The interaction between a Fe-oxides rich stockpile and the environment constrained with a multi-technique approach (Venice’s lagoon, Italy)

The stockpile of Mira is an 8 hectares area close to the Venice’s lagoon that arose between the 1960s and the 1970s as the result of the storing of 809600 tonnes of pyrite ashes, a residue made of Fe-oxides and hydroxides that comes from the industrial process of making sulphuric acid. The aim of this study is to investigate the interaction between a Fe-oxides rich stockpile and the environment with a multi-technique approach to evaluate and understand which contaminants are abundant and their mobilization mechanisms. To this end, new field data have been collected from a deep drillhole (PZ7) below the stockpile to reach the soils underneath it, a piezometer (PZ7P) was installed in the drillhole to sample the subsurface water and monitor the groundwater level. The solid samples PZ7-1/6 were analysed using X-ray diffraction to ascertain their mineralogical composition, while 4-Acids “near total” digestion, ICP-MS, and X-ray fluorescence spectroscopy were employed to determine the chemical composition. The groundwater samples (PZ7M and PZ7J) together with the eluates from the leaching test on the stockpile’s samples were analysed in two different laboratories employing ICP-OES and ion-chromatography. The interaction between the stockpile and the sediments was constrained using immobile element analyses and mass transfer calculations, and addition to the laboratory analytical techniques a geochemical modelling using PHREEQC Interactive software was employed to determine the groundwater speciation and to understand the adsorption mobilization mechanisms. It was found out that Fe, As, Mn, Zn and sulphates need to be monitored to understand their mechanisms of mobilization. In addition, some remediation procedures applicable to the site are indicated, even if further laboratory experiments would be necessary to try out the efficacy and the efficiency of these methods.