The notion that current engineering practices and works should be designed and carried out with consideration for posterity is a basic assumption of environmental ethics. In this regard, a sustainable approach to waste disposal should limit greenhouse gas emissions and, at the same time, avoid a detrimental impact on the quality of the groundwater in the long term. Reaching these goals requires adopting a risk assessment procedure that guarantees the protection of the groundwater resource, avoiding recourse to oversized lining systems, with relevant environmental and economic savings. This paper describes the procedure that has been recently introduced in the Italian regulations for landfills and suggests more advanced modelling approaches, which take into account the diffusive transport through the lining system and the contaminant distribution in the underlying aquifer. A numerical example is developed to show the application of the proposed analytical and numerical solutions to a hypothetical landfill for non-hazardous waste. The acceptable concentration of organic and inorganic contaminants contained in the landfill is determined for post-closure conditions both within the service life of the geomembrane and after a complete degradation of the geomembrane. A critical discussion about applying the precautionary principle and adopting excessively low concentration threshold values in the groundwater is presented.