# Geophysical Monitoring of Leachate Injection in Pretreated Waste Landfill
###### Abstract  
Landfill leachate recirculation is applied to manage the landfill as a bioreactor to enhance biogas production, limiting the impact due to greenhouse gas emissions and optimizing the energy recovery from biogas. This study deals with the geophysical monitoring of the leachate recirculation within the waste of a pretreated waste landfill. For this aim, electrical resistivity tomography (ERT) was adopted, detecting the main distribution of moisture within the waste both through the surface and through several boreholes. The electrical resistivity of waste mainly depends on water content, leachate salinity and temperature. The method is sensitive to the transient phenomena associated with leachate flow within the waste; moreover, the ERT long-term monitoring data suffer from anomalous and unexpected polarization phenomena induced by the measurements themselves. Results demonstrated the reliability of this approach to qualitatively detect the landfill volume affected by the leachate circulation. The effects of moisture changes by leachate infiltration on biogas production are still challenging, notwithstanding a positive effect on methane concentration in biogas itself is evident.  
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Footnote †: journal: Applied Sciences  
required to obtain optimal conditions can be supplied either by leachate recirculation or rainwater infiltration. Moreover, the leachate recirculation can make the spatial distribution of the indigenous microflora involved in the biogas generation process uniform.  
Especially in hot and dry climate areas, biodegradation of municipal solid waste in landfills is limited by moisture. Therefore, supplying adequate water quantity is crucial to enhancing bacterial activity and biogas production. Most of the conventional landfills reach moisture contents of 20-30% by weight, which is below the optimal range for waste biodegradation, around 40-70% by weight [6]. To achieve optimum moisture level in bioreactor landfills, typically described as moisture content at or above field capacity (40-70% by weight), water addition is required; that is to say, waste hydration must be carried out.  
The choice of the proper recirculation technique (vertical wells or horizontal pipeline network) is based on the assessment of its performance, which can be roughly determined by measuring the water content inside the influence zone of the injection system [7,8].  
The optimal performance and regulation of the frequency of leachate injection is based on the moisture content. The monitoring of the distribution of moisture content could be addressed by geophysical methods; most of them are based on the measurements and interpretation of electrical and electromagnetic parameters, such as electrical conductivity and polarizability, or electrical permittivity [9,10]. Methods such as neutron probes or time domain reflectometry (TDR) sensors provide punctual measurements of the fluid content and cannot be representative of the heterogeneous waste material [9,11,12].  
The most adopted method is electrical resistivity tomography (ERT), which allows one to detect the distribution of the electrical conductivity on a 2D section or to map it on a 3D iso-volume [13,14,15,16,17,18,19]. The non-intrusive, non-destructive and 3-dimensional response of the ERT method thus appears to be suitable for the study of the propagation of the leachate through the waste landfill.  
This study aimed to investigate the feasibility to monitor the leachate recirculation in a portion of a real-scale landfill for pretreated wastes by ERT and time-lapse inversion. At the same time, the biogas production and leachate properties were monitored, comparing the data with and without leachate recirculation.