Geo-electrical investigation for characterizing the shallow Quaternary aquifer parameters and vulnerability to contamination: a case study from semi-arid Khanasser Valley region, Syria

Water is an important and fundamental resource to the existence of life and also the most dominant in the earth system. It exists in various forms as streams, lakes, rivers, oceans, and groundwater. The water contained beneath the earth's surface in rocks and soil is called groundwater that is stored underground in aquifers. Groundwater is readily available as an alternative source of water for domestic, public and industrial usage, particularly in regions where there are limited surface water bodies and usually require minimum treatment to make it potable (Bayewua et al. 2018; Umar & Igwe 2019). The location of the groundwater in the weathered fractured rocks and soils in the pores presents many challenges for managing and quantifying the groundwater if compared with surface water. Contamination of the groundwater could pose a serious problem to the government and the people living in a given area, and can lead to diseases affecting human health. The degradation of groundwater quality could be caused by surface contaminants infiltration into the layers of the aquifer due to some environmental or geologic factors (Van Stempvoort et al. 2013; Obiora et al. 2015a, 2015b; George et al. 2016; Ibuot et al. 2017). As a result of the contamination of surface water bodies, there has been an increasing demand for groundwater to meet the world population growth (Reilly et al. 2008; George et al. 2015).

The availability of good-quality groundwater depends essentially on the permeability and porosity of the host rocks. The lithological units and the geological properties of a formation, which differ significantly from place to place, control the capacity of the hydrological unit to store groundwater.

The groundwater and electrical currents depend upon their potential gradients and are known to run from sites with higher potential to sites with lower potential. The electrical and hydraulic conductivities of an aquifer are therefore predicted to be influenced by similar variables and manner, as ions flow through water paths. The description and quantitative analysis of the aquifer units is a useful tool in both contamination migration modeling and groundwater research (Freeze & Cheery 1979; Asfahani 2007, 2013; Asfahani & Abou Zakhem 2013; George et al. 2015).

The cheap geoelectrical resistivity technique with its different possible applications and methodologies is widely used in groundwater exploration research, since it gives better and more rapid results in comparison with other geophysical methods (Zohdy et al. 1974). The resistivity which influences the artificial or natural electrical fields generated in the earth's subsurface depends on the lithology, the existence of water content, and the quality of water and pores.

The quantitative interpretation of vertical electrical sounding (VES) data constrained with logged borehole information is one of the approaches widely practiced for solving different hydrogeological and environmental problems (Oguama et al. 2019). Recent research demonstrates and documents the importance of using the VES technique in a hydrological domain. Hasan et al. (2019) applied an integrated approach including electrical resistivity with different geophysical methods to investigate the hard rock aquifers occurring in the weathered terrains of south China. They concluded that the high potential aquifers are contained within the fractured and weathered zones. Hasan et al. (2021) employed also VES technique in Huizhou ADS site in China to delineate the aquifer's potential zones contained within the weathered rocks. They used the available hydraulic conductivity and transmissivity data from the existing borehole pumping tests to establish empirical relationships with aquifer resistivity to be generalized on the entire study area.

De Almeida (2021) indicated the importance of applying VES technique for preliminary porous aquifer characterization, especially in the regions absent or with insufficient monitoring wells.

Akingboye et al. (2022) demonstrated the advantages of integrating different geophysical techniques to investigate the near-surface crustal architecture and geohydrodynamics of the crystalline basement terrain of Araromi, Southwestern Nigeria. Akingboye (2022) applied the VES technique with the Schlumberger array to assess the groundwater yield of aquifer units and their vulnerability to contaminants in Araromi, Southwestern Nigeria. Ikpe et al. (2022) applied VES to assess the protectivity of hydrogeological units in Ikot Ekpene Urban and its environs in Southern Nigeria, where valuable information has been provided to design effective groundwater and waste disposal management in the study area.

Aretouyap et al. (2022) used the VES technique with the contribution of the fuzzy algebraic model to the sustainable management of groundwater resources in the Adamawa watershed, where new hydrological insights combining six hydro-parameters contributing to groundwater occurrence in a GIS environment are discussed and documented to assess the groundwater potential (GWP) in the study region.

Abdulrazzaq et al. (2023) used also the VES technique to determine the optimum drilling sites for groundwater wells based on the hydro-geoelectrical parameters and weighted overlay approach via GIS in the Salah Al-Din area, central Iraq.

