The influence of geoelectrical parameters on the Quaternary aquifer and its protectivity and contamination: a case study from the Khanasser Valley Region, Northern Syria

The groundwater contained beneath the Earth's surface in rocks and soil and stored underground in aquifers is subjected to the contamination of the surface (Ikpe et al. 2021). The groundwater quality can be negatively affected by anthropogenic activities such as poor waste disposal management, which leads to leakages from the surface and underground storage tanks, sewage from oil spillage, mining activities and latrines and the movement of leachates from dumpsites into subsurface hydrogeological units (Oseji et al. 2018).

The hydrogeological aquifer system can be subsequently protected by the above overlying earth layers depending on their thicknesses, hydraulic conductivities and permeabilities. The aquifer protectivity in such a case can measure the aquifer overburden layers to behave as natural boundaries to filter percolating surface contaminated fluid (Adeniji et al. 2014). The major factors affecting the protectivity of the underlying hydrogeological aquifer are essentially thickness, porosity, grain size and permeability of the overburden-protecting layers (Mogaji et al. 2011; Adeniji et al. 2014; Ekanem 2020; Ekanem et al. 2020; Ikpe et al. 2021).

The permeable overlying layers, such as gravels and sand having high hydraulic conductivity and high resistivity, facilitate the easy movement and infiltration of surface fluid contaminants into the aquifer system. On the contrary, the impervious overburden units such as shale and clay slow and retard the rate of infiltration of surface contaminants into the subsurface, due to their low hydraulic conductivity and low resistivity, where the protectivity aquifer is considerably enhanced (Adeniji et al. 2014; Ayuk 2019; Ekanem et al. 2021).

The one-dimensional (1D) VES resistivity sounding data interpretation allows us to obtain the thicknesses and real resistivities of the subsurface lithological units penetrated by the electrical current passage under each VES point. The parameters of thicknesses and real resistivities are thereafter used to estimate the secondary geoelectric indice parameters (SGIP). The analysis of the SGIP of longitudinal conductance, reflection coefficient (RC), fractured constant (FC) and anisotropy coefficient is important in qualifying the aquifer protectivity and characterizing the aquifer overburden layers.

The total longitudinal conductance of the aquifer-protecting layers is proportional to aquifer protectivity (Abiola et al. 2009; Asfahani 2023).

The impervious materials’ overburden layers characterized by a total longitudinal conductance greater than one have good/excellent protectivity characteristics. On the contrary, the previous overburden layers, characterized by a total longitudinal conductance of less than 0.1, have poor/weak protection characteristics to the underlying aquifer systems.

An impermeable low-resistivity layer, such as shale or clay overlying a permeable layer of high resistivity, such as gravels and sands, yields a positive reflection resistivity coefficient, which can be used to determine the protectivity of aquifer systems. Positive values of reflection resistivity coefficient indicate good/excellent protectivity and vice versa.

The FC is big as much as the overburden-protecting impervious layer resistivity is low, where an inverse relationship exists between them.

The resistivity anisotropy coefficient (λ) of bigger than 1 indicates the materials’ resistivity variations according to the directions (Asfahani 2007, 2023; Yeboah-Forson & Whitman 2013). A sand–clay intercalation sequence can create such an anisotropy phenomenon, particularly in sedimentological environments (Asfahani 2007, 2023; Bala & Cichy 2015). λ coefficient can be, therefore, used as one of the important parameters affecting the aquifer protectivity and is related directly to the infiltration of surface water into the subsurface.

The overburden protective capacity (OPC, Ω⁻¹) of the Quaternary aquifer in the Khanasser Valley, Northern Syria has recently been assessed by analyzing the protecting layers and their longitudinal conductance of (Ω⁻¹) according to the classification of Henriet (1976); Asfahani (2023).

This paper concentrates basically on studying and analyzing the influence of several geoelectrical parameters affecting the aquifer protectivity and the OPC. A statistical correlation matrix is applied to evaluate the different mutual relationships between OPC and the treated geoelectrical parameters. Those parameters are overburden thickness (h_Overb), RC, FC and anisotropy coefficient (λ) of the subsurface hydrogeological layers.

