A comprehensive monitoring approach to evaluate the impact of an urban landfill on a naturally reducing aquifer

The processes leading to high levels of arsenic, iron, and manganese in a naturally reducing aquifer beneath a landfill are investigated. Traditional groundwater monitoring (physical-chemical parameters, major and trace inorganic compounds, organic compounds) has been complemented with the analysis of environmental isotopes of groundwater and of the dissolved gases (e.g., CH4 and CO2). The results suggest that landfill gas circulating in the aquifer (as demonstrated by 14C dating of methane) enhanced the naturally reducing conditions of the aquifer, eventually resulting in the observed high concentrations of redox-sensitive elements. At the same time, high pressure of CO2 at some piezometers locally lowers pH, hence increasing the dissolution of sedimentary carbonates and alkalinity. In addition, the reuse of water from leachate treatment to meet circular economy requirements was invoked to explain the high levels of tritium and 2H, associated with strongly significantly negative 13C, observed in a production well and in a nearby piezometer. Environmental isotopes can fruitfully complement traditional monitoring when the comprehension of processes is desired, but expert judgment is required.