Groundwater Detection Using Resistivity at Nubutautau Village in Viti Levu in Fiji

Abstract

A geophysical method, electrical resistivity tomography, was applied to identify potential groundwater-bearing zones around Nubutautau village on Viti Levu island, Fiji. Apparent resistivity data of the subsurface were collected through an electrode assembly along survey lines by injecting current into the subsurface using an ABEM Terrameter LS2. The apparent resistivity data were inverted using Res2DINVx64 software to produce the final electrical resistivity through an iterative process to compare the resistivity of layers and draw analogical hydrogeological results. Analysis revealed the presence of two potential groundwater-bearing zones as potential targets for future drilling. The two targets indicated the presence of potentially saturated vertical fractures through which infiltrating rainwater percolates through the volcanic rock towards a deeper basal aquifer. The identification of the two potential targets demonstrated great potential of this geophysical technique to effectively inform drilling operations. A scientific approach can increase the successful delivery of water security interventions in remote, drought-prone communities of the Pacific.

1. Introduction

Geophysical techniques provide preassessment of groundwater drilling work for making informed decisions on multiple platforms, such as technical, economic, and social, for the development of groundwater resources. Groundwater is an alternative source of water to the systematized, treated scheme for most communities globally, and it is still expanding boundaries in many facets of the development and implementation stages [1,2,3,4,5]. The detection of groundwater through multiple geophysical methods has developed over the years with the use of electronic instruments and consistent upgrades in technology. The resistivity survey method works with the inverse of conductivity principles to analyze geological zones correlating with hydrogeological parameters, leading to interpretations of where the best water-bearing zones might be. Resistivity comparisons provide scientific understanding of belowground parameters from which multiple inferences can be made, such as a predrilling assessment of water boreholes; locating buried objects such as metallic pipes; and identifying leaks, identifying aged boundary pegs and locators, and classifying and identifying different rocks within the geological layers by assigning them resistivity values [6,7,8].

Groundwater sources are extensively used in many parts of the globe due to their major advantage of being less prone to being affected by drought than systemized and surface water. Further advantages of groundwater are evident in many respects, such as consistent temperature, widespread availability, limited vulnerability, excellent natural quality, low cost of development, and reliability in times of drought. Other advantages of groundwater over surface water are that it is less affected by disasters and can be exploited when needed [9]. The effects of drought on groundwater may result in a temporary decrease in the groundwater table, which may result in a lower yield in terms of discharge than dams or natural rivers.

Case studies carried out in Nigeria revealed the success of geophysical resistivity exploration in locating fractured weathered zones that had been major groundwater development sites [10,11]. Studies carried out in India on sites such as Maheswaram watersheds in Andhra Pradesh using multiple electrode systems have further proved a success in locating water-bearing zones [12]. Researchers in southwestern Pacific countries such as New Zealand, which has many potential aquifers near the Canterbury region, have also used the same methods for groundwater exploration surveys to locate water resources and rechargeability through data interpretation [13,14].

Regional groundwater works have been carried out by the Secretariat of the Pacific Community (SPC), all through the use of geophysical surveys, to assist developing small-island countries. These include works carried out in Vanuatu after the passage of tropical cyclone Pam in the form of providing recommendations for the development of bore drilling [15]. Further works in strengthening water resilience have also been undertaken in certain other countries in the region, such as Kiribati and Solomons, to ensure the expansion of water resources. Locally, water assessment investigations through geophysical platforms have been run by the SPC in the western part of Viti Levu, Fiji, in the province of Ba and Rakiraki, as a response to the tropical cyclone recovery program. This program focused solely on the hydrogeological assessment of groundwater through resistivity methods [16].

Geophysical investigations carried out using standard survey procedures allow problem solving by identifying potential near-surface locations for future drilling locations. Surveys address the problem in the context of resistivity and conductivity by equating the parameters to hydrogeological logs for interpretation.

In this paper, we report on groundwater investigations that were carried out in Nubutautau village, Fiji to explore the possibility of potential water-bearing zones for the implementation of water development in the local community.

