CONTAMINAZIONE DELLE ACQUE SOTTERRANEE DA ARSENICO GEOGENICO: UNA PANORAMICA DEI MECCANISMI DI MOBILIZZAZIONE E DELLE TECNICHE DI INDIVIDUAZIONE DELLE SORGENTI

We aim at: (i) providing a general overview on the contamination of geogenic arsenic resulting from natural rock-water interactions in aquifer systems and (ii) suggesting a general approach to identifying possible source mechanisms. The presence of non-negligible concentrations of arsenic in groundwater is observed in many shallow and deep aquifers all over the world and affects more than 140 millions of people. Identifying the sources of geogenic arsenic remains one the most important contemporary challenges to minimize the portion of population exposed to arsenic hazard and to define possible mitigation strategies to contamination taking place in the system. Here, we present an overview of the geochemical mechanisms which are typically considered to be the driving source of geogenic arsenic in groundwater. These include: oxidation of arseno-pyrite and pyrite; reductive dissolution of iron hydroxides; adsorption and desorption; and dissolution of arseno-sulfides. For each mechanism, we provide a summary of the reaction network and of the boundary and environmental conditions that might trigger the release of arsenic in the natural water bodies. The description of the mechanisms is accomplished through a review of the methods and techniques that are commonly used to track back the geogenic source of arsenic and to define the conceptual model descriptive of arsenic mobilization: statistical analysis of field data; laboratory experiments; and numerical modeling. Strengths and limitations of these techniques are pinpointed and commented. The general picture presented in this work highlights the complexity and the difficulties that are typically faced during the investigations aimed at identifying the source of geogenic arsenic in natural subsurface water bodies. The critical discussion of source mechanisms and source identification techniques is finalized to suggest a possible structured approach, transferable to a variety of case studies, to define one or more conceptual models descriptive of the arsenic release in a case study of interest. This approach combines the diverse source identification techniques reviewed in this work and organizes these according to a set of sequential steps aiming at maximizing the benefit of their strengths and compensating possible limitations specific to a given technique.