Introduction

Water is only inorganic liquid that exists in three states, e.g., solid, liquid, and gas. It plays a significant role in occurrence of all aspects of life (Franks 2000; Makhmudov et al. 2021). Increase in population and climate change is severely affecting the water quality and quantity on planet earth. The situation gets worse in developing country like Pakistan which is among top ten countries severely affected by climate change (Hujakulova et al. 2021). The sub-Himalayan region due to its hard terrain and increasing population faces several challenges among which availability of potable water for domestic and industrial use is primary. Industrial growth is also introducing harmful chemicals in the local aquifer system that is deteriorating the quality of water (Nisar et al. 2021). Addressing these problems requires holistic approaches that combine science with social aspects.

Geophysical techniques have been used for centuries to demarcate water-bearing zones (Nisar et al. 2023). There are multiple surface-based geophysical techniques, which are used to explore subsurface water. These techniques include electrical resistivity, surface nuclear magnetic resonance, and telluric surveys (Singh and Sharma 2023). Geospatial techniques are also widely used for groundwater resource exploration and development (Gautam et al. 2023). The integrated geophysical techniques like vertical electrical sounding, time-domain electro-magnetic methods along with micro-gravity surveys are useful for the assessment of groundwater resources (Mohamed et al. 2023). These integrated studies involve several geological factors, such as fractures, faults, and lithologies that control the local groundwater distribution (Agyemang 2022; Nair et al. 2022). The vertical electrical sounding in conjunction with controlled source audio-frequency magnetotelluric (CSAMT) is most used in various exploration problems, such as subsurface structural and fault mapping as well as groundwater exploration around larger depths. This integration is mostly accurate and often requires much less budget and labor. In comparison, other geophysical methods have ambiguities and uncertainties unless exact fracture point is not given to them. The integration of these techniques can reduce the rate of unsuccessful drilling (Agyemang 2020; Chouteau et al. 1994; Kouadio et al. 2022; Li et al. 2017).

With increase in industrial activities the groundwater quality is also facing serious threat (Suleman et al. 2023). Most of the industrial zones introduce harmful chemicals into nearby streams or open spaces that eventually find their path to subsurface shallow aquifer system (Nisar et al. 2023). These contaminants range from microbial to heavy metals (Nisar et al. 2021). To understand the distribution of contamination in the subsurface, integrated approach of geophysical and hydrochemical methods is applied to identify the extent of subsurface contamination (Hasan et al. 2023).

The study area lies at the municipality of Rawalakot city and its adjoining areas, which is the part of District Poonch, Azad Kashmir (Fig. 1). Tectonically, the study area lies in Sub-Himalaya. The study area is about 20 square kilometers and located between latitude 33.855814°, 33.850082°; and longitude 73.70°, 73.737740°. With growing population, hard terrain, and soaring temperatures, the community of the area faces serious threat related to potable water availability for domestic as well as industrial purposes. The tough mountain terrain makes the water availability difficult, and the industrial development is making already available resources contaminated. The present study focuses in addressing these problems with the integration of electrical resistivity and audio-magnetotelluric methods for the demarcation of subsurface lithology, their lateral and vertical extension, depth, and thickness of different water-bearing layers, shallow and deep-seated rock features. Previous researchers in the area utilized an electrical resistivity method, which keeping in view the hilly terrain has several ambiguities and often required close integration with other data sets which at most of time was not available. This integration will help in identifying the new and accurate groundwater bearing zones. The geo-chemical analysis will be carried out to investigate the quality of groundwater. The contamination from industrial as well as domestic sources has been catered and classified in close coordination with the geophysical surveys. The research will be of particular interest to the scientific readers, Government decision-making authorities, and local community.