2. Materials and Methods

2.1. Site Description and Geology

Nubutautau village is situated on Viti Levu island, Fiji towards the inner highland, as shown in Figure 1. Nubutautau has an interesting underlying geology that exists close to the sharp boundary that divides the two different geologies across Viti Levu. Previous geological mapping and classification efforts suggest that the eastern part of the boundary is composed of thickly and thinly bedded sandstone, greywacke, and basal conglomerate, whilst the western part is covered by undifferentiated basaltic flows of the Ba volcanic group. Both these rock types are classified as being in volcanic groups [17]. The geological features of the underlying Vatukoro greywacke materials falling on the western part of the boundary includes sandstone with minor basalt and limestone. The geology represents aggressive magmatic events from the past in terms of strombolian and volcanic eruptions. The rock distributions over the near-surface points reveal that there were thick layers of lava spread across the area, with clay deposits sitting near the surface. It is interpreted from the topological features that lava domes were formed from eruptions and gave rise to many of the present structures. The Vatukoro greywacke was deposited in a series of smaller sub-basins from the erupting activity [18]. Crystalline volcanic mafic sandstone has been overlying certain other geological features, such as fractured uplift shales and siltstone [19]. Most of the exposed geological formations have weathered away due to the heat and convective activity from exposure to the tropical conditions [20].

2.2. Field Assessment

The field site was first assessed through lineament analysis using the Google Earth satellite tool. The analysis was used to view the orientation, topology, and known structural faults. Further, it required connecting points of the abrupt changes through geological features where the same orientations were observed, as shown in Figure 2. Three major lineaments were identified using Google Earth satellite imagery to match similarity in the features. Two of the lineaments were parallel, with an SW–NE trend, and one with an S–N trend, which was directly in line with a perennial spring located south of the village that discharges straight into Sigatoka River [17]. The usage of the lineament analysis was to match the topological and geological features using a desktop-based approach to narrow down the potential zones and kick-start the survey. The second stage of the assessment was based on physical inspection of zones picked by the lineament map, which required moving through the bushes, valleys, and mountains to assess the physical topography. This stage of the inspection was necessary to match the lineament analysis map and inconsistencies with the physical topological features. A better view in terms of the precise details about the topography provides a good analysis of existing features such as the perennial and intermittent creeks, dikes, ground slopes, and the type of soil drainage with soil density [21,22]. The analysis through the inspection of the target areas to work within was followed by the experimental setup, which required the assembly of the instruments with the specified procedure. Instrumentational field work was performed subsequently to check the desktop-based assessment and to verify and confirm a location.

2.3. Experimental Setup

The basic principle of operation in electrical resistivity methods is the injection of direct current (DC) into the ground using a pair of current electrodes. The voltage measured is converted into an apparent resistivity value for further interpretations. Different types of soil and geological formations have different corresponding resistivity responses as a function of the soil and rocks’ physical properties such as porosity, permeability, ionic charges of pore fluids, and clay mineralization. The resistivity was measured within different zones based on the lineament maps and physical assessment of the ground. The ABEM Terrameter LS2 resistivity setup consisted of four electrical cables rolls, each with 21 electrode takeouts positioning jumper connecting slots 5 m apart. The ‘roll-along’ technique was used to create seamless profiles longer than the four-cable spread along the survey lines. The resistivity method was used through the Multi-Gradient Electrode Array (MGEA) assembly, which stands as the electrode configuration to cover the selected zones. The major advantage of MGEA assembly is the favorable response of a good signal-to-noise ratio and better resolution to horizontal and vertical structures. The MGEA assembly reads the geoelectric data, performs self-computation through the loaded program, and stores them in the memory device, which are later retrieved for simulation purposes [23,24].

The experimental setup of 5 survey lines required positionings of an ABEM Terrameter LS2 termed as a station, which had 4 channels in operations connected to the cables overlaying the zones of the survey. Electrode pegs were 0.4 m in length and made of stainless steel materials to disallow any corrosion leading to the impurity of chemical reactions that may lead to pseudo anomalies in data. The ABEM Terrameter functioned as a mid-station, accommodating an equal number of cables to be laid on each side; typically, cables connected to the station on the left and right were termed as cable 2 and cable 3, respectively. Figure 3a shows the conceptualized diagram of 4 cable connections; the ABEM Terrameter is placed between cables 2 and 3. All 4 cables showed this general assembly set up for each new positions during the cable mechanics. The cable mechanics for the field movement required data stability and binding during each new positioning. It was noted that the movement between each consecutive position required 75% data binding or 3 out of 4 cables to remain pinned to the ground, while the 4th cable represented the 25% to be moved. This allows the data to be stable while the binding process continues throughout. The first position, which was noted, was to ensure the instrumentation responses and stability in terms of impedance response and current flow.

