# Geophysical Monitoring of Leachate Injection in Pretreated Waste Landfill
## 2 Materials and Methods
### Infiltration Test  
Tests were performed to estimate the infiltration rate in the uppermost part of the landfill before the final covering of the landfill itself. The pore networks and the anisotropic variability of hydraulic properties in landfill waste are highly complex. To check the waste permeability in the upper part of the landfill, an infiltration test was performed. For this aim, a controlled volume of water was infiltrated on the surface and the infiltration was monitored in time-lapse electrical resistivity tomography (yellow line in Figure 1). The infiltration system was designed to irrigate a surface with an area of about 6 m \(\times\) 3 m. The water was distributed through 5 sprinklers fed by a discharge pipe with a 2.5-inches diameter to homogenize the irrigation over the infiltration area. The irrigation was carried out at a flow rate of about 50 L/min lasting for two hours; the total amount of water that infiltrated was 6 m\({}^{3}\). The test was performed at the beginning of Spring (April), starting at 11 am and ending at 10 am the day after. The daily temperature ranged from 14 \({}^{\circ}\)C to 22 \({}^{\circ}\)C (sunny day); the minimum temperature during the night was 9 \({}^{\circ}\)C (with a clear sky). The relativity humidity ranged from 50 to 60%. The electrical conductivity of the injected water was measured just before the beginning of the test: the value was about 650 \(\mu\)S/cm, corresponding to the electrical resistivity of about 16 \(\Omega\)-m. During irrigation, a puddle having a 5 cm thickness formed; after 25 min of water supply, no further modifications occurred, and the infiltration process proceeded in steady-state condition until the pumping was stopped. Time-lapse monitoring of the subsurface processes was carried out by ERT to detect the hydraulic properties and estimate the influence volume of the infiltration. The background dataset was collected before the infiltration (at time t\({}_{0}\) = 0). Then, five other datasets were collected at the following times: t\({}_{1}\) = 0.25 h, t\({}_{2}\) = 1 h, t\({}_{3}\) = 2 h, t\({}_{4}\) = 4 h and t\({}_{5}\) = 23 h.  
Figure 1: Test site with the position of the transects of the ERT on the landfill (L1–L6); the coordinate of the lower-left corner of the image is 44\({}^{\circ}\)53\({}^{\prime}\)02 N–8\({}^{\circ}\)22\({}^{\prime}\)7 E. The image is aligned south to north along the vertical direction. Red lines refer to the ERT lines of the preliminary characterization (48 electrodes, spacing 2 m): the labels of each line indicate the position of the first electrode of each section (relative coordinate of 0 m). The yellow line refers to the position of the ERT adopted for the infiltration test (48 electrodes, spacing 1 m); the first electrode is located at the south-east side. The blue circle refers to the injection well of the test of leachate recirculation; the green circle refers to the position of the borehole, which is addressed with electrodes and used for monitoring the leachate recirculation process within the landfill. The position of the ERT line for infiltration test monitoring is highlighted in Figure 1, with yellow colour. Data acquisitions were carried out using a Wenner-Schlumberger array with 48 electrodes; in such a case, the adopted electrode spacing was 1 m. The survey line crossed the irrigation area at a distance between 28 and 31 m along the profile. Time-lapse data were inverted to monitor the infiltration process using the time-lapse difference inversion algorithm [28] that was implemented in the software R2 [27]. The spatial domain considered in the inversion procedure took into account the potential impact of the geomembrane on the reliability of the results, as previously indicated by [PERSON] et al. [29]. Particularly, we have checked through the forward modelling procedure that the geomembrane, located at an average depth of about 20 m from the top of the landfill, does not affect the reliability of the resistivity values achieved by the inversion procedure.