"Landfills form the final stage in waste processing and are sources of multiple environmentally harmful emissions. Due to the biodegradation of organic material, methane, a strong greenhouse gas, is released into the air and soluble pollutants, like ammonium or heavy metals, are flushed out of the landfill waste. In modern landfills, the waste body is isolated from the environment by an impermeable liner and both the gas and the leachate fluxes are collected and treated. However, sealing the landfill reduces the emissions on the short term but gives no sustainable solution as eternal active aftercare is needed and the emission potential is not reduced. The goal of the CURE project is to develop methods to enable the sustainable management of Dutch landfills. One of these methods is piloted at the Braambergen landfill, which is the in-situ aeration of the waste body with the goal of speeding up the biological degradation of organic matter to reduce the emission potential relatively quickly below the values in the Dutch regulations, such that the eternal active aftercare is not required anymore. The issue that the landfill operator, Afvalzorg, is facing is that in compartment 11Z of the landfill the aeration efficiency is poor due to ponding leachate. The objective of this thesis is to quantify the relation between the water level in the landfill and the efficiency of aeration, and to research the spatial and temporal variability of the water levels, leachate composition and hydraulic conductivity of the waste body to investigate the nature of the water and to find a suitable solution to mitigate the reduced aeration efficiency. To reach this goal, multiple types of field and lab measurements have been performed and the results of a water balance model have been used for a numerical evaluation. In the field, water tables have been measured as well the magnitude and composition of landfill gas extracted by the aeration wells. Pumping tests have been performed on the wells to assess the hydrological behaviour of the leachate surrounding the wells and to determine the horizontal hydraulic conductivity of the waste. Leachate samples from all the wells have been collected and analysed in the lab on pH, redox potential, electrical conductivity, DOC and ammonium concentration. Also were CTD-divers installed in some of the wells to monitor the response to rainfall and the medium long term behaviour of the water level and the electrical conductivity. The general conclusion of this thesis is that the studied compartment 11Z is homogenous in its heterogeneity. With all the different measurement types performed it was in some way the objective to see whether the behaviour or the properties of the landfill and the leachate can be grouped spatially, but this was not the case. Therefore can be concluded that the only scale for which a measurement can be representative for the entire compartment is on the scale of the compartment itself. This is in line with the expectations of large heterogeneity in landfills and the unique situation of aeration system on the Braambergen made it possible to proof this. The efficiency of the aeration system is particularly low in compartment 11Z because the conditions are unfavourable for both water and gas flow. The conditions in compartments 12 and especially 11N are more favourable because the waste, especially at the bottom of the compartments, is more permeable. Even if the amount of water in 11Z is reduced and the gas flow increases, the total amount of extracted LFG will not reach the same level as for the other compartments.