Each survey line had 20 electrodes, which injected current in the ground surface and received it back into the ABEM Terrameter LS2 in the memory storage device. The assembly of each cable with its sub connections to the electrode connections can be seen in Figure 3b. Cables 1 and 2 were on the left side of the ABEM resistivity device, while cables 3 and 4 were on the right. The cable mechanics were examined in two parts: one was in terms of being read by the ABEM Terameter and the other was being read in the field. The two readings were due to the ABEM Terrameter being coded such that it functions in between cables 2 and 3. The consecutive cables on each side were connected through a common electrode, which was jumpered twice with dual subconnections from survey cables (Figure 4) and was classified as an overlapping electrode (Figure 3a) for the continuity of the cable. Each cable catered 20 electrodes for the current to be injected into the ground surface; the 21st electrode functioned as an overlapping electrode on which the overlapping of the two cables from each side was conducted. The double jumping was to ensure the cable binding and free flow to current, countering the impedance found in different sections of the survey points. The cables were moved linearly through the cable mechanics along the path length of the ground until they covered the whole survey profile length; the mechanics required approximately 3–4 new positionings of the Terrameter LS2 device to complete each full profile. The ABEM Terrameter LS2 records the ground resistivity values and converts them to the apparent resistivity, which is later used by the Res2dinv64x software to produce multiple iterations with the least error in the results.

The profile lines were placed to collect geoelectric data through the locations on the field to obtain the cross-section of the zones in terms of resistivity and conductivity representations. The rationalization of using the survey lines was to scan the ground surface to 80 m bgl; this allowed the geological ground layer to be represented with significant features.

2.4. Data and Analysis

The apparent resistivity data were self-computed by the ABEM Terrameter while the field geoelectric data were being recorded simultaneously. These were the processed data in terms of device computation but still appeared in raw form to be downloaded on the hard drive for analysis by software. The files were in the DATs format, which required codes and formatting to open and process [25,26]. Specialized licensed software was used for handling the high volume of raw information (data) to produce resistivity tomograms. Res2Dinv64x software was used to assimilate and integrate elevation data from post-processed kinematics (PPK) picked up at 20 m intervals with geophysical data from the ABEM Terrameter LS2 to capture the exact geological profile for 75 m bgl. Certain technical steps were executed through the software, such as the application of the cut-off factor to adjust and eliminate outliers, followed by model discretization, inversion, and model refinement to allow data usage with various functions such as inversions to test responses of conductivity and resistivity. The volume of the data was reduced to the spacing electrode distance of half the value, from 5 m to 2.5 m, to accommodate the full data set for each of the profile lines. Statistical techniques were used to view the data graphically in terms of standard deviation and outliers, as a way forward for trimming is critically important to allow an efficient simulation within the software.

3. Results and Discussion

The second stage of the assessment was based on the physical inspection of the site, on which the actual verifications and recommendations were based. This stage of the inspection confirmed whether the lineament analysis map with probable features was consistent with the physical topological layers. The physical assessment topography highlighted certain features such as the perennial and intermittent springs, dikes, ground slopes, and the type of soil drainage and soil density [21,22]. The interpretations from the field lines revealed consistencies based on the near-surface point interactions and ground seepages.

The five resistivity survey lines revealed variable responses as influenced by the underlying geological rock formations through the presence of water saturation allowing conductivity variations. A total of four contrasting layers/areas were identified:

• A very low-resistivity layer, with a shallow depth down to 20 m (1–19 Ohm.m), identified as a low-permeability silty loam or clay or weathered siltstone that is partly or fully saturated, resulting in a low resistivity response.
• A zone of low resistivity (20–30 Ohm.m), interpreted as a fractured and/or weathered rock formation and represented as a vertical or near-vertical feature, expected to yield groundwater.
• A medium-resistivity layer (31–99 Ohm.m), suggesting a less-weathered volcanic or sandstone formation that may yield little to no groundwater.
• A high-resistivity layer (>100 Ohm.m) suggesting the presence of unweathered high-resistivity material, such as basaltic material, at the base was observed in survey lines 4 and 5, indicating low to no groundwater potential at depth [17]. The geophysical results displayed are in terms of the ERT along five survey lines, as presented in Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9.

3.1. Tomogram Observations

3.1.1. Survey Line 1

Survey line 1 was the first geophysical investigation carried out at Nubutautau village for geoelectric data collection; this was in the southwest to northeast direction (SW–NE), as shown in Figure 2. The total distance covered by the survey line was 500 m. The survey line started from the lower point on the lower hill and moved up slightly through the dry, sedimentary volcanic rocks where significant topological features were found such as dried creeks, dikes, etc. Figure 5 shows the resistivity profile of survey line 1 with the elevational topological features of the near-ground surface, which helped to match the significant features such as potential fractures formed by intrusive dikes associated with the volcanic, exposed areas. The entire survey line from 0 to 480 m showed low resistivity between the depths of 1 and 1.5 m bgl as a result of intense weathering of the top geological layers leading to the formation of thick, overlying clays that are moist in nature. The clays have a higher ability to hold moisture content revealed through the lower resistivity values of 1–10 ohm.m.

