The interaction between the leachate and the environmental media, in addition to spreading toxic compound in groundwater, leads to several hydrogeochemical and biological changes that, in turn, affect the natural conditions of aquifers. These changes also involve compounds that are generally considered slightly or not polluting, such as the major ions (Na, K, Ca, Mg, Cl, SO4, HCO3 + CO3), notwithstanding the Concentration Limits of Contamination (CLC) required by the current environmental legislation. In this general framework, the main objective of this research is to get a deeper insight into these processes, related to the leachate/groundwater interaction, in a landfill site where the CLC exceedances are limited. The selected study areas is a landfills located in Central Italy, within the scarcely permeable pliopleistocenic clayey deposits, that present a superficial alteration (siltyclayey lithologies). The hydrogeological characterization has been performed deepening different features: (1) the geological features have been described in detail, even with respect to the landfill volume, by means of geognostic boreholes; (2) the groundwater flow has been investigated by the hydraulic head timeseries analysis and the permeability tests performed in site; (3) the chemical analyses of surface water, groundwater and leachate, analyzed as water (i.e. the same units and detection limits), have been compared to assess the contamination extent and to interpret the changes related to leachate leaks in the aquifer. The preliminary results highlight that the clayey bedrock alteration (up to 15 m thick) favors, where the landfill is unlined, the mixing between leachate and groundwater that flows toward the stream. The hydraulic conductivity of this alteration is an order of magnitude higher (~108 m/s) than the clayey bedrock one (~109 m/s). The contaminated groundwater, in addition to showing CLC exceedances related to some organochlorinated solvents, are characterized by high concentrations of redoxsensitive ions (e.g. Fe e Mn), due to the anoxic conditions caused by the dissolved organic matter. Furthermore, the contaminated groundwater is enriched in ions Na, K, Cl, SO4, HCO3 + CO3 and depleted in in Ca and Mg respect to the natural groundwater. This effect can be considered a combination of intermediate redox conditions, that bring to the S species oxidation, and mixing and/or cation exchange, attributable to the clayey minerals in the aquifer. The multidisciplinary approach has enabled to clarify a complex environmental problem. In particular, the comparison between the chemical analyses of the landfill leachate, analyzed as water, and the and natural and contaminated groundwater has allowed to get a deeper insight into the hydrogeochemical processes that change the groundwater chemistry when a landfill leachate contamination occurs
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