In the Khanasser Valley, three main aquifers are used for groundwater extraction and the deepest one is the upper Cretaceous, located at 400 m under the ground level. The second low-productive aquifer of Paleocene-Lower Eocene limestone is found above the Maastrichtian (ACSAD 1984). The average hydraulic conductivity (k) of this aquifer is 0.0054 m/day (Schweers et al. 2002). The Paleogene strata of around 50 m of Paleocene and lower Eocene in the central part of the Khanasser valley are not sufficiently thick over the Maestrichtian formation. The Quaternary aquifer, the most transmissive structure in the study region, is located near the surface and covered by proluvial and alluvial sediments of about 10 m. The main direct recharge of this aquifer is provided by the rainwater, the infiltrating runoff and the subsurface flow from the Jabal Shbeith and Jabal Al Hoss slopes.

The Schlumberger electrode configuration gradually increases the current electrode distance separation of AB and occasionally separates the potential electrode distance MN about the centre of the configuration to penetrate deeper layers and complete the field VES resistivity curve. AB/2 is ranged between 3 and 500 m, while MN is ranged between 0.5 and 20 m. The electrode spacing in all the VES sounding points was such as AB≥5 MN in accordance with the potential gradient assumption (Keller & Frischknecht 1966; Dobrin & Savit 1988).

Thirty-four VES sounding points were carried out in the study of the Khanasser area, as shown in Figure 2.

Some shallow available boreholes with their lithological descriptions close to some VES points are used to calibrate the VES sounding interpretations and help in the modeling interpretation stage of the acquired data.

The field apparent resistivity values (ρ_a) computed and obtained for each VES sounding location were plotted against (AB/2) on a log–log scale to produce the complete VES sounding curves.

This ID inversion interpretative technique allows us to get the final optimum inverted model curves with their first-order geoelectric indices, which are the true layer resistivity, thickness and depth accordingly.

Five available shallow boreholes with their lithological descriptions in the Khanasser area were used to constrain the inversion schema.

The overlying materials controls mainly the protectivity of a given hydrogeological unit. The protectivity in this case research is the ability of the overlying layers to act as protecting layers to retard and filter percolating surface fluid (Ekanem et al. 2021). The key factors affecting the protectivity of the underlying hydrogeological units are the permeability, grain size, porosity and thickness of the protecting layers (Mogaji et al. 2011; Adeniji et al. 2014; Ekanem 2020; Ekanem et al. 2021). In fact, sufficient impermeable and thick overlying protecting layers with low hydraulic conductivity contribute largely to retarding the infiltration of water into the subsurface and hence improve the aquifer system protectivity (Adeniji et al. 2014; Ekanem et al. 2021).

The overburden layer resistivity and thickness are the two basic primary geoelectric parameters that can be used conjointly according to (Henriet 1976) to evaluate the longitudinal conductance (Ω⁻¹), which is proportional to the aquifer protectivity (Henriet 1976; Abiola et al. 2009; Asfahani 2023).

The resistivity RC, the FC and the anisotropy coefficient (λ) are three main geoelectrical parameters that take into consideration the resistivity variations between the aquifer layer and its overlying protecting layers. The main targeting objective of this paper is to establish the mutual empirical relationships between those parameters and aquifer protectivity (OPC).

FC is inversely proportional to the overburden resistivity (ρ_n−1). The impervious shale and clay with low-resistivity values contribute to increasing FC.

λ is evaluated by knowing the total transverse resistivity (ρ_t) and the total longitudinal resistivity (ρ_l) of the subsurface hydrogeological units penetrated by the electrical current passage (Asfahani 2023).