Fig. 1
figure 1

a Location map of Azad Kashmir in Pakistan shown by red polygon. b Figure showing Azad Kashmir with Poonch District represented by yellow color polygon. c The tectonic map of northern Pakistan with study area compiled after (Calkins et al. 1975; Chaudhry et al. 1997; Hameed et al. 2023; Khan et al. 2016; Searle et al. 1996; Wadia 1931). d The geological map of the study area (compiled and modified after Hussain et al. 2014, 2004)

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Study area

Geological and tectonic setting of the study area

The current study is part of District Poonch, Azad Kashmir. Tectonically, the study area is part of Sub-Himalaya, Pakistan (Fig. 1c). Multiple active fault lines pass through the vicinity of the study area. The northern Pakistan has number of disastrous earthquakes especially the 8th October, 2005 earthquake which initiated with Magnitude 7.5 and jolted the Muzaffarabad, Bagh, and Rawalakot areas of Azad Kashmir (Lisa et al. 2006). The Riasi thrust passes near the study area. The recent mapping demarcated the presence of active emergent thrust faulting occurring within the fold-and-thrust belt north of the deformation thrust front in the NW Himalaya. The > 60-km-long Riasi fault system is the southeastern most segment of a seismically active regional fault system that extends more than 200 km stepwise to the southeast from the Balakot-Bagh fault in Pakistan into northwestern India (Gavillot et al. 2016).

The study area consists of a narrow valley surrounded by hills and mountains with elevation ranging from 1544 to 1790 m from the sea level. The main source of the groundwater recharge is the meteoric water with maximum of 476 mm in the summer season. Two geological formations lie in the study area which is Murree Formation (Rawalpindi Group) of Early Miocene age and surficial deposits of Recent age. The Murree Formation is mainly composed of red to purple and greenish grey sandstone, siltstone, purple to reddish brown mudstone and conglomerate, while the surficial deposits are composed of unconsolidated deposits of sand, silt, clay, and gravels (Latif 1970; Shah 2009). The conglomerates of Murree Formation and gravels of surficial deposits act as aquifer in the study area.

Methodology

The present work involves extensive resistivity, audio-magnetotelluric and hydrochemical data acquisition. ABEM SAS Terrameter 4000 was used for the acquisition of the 1D electrical resistivity survey, while the PQWT TC-500 was used for the acquisition of the 2D audio-magnetotelluric data in the study area. A total number of 67 Vertical Electrical Soundings, 34 AMT Profiles, and 40 water samples were acquired from the field to investigate the groundwater potential and water quality of study area. Different computer softwares were used for the processing of ERS and AMT data. Arc GIS (10.4.1) was used for the compilation of geological maps, Rockworks (16) was used for the generation of 3D models of study area, and the Surfer (16) Golden Software was used for making different isopach apparent resistivity and magnetotelluric data as well as the 2D profile. Similarly, Strater (5) Golden Software was used for making the interpreted lithologs, IPI2WIN software (Bobachev 2003) was used for the processing of 1D ERS data, and Google Earth Pro was used for plotting the field data in the laboratory.

The whole work was divided into three phases. Phase 1 was to conduct the 1D Vertical Electrical Sounding (VES) in the study area within the space that was available. A large part of the city was covered with construction; however, the VES stations were established at available space intervals. The ERS data were processed to generate apparent resistivity curves, apparent resistivity maps at different depths, 3D lithological models, 3D inversion model, and pseudo-resistivity sections. The apparent resistivity maps at MN and AB (current and potential electrodes) electrode spacings were prepared using the Golden Software (Surfer 16) using the minimum curvature and convex hull as the study area was in hilly terrain and maximum space was covered. The second phase was comprised of Audio-Magnetotelluric survey data compilation. The time was selected for the acquisition of 2D-AMT data as time factor majorly influence the quality of the AMT data (Hermance et al. 1975; Kelbert et al. 2017; Kirkby 2019; Rosas-Carbajal et al. 2015; Simpson et al. 2021). The 2D profile data were then processed to convert 1D data to co-relate with 1D VES results. Several isopach maps of AMT data and 3D inversion model of AMT data were also generated. The third phase was comprised of collection of water samples from the municipality of Rawalakot city according to the International standards (Gelsey et al. 2023; Hayder et al. 2023; Hespanhol et al. 1994; Javaid et al. 2023; Lee et al. 1982; Nyambar et al. 2023; Rahman et al. 2023). Thirty-nine water samples were collected from the study area and its adjoining areas. The chemical analysis including color, electrical conductivity, pH, turbidity, bicarbonate, calcium, hardness, chloride, magnesium, sulphate, nitrate, sodium, potassium, alkalinity, and total coliforms were performed to ensure the quality of groundwater.

