Introduction

Potable water is one of the most essential natural resource for the survival of living beings on earth and is valuable for human development (Memon et al., 2019). Groundwater yield potential is efficiently used to map and manage groundwater productivity and groundwater exploration at both the local and the regional level (Soomro et al., 2019a). A number of factors can affect the quality of a groundwater reservoir; such as contamination by the salt-water intrusion. Harding (1991) reported that groundwater is a significant source that is not only withdrawn for drinking purposes but also for irrigation purposes. Seawater intrusions (SI) commonly occur in coastal aquifers due to density differences between seawater and freshwater (Arslan et al., 2017). Unconfined aquifer has widely been known as the most affected aquifer by SI in coastal areas due to its direct connection to the sea (Najib et al., 2017). Indus delta is one of the world largest river delta which covers 0.6 million hectares area, consisting World’s largest and unique arid climate (Spalding et al., 2010). Indus River Estuary (IRE) has been facing a Seawater intrusion problem for decades across the whole Indus delta area. The previous studies reveal that hydrological and metrological data were analyzed to interpret the causes and consequences of saline water intrusion in the IRE. The main problems in the Indus delta are seawater intrusions and coastal erosion, having an overwhelming environmental and social impacts on the whole ecosystem (Kalhoro et al., 2016). Percolation of freshwater from river Indus and its tributaries on the left bank has resulted in decreasing the salinity of aquifers and increasing the electrical resistivity (elevate up to 4.2 Am to 5.2 Am in the wet season) of the same aquifers (Bablani & Soomro, 2006). Due to the water shortage in the river, the seasonal flow from Kotri downstream towards delta area reduces or decreases the sediment and freshwater supply allowing the subsurface seawater intrusion into landward. This water shortage and mismanagement are causing one of the largest ecosystems in the world shrinking and degrading (Majeed et al., 2010; Rasul et al., 2012). Indus delta mostly consists of lithology of sand, sandy clay and clay loam (Majeed et al., 2010). Indus Delta provides livelihood to nearly 0.9 million coastal populations (Wood et al., 2013). The lower riparian on the Indus River are demanding river water to sustain the Indus Delta, its ecosystem and millions of people living in the Indus Basin, while the upper riparian are denying the facts of coastal degradation (Mountjoy, 2004).

The geoelectrical imaging method is a tool for mapping the intrusion boundary between freshwater and saline water due to its inherent capability to delineate the lateral changes in pore-water salinity (Nassir et al., 2000; Zeyad & Muhsen, 2016), employed VES to delineate seawater intrusion in the freshwater aquifer in Gaza strip Palestine. They conducted 14 VES and dug 12 boreholes to integrate the results and found seawater with resistivity 2 Ωm dominant along the section parallel to the coast at a shallow depth of 3 to 10 m. (Emekeme et al., (2017), employed geophysical and geochemical methods to identify underground water conditions in Jeddo, Nigeria; their study revealed the soil formed of clay, clayey sand, and fine to coarse-grained sand, while the aquifer was found in a 6% area at the depth of 12.7 m, underground water movement direction as northeast-southwest, and the geochemical analysis proved water samples as of poor quality with reference to WHO and WQI standards. Usama et al., (2015), conducted a similar study on the Egyptian coast to outline the freshwater layer floating over saline water body incorporating 1D and 3D interpretation of 11 VES data. The method successfully identified the freshwater slice with depth to locate a suitable location for drilling water wells for irrigation purposes.

The difference between the previous work done employing VES and presently reported work is that the current work does not only focus on the identification of saline and freshwater zones but also focusing to explore groundwater quality with respect to the possible influence of seepage from water bodies.

Materials and methods

Study area

The Indus River forms its delta when it empties into the Arabian Sea. It stretches into three administrative districts of Southern Sindh province of Pakistan, i.e. Badin, Thatta and Sujawal districts. Current study area is located in the district Badin Sindh, Pakistan (Fig. 1) the study has been conducted in the southern part of the lower Indus Basin stretching towards the Indus delta (Fig. 1). The study area shares the International boundary with India in the south-east. The Arabian Sea lies in the South and Karachi in the west. Indus Delta is prominent for its unique arid species of mangrove, and nesting place for thousands of migratory birds from Siberia and hundreds of species of flora and fauna. Indus River was once one of the mighty rivers which carried a huge sediment load and occupied vast alluvial areas from the Sea.

