The fundamental role of environmental isotopes in the characterization of landfill impact on groundwater

<p>Modern urban landfills are useful instruments for the safe disposal of everyday waste, especially when associated to a correct separate waste collection and circular economy best practices. Nevertheless, environmental pollution in the surrounding of the disposal area is always a major threat. The leachate and gas produced during the waste maturation must be carefully collected and conveyed to appropriate treatments or uses e.g. for energy production, in order to avoid harmful pollutants from migrating to groundwater or other natural matrices.</p><p>Appropriate monitoring practices are required to intervene promptly at the first sign of inefficiency of the protective barriers or leachate and gas collection systems. As regards groundwater, the monitoring network must include at least 3 observation points. The parameters to be analyzed, required by the legislation, aim at detecting the passage of specific contaminants or indicators of pollution, including inorganic elements and organic contaminants.</p><p>Very often, reducing conditions are observed in the groundwater underlying landfills, which trigger the reductive dissolution of iron and manganese (hydro)oxides. Reasons for this include: natural conditions of the aquifer, leachate pollution, the interaction of groundwater with landfill gas migrating from the plant.</p><p>Groundwater monitoring campaigns have been conducted for several years for the characterization of landfill impact on groundwater in central Italy and several case studies have been analyzed. Natural background levels have been applied, when possible, to distinguish the presence in groundwater of metals due to natural conditions from exceedances related to anthropogenic impact. Traditional groundwater monitoring has been complemented with the analysis of environmental isotopes including tritium and 13-carbon . Tritium is an excellent tracer of landfill pollution because its concentration is particularly high in both leachate and landfill gas.</p><p>The aim of this communication is to present some successful examples of isotope application to resolve doubts about the origin of high levels of inorganic compounds in groundwater, as well as traces of organic compounds, which are of concern as a possible sign of failure of the protective barriers of the plant.</p><p>In particular, we compare the results of the monitoring activities at two landfills, one currently active and one that has been operating in the past and is now completely dismissed. Field parameters (T, EC, pH, DO, ORP) were measured with probes in a flow-through cell. Ammonia, nitrite, sulfur and cyanide were measured in the field (UV-VIS). Quality control includes blind samples, field blanks and equipment blanks. Lab analysis were performed for major and trace elements, environmental isotopes (δ<sup>18</sup>O, δ<sup>2</sup>H, Tritium, δ<sup>13</sup>C), DOC, VOC with standard procedures.</p><p>The hydrochemical, hydrogeological and isotope data indicate a slow and modest groundwater contamination that is taking place in the old plant. The traces of organic compounds observed there were ascribed to the leachate, which is still produced and collected, which possibly infiltrates the groundwater. In the active plant no indication of pollution was found and some anomalous data regarding sulfur and chloride were provisionally ascribed to a geogenic origin.</p>