## 2 Materials and Methods  
### Methodology Used in the Study  
Due to economic factors and efficiency, the recommended method for the environmental investigation of a landfill site is the electrical resistivity test. The advantages of this technique are: (1) the precision in measuring the electromagnetic excitation and neutral field, including the continuous monitoring of such parameters; (2) close relationships between the parameter magnitude, chemical composition, and mechanical properties; (3) the established and verified methods of complex electrical resistivity and geochemical testing; (4) allowing a physical transformation of the investigated material, into chemical characteristics; (5) the capability of geochemical and hydro-geochemical change monitoring, as well as a stress determination of the soil layer; and (6) the non-invasive character of the tests, free of an environmental impact and causing no changes in the soil hydrology and structure. Furthermore, the method allows: (1) the identification of the lithology of a landfill subsoil; (2) the determination of the groundwater table depth; (3) the determination of the distribution of the contamination zones and the direction of the pollutant migration; and (4) the evaluation of waste thickness disposed at a landfill site. The resistivity method has been of great use over the last 50 years [7; 11; 19; 20; 21; 22]. In practice, the electrical resistivity of the soil is measured using a four-electrode configuration. In this method, an electrical current is injected into the soil using two electrodes (A and B), whereas M and N electrodes measure its potential. The apparent resistivity of the soil can be determined, based on the known difference between the electric field potential (\(\Delta V\)) and the current (\(I\)), and the distance between the electrodes [11]. The resistivity is given by the equation:  
\[\rho_{a}=k\frac{\Delta V}{I} \tag{1}\]  
where:  
\(\rho_{a}\)--resistivity of a bedrock,  
\(\mid\)--intensity of current applied to the soil by electrodes AB (mA),  
\(\Delta V\)--differential potential between electrodes MN (mV),  
\(k\)--geometrical coefficient of electrode positioning (m). The geometrical factor \(k\) is dependent on the distribution geometry of the electrodes, as follows:  
\[k=\frac{2\pi}{\left(\frac{1}{\Delta\Delta}-\frac{1}{\Delta N}-\frac{1}{ \Delta N}+\frac{1}{\Delta N}\right)} \tag{2}\]  
where:  
\(\overline{AM}\), \(\overline{BM}\), \(\overline{AN}\), \(\overline{BN}\)--distance between electrodes (m). The resistivity does not reflect the properties of one particular soil type; however, it presents the capability of electrical conductivity measurements for the investigated subsoil. Such resistivity is called apparent resistivity, and depends on the resistivity of a particular soil type, its deposition, and the distance and distribution between the electrodes. The geoelectrical investigation is based on the measurement of apparent resistivity, accompanied by a symmetrical increase of distance between charged electrodes A and B. The width of the distance dipper for the electric field penetration of the subsoil, significantly influences the investigation depth. For the purpose of the present study, the Wenner and Schlumberger survey was used. Resistivity imaging was carried out as a number of 1D (1 st and 4 th case study--Vertical Electrical Sounding) and 2D sections (2 nd and 3 rd site--Electrical Resistivity Tomography). Each single Electrical Resistivity Tomography (ERT) test was based on a Wenner array measurement of the electrical potential between two inner electrodes (M and N), resulting from a constant current injected through two outer electrodes (A and B). Each VES test were performed using the Schlumberger method. In both cases, the apparent electrical conductivity was calculated as the ratio of the injected current to the measured potential, with a correction applied for the geometrical effects of the electrode spacing which were appropriate for each method.