The permeable gravels and sands have high-resistivity values and low longitudinal conductance values. On the contrary, the impervious clay and shale have high longitudinal conductance and low-resistivity values (Adeniji et al. 2014; Ayuk 2019). The materials characterized by high values of longitudinal conductance greater than 1 are accompanied by good/excellent protectivity, while those characterized by longitudinal conductance of less than 0.1 are accompanied generally by poor/weak aquifer protectivity (Henriet 1976; Abiola et al. 2009; Asfahani 2023).

The anisotropy can be regarded as a result of a thin sequence of sand–clay intercalation (Asfahani 2007, 2023; Bala & Cichy 2015; Ekanem 2020).

Table 1 includes the resulting geoelectrical parameters of the RC, FC, anisotropy of the subsurface hydrogeological layers (λ) and the overburden-protecting layers (h_Overb).

The statistical correlation matrix applied, as shown in Table 2, allows us to derive and establish the mutual empirical relationships between those different geoelectrical parameters and the OPC.

The higher FC values are concentrated on VES 10-2 in the extreme northern part of the study area as for RC. The area with lower FC values is regarded as a good indicator of a high water potential weathered area (Olayinka 1996; Asfahani 2021).

An empirical linear equation is found between the OPC and the FC, as indicated in Figure 13.

FC is bigger as the overlying layer resistivity is low, as indicated by Equation (5).

The three empirical trends are expressed by exponential equations as follows:

The negative trends indicate different hydraulic systems in the Khanasser Valley region and show that as much λ is bigger as the OPC is lower.

The correlation matrix shown in Table 2 indicates an acceptable correlation between h_Overb and OPC (0.47). Table 3 summarizes the results of the geoelectrical parameters obtained by studying and analyzing the 34 VES points in the study region, for assessing the different parameters having their influences in affecting the protectivity of the Quaternary aquifer from the fluid contamination originating in the Khanasser Valley, Syria.

The VES technique is used in this paper to assess and evaluate the protectivity of the hydrogeological units of the Khanasser Valley region, Northern Syria. The influence of some geoelectrical variables that considerably affect the protectivity of the Quaternary aquifer in the study region is analyzed and documented.

The 34 VES points were interpreted quantitatively in terms of 1D structure by the curve matching technique to obtain an initial model of the corresponding thicknesses and resistivities. This model is thereafter used as an input in the WINRESIST computer software program to obtain the final geoelectrical inverted optimum model for every treated VES point.

The available shallow borehole lithological data in the study region and their correlations with the quantitative VES data interpretations confirm that the geoelectrical layers in the study area are mainly composed of sandy layers (fine, coarse and gravelly) with clay intercalations. Such a lithological sequence dominates the geoelectrical variables that control the OPC of the Quaternary aquifer. Those variables are the overburden-protecting layers (h_Overb), RC, FC and anisotropy of the subsurface hydrogeological layers (λ). The OPC was already assessed by evaluating the longitudinal conductance of the overburden-protecting layers (Asfahani 2023). The study area has been classified according to the distribution of the longitudinal conductance and OPC(Ω⁻¹) as 56% poor, indicating high vulnerability to contamination, 5.9% weak, 14.7% moderate, 20.6% good and 2.9% excellent (Asfahani 2023).

The statistical correlation matrix used in this paper allows us to obtain and establish different mutual empirical equations between (RC, FC, λ and h_Overb). These empirical equations indicate the considerable influence of the mentioned geoelectrical variables on the spatial distribution of the OPC.

The different geoelectrical results obtained and documented provide useful information that largely helps in mapping out and locating the effectively protected groundwater area. The approach developed in this research paper is applied for the first time in Syria and can be easily practised worldwide with its different steps to assess the different protectivity conditions surrounding an aquifer.

The author would like to thank Dr I. Othman, General Director of the 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. Prof. Late Rieser Armins (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 great help during the different stages of the project.

The three competent reviewers are cordially thanked for their professional remarks, critics and suggestions that considerably improve the final version of this paper. The Editor-in-Chief and the staff of WPT are cordially thanked for their assistance during the different stages of paper publication.

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

The authors declare there is no conflict.