Moderate impact of landfill gas on a naturally reducing aquifer should not be confused with leachate pollution

The processes leading to high levels of arsenic, iron, and manganese in a naturally reducing aquifer beneath a landfill are investigated. Between 2016 and 2022, groundwater monitoring (physical-chemical parameters, major and trace inorganic compounds) has been complemented with the analysis of environmental isotopes (tritium, δ²H, and δ¹³C) of groundwater and of the dissolved gases (δ¹³C of CH₄ and CO₂, ¹⁴C of CH₄). Statistics, including Pearson/Spearman correlation and PCA, were used to define the main correlation among variables. The presence of methane and carbon dioxide was attributed to landfill gas migration from the waste as ¹⁴C dating confirmed that methane is modern (F¹⁴C = 1.0684) and likely produced by methyl fermentation within the waste. While methane, enhancing the naturally reducing conditions of the aquifer, appears to be the driver of the high concentration of Fe and As, Mn appears to be governed by carbon dioxide. At the same time, CO₂ may locally lower the pH, thus increasing the dissolution of sedimentary carbonates and ultimately producing high alkalinity and salinity. Furthermore, the reuse of water from leachate treatment to meet circular economy requirements was invoked to explain the elevated levels of tritium and ²H, associated with significantly negative ¹³C, observed in a production well and in a nearby piezometer. The integration of environmental isotopes and geochemical parameters allowed to exclude leachate contamination: tritium, δ²H, and δ¹³C were within the expected range for natural groundwater. No compounds typical of leachate contamination were detected. Environmental isotopes can fruitfully complement traditional monitoring when the comprehension of processes is desired, but this requires an expert judgment and a solid conceptual hydrogeological model.