The use of water isotopes as environmental tracers in contamination phenomena between groundwater and leachate

Nowadays, one of the serious environmental threats is groundwater contamination due to leachate from municipal solid waste landfills. In recent years, the use of stable isotopes as environmental tracers to identify potential groundwater contamination phenomena has found increasing application in environmental engineering. Deuterium (²H) and oxygen (¹⁸O) isotopes have been successfully used to determine contamination phenomena, when groundwater interact with leachate from municipal solid waste landfills with a significant organic amount. In these cases, groundwater present isotopic compositions relating to ²H and ¹⁸O, which highlight an enrichment of δ²H, an enrichment probably caused by methanogenesis phenomena, during which the bacteria preferentially use the "lighter" isotope of hydrogen (¹H) and the remaining part enriched in the "heavier" isotope (²H). A parameter that influences the isotopic content of deuterium and oxygen18 is the deuterium excess (d or d-excess). An index F is then identified as a percentage change in d-excess, which allows the definition of a system of alert levels to evaluate and control the contamination of aquifers by leachate. F index values higher than 1.1 highlight possible phenomena of contamination of the aquifers due to leachate.

In the case study, the results of the isotopic analyses of oxygen18, deuterium and tritium show mixing phenomena of the leachate and groundwater. The influence of the leachate, following mixing phenomena with groundwater, can determine a reduction in the ORP redox potential in groundwater such that it takes on low and sometimes negative values. This condition ensures the establishment of anaerobic environments and reduces the mobilization of metals, such as Mn, in groundwater. Therefore, in the study area, the overlap of these two phenomena seems evident, such as causing the deterioration of groundwater.