## 3 Results and Discussion
### Municipal Landfill with Liners Installed-Leakage Investigation
It is expressed as a mass of sediment left after evaporating 1 dm\({}^{3}\) of water at a temperature of 105 \({}^{\circ}\)C, and drying the residue in a temperature which does not exceed 105 \({}^{\circ}\)C. The dry residue, which is also an indicator of water mineralization, is expressed in mg/dm\({}^{3}\). The mineral matter content, which is the dry residue, is a principal classification of highly mineralized water, with the content exceeding 1500 mg/dm\({}^{3}\). Pumping out 12,930 dm\({}^{3}\) of water did not result in a significant increase in the water quality in piezometer P-2. At the beginning, pumping was carried out at a capacity of 40 L per day (blue line at Figure 5). The values of EC within this time decreased from 6961 to 4752 \(\upmu\)S/cm, and the mass of dry residue (dried sewage sludge) decreased from 2804 to 1944 mg/dm\({}^{3}\). Following this, the pumping capacity was tripled to 120 L per day. As a result, the EC increased to 8589 \(\upmu\)S/cm, and the mass of the dry residue increased to 3916 mg/dm\({}^{3}\). The increase of EC after the intensification of pumping could be a result of:  
* too much water being pumped, causing the chloride concentration to increase (lower dissolution);
* seasonal changes of the groundwater table and a wash out of the soil elements. There is a possibility that both of these reasons resulted in an increased contamination. The position of the groundwater levels at that time fluctuated by about 1 m.  
The lack of treatment effects during the pumping from the piezometer forced the landfill's owner to apply further reclamation solutions, which involved the construction of another depression well to catch all of the stream of leachate from the landfill. Moreover, by using an injection method, we were able to isolate the landfill's base, remove some of the waste from the landfill body, and repair the isolation layer in place. The main factors affecting the soil resistivity, apart from the factors specified above, are the soil's water content and the concentration of salts dissolved in the water. The effect of groundwater salinity significantly affects the resistivity of soil and ground water. It could cause the differences in the resistivity of soils contaminated with the landfill leachates. The use of electrical resistivity tests for monitoring the mechanical condition of a landfill containment system, contouring the zone of soil contamination, and determining migration routes for the contaminants, is fully justified. The test also allows a determination of the specific test locations, including boreholes, standpipe piezometer installation points, and chemical analysis sampling points. The second case study is an active MSW landfill located in the eastern part of Poland, in a post-mining sand and gravel pit. It is a sub-horizon landfill of a depth up to 12 m below the ground level. The landfill base and slopes are sealed with a 1.5 mm thick HDPE geomembrane, laid down on the 0.5 m thick clay layer. The geomembrane is covered with the protective layer, and subsoil drainage is made of perforated vitrified clay pipes. The liner was installed at the bottom of the landfill. An occurrence of the glacial clay and sandy clay loam deposits of a thickness of approximately 4 m, lined with sands and gravel with an unknown floor depth was determined, based on the data from the boreholes. A free water table was observed at 16.3 and 17.5 m below the ground level. The electrical resistivity tomography (Figure 6) was performed by using electrodes spaced 4.8 m apart and a total measured length of 72 m.  
In the middle and right side of the profile, the electrical resistivity was 150-200 \(\Omega\)m, mainly due to the location of an access road to the landfill facilities. An old waste layer with an electrical resistivity of approximately 30-40 \(\Omega\)m is located just below the road. The facilities, where the deposited municipal wastes are mixed, featured an electrical resistivity of less than 50 \(\Omega\)m, on the left side of the profile.