Given the above, this paper simulates the recent hydrogeological trends and therefore employs the VES and regression modeling techniques to determine and predict the spatial variations of the different aquifer parameters and groundwater quality in shallow Quaternary aquifers in the Khanasser region, Northern Syria.

The original news in the paper is to characterize the shallow Quaternary aquifer in the Khanasser Valley area by using the regression analysis modeling technique and developing the Dar-Zarrouk parameters (DZPs), resulting from interpreting the VES datasets. Those (DZP) allow us to predict the water resistivities even in locations where no samples of water are available. The first new predicting procedure developed in this paper permits consequently to characterize the parameters of electrical conductivity (EC), total dissolved solids (TDS), formation factor (F), and porosity (Ф%) of the Quaternary aquifer, and follow their spatial variations in the study region. The second new aspect in this research paper is an attempt to characterize the aquifer's vulnerability to contamination and corrosion by adopting a specific methodology approach, aimed at evaluating and identifying both the overburden protective capacity (OPC) and the corrosion of the aquifer (Corr). This new aspect, highly important for both industry and the environment is applied for the first time in Syria. The environmental topic of this paper is oriented therefore toward achieving and determining safe drinking water locations in accordance with sustainable development goals (SDGs).

The main objectives of this paper are therefore to determine and estimate the geoelectrical characteristics of the subsurface lithological units in the Khanasser Valley area by applying an integrated VES methodology approach. This application is to get information about the subsurface layer descriptions and the vulnerability of the shallow Quaternary aquifer to contamination as follows:

• Characterizing the subsurface geoelectrical layers through interpreting 34 VES data points, and their related DZP of total resistance (TR), transverse resistivity (ρ_t), total conductance (S), longitudinal resistivity (ρ_l), and anisotropy (λ).

• Establishing a calibrated equation line between transverse resistance (TR) related specifically to the shallow Quaternary aquifer and modified transverse resistance (MTR). This calibration line allows us to extrapolate and deduce the values of water resistivity samples in the VES points, where water samples do not exist. The EC, TDS, F, and Ø% are consequently derived for the shallow Quaternary aquifer in the study area.

• Determining the corrosion (Corr) and the vulnerability to contamination through analyzing the longitudinal conductance (Ω⁻¹) and shallow aquifer overburden capacity protection (OPC).

• Mapping the different hydrogeophysical parameters related to the shallow Quaternary aquifer (EC, TDS, F,Ф, TR, S, ρ_t, ρ_l, λ, OPC, and Corr) in the study Khanasser Valley region.

The groundwater extraction in the Khanasser Valley is carried out through three existing aquifers. The deepest aquifer is the upper Cretaceous, 400 m below the ground level. The second aquifer of Paleocene-Lower Eocene limestone is located above the Maestrichtian and is of low productivity (ACSAD 1984). The average hydraulic conductivity (k) of this aquifer is equal to 0.0054 m/day (Schweers et al. 2002). In the central part of the Khanasser Valley, the paleogene strata of around 50 m of Paleocene and lower Eocene is not too thick over the Maestrichtian formation.

Figure 2(a) shows a geological cross-section along the AB profile indicated in Figure 2.

In total, 34 vertical electrical soundings (VES) disseminated on three longitudinal profiles (LP1, LP2, and LP3) were performed across the Khanasser Valley study area, with the use of the Schlumberger electrode configuration shown in Figure 2(b).

The complete field apparent resistivity curve is obtained by increasing the electrode spacing AB/2 about a fixed point. Conventional manual curves (CMC) and auxiliary charts were used to smooth this field apparent resistivity curve to eliminate the effects of lateral inhomogeneities, and to quantitatively interpret it in terms of true resistivity and thickness (Orellana & Mooney 1966; Zohdy et al. 1974). To improve upon the initial manually interpreted VES data, the initial parameters of the approximate model are thereafter interpreted by an inversion WINRESIST software of Velpen (2004) to accurately get the final optimum model, in which a good fit between the final theoretical regenerated curve and the field apparent resistivity curve was obtained (Zohdy 1989; Zohdy & Bisdorf 1989). The final optimum model includes the values of real resistivity, depth, and thickness of each geoelectrical layer. Both layer resistivity (ρ) and thickness (h) are two fundamental parameters for understanding and interpreting the geoelectrical models for characterizing and describing the subsurface hydrogeological units under each studied VES.

