For protection of HHE a site specific flux based approach considering the hydrology of the landfill and its environment is probably inevitable. The main questions are: do low concentrations remain low in the far future and how can we extrapolate from one landfill to another with limited data?
The scientific community seems to favour an aftercare completion approach where site-specific emission criteria are developed based on local assessment of risk. It would be useful if models could be developed to assess the risk and the impact of a (passive) control mechanism failing in the future.
Laner D., Crest M., Scharff H., Morris J.W.F., Barlaz M.A. (2012) A review of approaches for the long-term management of municipal waste landfills. Waste Management 32, 498-512.