Most regions covered by survey line 1 showed a resistivity of 40–50 ohm.m and greater within the full horizontal profile of 500 m to a depth of 75 m bgl. This was classified as medium resistivity, which in the geological log indicates some possibility of groundwater presence within the zone. The possibility of freshwater existence may be present but is not fully indicative due to the equal distribution of resistivity. Portions of the survey lines with the depth extending as close to 25 m bgl with the elevation of 568–598 m msl revealed resistivity values ranging from 45 to 55 ohm.m. This could be due to the compact and impermeable characteristics of basalts, which do not allow groundwater to be held within their pores. It is, therefore, quite probable that shallow, low-resistivity zones near the ground surface contain some groundwater held within the clay by capillary forces [15].

The region “X” in the tomography reveals a deeper and organized dimension with a portion of lower resistivity, indicating hydrological features of the volcanic or lateral part of the basement aquifer in the confined zone. The defined enlarged region represented at point “X” has a resistivity value of 30 ohm.m at 25 m bgl corresponding to the length of 240–280 m within the survey line. The resistivity value of 30 ohms suggests weathered basalt with freshwater. In comparison with the other portions of the ERT for the same survey line, a significant shift in the resistivity values to a lower range reveals the possibility of existence for a water containment unit. Depiction of an obvious occurrence of the groundwater features corresponding to point “X” near the ground surface within the survey range of 230–280 m of the horizontal profile is obvious in the figure. The lower resistivity found at point X can be termed as a possibility of a confined aquifer with basaltic rocks and clays containing freshwater being revealed by a smaller portion of the adjacent aquifer captured in the ERT. This confined aquifer portion is overlaid by a thick bed of volcaniclastic geological rocks and clays. Hydrogeologically, it is inferred that there is the existence of a main aquifer adjacent to survey line 1.

3.1.2. Survey Line 2

Survey line 2 was laid in the southeast to northwest direction (SE–NW) orthogonal to survey line 1 and intersecting at 285 m (Figure 2). The tomogram showed a possibility of groundwater presence as per the lower resistivity within the range from 320 to 360 m shown in Figure 6 with the resistivity values ranging from 25 to 30 ohm.m. The orthogonal direction was chosen to cross checkpoint X from survey line 1 with the hypothetical approach and assumption that a part of the aquifer was captured but not fully indicated. Alternatively, it was also executed to verify if there was any other occurrence of groundwater in a region of lower resistivity adjacent to survey line 1. Substantial groundwater accumulation with promising drilling potential was interpreted at 240–360 m along the profile within the deeper half of the section to about 37.5–65 m bgl, as shown in the same figure. The resistivity range indicates gravels saturated with fresh water and weathered basalt, with freshwater present at the point. This revealed some interesting features with resistivity variation in the suitable range for groundwater development, labeled as point Y. The lower resistivity represented by point Y is connected to point X of the first survey line, demonstrating a conceptualized groundwater-bearing containment unit in a volumetric 3D existence of the aquifer through the very low resistivity value of about 15–30 ohm.m, revealing a water-bearing unit with sharp boundary features. It is certain that if drilling is performed at any point within the range of 320–360 m, a water table would be reached upon the drill rig reaching an average depth of 60–65 m bgl. All the other regions within the survey line revealed a high resistivity of 45–55 ohm.m and low-conductivity geological elements like basaltic materials [27]. Inferences about the water presence as detected and revealed by survey line 2 showed a water-bearing zone that has a possibility of higher porosity, storativity, and permeability within the confined aquifer, which may have a higher tendency to hold and retain water in large quantities and volumes [28,29,30,31]. The 340 m point with a range of 320–260 m is suitable for the placement of target 2, due to considerations to avoid destruction to vegetation near the adjacent perennial creek found closer to the endpoint of the survey line. It is possible during the future drilling of the point of interest at 340 m locations that water extraction would be obtained upon the drill rig reaching the minimum depth of approximately 40–60 m bgl.

The overlying layer from the near-surface depth of 0–50 m bgl has a higher level of resistivity within 0–320 m along the survey line, which consists of rock materials such as siltstone and conglomerates indicative of a drier zone with higher resistivity. Beyond the linear zone of 320 m of the survey line, the ground had a medium resistivity of 31–99 ohm.m to a depth of about 25 m bgl, as revealed by the green color image. This suggests that the water found within the depth of 40–60 m is rarely from the precipitation and surface infiltration mechanism due to the higher resistivity layers indicating poor ground permeability from the near-surface point [32]. As seen in target 1, the location is close to the sharp vegetation boundary of the downward slope, which provides evidence and an indication of how the internal groundwater flow feeds this valley leading to rich vegetation boundaries [33]. It is assumed that the water found within the adjacent perennial creek was from the seepage of the aquifer picked by survey line 2.