The methodology approach applied in this research consists of two parts. The first part characterizes the shallow Quaternary aquifer in the study Khanasser Valley area, by determining the spatial variations of EC, TDS, F, and Ø%.

The second part evaluates the OPC of the Quaternary aquifer, and the corrosion (Corr) parameter in the study region.

This methodology approach in its two parts is based essentially on the second geoelectrical indices of DZP as described below.

The fundamental parameters describing the geoelectrical layers are derived from the quantitative interpretation of the VES datasets, as the real resistivity (ρ_i) and thickness (h_i) values along the study area; where the subscript ‘i’ refers to the layer position in the geoelectrical section. Some geoelectrical parameters can be also derived from these basic resistivity and thickness parameters, such as the total longitudinal conductance (S) and total transverse resistance (TR). These have been illustrated as the DZP (Maillet 1974; Oladapo & Akintorinwa 2007; Asfahani 2016; IKpe et al. 2022).

A unit square cross-section area cut out of a group of n-layers of infinite lateral extent is taken into consideration in order to obtain the hydro-geoelectrical parameters as follows:

Generally, the longitudinal resistivity (ρ_l) is less than the transverse resistivity (ρ_t) unless the medium is uniform (Flathe 1955). Keller (1987) also suggested that the more conductive layers dominate the distribution of (ρ_l) (the clay and weathered/sediments dominate in the present case study, even if a small fraction of resistive layers is present).

The DZPs are developed in the present paper in order to characterize as precisely as possible the shallow Quaternary aquifer and its vulnerability to contamination in the Khanasser Valley, Northern Syria. In fact, those parameters help in interpreting and analyzing the structural characteristics and subsurface lithology with reduced uncertainties (Maillet 1974). In addition, the anisotropy parameter plays an important role in aquifer vulnerability and groundwater assessment.

In which is the average water resistivity value of the available 15 water samples (3.35 Ω m), and (ρ_w)_i is the water resistivity at the location of the (VES)_i point.

In which ρ_e is the saturated Quaternary aquifer resistivity estimated from the quantitative interpretation of (VES), and ρ_w is the pore fluid resistivity.

The dimensionless coefficients a and m depend on the rock type.

The values of a = 0.88 and m = 1.37 were taken in this study following the values given by Keller (1988) for sands, sandstone, and some limestone (Asfahani 2007).

The OPC of an aquifer is defined as the ratio of the overburden thickness to its resistivity (Omoyoloy et al. 2008). The greater the protective capacity the greater the overburden longitudinal conductance (Ω⁻¹).

DZPs resulting from the quantitative interpretation of VES soundings are used herein to evaluate the shallow Quaternary aquifer protective capacity, and longitudinal conductance of overburden units in the Khanasser Valley region. The identification of the groundwater areas vulnerable to contamination or could be contaminated by leaching is an important factor in subsurface exploration hydrogeology. The groundwater contamination depends on porosity, permeability, and the overburden thickness of the geological layers. The unconsolidated and un-compacted coarse sand is an example of underlying material that helps and favorites the capability of polluting influents to infiltrate and escape into the subsurface to contaminate the groundwater, form a polluting plume extending hundreds of meters, and render the soil corrosive (Keswick et al. 1982).

A leakage, rupture pipeline, or failure pipeline could create serious hazards to the assets, environment, and even humans due to the leakage and explosion (Yahaya et al. 2009). The environment impacts can be easily evaluated by using geoelectrical techniques without interfering with the hydrogeologic system (Mogaji et al. 2007).

Henriet (1976) explained that the combination of aquifer layer thickness and resistivity using DZPs of transverse resistance and longitudinal conductance may be of a direct application in aquifer protection research, where hydrological properties of aquifers and also the protective capacity of clayey aquifer overburden can be evaluated. The longitudinal conductance (DZP) is used in this paper as a criterion for determining the shallow Quaternary aquifer protective capacity rating according to the rating shown in Table 1.

Soil corrosivity (Corr) is estimated by taking into consideration the resistivity value of the first layer for each VES location in the study region, and by refereeing and comparing with the corrosivity rating indicated in Table 2.

The earth's medium acts as a natural filter for the percolation of fluid. The ability of the earth to accelerate or retard and filter fluid percolation is a measured parameter of its protective capacity (Barker et al. 2001). The zones of poor and weak protective capacity are those vulnerable to surface contaminant materials. The appreciable overburden thickness with clayey columns, which are thick enough to represent the moderate to good protective capacity zones. Those overburden layers protect the shallow aquifer in the study area from surface polluting fluid.

