## 5 Discussion  
### Source and Fate of Arsenic Contamination  
The main source of As contamination is geogenic. The weathering of the As-bearing mineral phases outcropping upstream the Baccatoio catchment, followed by erosion and transport during the evolution of the alluvial plain, may have a key role. In particular, fluctuations in the groundwater lever and/or the introduction of oxygenated recharge water through hydraulic connections may represent the hydrogeological control for the oxidative dissolution of spatially distributed lenses of sulfides, triggering As release and mobility in the aquifer systems under dynamic redox conditions ([60] and references therein). Indeed, As-bearing pyrite oxidation causes the concomitant release of Fe and S, and the fate of As is determined by the processes affecting the mineralogical characteristics of the aquifer media, in particular the precipitation of newly formed hydrous ferric oxides from supersaturated waters acting as As sorbents. In addition, the distribution of As in groundwater indicates that the most contaminated wells are located in the intermediate zone of the plain (Figure 8), characterized by sediments from a lacustrine/marshy environment including peats and peaty clays (Figure 4), suggesting the possibility that natural organic matter (NOM) in the aquifer assists As's release and fate. Indeed, NOM serves as a geochemical trap for arsenic and may drive the dissolution of As-bearing iron oxyhydroxides in the aquifer sediments in a reducing environment, allowing the transformation of arsenate species to the more mobile arsenite. Along with oxidation, the As immobilized by NOM can be released and partly adsorbed on newly formed metastable iron oxyhydroxides, and the dissolution of As-hosting Fe hydroxides takes place when reducing conditions prevail. The complex scenario for As sequestration and release by high-iron sediments and organic matter not uniformly distributed in the aquifer, and the role of subsurface bacterial communities, results in the decoupling of As and Fe in groundwater. \begin{table}
\begin{tabular}{c c c c c c c c}  & \begin{tabular}{c} **Soil** \\ **Ingestion** \\ \end{tabular}  & \begin{tabular}{c} **Dermal** \\ **Contact** \\ \end{tabular}  & \begin{tabular}{c} **Inhalation** \\ **Dust** \\ \end{tabular}  & \begin{tabular}{c} **Hi** \\ **Hii** \\ \end{tabular}  & \begin{tabular}{c} **Ri** \\ **Ingestion** \\ **(Tomato)** \\ \end{tabular}  & \ \begin{tabular}{c} **Vegetable** \\ **Ingestion** \\ **(Cabbage)** \\ \end{tabular}  & \begin{tabular}{c} **Vegetable** \\ **Ingestion** \\ **Exposures** \\ \end{tabular}  \\ \hline HQ\({}^{\star}\) & \( \begin{array}{c}1.19\\ (0.127)\\ \end{array} \) & \( \begin{array}{c}0.166\\ (2.54\times 10^{-2})\\ \end{array} \) & \( \begin{array}{c}2.05\times 10^{-5}\\ (2.54\times 10^{-2})\\ \end{array} \) & \(\begin{array  
It must also be noted that a hydraulic connection exists between the superficial aquifer horizons and the Baccatoio stream, especially in the upper and middle part of the alluvial fan. This is supported by field observations which indicate that, when it is minimally fed by springs, the Baccatoio stream loses water and almost vanishes, reducing the streamflow and recharging the aquifer, as expected in these geomorphological settings. The Baccato stream waters are supersaturated with respect to a number of iron minerals, allowing yellowish and reddish precipitates to form and determining a high As concentration in bed-stream sediments [61]. When As-hosting iron oxyhydroxides become buried in the Baccatoio streambed sediments in the reducing conditions of the hyporheic zone, arsenic may be released via reductive dissolution. The hyporheic zone may hence act as an interface for As transport between the surface and the shallower groundwater in the stream-aquifer system, and As can subsequently infiltrate through the aquifer sediments.