3.1.3. Survey Line 3

Survey line 3 was located close to the village cemetery running from the east to southwest direction (E–SW) direction of true north. The survey line was 400 m in length and 0.5 km from the village in the SW direction. The surface geological location along the line showed good clay and sand distributions within the first 100 m zone from the starting point and the later part varied slightly from rocks to other volcanic materials. Most of the near-surface ground features showed lower resistivity, from 0 to 19 ohms.m to a depth of 5 m bgl, for the entire length of the survey track due to the surface weathering of the rocks of the upper lying layer, which accounts for efficient infiltrations and ground saturation [34,35]. The geological area consisted of potential groundwater impoundment at the dikes, which was near the fractured zone at a point of 80 m. According to [36], surficial expressions such as eruptive fissures and cones may indicate the position of subsurface dikes. Moreover, when cones are elongated or aligned topographically, as observed in the study area, they are likely to reveal the presence of dike swarms [37]. Eruptive fissures and cones may have resulted in some fracturing along their edges, which may allow groundwater to occur at shallow depths. The small zones revealing a lower resistivity beyond 160 m onward with the near-surface groundwater movement flowing perpendicular to the survey line direction in the form of dry creeks or dikes are termed as perched water [38]. The hill existence was in the form of an elevated plateau, adjacent to survey line 3, and consisted of the village burial ground, which contained hard volcanic rocks and sandstone materials exposed to weathering with poor vegetation. This revealed the structure of the erupted volcanic features in the form of volcanic necks. The rock featured an exposed elevated plateau, which accounts for surface runoffs during severe weather conditions, allowing water to flow down to the lower part and feed into the region of 160–240 m of the survey line. The rain infiltration through the surface runoffs catches and retains water, keeping the clay moist, and eventually feeds into the perched aquifer leading to surface interactions. Within the first 45 m range, the lower resistivity indicates water presence to the depth of 15–18 m bgl, equating to 470 m msl. The survey range within the 60–120 m, as shown in Figure 7, indicates a deeper level of lower resistivity in terms of the huge lens of water due to the fractured feature of the vertical passage for water movement to the surface. The fractured point accounts for the lower resistivity and water presence forming potential target point 2. In the 60–120 m range, the 85 m point was assessed to be the best location with a lower resistivity of 0–19 ohm.m and an elevation height of approximately 485 m msl, as shown in the same figure. This fractured zone present at the site was due to the uplifting of the ground geology resulting from viscodynamic forces and compressional tension, and horizontal stresses leading to rock deformations and splitting [39,40]. The presence of high-resistivity, vertically oriented features cutting through the older volcanic deposits suggests recent volcanic activity, which has resulted in surficial, less-weathered volcanic deposits [16]

3.1.4. Survey Line 4

Survey line 4 and survey line 5 were run in the same area and in perpendicular directions to each other. Survey line 4 ran in the east to northwest direction (E–NW) direction, 400 m in total length, and was adjacent to the location where the old borehole was installed. Survey line 4 was indicative of a clay geological presence near the ground or on the surface, as shown by the resistivity value of 5 ohm.m (Figure 8). The thick clay distribution was mainly from the silty loam observed during the old borehole drilling within the depth of about 0–6 m bgl, spreading linearly to a range of about 150 m of the survey line and then decreasing in depth beyond a range of 160 m. The old borehole was drilled at the location of 160 m; the well was poorly inefficient in yielding a good rate of a water leak, before completely drying out. Lithological changes within the depth profile of the borehole have been observed and documented by SPC [17], as shown in

Table 1. Lithological changes with the variation in the geological profile for the old borehole near survey line 4 [17].

