Introduction

The growths of agriculture at alarming rates in arid and semi-arid regions throughout the globe are the major causes of irrigation problems particularly in the dry seasons. Most of the desert areas of Punjab Province, Pakistan were cultivated by the diversion of rivers water in the form of canals (Hussain 2014). The canal water is not compatible with the growth of agriculture, to fulfill these demands groundwater is used. Groundwater is present in the voids which fully saturates it and it is recharged by the rivers, canals seepage and irrigation return flows in TDA scenario (Hussain et al. 2016a, 2017). Groundwater is again pumped by the tubewells for agricultural and domestic uses. The entire Punjab province of Pakistan is suffering from rapid growths of tubewells that are the main cause of various environmental managerial problems like NPS and SS. During just one decade (2000–2010), 0.94 million tubewells were installed in Punjab province most of them were installed in poor to moderate groundwater quality zones which bring large amounts of salts on the surface. These excessive salt concentrations alter the soil composition and gradually turned it into a completely unproductive soil. The overexploitation of this valuable new resource has led to high salinity because of the up coning of under lying naturally saline groundwater. This fact has also been approved by a soil fertility study conducted by Khan et al. (2012) in which 315 samples were collected from 32 union councils of Tehsil Kot Adhu. These samples were tested for sodium absorption ration, electrical conductivity (EC), RSC and chloride ions. From that analysis authors have concluded that 30% of total samples were fit, 8% marginally fit and 62% were unfit for irrigation. In another study, Ashfaq et al. (2009) have found 50–60% of Punjab’s tubewells water as brackish. The major hydrological impacts of these growth rates are the deterioration of soil conditions, water table fluctuations, groundwater mining and quality problems. This imbalance of groundwater abstraction-recharge will hurt food chain of Pakistan in near future and requires interventions from law enforcement agencies for sustainable land use development.

The electrical resistivity method is the most effective method used in many hydrogeological studies specifically referring to groundwater quality prospects. This method not only gives information about the subsurface lithology, but also provides an indication of the presence of water with the estimation of salt concentration in it. In spite of enormous utilities of electrical resistivity survey, it can also be used for groundwater quality assessment. There is a relationship between earth resistivity and groundwater resistivity which can be utilized for the achievement of these objectives. The studies throughout the globe reveal the importance of vertical electrical sounding (VES) in groundwater related problem solving particular in arid regions (Kelly and Stanislav 1993). VES has been used in various groundwater studies for the achievement of multiple objectives in numerous hydrogeological conditions (Mosuro et al. 2016). Some of the important studies are, groundwater quality (Sonkamble 2014), delineation of fresh and saline water boundary (Swartz 1937), for imaging the industrial contamination plume (de Lima et al. 1995), determination of aquifer parameters (Sikandar et al. 2010; Srinivasan 2013; Akhter and Hasan 2016), hydraulic conductivity and transmissivity (Amos-Uhegbu et al. 2014; Mbonu et al. 1991), and protective capacity assessment of vadose zone material (Hussain et al. 2016b; Braga 2008), depth to bed rock and status of earth dams (Bogoslovsky and Ogilvy 1973), in miming (Moreira et al. 2016) and biogas accumulation in landfill (Moreira et al. 2015; Georgaki et al. 2008).

The present study applies VES with Schlumberger configuration to divide the study area into different tubewell suitability classes based on the electrical conductivity values using Water and Power Development Authority (WAPDA) (1981) groundwater usage criteria, which can be used to optimize both depths and positions of tubewells for sustainable agriculture and a balanced environment through the establishment of eco-friendly agricultural growth rates.

Material and methods

Study area

The study area is situated in the Punjab province having geographic coordinates longitude, 30°05′39″–30°45′57″ north and latitude 70°47′35″–71°33′37″ east. The River Indus flows on the western side and River Chenab flows on the eastern side (Fig. 1). This area lies in an arid climatic region where rainfall is less than evapotranspiration. The dust storms are common climatic features of the region (Hussain 2014). The contrast between summer and winter is well marked, and these two primary seasons are associated with two main crop seasons Kharif (Fall) and Rabi (Spring), respectively (Gosal 2004). The mean annual rainfall at Mulatn is 195 mm with the dominance of summer monsoon. The area is relatively flat with maximum elevation 154 m and minimum elevation 114 m from the mean sea level. The soil of the region is mostly alluvial in nature and was brought by the rivers from Himalayans Mountains which lie at the northern side of Punjab (Greenman et al. 1967).

