Livelli di fondo naturale, origine ed evoluzione di alcuni elementi potenzialmente tossici nelle acque sotterranee

The research deals with the study of Natural Background Levels (NBLs) of potentially toxic elements in groundwater, accompanied by an investigation on the origin and processes that determine their presence in the aquifers. Starting point is a regional framework on the presence of arsenic and fluorine in groundwaters of central-northern Latium. The main target is to define, through a geochemical, statistical and geospatial approach, the distribution of the two elements, the most critical areas and the possible processes that determine their co-contamination at regional/groundwater body scale. This framework is followed by a downscaling of the problem from the regional to the local scale, investigating the arsenic presence in a more restricted study transept. The identification of the possible mineralogical sources of arsenic in the aquifer and the main mechanisms that govern the As release are investigated through different types of laboratory tests and analysis. The work is concluded by the proposal of a procedure to derive natural background values, with application to the same focus area. The main objective is the identification of a methodology able to integrate the approaches frequently used today for the definition of NBLs. It is a more operative aspect of the research, crucial especially in areas where critical elements such as As and F show frequent overruns of the groundwater law limits. Although probably there are important inputs related to deep-rising geothermal fluids, in addition to anomalous areas characterized by specific processes for As or F, in central-northern Latium there is a diffused geochemical background for both elements, resulting from water-rock interaction processes and responsible for their good correlation in groundwater. Laboratory analysis performed on a representative solid matrix (Via Tiberina Yellow Tuff) showed that, although most of As is linked to low crystalline Fe oxy-hydroxides, the mobilization in oxidizing conditions of groundwaters mainly derives from desorption phenomena in presence of specific exchanger (e.g. phosphates) and/or from calcite. Fe oxy-hydroxides maintain a key role for As mobility both in alkaline (arsenate desorption/adsorption) and reducing conditions (Fe-oxides reductive dissolution). The proposed methodology for NBLs assessment, which includes the pre-selection of uncontaminated samples and the analysis of the residual distribution, confirmed also at local scale the existence of a high As-F geochemical background in groundwater, well above the standard limits for human consumption.