Formation	Geological Characteristics/Contrast	Depth Range (m)
Soil and weathered bedrock	Silty loam with highly or completely weathered siltstone	0–6
Fresh siltstone with minor fractures	Predominately gray, fine grained, well-sorted formation with low strength; some weathered material observed between 12 and 14 m indicating minor fracturing and potential groundwater sources	6–24
Weathered volcaniclastic formations	Moderate weathered and fractured formation with midrate strength characteristics by noticeably disclosed volcanic clast with brown to reddish brown matrix suggesting groundwater movement and occurrence	24–30
Fresh volcaniclastic formation with minor fractures	Hard and fresh volcaniclastic zones dominated by the volcaniclastic clast with minor fractures observed between 37 and 45 m; some weathered materials denoted	30–51

. This hydrogeologically suggests that a small amount of water obtained during drilling existed within a certain range of the sub geological profiles through seepages from the other sources, and this was separated by the impermeable rock layers [41]. Figure 8 reveals different layers, with the lower resistivity values ranging from 20 to 25 ohm.m from the surface to 20 m bgl depth, indicated by the pale blue indicator. This shows an interesting feature of multiple layers with increasing trends of resistivity values lying adjacent to one another being indicative of the possibility of the permeability and non-permeability of existing water movements through different layers. The previous groundwater investigation at the northeastern part of the village showed the underlying rock formation composed of siltstone up to 24 m of the survey range before a weathered and fresh basaltic unit was encountered between 24 and 51 m. The tomography also shows that the thicker layer of higher resistivity, varying 100–200 ohm.m from 30 to 40 m bgl, is interpreted as having almost zero water occurrence. The bottom high resistivity from 40 to 75 m bgl accounts for the heavy basaltic rock with a resistivity of >200 ohm.m, which later channels into the Sigatoka river and forms a thick bed of basaltic pillow lava with interbedded siltstone and sandstone [42]. The groundwater potential along this survey line is not recommended.

3.1.5. Survey Line 5

Survey line 5 was run in a similar zone to survey line 4 and in a perpendicular direction intersecting survey line 4 at 180 m (Figure 2). This line was laid in the NE–SW direction and was the shortest profile with a length of 385 m. The orthogonal direction was used to cross-check the possibility of any adjacent aquifer present from which water was moving towards the old borehole. The entire full profile of the survey line showed overlaying layers of thick clays, visible during the entire field survey work. As shown in Figure 9, the resistivity varied 20–25 ohm.m within the length of 0–80 m, detecting the presence of clay, silty sandstone, and surface freshwater in the form of perched features of water seepage. The range length of 80–385 m resistivity varied between 5 and 15 ohm.m; at approximately 50 m on the line was a region represented by rich clay and the presence of brackish water as there existed a small flowing creek and drain structure with little existence of water content. The water existence demonstrated was an indication of the surface water, which is seen as a slight pigmented point at approximately 50 m of the survey line on the resistivity profile. It reveals a thin, shallow, and unconfined groundwater body in existence and is believed to be a localized perched aquifer overlaying low-permeability weathered lava flows. Beyond the depth of 14–75 m bgl of the entire 385 m survey range, the resistivity values increased gradually, indicating the presence of zero water occurrences. The thicker bed of portion located at point “K” ranging between 120 and 280 m at approximately 30 m bgl reveals the presence of very high igneous metamorphic rocks such as those containing basaltic material. This bed of rock continues at the deeper level within the geology of Viti Levu, common to survey line 4, and is seen outcropping in the Sigatoka River [42]. The possibility of any groundwater occurrences on survey line 5 was absent, with zero recommendations for future drilling tasks.

The corresponding values of the resistivities of the geological ground materials can be equated to

Table 2. The resistivity value ranges in correspondence with the geological materials present below ground level (bgl) [16].

Rocks and Sediments Type	Resistivity Range (Ohm.m)
Clay containing brackish to saline water	<3
Clay containing brackish to fresh water	5–8
Clay, silty sand, and some gravel saturated with fresh water	11–25
Weathered basalt containing fresh water	30–60
Fresh basalt saturated with saline water	30–40
Fresh basalt saturated with fresh water	300–700
Dry coral sediments	500–1000

based on the rocks and sediment types present in the survey within 75 m depth.

The usage of survey profiles enabled the cross-feature analysis of below-ground-level aquifer identifications. The profiles revealed the resistivity response of the similar conductivity features of the water-bearing zones. Two potential locations were identified by the survey as recommended points of interest for the future. From this, it can be inferred that the assumption made based on the desktop study to form an analysis using the Qgis and satellite pictures to match the similar topological and geological features is supported.

3.2. Hydrochemistry of Selected Aquifers

The presence of springs within the investigation area suggests discharge of dike-impounded groundwater; these springs were surrounded by large vegetations and thick clay layers. The investigations locating the below-ground surface with the existing aquifer compartments was impossible due to the poor and insignificant surface leaks at a steady phase from below.

