Long- and short-term hydrogeochemical monitoring for improving the comprehension of geochemical processes in a waste landfill area

In this study, we investigated the processes leading to anomalously high val-ues of some metalloids and minor elements (As, F, Fe, Mn) in groundwater in a porous/fractured pyroclastic aquifer at a closed municipal solid waste landfill site. The goal of the study is to distinguish between natural enrich-ments (e.g., geogenic sources) and anthropogenic pressures (e.g., leakages from the landfill body). This study takes the advantages of the synergy be-tween scientists, landfill operators and local authorities. The long-term moni-toring activity on groundwater quality conducted by the landfill operators was complemented by a short in-depth groundwater monitoring of physical-chemical parameters, inorganic compounds, water stable isotopes, carbon isotopes and tritium. Long-term monitoring shows a general stability or a decrease in the concen-tration of the elements of concern. Instead, short-term monitoring shows fluctuations in their concentrations, especially of Fe and Mn. The presence of As and F in groundwater is natural and associated with adsorption-desorption processes that occur in the pyroclastic aquifer or with the rise of hydrother-mal fluids - enriched in metals - from a nearby active volcanic area. Instead, the presence or absence of Fe and Mn can be attributed to local redox condi-tions that favor or inhibit the mobilization of these elements. These condi-tions may be related to the combined effect of natural phenomena, such as the rise of deep fluids, and interference related to the presence of the landfill. In fact, the landfill body may act as a barrier between the saturated zone and the atmosphere, favoring anoxic/abiotic conditions. These conditions create a more reducing chemical environment below and downgradient of the land-fill. Possibly due to the occurrence of oxidizing conditions, no other wells downgradient the landfill show physical-chemical anomalies. The coupled use of physical-chemical and isotopic analyses, as complemen-tary of a long-term monitoring activity, effectively supported the comprehen-sion of the geochemical processes occurring in the aquifer in presence of a municipal landfill. The outcomes of this study will help decision makers in planning and managing present and future activities at the landfill aimed at reducing the impact on groundwater resources, also inspiring social, eco-nomic and environmentally sustainable strategies.