Results and discussion

Borehole lithologs

The borehole lithologs were acquired from five locations in the study area represented by BH-01–05 (Fig. 1d). The borehole BH-01 was at the northern portion of the study area and situated in the vicinity of Officers Colony. The borehole BH-02 was at the western portion of the study area near the Khrick Road, the borehole BH-03 and BH-04 were at the middle portion of study area near Ghazi-Millat Road and Green Town areas, respectively. Above-mentioned all four boreholes were on Murree Formation and contain topsoil, boulder clay, sandstone, sandstone (weathered), dry sandy soil, sandy clay, and mudstone. The borehole BH-05 was at the southeastern portion of the study area near the University of Poonch, Rawalakot. This borehole was at the surficial deposits and comprised of unconsolidated deposits of clay, silt, sand, and gravel, respectively. All borehole lithologs with their depth and lithology are shown in Fig. 2. The weathered sandstone of BH-01, BH-02, BH-03, and BH-04 acts as an aquifer (water-bearing potential rock), while the sand and gravels (saturated) are acting as an aquifer (water-bearing rock unit).

Fig. 2
figure 2

Lithologs of boreholes obtained from the study area

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1D electrical resistivity survey

The analysis of 1D vertical electrical sounding revealed the presence of three-to-five subsurface lithological layers. The coordinates along with true resistivity, thickness, depth, and different geoelectrical parameters are given in Table 1. The different subsurface lithological layers include topsoil, clay, clay (very dry), boulder clay, clayey sand, sandy clay, dry sandy soil, siltstone, mudstone, sandstone, sandstone (weathered), gravel (saturated), and sand and gravel (saturated). The apparent resistivity values from 0 to 10 Ωm are interpreted as clays, while the lithological layers with 15–35 Ωm resistivity values are interpreted as layers of boulder clays. The layers with resistivity values ranging from 20 to 150 Ωm are interpreted as siltstone and mudstone, respectively. The apparent resistivity values ranging from 50 to 200 Ωm are interpreted as weathered sandstone, whereas the subsurface lithological bodies with resistivity values from 200 to 5000 are interpreted as layers of sandstone. The aquifer thickness values obtained by VES range from 5.28 to 184.38 m at VES 53 and VES 9, respectively. The high and low resistivity values are interpreted in terms of low and high groundwater potential in the study area (Niaz et al. 2021, 2016, 2018, 2017).

Table 1 Modified longitudinal conductance/protective capacity rating (Nisar et al. 2018)
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Qualitative Interpretation

Apparent resistivity maps

The presence of groundwater can be efficiently predicted by the apparent resistivity values in the subsurface. The various iso-apparent resistivity contour maps can represent the groundwater potential at different depth ranges (Moulds et al. 2023). Different isopach apparent resistivity maps including 2, 6, 10, 20, 30, 40, 50, and 80 m, respectively are shown in Fig. 3. All the apparent resistivity maps show that there are intermediate-to-hard rocks on the northwestern side of the study area. Low resistivity rocks which are associated with surficial deposits are present at the southeastern part of study area which lead to the groundwater potential zones. The subsurface layers as well as their thickness and curve types are shown in Table 1, whereas the geoelectrical and geohydrological parameters are given in Table 2.

Fig. 3
figure 3

Iso-apparent resistivity map at various electrodes spacing: a 2 m, b 6 m, c 10 m, d 20 m, e 30 m, f 40 m, g 50 m, and h 80 m

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Table 2 Source wise distribution of water samples
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Quantitative interpretation

Dar–Zarrouk parameters

The Dar-Zarrouk parameters were evaluated through electrical resistivity survey outcomes. This survey is very useful for demarcating subsurface lithological features; however, same lithologies may exist which may increase the ambiguities and uncertainties in results (Hasan et al. 2019). To minimize these ambiguities, the Dar-Zarrouk parameters named as longitudinal conductance and transverse resistance have been successfully applied to the present research work. The combination of geoelectrical layers thickness and resistivity can be used in aquifer protection assessment as well as evaluation of hydrologic properties of an aquifer (Henriet 1976). The protective capacity of clayey overburden is directly related to the parameter of longitudinal conductance which is denoted as “S”.