Three nearby springs were visited to collect water samples to test the geochemistry and the hydrochemistry of the existing aquifers for the Nubutautau location. These found springs were representative of the area of survey zones with similar geological features; the springs were the Nubutautau, Tauboto, and Yavuilagi springs. Streams connecting to these springs were generally dry except during strong rainfall events, and the unconsolidated colluvial deposits lying in streambeds act as infiltration areas for rainwater. All three springs showed similarity in the water constituents in terms of chemical composition. Ten chemical elements were tested to check the groundwater hydrochemistry charged by the chemical reactions of the rock water interactions passing through different passages within the aquifer.

Table 3. List of the chemical parameters that were tested from the three springs. The tested parameters were analyzed at the Institute of Applied Science laboratory at The University of the South Pacific, Fiji.

Customer ID Lab No	Spring 1 Nubutautau Spring 2020/3851	Spring 2 Tauboto Spring 2020/3852	Spring 3 Yavulagi Spring 2020/2853	Date Analyzed	Method Ref. No
Alkalinity (mg/L)	108	97.7	71.2	14/01/20	AP 2320B
Chloride (mg/L)	90	120	95	20/10/20	AP 4500-${\mathrm{C}\mathrm{l}}^{-}$B
Calcium (ug/L)	13.5	12.2	16.4	14/10/20	AP3111B
Electrical conductivity (uS/cm)	213	147	185	20/10/20	AP2510B
Iron (mg/L)	3.72	1.08	1.90	15/10/20	AP3113B
Magnesium (mg/L)	5.41	7.61	6.75	14/10/20	AP3111B
Manganese (mg/L)	<1	<1	1.17	15/10/20	AP 3113B
Silica (mg/L)	28.8	43.9	46.9	04/10/20	AP 4500-Si${\mathrm{O}}_2$D
Sodium (mg/L)	13.9	2.30	9.85	14/10/20	AP 3500-Na B
Sulphate (mg/L)	2.45	4.29	10.4	12/11/20	AP 4500-S${\mathrm{O}}_4^{2-}$ E

shows the list of chemical elements tested from all three springs, with amounts presented in mg/L. The laboratory analysis was performed at the University of the South Pacific, Suva, Fiji at the Institute of Applied Science. Ionic charges obtained were indicators of certain minerals present within the rocks that had broken into charges through the dissociation process. It is evident that this process yielded reactions within the geochemical aqueous medium in the splitting of the chemical components from compounds of minerals [43]. The chemical composition of the spring water samples reflects mainly the interaction of groundwater with feldspars and plagioclase feldspars contained in the sandstone and siltstone deposits, and possibly the dissolution of the underlying basaltic rock. The relatively high silica (Si) and total dissolved solids (TDS) content are likely attributed to groundwater residence time within the sedimentary and volcaniclastic formations where plagioclase feldspar may have been weathered into kaolinite clay, releasing cations and dissolved Si into the groundwater. The release of Ca, Mg, Cl, and Fe may also derive from the dissolution of basaltic rock contained in the weathered volcaniclastic rocks.

Elements constituents were all in close range to one another, suggesting that the groundwater in the confined zones had no contamination from any external pollutant activity due to the non-existence of chemical anomalies. It can also be interpreted that the three springs are from the common aquifers below ground level. Water found within the same aquifer have similar chemical reactions due to the same exposure and the amount of time they have been passing over the rocks and the soil materials [21,44]. Reactions within the rock–water interactions contain the cationic and anionic charges exchanged to form chemically different compounds. The conductivity or salinity of the groundwater samples indicates a moderate range, which further suggests that the water is acceptable for all purposes at a community level.

The chemical properties of the ground and the water represent the degree of reactions, which determine the quality of water in terms of the dissociated ions. The atmospheric precipitation of the rainfall allows certain C${\mathrm{O}}_2$ from the atmosphere to be dissolved, which gets percolated in the ground surface, picking up more C${\mathrm{O}}_2$ and forming a weak solution of carbonic acid. Carbonic acids chemically dissolve in water, changing the PH range to a lower scale and forming an acidic property of the infiltrated water [45]. In cases of acidic nature of groundwater, the volcanic rocks act as a neutralizing agent to effectively buffer the acidity.

