Metallic contaminants in areas of mining drainage: the sediments of the Lake Flumendosa, Sardinia, Italy

The Lake Flumendosa is one of the main basins of an hydraulic system of artificial lakes that is the major water resource for southern Sardinia. It is situated in ore-rich terrains which are responsible for the supply of different heavy metals in the bottom sediments forming in this lake. The chemical and mineralogical composition of these materials reflects the lithology of the country rocks and is strongly influenced by the surrounding ore mineral occurrences. At many locations mining has been active for several decades affecting not only important ore bodies, but also minor occurrences and mineralised outcrops so that mining excavations, trenches, galleries and exploitations gave origin to numerous dumps which, along with the unexplored mineralised outcrops, supply metals to the lake sediments. In recent years the fluxes of many trace elements from terrestrial or atmospheric sources to the aquatic environment of these reservoirs has increased either as a direct action of mining and other industrial operations or as an indirect consequence owing to acidification of the rains. Although the period of most intense sulphide oxidation and associated release of dissolved metals has passed the transports of contaminants through underlying aquifers will continue. Furthermore, because of the semiarid climate of this island, along with the uneven distributions of rains, water levels in this lakes are prone to broad yearly variations and consequently their sediments are periodically prone to the action of the meteoric waters and of the atmosphere which causes oxidation and remobilisation of most metals. However the metal-rich sediments of the lake appear at present sufficiently stable as elemental concentrations in the water reaching the treatment plants are normally below the permissible limits for drinking use. Metals reprecipitate in less soluble forms and accumulate in the deepest parts of the sedimentary piles which would act as a trap for the metals. Data on metal contents in the sediments are examined, as well as deriving pollution risks. Distribution maps have been drawn for each element to assist in the assessment of the dispersion and accumulation effects. The mapping, together with statistical treatment of the results and comparison with previous data has allowed hypotheses to be advanced about the origin and fate of the various pollutants.