Longitudinal conductance

The term longitudinal conductance is one of the most commonly used geoelectrical parameter which means the total conductance that took place in the direction of bedding plane of 1 m (Mohammed et al. 2023). The symbol “S” is generally used for the representation of the longitudinal conductance (Niaz et al. 2013). The longitudinal conductance is represented by the following formula:

$$S=\frac{h1}{\uprho 1}=\frac{h2}{\uprho 2}$$
(1)
$$S={\sum }_{n=1}^{n}\frac{hn}{\mathrm{\rho n}},$$
(2)

where “h” and “ρ” stand for thickness and true resistivity of each layer, respectively. The longitudinal conductance is of vital importance as it is reciprocal of resistance and indicates the groundwater potential in the subsurface (Adeniji et al. 2023). The longitudinal conductance values range from 0.01 to 280 Siemens (Fig. 4a). The longitudinal conductance map of the study area is divided into five lows and six high valued areas. The H1, H2, H3, H4, H5, and H6 are areas with high longitudinal conductance values, while L1, L2, L3, L4, and L5 are areas with low longitudinal conductance values. The high values of longitudinal conductance are distributed at central and southern portion of study area showing the groundwater potential while the low values are distributed at northern portion of the study area.

Fig. 4
figure 4

Geo-electrical parameters of the study area. a longitudinal conductance, b aquifer thickness, c transmissivity, and d transverse resistance. Geo-electrical parameters of the study area. e Unit longitudinal conductance. f Hydraulic conductivity

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Transverse resistance

The transverse resistance of any resistive layer and conductance of conducting layer are measured in the terms of transverse resistance and longitudinal conductance (Nisar et al. 2018). It is represented by the alphabetical letter “T.” It is represented mathematically as follows:

$$T = {h_1}{\rho_1} + {h_2}{\rho_2} + \ldots \ldots {h_n}{p_n}$$
(3)
$$S={\sum }_{n=1}^{n}hn\times \rho n,$$
(4)

where “ρ” and “h” stand for true resistivity and thickness of each layer in the subsurface (Ako et al. 1986; Arulprakasam et al. 2013; Nwachukwu et al. 2019). The transverse resistance map is shown in Fig. 4d. The transverse resistance map of the study area is divided into four highs and five lows. The areas with high values of transverse resistance, e.g., H1, H2, H3, and H4, are distributed on northwestern and southwestern portion of study area which are associated with the low groundwater potential. Whereas, the areas with low values of transverse resistance, e.g., L1, L2, L3, L4, and L5, are distributed at central and southern portion of study area which are associated with the good groundwater potential (Azeem et al. 2021; Mahmud et al. 2022; S. Singh et al. 2021).

Unit longitudinal conductance

The unit longitudinal conductance plays an important role in the overburden protective capacity ratings of the area (Obiora et al. 2015). The unit longitudinal conductance map of the area is classified into 6 lows and three highs. The H1 area is situated on the northwestern portion of study area with high value of unit longitudinal conductance, H2 and H3 areas are distributed on the central portions of study area. These areas with high values of unit longitudinal conductance are categorized as good overburden protective capacity ratings which are associated with less vulnerable to leached toxic fluids from surface (Nisar et al. 2021) (Fig. 4e). Whereas the L1, L2, L3, L4, L5, and L6 areas are distributed on the central and southern portions of study area. These areas with low values of unit longitudinal conductance are categorized as the low or poor overburden protective capacity ratings which are associated with high vulnerability of leached toxic fluids from the surface (Table 1) (Chukwuma et al. 2015).