Fig. 1
figure 1

Regional location of the study area (top right) on Pakistan map, Agro-climatic zones, exploratory wells (bottom right) and VES probes, digital elevation model major Canals and Rivers of Tehsil Kot Adhu (left)

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Physiography and hydrogeology

The study area is a part of greater Punjab plain, where geology is related to physiography (Hussain 2014). The study area contains thick alluvium and aeolian deposits of quaternary age which are characterized based on the nature of depositing agents and their relation to the current positions of the depositing agents like active flood plain, abandon flood plain and sand dunes. The active flood plain of the Indus River is very important in terms of availability of fresh groundwater for this region. The abandon flood plain is further away from the river, this physiographic unit has a good quality of groundwater reservoir which is used for healthy and productive agriculture. The flood plains of the Chanab and the Jhelum are wider on eastern side while they have insignificant width westward. The flood plains are formed as a result of deposition of sediments by rivers during rainy seasons. Their width varies from 10 to 15 km (Gosal 2004). The Indus River has played a dominant role in development of major physiological features of the area. These units contain sand, silt, clays and gravels containing good hydrogeological characteristics. The clay and silts usually occur in the form of small lens which are difficult to map (Hussain et al. 2016c).

The study area is a part of Thal Doab Aquifer (TDA) which lies in greater Punjab plain irrigated by the Indus River and its tributaries like Jhelum and Chenab (Fig. 1). The availability of surface and Groundwater makes this area as an ideal place for agriculture. TDA consists of thick alluvium, composition wise it is a mixture of sand, silt and clay. The coefficient of permeability is in the range of 0.05 to 1.2 m/s (Khan et al. 2014). Good recharge potential of region’s lithology reduces the surface runoff.

The area is irrigated by world’s largest contiguous irrigation system which is called Indus Basin Irrigation System (IBIS). This system consists of a well-developed canal irrigation system. There is a well-developed canal system in central, eastern and western parts while there is no irrigation system in northern parts of the study area near Chenab River. Taunsa Barrage which was built on the Indus River is the main source of water in the canals. Recharge is from rivers, canals and rainfall. The direction of groundwater flow is towards south–east (Nickson et al. 2005). Hydrogeology described in detail by Hussain et al. (2017).

Sources of contaminations

Kot Adhu Scenario (KAS) is described interm of two environmental variables like surface NPS contaminants and subsurface secondary salinity. Both of these hydrological menaces are managerial issues and need special policies from the law enforcement agencies. These NPS contaminants percolate with irrigation water and pollute the underlying aquifer. The pollution potential assessment of underlying aquifer because of these NPS lies beyond the scope of this study and has already been well explained by Hussain et al. (2016a, 2017), where the authors has linked the aquifer pollution potential to the aquifer recharge and the intensity of agricultural activities there.

The problem which is being highlighted in this study is the secondary salinity the source of which are unchecked pumping of groundwater by the tubewells (groundwater development). In KAS the problem of SS is depicted in the form of a conceptual model shown in Fig. 2. In the absence of managerial works, most of these tubewells are installed in those areas where groundwater is low quality fresh to marginally fit for agriculture. This secondary salinity alters the soil composition by reducing its permeability and hydraulic conductivity and finally, turned it into a barren land. These salt-affected soils are becoming an important ecological entity in the Indus Basin of Pakistan and making it questionable for agricultural use (Qureshi et al. 2003).

Fig. 2
figure 2

Conceptual model of secondary salinity and Source-Pathway-Receptor model in Kot Adhu Scenario

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Electrical resistivity survey

In electrical resistivity survey (ERS), evaluation of the resistivity of the ground is accomplished by transmitting a known amount of current (I) between two electrodes (current electrode) nailed into the ground and measuring the potential (V) between two other electrodes (potential electrodes). Direct current (DC) or an alternating current (AC) of very low frequency is applied, and the phenomenon is often called DC-resistivity. The resistance (R) is calculated using Ohm’s law. The geoelectrical method is described in details in many text books (e.g. Sheriff 1989; Sharma 1997; Keller and Frischknecht 1970; Kearey and Brooks 1984).

The apparent resistivity of ground can be calculated using:

$$\text{ }\!\!\rho\!\!\text{ a}=\text{KR,}$$
(1)

where K is the geometric factor and R, the ground resistance. MN and AB are the current electrode and potential electrodes, respectively.