Fig. 1
figure 1

Location map of study area Showing VES locations

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General geology of the study area

The geology of the lower Indus basin particularly along southern Sindh chiefly comprises Cretaceous to Tertiary formations with major lithologies of shale, sandstone, limestone, dolomite, clay, and marl (Shah, 2009). Badin area is located in the southwestern periphery of the Indian plate, in the lower Indus basin. The Chiltan Limestone of the Jurassic age is the oldest formation in the Badin block (Mozaffar et al., 2002) and is overlain by the Lower Cretaceous marine sand-shale sequence of the Sembar and Goru Formations (Wasimuddin et al., 2005). Meanwhile, the surface area of the Badin district comprises alluvium and sub-recent deposits without exposure to any outcrop. The affected areas of saline water intrusion in the Badin district are Kadhan, Bhugra Memon, Golo Mandhro, Haij Haijam, Shaikh Keerio, and Bhandadri (Fig. 2).

Fig. 2
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Geological Map of Study area (modified and reproduced after HSC, 1961)

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Hydrogeology of the study area

The Indus river forms the world’s largest irrigation network system on the surface and a deep alluvium porous and permeable rock holding aquifer resulting in a higher water table in the subsurface. Currently, this area is at threat of groundwater aquifer contamination due to rapid landward intrusion of saline water and disturbing subsurface aquifers (Shahab et al., 2016). The depth of freshwater aquifers in the lower Indus basin range from 30 to 50 m, beneath these aquifers are brackish to saline waters. The Badin district is mostly drained by the Indus River through Akram Canal and Phuleli Canal. In the study area, main tributaries are flowing from these canals are Raj wah, Sani guni wah, Char Kadhan wah, Escape canal, Ali wah, Dalel Khab Chandia, and Nur wah (Fig. 3). These all tributaries flow towards agricultural land in profile (1) while southward in profile (2) no any tributary exist. (Fig. 3).

Fig. 3
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Canal water tributary map of the study area

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Electrical resistivity imaging

The function of the electrical resistivity survey is to calculate the underground resistivity distribution for measuring current and potential values in the field of study at the surface of the earth (Soomro et al., 2019b). Electrical resistivity imaging technique is used to determine the resistivity of different materials at the subsurface (Dahlin, 2001; Herman, 2001; Samouelian et al., 2005). Primarily an electrical current is introduced into the subsurface through a pair of current electrodes C1 and C2 and a potential difference is calculated through a pair of potential electrodes P1 and P2. A multi-channel resistivity configuration survey is a combination of electrodes connected to a Terrameter system via a multi-core cable. The apparent resistivity (ρa) is calculated for every electrode quadrupole by Eq. (1). The SAS4000 Terrameter of ABEM was used in this study for the acquisition of resistivity data.

$$\rho a = k \, \left( {V/I} \right)$$
(1)

Here I represents current, V represents voltage and k stands for geometric factor.

Wenner array configuration

The configurations used in the field are Wenner array configurations. The Vertical electrical sounding method applies Wenner array configuration to keep the center point constant and just increasing separation (a) at each reading at different positions, increase in the distance interval between the separations enables the current to penetrate deeper into the subsurface thus enabling the investigations of vertical depth profiles (Fig. 4). The Constant separation traversing method maintains the separation (a) constant and just increase the complete array over the earth surface. Using Eq. (2) the apparent resistivity through Wenner array configuration was calculated (Fig. 5).

$$\rho a = 2\pi a \times V/I$$
(2)
Fig. 4
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Configuration of electrodes for the Wenner array

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Fig. 5
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Comparing the electrical resistivity and conductivity values found for Earth materials with common earth materials adopted from Palacky (1988)

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Results and discussion

The saline water intrusion was observed formerly in the water exploitation wells in Kadhan area and adjoining areas of Indus delta district Badin Sindh. In this regard, Vertical Electrical Sounding (VES) survey was carried out and the results are described below:

Geo-electrical resistivity profiling

Total of ten vertical electrical soundings were carried along two profiles, with 05 soundings at each profile representing resistivity alteration of various subsurface lithology. IPi2-win software was used to process and analyze the resistivity data at different depths and layers. A graphic view of all the units representing different lithological elements across the area up to a depth of 67 m that comprise an Aluvium, Sandy Clay and Sand were achieved (Table 1).

Table 1 GPS locations of VES points
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Kadhan Profile-1

The VES data of Kadhan Profile-1 is given in Table 2. The geoelectric sections of Kadhan Profile-1 are shown in Fig. 6.