Aquifer thickness

The aquifer thickness map of the study area is represented in Fig. 4b. The total aquifer thickness map was prepared by processing of apparent resistivity data (Nowroozi et al. 1999; Muchingami et al. 2012). The aquifer thickness map can be visualized in terms of the groundwater volume controlled by subsurface rocks (Arshad et al. 2007; Hamzah et al. 2006; Riwayat et al. 2018). The aquifer thickness map is divided into four high and three low areas. The H1, H2, H3, and H4 areas are classified as good groundwater potential zones which are associated with good depth for accumulation of groundwater. These high groundwater potential zones have groundwater depth ranging from 70 to 180 m. Whereas, on the other side, the L1, L2, and L3 areas are classified as low groundwater potential zones which are associated with low depths for the accumulation of groundwater. These low groundwater potential zones have depth ranging from 20 to 70 m in the subsurface. Most of the aquifers were demarcated as unconfined in the study area.

Hydrogeological parameters

Total transmissivity

The total transmissivity map of the study area is shown in Fig. 4c. The total transmissivity of any area play a vital role in the hydro-geological properties of area (Ako et al. 1986); (Torrese et al. 2013); (Utom et al. 2012). The total transmissivity map of the study area is classified into four highs and low areas. The H1, H2, H3, and H4 are areas with high values of transmissivity and situated on the central and southern portion of the study area. These high values of transmissivity show the good groundwater potential due to the presence of unconsolidated surficial deposits exposed on the southern portion of study area. The L1, L2, L3, and L4 are areas with low values of transmissivity values. These areas are distributed on the central and northern portion of study area demarcating the lack of groundwater potential in these areas (Yeh et al. 2006).

Total hydraulic conductivity

The groundwater potential of any area mainly depends upon the hydraulic conductivity which is an important hydrogeological parameter (Mualem 1986). The hydraulic conductivity has strong relationship with groundwater bodies that helps us to understand the groundwater system in the subsurface (Gernez et al. 2019; Lu et al. 2021; Slater 2007; Troisi et al. 2000). The current study area is classified into four highs and six lows (Fig. 4f). The H1, H2, H3, and H4 areas are classified as high groundwater potential zones due to the high values of hydraulic conductivity, whereas the L1, L2, L3, L4, L5, and L6 are the areas with low hydraulic conductivity values that leads to the low groundwater potential in the study area. The elevated values of hydraulic conductivity are associated with unconsolidated surficial rocks deposited in the southern portion of study area, while the low values of hydraulic conductivity are associated to rocks with low permeability.

Lithology models

The 3D lithology model was prepared on the Rockworks (16) (RockWare 2023). The interpreted model based on the resistivity data revealed the presence of 3–5 subsurface lithological layers which are boulder clay, clay, dry clay, clayey sand, dry sandy soil, gravel (saturated), mudstone, sand and gravel (saturated), sandstone, sandstone (weathered), sandy clay, siltstone, and top soil (Fig. 5a). Confined aquifers are observed on the northeastern side of the study area, whereas unconfined aquifers are demarcated mostly on southwestern side of the study area. The inversion of ERS data was executed in Arc-GIS Pro software to view the image of subsurface rocks by the means of electrical resistivity, as shown in Fig. 5b. The inversion model shows the distribution of different rock materials in subsurface up to the depth of 80 m. The interpreted lithological model and inversion model match closely to each other. The mudstone is distributed on the central and eastern parts of study area as shown by the lithology model in Fig. 5a.

Fig. 5
figure 5

a Lithological model of the study area. b Apparent resistivity inversion model of the study area

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Lithology projected sections

Figure 6 shows the base map of the study area showing all VES stations with both profiles A–A′ and B–B′. The profile A–A′ is taken in north to south direction, while B–B′ profile is taken from west to east direction. The lithological units demarcated in the subsurface lithology projected sections are boulder clay, clay, clay (very dry), clayey sand, dry sandy soil, gravel (saturated), mudstone, sand and gravel (saturated), sandstone, sandstone (weathered), sandy clay, siltstone, and topsoil (Fig. 5b) (Hidayatika et al. 2022; Sajadi et al. 2013; Uwiringiyimana 2019). Both profiles also represent the subsurface lithology with best fit of all VES stations lithology with maximum sandstone, mudstone, and clay.

