# Geophysical Monitoring of Leachate Injection in Pretreated Waste Landfill
## 2 Materials and Methods  
The experimental study was performed in a landfill in northwest Italy. The landfill is composed of two hydraulically independent cells filled with pre-treated wastes: each cell has its independent systems for biogas and leachate extraction. The work was organized into the following parts:  
* preliminary geophysical characterization of the waste material in the uppermost part of the landfill;
* application of electrical resistivity tomography in time-lapse modality to estimate the hydraulic properties of the waste;
* long-term monitoring of the leachate recirculation. ERT is a non-destructive method where the measurements are performed on the surface of the media through several electrodes. An electrical current is injected into the media through two electrodes, and a set of electrodes measures the resulting electrical potential. Several electrical arrays can be adopted based on the different ways to mutually arrange the current and voltage electrodes; the array also differs according to a different sensitivity of the measurements to the observed volume. The Wenner and Schlumberger array is more sensitive to vertical variations of the electrical potential, while the dipole-dipole array is better suggested to detect lateral changes in the observed parameters. This issue has been previously demonstrated according to a theoretical point of view and experimental approach in near-surface geophysical applications [20,21]. The measurements are then converted into apparent electrical resistivity values. The subsequent tomographic data processing involves the interpretation of all the data set to convert the observed data (apparent resistivity values) into a distribution of true electrical resistivity (or the inverse, the electrical conductivity). The main factors that control the electrical conductivity in the porous material are the fluid content, its salinity and temperature. In landfill investigation, the electrical conductivity could be related to the distribution of leachate within the waste body. Typical values of electrical resistivity of the leachate of municipal waste landfills are around 1 \(\Omega\cdot\)m, which makes the volume partially saturated by leachate highly conductive volumes. Electrical tomography was applied for the detection of the flow path of the leachate through municipal solid waste (MSW), both for increasing the knowledge of MSW behaviour and for using the results to improve the design of the leachate injection systems [18; 19; 20; 22]. More generally, the approach is widely adopted to study the infiltration process in porous media [18; 23; 24]. During the injection of water into porous media, ERT time-lapse monitoring (TLERT) is adopted, and the changes in the electrical resistivity can be considered sensitive to the changes in fluid content if the temperature and the porosity are constant during the infiltration. The TLERT method is based on the analysis of a dataset; in our case, on 2D resistivity images collected at different time intervals; the inversion procedure of the TLERT dataset could be performed according to several approaches [25]. The simpler approach is based on the analysis of the differences between the resistivity images independently inverted (independent approach). This approach could provide for amplification of the ghost and artefacts of every single inversion. The method of difference inversion of the ERT data is based on the inversion of the differences between the reference resistivity dataset images and the resistivity datasets collected at different times. Finally, the ratio inversion adopts the ratio between two successive ERT measurements multiplied with a resistivity value representing a homogenous subsurface model. In the present work, we adopt the difference inversion approach to invert the datasets of resistivity data collected during the infiltration test.