Table 2 The VES results of Kadhan Profile -1
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Fig. 6
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Geo-electrical section of (VES-1, 2, 3, 4) of Profile-1

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Kadhan Profile-2

The VES data of Kadhan Profile-2 is given in Table 3. The geoelectric sections of Kadhan Profile-2 are shown in Fig. 7.

Table 3 The VES results of Kadhan Profile-2
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Fig. 7
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Geo-electrical section of (VES-6, 7, 8, 9) of Profile-2

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Correlation of lithological logs of Profile 1 and 2

The total thickness covered in both the profiles is 67 m below the ground. The lithological correlation of both profiles shows the first topmost layer is showing alluvium lithology and has an average thickness of 3.034 m in profile 1 and 1.814 m in profile 2. After alluvium layer in VES 1 the sandy clay with a thickness of 2.12 m is observed while, at VES-02, 03, 04, and 05 the shallow freshwater aquifer in sand lithology is present and has an average thickness of 11.095 m. In profile 2 the second layer of VES-7, 8, and 10 show lithology of sandy clay and has an average thickness of 3.36 m and the remaining VES- 6, 9 show lithology of sand or sandstone contains brackish water aquifer and having an average thickness of 16 m, below this depth brackish water and saline water aquifers are present in both profile 1 and 2, mostly the subsurface is intruded by saline water from Arabian sea and has contaminated subsurface aquifer. The apparent resistivity readings of 1–10 Ωm are recorded in this intrusions zone (Figs. 8 and 9).

Fig. 8
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Lithological correlation of log at Profile-1

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Fig. 9
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Lithological correlation of log at Profile-2

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Pseudo cross-section and resistivity cross-section

It is an obvious representation of resistivity scattering and differences at different thicknesses and depths. Each profile has a different pseudo-section and resistivity section of all VES.

Kadhan Profile-1: The pseudo section of profile one displays variations in zones such as the low resistivity zone or Saline water zone (0–6 Ωm), moderate resistivity zone or brackish water zone (6–17 Ωm) and the high resistive freshwater zone or alluvium (17–36 Ωm). In the study area, shallow aquifers of fresh water are present which get recharged by small fish hatcheries, canal and tributaries which run along these small shallow aquifers. The high resistivity zone contains an alluvium and sand bearing fresh water with a resistivity of 30–36 Ωm at depth of from 5 to 14 m and an alluvium of recent deposit which lies up to the depth of 4.39 m. Low resistivity zone/saline water zone consists of sandy clay found comparatively deeper than other lithologies in the section, the moderate resistivity zone contains brackish water or saline margin (Fig. 10).

Fig. 10
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Pseudo section of Profile-1

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The general style of the cross resistivity section shows decreasing downwards style with a vertical drop. It is concluded that with the help of interpretation of pseudo-section, the installation of a tube well in a high resistivity zone has a high potential for a good freshwater aquifer (Fig. 11).

Fig. 11
figure 11

Resistivity section of Profile-1

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Kadhan Profile-2: The pseudo section of profile-2 also varies in zones as the low resistivity zone (0–2 Ωm), moderate resistivity zone (2–4 Ωm) and the high resistivity zone (4–8 Ωm). High resistivity zone is interpreted as an alluvium and brackish water zone (4–8 Ωm). The high resistivity zone contains sand that bears brackish water with a resistivity of 4–8 Ωm at depth of 2–20 m and an alluvium of recent deposit which lies at the depth of 2 m with a resistivity of 4–8 Ωm. Low resistivity zone consists of sandy clay. And in moderate resistivity zone also contains saline water (Fig. 12).

Fig. 12
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Pseudo section of Profile -2

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The resistivity cross section describes the altering of accurate resistivity with respect to depth. The general style of the cross resistivity section shows decreasing downwards style with a vertical drop. It is concluded that with the help of interpretation of pseudo-section, and resistivity section, freshwater zone is not found (Fig. 13).

Fig. 13
figure 13

Resistivity section of Profile-2

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Conclusions

It is concluded on the basis of the above results that:

The main focus of the present study in Kadhan area of district Badin (Indus delta) reveals the existence of shallow water aquifers in only those area where canal water infiltrate and recharge shallow aquifers whereas, there is no existence of freshwater aquifer in the area that is away from the surface recharge in the form of water canal. The surface recharge of freshwater plays important role in encountering saline intrusions also. The only possible way to stop saline water intrusion in the coastal area is to introduce canal water system in the whole coastal area of the Badin, Sujawal and Thatta districts. It is concluded that shallow freshwater aquifers are encountered in profile-1 area at the average depth of 11 m, and recommended for the installation of hand pumps and tube wells for freshwater.