Fig. 6
figure 6

a Base map of the study area showing all VES stations with both profiles A–A′ and B–B′. b Lithology projected sections A–A’ (VES 01, 26, 57, 34, and 23) and B–B’ (VES 38, 06, 52, 51, 55, 62, and 63)

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Audio-magnetotelluric survey

2D audio-magnetotelluric profiles

The magnetotelluric survey depicts the subsurface image of rocks and their structures by inferring the total variations in their electric and magnetic fields. This variation makes the interpreter enable to observe subsurface lithological bodies and their structures, including contacts, fractures, etc., that may lead to the accumulation of the groundwater system, especially in the hilly terrain (Aizawa et al. 2009; Huang et al. 2021; Mekkawi et al. 2022; Xu et al. 2020). A total of 34 audio-magnetotelluric profiles were taken in the study area to validate the results of electrical resistivity survey (Fig. 7a). The seven AMT profiles (A-6, A-9, A-7, A-8, A-11, A-19, and A-1) were joined to form southwest–northeast oriented W–W′ profile, the six AMT profiles (A-3, A-15, A-12, A-8, A-26, and A-30) were joined to form north to south oriented X–X′ profile, and the seven AMT profiles (A-33, A-17, A-10, A-7, A-26, A-23 and A-24) were joined to form northwest–southeast oriented Y–Y′ profile, while the five AMT profiles (A-30, A-22, A-23, A-27, and A-28) were joined to form southwest-to-northeast oriented Z–Z′ profile. These four AMT profiles were plotted in the Move 2D software (Bremmer et al. 2016) to understand the lithology and structural elements of subsurface rocks that may lead to the groundwater accumulation (Fig. 7).

Fig. 7
figure 7

a The base map of study area with W–W′, X–X′, Y–Y′, and Z–Z′ 2D audio-magnetotelluric profiles; b overlay of above-mentioned four profiles

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Figure 7b shows the 3D lithological diagram of subsurface rocks as well as their structures and fractures and contacts of different lithological units. Each profile is presented separately, to understand the subsurface lithological units as well as their horizontal and vertical discontinuities. Figure 7b represents the iso-anomaly contour map of southwest-to-northeast oriented Profile W–W′. The loose to intermediate strata is distributed over the profile from start to end with depth ranging from 20 to 150 m in the subsurface. The hard strata (sandstone) are distributed over the profile with depth ranging from 100 to 400 m in the subsurface. The second iso-anomaly contour map of Profile X-X’, which was north to south oriented, shows the potential difference (mV) values from 0.003 to 20. This profile contains almost hard strata up to the depth of 400 m in the subsurface with no lateral or vertical discontinuities.

The third iso-anomaly contour map of Profile Y-Y’ was established in west-to-east direction (Fig. 7b). The values of potential differences range from 0.01 to 20 mV all over the profile. Different zones are marked on the profile map. The loose material/low resistivity material was demarcated from Point Number 57 to Point Number 77 on the profile map. This low resistivity material extends up to the depth of 300 m in the subsurface. There was a cavity which was identified at the depth of 250 m at Point Number 29 of the profile map. Similarly, the fourth iso-anomaly contour map of Profile Z–Z’ was conducted in southwest-to-northeast direction. The values of potential differences range from 0.003 to 50 mV with highest values towards the end portion of the profile map. The low resistivity material is distributed from Point Number 21 to Point Number 54 up to the maximum depth of 100 m in the subsurface.

1D audio-magnetotelluric survey

The 2D audio-magnetotelluric profiles were converted into 1D data to prepare the iso-anomaly contour maps at different depths in the study area. These iso-anomaly maps at different depths enabled us to observe the subsurface lithology from shallow surface up to the depth of 500 m. Keeping in view the other active geophysical survey (1D electrical resistivity survey), their limitations, and applications, the AMT survey was immensely helpful to understand the subsurface lithology and to compare the results of lithologs and different subsurface lithological layers with each other. Figure 8 shows the contour and iso-anomaly contour maps of all AMT stations. These maps give valuable information about the change of electric field which is associated with diverse types of sedimentary rocks in the study area. All these contour maps and grid vector hill shaded relief maps show that in the shallow surface, the groundwater potential is present at the southeastern part of the study area. As the depth increases, this closure of low resistivity tends to increase up to the depth of 450 m which is associated with good groundwater potential in the study area.