This calibration equation line is used thereafter to predict and extrapolate the values of the resistivity water samples ρ_w(Ω.m) in the 19 VES points, where no water samples are available, as shown and indicated in Table 4.

The conjoint VES data (15 + 19) presented in Tables 3 and 4 will be used to characterize the spatial variations of the treated parameters for the entire Quaternary aquifer in the study area.

Table 5 shows the DZPs of TR, S, ρ_t, ρ_l, and λ computed according to Equations (2), (3), (4), (5) and (6), respectively, for the 34 VES points in the Khanasser Valley region.

The longitudinal resistivity ρ_l varies between a minimum of 1.86 Ω m at V2-4 and a maximum of 15.7 Ω m at Sh13, with an average of 5.68 Ω m and a standard deviation of 3 Ω m as indicated by Figure 6(b).

The transverse resistivity ρ_t varies between a minimum of 2.94 Ω · m at V2-4 and a maximum of 25.5 Ω · m at V2-5, with an average of 8.41 Ω · m and a standard deviation of 4.9 Ω · m as indicated by Figure 7(b).

The sedimentological nature of the Quaternary aquifer composed of unconsolidated materials such as alluvial, proluvial, and lacustrine deposits provokes such high anisotropy values of λ (Asfahani 2013). The anisotropy results reported in this paper are in good agreement with those obtained by azimuthal VES configuration already applied for studying four VES points (V2-3, V8-2, V10-2, and V6-3) in the Khanaser valley (Asfahani 2013).

Figure 9(b) shows the cross plot between the anisotropy (λ) and the transverse resistance (TR) of the Quaternary aquifer. One can notice again the presence of three positive trends between λ and TR, reflecting different hydraulic systems in the study Khanasser region.

The distribution of the TDS of the Quaternary aquifer in the study Khanasser Valley obtained by integrating the 34 VES points (Tables 3 and 4) is shown in Figure 10(b).

TDS varies between a minimum of 860 ppm at the VES location Sh12 and a maximum of 6,400 ppm at the VES location, V6-2 with an average of 2,592 ppm and a standard deviation of 1,208 ppm.

Asfahani (2007) has already developed different empirical relationships between earth resistivity, water resistivity and the available TDS data to predict and derive the apparent salinity TDS of only 19 VES points in the study region for different AB/2 of 70, 100, and 150 m (Figure 8). While the established TDS values in this paper are real and more accurate than previously for the following reasons:

1. They are based on using 34 VES data points (19 + 15) according to the methodology developed and described in this paper, with the use of basically Dar-Zarrouck parameters.

2. The real resistivity and thickness of the 34 VES data points related to the Quaternary aquifer are taken into consideration while deriving the real TDS values.

The distribution of the Ø% of the Quaternary aquifer in the study Khanasser Valley obtained by integrating the 34 VES points (Tables 3 and 4) is shown in Figure 11(b). Ø% varies between a minimum of 2.8% at the VES location V6-2 and a maximum of 44% at the VES location V3-1, with an average of 16.9% and a standard deviation of 11.8%.

Table 6 summarizes the geoelectrical and hydrological parameters obtained through studying and analyzing 34 VES points for characterizing the Quaternary aquifer in the Khanasser Valley, Syria.

The OPC of the shallow Quaternary aquifer is classified according to Tables 1, 3 and 4 as 56% poor, indicating high vulnerability to contamination in the VES locations of V9-3, V2-5, Sh13, V8-3, V3-1, V3-5, V5-4, V10-4, V10-1, V9-1, V9-4, V8-2, V5-3, V5-5, V4-3, V3-3, V2-3, V2-4, and V1-2, 5.9% weak in the VES locations of V6-1 and V3-4, 14.7% moderate in the VES locations of V3-2, V7-2, V7-3, V9-2, V2-1, and V2-2, 20.6% good in the VES locations of V1-1, Sh11, Sh12, V6-2, V10-3, and V6-3, and 2.9% as excellent in the VES locations of V10-4.

The percolation of the fluid is originated from the earth's medium that acts as a natural filter. The OPC is the measure of the earth's ability to accelerate or retard and filter fluid percolation (Barker et al. 2001).

