L'interrimento dei bacini artificiali: il caso del Lago delle Fate (Val Quarazza, VCO)

The following work was conducted with the aim of studying the phenomenon of the artificial basins' silting and the management of the materials extracted. The basin studied is the ¿Lago delle Fate¿, located in the town of Macugnaga (VCO), which has a watershed of 25.8 km2 (Quarazza basin). Besides the silting that has reduced the volume of the reservoir (from 120,000 m3 to 40,000 m3), the lake shows an environmental problem: arsenic pollution in the environmental matrices mainly caused by mining activities practiced over the last century. To reach the goal, investigations were carried out aiming at: estimate the volume of sediment produced at the basin scale with the methods USLE, PSIAC and Gavrilovic (the values obtained respectively are 34,900, 9,800 and 5,630 m3/year), sample the surface water of the basin above the dam; and, finally, analyze the chemical-physical characteristics of the sediment inside the reservoir. Comparing the estimated values of sediment with the real ones (these last ones were included in a range between 1,333 and 7,273 m3/year), it results that the most reliable method was the Gavrilovic (5630 m3/year is the only value matching with the interval). The chemical analyzes carried out have confirmed the presence of arsenic in the ground, although at concentrations above the CSC (due to the D.L. 152/2006) only for those placed upstream of the reservoir (the maximum value found is 36 mg/Kg). The water's analysis results show the presence of arsenic even if the concentration is below the law's limits (due to the D.L. 31/01). Considering the materials' data in terms of quality and quantity in the reservoir, it was possible to provide operational proposals for their management: to use them within the village of Macugnaga since the value of the natural background, set at 477 mg / kg, it's higher than the one measured; to extract and treat them separately (those polluted as waste and those "clean" as a by-product); to bring contaminated materials to a dumping ground and reuse them (due to the D.M. 161/2012).