Fig. 8
figure 8

Overlay of 1D AMT maps at 9 m, 36 m, 54 m, 80 m, 107 m, 143 m, 170 m, 205 m, 250 m, 286 m, 330 m, 375 m, 420 m, 455 m, 480 m, and 500 m

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Figure 8 represents the iso-anomaly contour maps of audio-magnetotelluric data from 9 m to the depth of 80 m. The iso-anomaly contour map of 8 m shows one closure of high electric field values in the extreme northern part of study area which is associated with the presence of hard/compact rocks. It also has the one closure of low electric field values at the central part of the study area which are associated with the presence of low resistivity/unconsolidated rocks. Similarly, the iso-anomaly contour maps of 36 m, 54 m, and 80 m have low resistivity closures in the southern portion of study area which are gradually decreasing from shallow surface to 80 m. The iso-anomaly contour maps of 107 m, 143 m, 170 m, and 205 m depths have the closure of high electric field values in the northern portion of study area which are associated with the presence of hard rock, i.e., sandstone. Similarly, the closure of low electric field values is found in the middle portion of study area as shown in the 107 m depth map, which is gradually increasing towards the south that can be observed in 143 m, 170 m, and 205 m in the subsurface.

The iso-anomaly contour maps at 250 m, 286 m, 330 m, and 375 m depths have the closure of high electric field values in the northern portion of study area which are associated with the presence of hard rock, i.e., sandstone. Similarly, the patch of low electric field values is found in the middle portion of study area as shown in all figures which is gradually increasing towards the south. Similarly, in the iso-anomaly contour maps of audio-magnetotelluric data from 420 m to the depth of 500 m, the patch of low electric field values gradually increases from 420 m depth to 482 m. Then, this path shifts to the extreme southern portion of the study area which shows the presence of hard rocks in the central part of study area at a depth of 500 m. Comparable results were revealed by the 1D Vertical Electrical Sounding observations.

3D audio-magnetotelluric survey model

The three-dimensional model of audio-magnetotelluric survey of study area was prepared using Arc GIS Pro software. The total depth achieved in the survey was 80 m. Total number of 34 AMT stations were established in the study area. The excel database was imported in the Arc GIS Pro. The “Empirical Bayesian Krigging 3D” tool of “geoprocessing” was used to prepare the 2D contour maps at different depths. Then, “GD Layer 3D to NetCDF” tool was used to prepare the 3D model of the study area up to the depth of 500 m in subsurface. Figure 9 represents the 3D model of AMT data of study area. This model validates the findings of ERS model and shows the distribution of intermediate-to-hard rocks on the northern part of study area due to the presence of sandstone, mudstone, and siltstone of Murree Formation. Whereas, the souhern part of study area shows the groundwater potential due to the presence of unconsolidated rocks of surficial deposits like clay, silt, sand, and gravels.

Fig. 9
figure 9

3D inversion model of audio-magnetotelluric data of the study area

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Comparison of 1D electrical resistivity survey and 1D audio-magnetotelluric survey

The comparison of 1D electrical resistivity survey and 1D audio-magnetotelluric survey was made to understand the distribution of subsurface rocks in the study area. The total number of 1D ERS stations was 67, while that of AMT survey was 34. Figure 10 demonstrates the comparison of ERS and AMT surveys. The left portion of diagram shows the apparent resistivity maps at 2, 6, 10, 20, 30, 40, 50, and 80 m depth, while the right portion of diagram show the iso-electric field maps at 9, 36, 54, 80, 107, 143, 176, 205, 250, 420, and 500 m, respectively. The iso-resistivity maps show the distribution of intermediate-to-hard rocks on the northern portion of study area which are associated with sandstone and mudstone of Murree Formation, while low resistivity rocks are distributed in the southern portion of study area which are associated with unconsolidated clay, silt, sand, and gravels of surficial deposits.