The resistivity of the first layer in the study area is used to determine the soil corrosivity according to Tables 2–4.

The corrosivity ratings of the study area are classified as 26.5% slightly corrosive (SC) in the VES locations of V9-3, V8-3, V3-5, V5-4, V5-5, V4-3, V3-3, V2-3, and V1-2 where the resistivity of this first layer is between 60 and 180 Ω · m.

61.8% moderately corrosive (MC) in the VES locations of V6-1, V1-1, Sh11, Sh12, Sh13, V3-1, V3-2, V7-2, V7-3, V6-2, V10-4, V10-1, V9-2, V9-4, V8-2, V6-3, V5-3, V3-4, V2-1, V2-2, and V2-4 where the resistivity of this first layer is between 10 and 60 Ω · m.

The different information obtained through the Dar-Zarrouk geoelectrical parameters is highly important for the industries and for the locations of iron pipes to assuring the safeguarding of the hydrological setting for resident's safety in the study area. The suitable site locations for drilling boreholes must be selected in the regions with moderate/good protective capacity.

The methodology described in this paper is applied to a case study from the Khanasser Valley region. It is practiced for the first time in Syria for characterizing the Quaternary aquifer in the study region, and can be easily transferred to cover and deal with different aquifers in the country. Its novelty and originality are proven, where it can be extended to other worldwide applications with different geological contexts.

The electrical resistivity (VES) technique is an efficient tool for most groundwater studies. It is used in this paper to investigate the characteristics of hydrogeological parameters of the Quaternary aquifer in the Khanasser Valley region, Northern Syria. An integrated methodology approach based on the DZPs is developed in this paper to investigate the parameters of EC, TDS, F, Ø%, TR, S, ρ_t, ρ_l, λ, OPC, and Corr. The calibrated empirical equation established between transverse resistance TR and the MTR allows us to extrapolate the values of the water resistivity in the VES locations where no water samples are available. This new calibration proceeding permits getting the EC, TDS, F, and the Ø% of the Quaternary aquifer in the entire study area. The management and modeling of water resources in the Khanasser Valley require such calibrated information.

The longitudinal conductance (DZP) is used to determine the OPC of the shallow Quaternary aquifer in the study area. The hydrological properties of the Quaternary aquifer and the protective capacity of a clayey aquifer overburden are also evaluated through the combination of aquifer layer thickness and resistivity using DZPs of transverse resistance and longitudinal conductance. The resistivity of the first layer is used to evaluate the corrosivity (Corr) in the study area.

This paper shows the importance of the electrical DZPs of transverse resistance, longitudinal conductance, and anisotropy in solving and obtaining the different constrained hydrogeological parameters. The integrated geoelectrical results obtained in this paper reasonably provide information on areas where industries can be sited, and iron pipes can be laid in order to safeguard the hydrological setting for resident's safety in the study area. Regions with moderate/good protective capacity are good sites for locating safe boreholes available for drinking water. The paper as designed through its specific useful methodology stimulates new insights and important environmental questions on the relations between aquifer depth and vulnerability of the water wells at certain depths, as a result of pollution related to the different human activities. It was also shown the importance of determining safe groundwater locations to get suitable drinking water in accordance with SDGs. One of the SDG is to ensure human health as regards water health and sanitation (WASH).

The novelty and the importance of the integrated methodology approach developed and applied in this paper are well proven and demonstrated at both industry and environmental levels. This new methodology approach can be therefore easily practiced and used worldwide in other similar geological contexts.

The author would like to thank Dr I. Othman, General Director of Syrian Atomic Energy Commission for permission to publish this research work. The German Ministry of Economic Cooperation and Development (BMZ) and German Agency for Technical Cooperation (GTZ) are acknowledged for financial and administrative support to the Khanasser Valley Integrated Research Site (KVIRS) project. The late Professor Armin Rieser (coordinator of the project) from Bonn University, Germany is deeply thanked for many useful discussions during the preparation stages of this project.

ICARDA is highly thanked for providing the facilities and the logistics during the realization of the Khanasser Valley project. Dr Fares Asfari from ICARDA is cordially thanked for his many help during the different stages of the project. The two competent reviewers are cordially thanked for their professional critics, remarks, and suggestions that considerably improved the final version of this paper.

This work is part of an international scientific research under the No. 97-2001, which is totally funded by the authority of the Atomic Energy Commission of Syria.

This article does not contain studies with human or animal subjects.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.