 7, suggests spatial variability in subsurface properties, potentially controlled by stratigraphic or structural heterogeneity. Comparable resistivity patterns have been reported in tectonically active regions, where variations in lithology, weathering, and discontinuities influence subsurface moisture distribution, although such interpretations remain inherently non-unique [3,22,23]. The VES results along lines 5 and 6 show broadly consistent resistivity characteristics, indicating laterally comparable subsurface conditions within this sector of the study area. Deviations in the VES curves below ~14 m depth may reflect changes in material properties, such as increased weathering or fracturing; however, these features cannot be uniquely attributed to enhanced permeability or groundwater pathways without corroborating geological or hydrogeological data. Given the inherent ambiguity of resistivity interpretation, low-resistivity anomalies may arise from multiple causes, including saturated soils, clay-rich units, fine-grained sediments, or water-bearing discontinuities [41]. Therefore, the interpretations presented here are intentionally conservative and should be considered as working hypotheses rather than definitive conclusions.

To reduce interpretational uncertainty, future investigations will include repeat resistivity measurements using a time-lapse approach to evaluate temporal variability in subsurface conditions. Integration with borehole data, lithological logging, and direct hydrological measurements will be essential for validating the inferred features and for developing a more robust understanding of the processes influencing water loss from Nagi Lake. From a management perspective, potential interventions such as clay lining, targeted grouting, or enhancement of groundwater recharge may be considered. However, such measures should only be implemented following detailed, site-specific hydrogeological investigations to ensure their appropriateness and effectiveness.

6. Conclusions

The following conclusions can be drawn from the present investigation:

• Electrical resistivity profiling conducted at Nagi Lake successfully delineated subsurface zones with contrasting moisture conditions and permeability characteristics, demonstrating the utility of geoelectrical methods for investigating lake–groundwater interactions in data-scarce Himalayan environments.
• Shallow low-resistivity zones observed along profiles L1–L4 and L7 are primarily confined to the upper ~5 m and are consistent with near-surface moisture retention and infiltration-driven percolation under prevailing hydrological conditions. However, these interpretations remain non-unique and may also reflect lithological influences.
• In contrast, profiles L5 and L6 exhibit relatively lower resistivity values extending to depths of ~9 m. These features may indicate localized subsurface moisture accumulation; however, alternative explanations such as lithological heterogeneity, clay-rich layers, or variations in porosity and permeability are equally plausible. No definitive evidence is found to support upward-directed or pressurized groundwater discharge.
• Vertical Electrical Sounding (VES) results, particularly beneath profiles L5 and L6, reveal subsurface resistivity anomalies below ~14 m depth with apparent resistivity values ranging from approximately 1504 to 2234 Ω·m. These features are interpreted as possible fractured or discontinuous bedrock zones that may facilitate preferential groundwater and moisture movement beneath the lake basin.
• The spatial confinement of conductive anomalies to specific survey lines indicates that subsurface moisture distribution and potential seepage pathways beneath Nagi Lake are heterogeneous and likely controlled by localized structural or stratigraphic factors, with limited lateral continuity across the basin.
• The study also highlights the inherent non-uniqueness of geoelectrical interpretations. Variations in resistivity may result from multiple factors, including lithology, moisture content, clay fraction, and pore fluid conductivity; therefore, the identified anomalies should be considered indicative rather than definitive evidence of subsurface seepage pathways.
• A key limitation of the present study is the reliance on single-season geophysical measurements without independent hydrogeological validation such as borehole data, lithological logging, or direct seepage measurements. Consequently, the interpretations remain preliminary and require further verification.
• Future investigations should integrate time-lapse resistivity surveys, drilling and lithological characterization, tracer studies, and continuous hydrological monitoring to better constrain seasonal variability and validate the inferred subsurface structures. Repeated resistivity profiling along profiles L5 and L6 during subsequent non-rainy seasons would be particularly valuable in distinguishing transient rainfall-induced anomalies from persistent subsurface hydrological features.