Fig. 10
figure 10

Comparison of ERS and AMT results

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Similarly, the iso-electric field audio-magnetotelluric maps from shallow depth to 500 m were prepared to obtain information about subsurface rocks. The audio-magnetotelluric maps show the distribution of intermediate-to-hard rocks on the northern portion of study area which are associated with sandstone and mudstone of Murree Fomration. Whereas, the patch of low electric field can be observed in the southern portion of study area which are associated with the unconsolidated rocks like clay, silt, sand, and gravels of surficial deposits. The electrical resistivity survey and audio-magnetotelluric survey cross-validate and match closely to each other.

Physiochemical parameters

A total 39 water samples were analyzed from the Rawalakot city municipality to investigate their chemical and biological properties. Among which, 23 water samples were found to be safe, whereas 16 water samples were classified as unsafe for drinking purpose. All 23 water samples have found “Positive” results of “Total Coliforms”. The total coliforms are normally found in soil, plants, and warm-blooded animals. In case of drinking water, their presence typically points out towards the potential contaminations of pathogens. The water samples with total coliforms should be disinfected instantly for drinking purpose. The sample no. 3, 7, 11, 14, 16, 17, 18, 21, 22, 24, 27, 30, 31, 32, 33, and 34 are marked as unsafe due to the micro-biological contamination. The source-wise distribution of water samples and their physio-chemical results along with their locations are given in Tables 2 and 3, respectively. It is observed that drinking water samples obtained from western and southeastern part of study area are mainly contaminated, whereas the drinking water samples obtained from central and northeastern part of study area are safe for drinking purpose. The groundwater contamination was found in Galiat, Tarar, Chaitri, Kowas, Khrick, Bathnara, and Pothi-Bala areas. The first 38 groundwater samples belong to the Ca–HCO3 water type, while last groundwater sample belongs to the Mg–HCO3 water type. Moreover, considering the irrigation scope, 26 groundwater samples wer marked with high salinity hazard, while 13 groundwater samples were marked with medium salinity hazard in the study area. The groundwater quality map with the help of dot density map is given in Fig. 11. Table 2 explains the source-wise distribution of water samples from study area.

Table 3 Physciochemical parameters of water samples from study area
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Fig. 11
figure 11

Groundwater quality map of study area with marked safe and unsafe drinking water samples

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Conclusion

The integrated aproach of two geophysical methods, e.g., 1D electrical resistivity survey and 2D audio-magnetotelluric survey, has revealed the demarcation of groundwater potential from shallow depth upto 180 m at Rawalakot city municipality, Azad Kashmir. The integration of both techniques has minimzed the limitations and ambiguities of geophysical methods. The electrical resistivity survey demarcated the low resistivity material at the central part of study area at depth of 2 m. This patch of low resistivity material extends further up to the depth of 10 m in the subsurface. The low resistivity patch further extends towards the southern portion of study area up to the depth of 50 m. Finally, at the depth of 80 m, the good groundwater potential was demarcated which was associated with the low resistivity material at central portion of study area. Similarly, the audio-magnetotelluric maps were acquired up to the depth of 500 m. The iso-electric field maps of AMT data revealed the presence of low resistivity material at central and southern part of the study area. Both surveys revealed that there is a good groundwater potential at the central and southeastern part of the study area with three-to-five subsurface layers which are boulder clay, clay, clay (very dry), clayey sand, dry sandy soil, gravel (saturated), mudstone, sand and gravel (saturated), sandstone, sandstone (weathered), sandy clay, siltstone, and top soil. The borehole data acquired from study area match closely with the interpreted lithology. The deep groundwater potential zones were demarcated from 50 m in the subsurface to 480 m as interpreted by AMT data in the southern part of study area. Different fractures were also delinated in the study area with depth ranging fom 50 m to 300 m in the subsurface. Similarly, the 1D iso-anomaly contour maps of ERS data were compared with the 1D iso-anomaly contour maps of AMT data. Their comparison leads to the accuracy and validation of both surveys. In this way, the 1D and 2D data demarcated the presence of subsurface groundwater potential at the central and southeastern portion of the study area. The water samples acquired from the study area revealed that among 39 water samples 16 water smples were found to be unsafe for drinking purpose due to the total coliforms’ contamination. While, 23 water samples were found to be safe for drinking purpose.