There are different types of ERS depending on the way the electrodes are installed on the surface, but the most commonly used is Schlumberger electrode configuration (Yadav et al. 2010). This array is preferred for deeper penetrations and is relatively less labor extensive (Srinivasan 2013). In this configuration, two metal electrodes which are treated as potential electrodes are fixed at center and two other outer metal electrodes (Fig. 3) change their distance in order to target different depths in the subsurface.

Fig. 3
figure 3

Sketch of Schlumberger configuration

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Data acquisition and processing

The detailed investigation of the study area was carried out by VES at 5 km × 5 km grid with the investigated depth of 300 m as the main aquifer lies at that depth (Hussain 2014).

The earth’s resistivity obtained from resistivity curves are called apparent resistivity. In order to obtained useful information from these resistivity values they need to be correlated with some known hydrogeological conditions usually obtained from the bore holes, in Kot Adhu scenario as discussed in “Physiography and hydrogeology”, the alluvium is difficult to further differentiate into clay, sand and silt at available geological information. The required information for the conversion of geoelectrical section to hydrogeological sections is taken from PCRWR’s exploratory wells drilled at a regular grid of 25 km × 25 km in the entire TDA (Fig. 1). The percussion rigs drilling method was used to drill these exploratory wells with 450 mm diameter (Shah and Ahmad 2015, 2016). This survey was a result of joint project between the Pakistan Council of Research in Water Resources (PCRWR) and the Ministry of Science and Technology (Government of Pakistan) aiming at demarcating the groundwater quality zones in the Indus Plain and marginal areas for sustainable development and management from June 2003 to June 2008. About 1860 samples were tested for the water quality and lithological variations throughout the Thal Doab. The details of this project can be found at http://www.pcrwr.pk. There are very few exploratory wells lie in the study area so instead of considering Tehsil Kot Adhu, the data of all the wells in Thal Doab are taken for the regression plot. This will increase the data points and so the relation has more reliability. Below water table the resistivity is correlated with water quality at the regular interval of 3 m upto 90 m. As the depth to water table varies from point to point in the area therefore instead of taking only upper level of the aquifer the shallow water quality is mapped up to the depth of 50 m. The values of electrical conductivities of water samples taken from exploratory well and the bulk resistivity taken from the near by VES probe was plotted in excel and 3rd order polynomial trend is developed for the whole TDA as shown in Fig. 4. This polynomial equation is used to convert the resistivity values of all the VES probes to respective electrical conductivity.

Fig. 4
figure 4

Regression plot between earth resistivity (from resistivity models) and water resistivity (from exploratory wells) (PCRWR 2014)

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The combination of resistivity data with in situ total dissolved solids (TDS) or electrical conductivity measurements in wells can help identify shallow contaminated zones (Sonkamble 2014). The literature on the subject shows the possibilities of use of regression relation between earth resistivity and groundwater electrical conductivity (Cartwright and Sherman 1972; Peinado-Guevara et al. 2012; Sainato and Losinno 2006).

ArcGIS (Version 10.1) is used to map and analyze the data for the evaluation of groundwater quality. Inverse Distance Weighting (IDW) algorithm is used for the spatial interpolations of EC values at all six depths zones. Manual classification is used in which a single value is assigned to each class.

Quantitative interpretation of field curves is done by IX1D resistivity modelling software which uses standard curves and computer iteration techniques. The detailed description of the modelling can be assessed at Farid et al. (2017). Few modelled curves are shown in Fig. 5.

Fig. 5
figure 5

ID modeled resistivity curves

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For the representation of TWS classes a traffic color scheme is used. The green color indicates the high suitability of groundwater for tubewells installation, brown and red colors are representative of the moderate and low suitability zones, respectively. The GIS maps are developed at six depth zones as: 0–50, 50–100, 100–150, 150–200, 200–250 and 250–300 m. The developed maps are shown in Fig. 6a–f.

Fig. 6
figure 6

Tubewells suitability classes at various depths zones 0–50, 50–100, 100–150,150–200, 200–250, 250–300 meters. Legnd for all the maps is same

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Results and discussions

The interpreted sub-surface hydrogeological conditions were classified into three basic resistivity classes based on the criteria developed by WAPDA (1981) for groundwater quality. From the groundwater sample analysis, it is concluded that groundwater quality in subsurface layers vary from 0.5 to 4.6 dS/m for different resistivity values as shown in Fig. 6.

