# Geophysical Monitoring of Leachate Injection in Pretreated Waste Landfill
## 4 Discussion  
The preliminary geophysical characterization of the landfill aimed to point out the electrical response of the landfill and the sensitivity of ERT to detect the waste volume affected by leachate recirculation. We pointed out low values of electrical resistivity all around the landfill, with a decrease of values at a deeper level, according to an increase of temperature and moisture with depth. The preliminary infiltration tests pointed out that the infiltration volume spreads out mainly horizontally, according to the peculiarity of the landfill, and is made by layers of pre-treated waste blocks separated with thinner layers of compost at very low permeability. The survey permitted the design of the best monitoring strategy by integrating cross-hole and surface electrical resistivity tomography during the leachate recirculation into the cell. The monitoring of the leachate recirculation demonstrated the relevance of the indirect monitoring (by geophysical survey) of the spatial and temporal distribution of recirculating leachate into the waste so that the whole bioreactor landfill performance can be optimized. The sensitivity of the electrical resistivity of bioreactor landfills to the properties of solid waste, such as moisture content, leachate ionic strength and temperature, is straightforward [17]. Some authors pointed out that an increase in electrical resistivity indicates a decrease in water content and a decrease in electrical resistivity indicates infiltration of a liquid [17; 31]. Figure 8: Temperature and pH of the recirculated leachate. Figure 9: COD, chlorides and N-NH4\({}^{+}\) content of the recirculated leachate. In the present study, the temperature effect seems to play a less important role because rather constant values below 3-4 m from the top were measured (46-48 \({}^{\circ}\)C). Therefore, the response change is mainly caused by the variations in water content within the waste. ERT was applied for the monitoring of leachate recirculation at a bioreactor landfill. This method is sensitive to the transient phenomena associated with leachate movement within the waste. In the monitored injection-extraction location, leachate flows following patterns that cannot be well resolved down to a depth of 6-7 m with preferential channels through macropores in the waste. The data on biogas production extracted from the tested landfill cells (with and without leachate recirculation) gave relevant information about the influence of waste moisture on biological processes occurring inside the landfill. Without leachate recirculation, the biogas has a content of methane and carbon dioxide giving a ratio of CH4/CO2 around 1.3-1.4. As shown in Figure 7, higher values were found during the leachate recirculation periods; notwithstanding, the methane flow rate did not grow compared to the ones without recirculation. The biogas generation does not seem influenced by seasonal changes in the leachate temperature, that is to say, the liquid injection does not constitute an evident disturbance to the biological process; in any situation, this could be verified by the thermal balance to the system (landfill), usually at least in mesophilic conditions (over 35 \({}^{\circ}\)C). Even with winter temperatures (close to 0 \({}^{\circ}\)C), the leachate flow rate is discontinuous, and this feed does not constitute a thermal disturbance on the huge mass of landfill. Very probably, one positive effect of leachate recirculation is the transport and distribution of substrates and microorganisms. Similar results were found by other authors:  
* [PERSON] et al. [32] studied the effect of leachate recirculation and compaction in lysimeters for municipal solid wastes. Each lysimeter contained 9 kg of waste, and the test lasted more than 5 months. In both cases, the ratio CH4/CO2 was higher than two, showing the effect of waste moisture on the biodegradation process. Moreover, these authors found that leachate COD decreased during the recirculation period, whereas in our study this trend is not so well defined.