Groundwater contamination due to municipal solid waste landfills leachate is a serious
environmental threat. During recent years, the use of stable isotopes as environmental tracers
to identify groundwater contamination phenomena has found application to environmental engineering.
Deuterium (2H) and oxygen (18O) isotopes have successfully used to identify groundwater
contamination phenomena if submitted to interactions with municipal solid waste landfills leachate,
with a significant organic amount. The paper shows two case studies, in central and southern Italy,
where potential contamination phenomenon of groundwater under municipal solid waste landfills
occurred. In both cases, isotope compositions referred to 2H and 18O highlight a δ2H enrichment for
some groundwater samples taken in wells, located near leachate storage wells. The δ2H enrichment is
probably caused by methanogenesis phenomena, during which the bacteria use preferentially the
hydrogen “lighter” isotope (1H), and the remaining enriched the “heavier” isotope (2H). The study
of the isotope composition variation, combined with the spatial trend of some analytes (Fe, Mn, Ni)
concentrations, allowed to identify interaction phenomena between the municipal solid waste
landfills leachate and groundwater in both case histories. Therefore, these results confirm the
effectiveness of 2H isotopes application as environmental tracer of groundwater contamination
phenomena due to mixing with municipal solid waste landfills leachate