The first zone which start at the depth of water table has area wise proportions of TWSs as 36, 23.8 and 46.2% high, moderate and low stabilities, respectively (Fig. 7a). The availability of fresh water is high near the Indus River having electrical conductivity value less than 1.5 dS/m. A small portion of the study area under moderate suitability (23.8%). The electrical conductivity of this class ranges from 1.5 to 2.2 dS/m. Low suitability class has less importance because there is no groundwater development in it. Groundwater with such high amounts of salts is neither suitable for irrigation nor for domestic use. The quality of groundwater of this zone can be improved by decreasing the groundwater abstraction through tubewells and by increasing the groundwater recharge from surface water.

Fig. 7
figure 7

The area wise proportions of all suitability classes at each depth zone (a–f)

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In depth zone raging from 50 to 100 m the area wise portions are 56.4, 22.5 and 21.4% with suitabilities for tubewells installation as high, moderate and low, respectively. It can be seen from the Fig. 7b that proportion of fresh water has increased from 36 to 56.4% with depth of 50 m.

In third depth zone (100–150 m), the high suitability class has shrinked to 40.85% the proportions of other two zones are 16.83 moderate and 42.35% as low. It can be seen from Fig. 7 that the moderate suitability has reduced to low greatly which is mainly because of high abstraction rates than the recharge form canals and the rivers.

From 150 to 200 m depth the fresh water decreases to 34% and moderate to 15.5 while low suitability zone increases greatly with an area proportion as 50.5% of the total area.

At depth from 200 to 250 m the high suitability zone has decreased to 27.4% the Fig. 6 shows that this zone become narrower with depth and shrined to lower south western edge of the Kot Adhu. The moderate and low suitabilities have areas percentages as 17.5 and 55.1, respectively in this depth zone.

In last zone (250–300 m), the situation is quite the same with area wise proportions of classes are 27, 16.48 and 56.55% as high, moderate and low suitabilities for tubewells installation, respectively.

At the active flood plain of the Indus River the low conductivity class is quite consistent even at greater depths because of the availability of surface water at greater depth. Along with this fresh groundwater quality at greater depth, the flood plains of the Indus River have favorable geological materials (good transmissive and hydraulic conductivities) for aquifer recharge and is a home of intense agriculture. The distance from the flood plain in eastern parts towards the Chenab River where recharge source is only precipitation, water quality deteriorates greatly.

The EC variations with depth show that there are very little variations of groundwater quality near the Indus River and its canal system while this quality goes on deteriorating in non-canals command areas of Tehsil Kot Adhu. These results are matched well with the results obtained from the works of other researchers in the area (e.g. Khan et al. 2012). These results differ from the results of Hussain et al. (2017), where authors had concluded that groundwater vulnerability to surface contaminates in the TDA are related to the availability of fresh water form the canals and rivers. One possible reason of these contradictions between these two researches is that, the previous study discussed the probability of vertical movement of the contaminants from the surface to groundwater which is completely a different mechanism as discussed in the present study. This study is empirical as contrast to previous probabilistic studies done by Hussain et al. (2017).

In KAS the tubewells and canal densities are related directly and inversely to the aquifer vulnerability and secondary salinity, respectively. Similar affects of recharge on the arsenic concentration were observed by Hussain et al. 2016c in the District Layyah which lies at the northern side of the study area having similar hydrological characteristics. The concentration of Arsenic in groundwater was found related positively with the amount of recharge from the Indus River.

Conclusions

This study was carried out to divide the study area into different tubewell suitability classes based of the EC values derived from vertical electrical soundings. From this analysis it is clear that the availability of fresh water is directly related to recharge from rivers and canals. The results of this study can be utilized by researchers and administrative authorities both at local and regional scales for an environmentally sustainable agricultural growths and thus a check on the unplanned growths of tubewells in the region. The tubewell installation must be balanced in fresh water regions and be avoided in both moderate and low suitability classes.

Following are the main conclusions drawn from this analysis:

  • Electrical resistivity variations in the area are mainly because of variations in groundwater quality.

  • Fresh water occurs in the form layer over saline water whose quality varies with the depth from the surface and distances from the recharging agents like rivers and canals.

  • Central and eastern parts of Kot Adhu have high salinity values because of the absence of well developed canal system as well as distance from the Indus River.