Porosity and permeability in geological materials depend on the openings related to scoria deposits, breccia zones and cavities between flows, lava tubes, fractures, and lineament [14]. Hydrogeological parameters such as porosity and hydraulic conductivity tend to fluctuate depending on climate responses to hydrogeological rechargeability. Borehole yield in volcanic rocks typically depends on the rock type present based on other parameters such as well locations, topography, and geological responses to the hydrometeorological elements. These aquifer properties can only be calculated from pumping test data, which is beyond the scope of the survey [15]

3.3. Hydrogeological Assessment

The hydrogeological system investigated at Nubutautau village revealed a potential groundwater association with geological features such as dikes, and intermittent and perennials creeks. The vertical feature investigated at target 2 (Survey line 3) has shown an obvious indication of the groundwater moving up the fracture passage and water seeping out in a horizontal pattern to near-surface points into the shape vegetation. Target 1 (Survey line 2) reveals a large, shallow aquifer with a depth of approximately 37.5–65 m bgl, which is expected to hold a substantial amount of fresh groundwater. This target was found on the sloped terrain, starting from the upper point to the lower point towards the cliff and large valley river basin creek. It can be hydrogeologically conceptualized, water from the shallow aquifer moves towards the lower part of the survey line closer to the adjacent creek, which holds water from the seepages and runoffs.

Low-permeability layers (weathered or massive volcanic) allow groundwater to be held between the dikes or fine materials in filling these subvertical features. Groundwater potential in these contacts may vary over space and time and will be dependent on the storage capacity of the sedimentary formation towards the weathered interface. It can be conceptualized through the lineament topological near-surface vegetation features and the existence of the other shallow aquifers present within the vicinity of Nubutautau village, and this may be compartmentalized with poor flows due to partitioning from vegetation and geological settings [12].

From the drill log data of the old borehole mentioned in Table 1, it appears that fresh siltstone with minor fractures overlies a weathered volcaniclastic formation, which gradually becomes harder with depth (fresh basalt seen in survey lines 4 and 5). Groundwater occurrences within these units is likely controlled by the bedding of sedimentary units, as observed in the existing springs, and within the fractures and structures within the volcaniclastic units, as supported by the geophysical investigation conducted. The interface between weathered and fresh volcaniclastic formations extend around the area where survey lines 1, 2, and 3 seem to be particularly suitable for groundwater accumulation, as inferred by the survey. This formation, which is mainly composed of sandstone and siltstone, is likely thicker and much more weathered, allowing for a higher degree of saturation, as inferred by the resistivity ranges observed and tabulated in Table 2.

The shallow aquifers are the elongated points from the deeper indwelling groundwater-bearing units on which the interior of Viti Levu is placed. The hydrochemical analysis also supports the conceptualized groundwater interactions with the surface seepage flows from the main aquifers below ground level. This generalized water is contained in a single, deeper unit, and the targets are the elongated features that form the shallow aquifer [17,46]. The groundwater sampled in Nubutautau indicates a different, probably slower and shallower flow path, involving interaction with soils and volcaniclastic deposits, which are more effective in neutralizing rainwater acidity. This is probably associated with dike-impounded or perched groundwater exfiltrating through contact or a break-slope spring. These types of springs are, therefore, good indications of the quality of groundwater bodies with a high potential for development.

4. Conclusions

Electrical Resistivity Tomography (ERT) was used to study the water-bearing zones and features through the Multi-Gradient Electrode Array (MGEA) assembly. Resistivity variations along five survey lines near Nubutautau Village within Viti Levu (island) in Fiji were obtained. The below ground level (bgl) features were studied and inferences about groundwater potential points were made. The MGEA assembly proved to be reliable in terms of capturing a complete resistivity profile at depth levels for the target points as the predrilling assessment.

Inferences from the ERT have been used to locate two potential groundwater-bearing zones with significant features containing water for future drilling targets. These include the following:

A low-resistivity zone identified along survey line 2 within a 320–400 m profile distance and at 40–60 m depth below ground level. It is suggested that this may represent the presence of a fracture zone and may be part of a bigger SW–NE fracture zone adjacent to the site, as revealed by the near-surface topography features of vegetation.

A prominent vertical feature of low resistivity at survey line 3, from 65 to 125 m profile distance and from 20 m depth extending down to 80 m depth below ground level, also indicating a groundwater-bearing fracture zone and connected to a spring source.

Both the targets are reliable and consistent with the near-surface topological and geological features in the yield of the water given the geological and hydrogeological interpretations.

Hydrochemical assessment revealed the existence of a common aquifer at the Nubutautau location through the similarity in the water constituents. This aquifer is assumed to be huge and covers a wider area. The flow rates of the three springs had been dropping significantly, which is mainly due to the insufficient rain replenishment, which accounts for the dry conditions and significant drop in the water table.

Future research investigations of ground resistivity may involve the second stage of borehole implementation in which drilling of the targets could be carried out. This would involve further findings related to hydraulic testing such as carrying out pumping tests of the groundwater to check the efficiency of the aquifer yield, constructional methods of the borewell and sustainability measures for a longer run. Furthermore, it would require analyzing the chemical constituents of the ground-extracted water for portability in the community.