The controlled landfill technology is now adopting passive attenuation techniques as an increasing number of landfill sites reach the post-closure phase. During the post-closure phase, landfill operators need to convince environmental authorities that landfills no longer pose a threat to health or the environment. The demonstration of such absence of risk should rely in particular on data collected during environmental monitoring in addition to modelling of possible future evolutions of environmental concentrations. One difficulty that is typically encountered in France is related to the fact that groundwater monitoring systems around landfills are often insufficiently detailed to provide conclusive evidence of natural attenuation mechanisms. This paper presents data on groundwater quality in the vicinity of an old landfill located in a complex aquifer system. While isotopic data suggest a signature of the landfill leachate in the groundwater in the vicinity of the landfill, chemical analyses do not show a strong influence, which could be indicative of natural attenuation mechanisms. However, the complexity of the groundwater system in this area is such that it cannot be excluded that emissions to the groundwater are not being overlooked. Implications of demonstrating natural attenuation during the landfill post-closure phase, with respect to groundwater monitoring, are discussed.