Emissione e dispersione in atmosfera di polveri derivanti da sorgenti diffuse nelle attività estrattive e di ripristino ambientale

The research deals with the emissions of particulate matter, arising from fugitive dust sources in mining remediation activities, and the analysis of dust atmospheric dispersion. The thesis is based on field measurements carried out in some abandoned mines in the south of Sardinia, where reclamation activities are currently in progress. The sampling campaign was performed in order to characterize typical emission sources and assess the dust exposure of the workers employed in the sampled activities. The environmental impact of the activities under examination has been studied by using a dispersion model, which has allowed to predict ground level dust concentrations on potential receptors located in surrounding areas. The on-site measurements, aimed at characterizing dust emissions, were performed during transit along a haulage road, operations of material handling and maintenance of tracks and yards. The application of the exposure profile method during the transit along the road has allowed to calculate emission factors, in terms of grams for travelled kilometre, both for PM2.5 and PM10 fractions. The results were comparable with the emission factors suggested by U.S. EPA (U.S. Environmental Protection Agency). By considering the overall results, the emissions resulted widely variable, depending on the particle size distribution and the moisture content of the surface material. For all sources under examination, the emissions comprise a relevant percentage of coarse particles, with an aerodynamic diameter greater than 10 μm. As regards working exposure conditions, dust samples, collected during typical work tasks, have been analysed in order to detect silica and metals content. Silica has been found in a few number of samples; the content of As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Sn, Tl, Zn has been measured in inhalable dust samples. The contents of lead, zinc and chromium were found to be the highest but all metal concentrations are lower than suggested limit values. A limited number of samples has been analysed in order to study size distribution of metals in the collected dust; all elements under examination tends to concentrate in the finest fraction of the particulate. The dust dispersion phenomenon, arising from the sampled activities, was analysed by using the Gaussian model ADMS. The predicted ground level dust concentrations near potential receptors were lower than limit values recommended by the current regulations, both for PM2.5 and PM10 fractions. The results can be explained by considering the low emission values, assumed to be representative for the analysed activities, and the instable atmospheric conditions, occurring during the on-site measurements. These conditions aid plume vertical dispersion, resulting in low ground level concentrations. The case study under exam does not have important consequences, either for workers health or for environmental impact; the dust concentration values are lower than limit values suggested by current regulations. However, the high content of metals detected in collected dust suggests the need of a continuous monitoring, aimed at assessing occupational exposure and potential environmental impacts on the surrounding areas. The present study confirms the findings of previous researches: the emissions of fugitive dust sources are difficult to quantify because of the diffuse nature of these sources and the large number of parameters which influence the dust emission rates. Moreover, the atmospheric dispersion phenomenon of fugitive dust is strongly affected by the topography, because the emitting sources have low release heights.