Dalla Libera, Nico (2019) The Natural Background Level problem: A Hydro-geochemical study on the natural occurrence of Arsenic located in the Veneto region (NE Italy). [Ph.D. thesis]
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Abstract (italian or english)
Excess of arsenic in groundwater is a worldwide problem threatening the health of the millions of people directly exposed to Arsenic-rich water intake. The problem is particularly acute in naturally occurring unconsolidated aquifers where Arsenic-rich groundwater is an easily accessible resource of drinking water, such arsenic in India, Bangladesh and Vietnam. In Italy, arsenic is found in aquifers from north to south and it is associated to different geological settings (i.e. volcanic areas and alluvial plains). The shallow aquifer of Venetian Alluvial Plain (VAP) is notoriously affected by arsenic contamination, characterized by a patchy distribution with variable extensions and concentrations. With concentrations exceeding the WHO limit of 10 μg/L, this metal poses a risk for locals, arsenic the aquifer is exploited for agricultural purposes. Although empirical evidences exist about the relationship between arsenic occurrence and other factors, several aspects regarding the physical and geochemical processes controlling arsenic in the VAP aquifers remain unclear. In this line, the present study aims to elucidate both the geochemical processes fostering arsenic mobility and their correlation with the subsurface heterogeneity, in order to improve the knowledge about arsenic-controlling processes and use them arsenic aid for the environmental management. To this end, we focused in detail on an agricultural zone nearby the Venice lagoon, affected by arsenic contamination (called “Western Agricultural Areas”, WAA). The available data, collected by several hydrogeological surveys, show a spatial and temporal variability of arsenic concentration, which can be associated to a variety of hydro-geochemical processes such arsenic redox variations, sorption or reductive dissolution of Arsenic-rich iron oxy-hydroxides. In order to point out the consistency and the importance of these processes, we structured the study in two main phases: 1) evaluation of geochemical processes by means of a PHREEQC batch-like reactive model and 2) creation of a 3D reactive transport model based on previous results. The former allows us for testing whether the supposed geochemical processes are able to fit the geochemical conditions of the study system, identifying the main actors for arsenic mobility. The latter couples the obtained reactive processes with a 3D flow model, in order to evaluate the spatial and temporal distribution of dissolved arsenic as function of the subsurface heterogeneity. The results highlighted a strong effect of oxy-reductive potential on arsenic mobility, and it seems to be strictly correlated to organic matter degradation. The uprising of reduced condition, then, affects other mechanism such arsenic reductive dissolution of iron hydroxides, ion exchange and sorption processes, causing arsenic mobilization. Moreover, this study shaded light on the existence of oxygen ingress arsenic function of local water recharge events, which seems to be responsible of space/time redox variation. The 3D reactive transport model showed a strong dependence between the aforementioned processes and subsurface heterogeneity. The material distribution, indeed, plays an import role affecting the arising of the main chemical reactions.
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