Environmental Impact of Abandoned Mine Waste: a review
Yüklə 0,52 Mb.
|
- Bu səhifədəki naviqasiya:
- 4.5.1. Valle Imperina
4.5. Venetian TerritoryThe Venetian territory has been inhabited since long time, and was colonized intensively by Celtic and Roman people. Ore exploitation, however, took place especially during the late Middle Age and the Renaissance, when the Venice Republic dominated the most part of North-Eastern territories, where forests, for building houses and ships, and metals, smelted for the coinage and the armaments, were easily available. In the territory there is a number of small ancient mines (Gares, Forno di Canale, Vallalta, Zoldo), located in mountain areas of difficult access, with the exception of the one in the Imperina creek valley, all closed since the end of XIX century. 4.5.1. Valle ImperinaValle Imperina, a Cu-Fe-Zn-(Pb) mixed sulphide ore deposit, was the most important mine site in the Venetian territory since 1400, and during five centuries, until final closure in 1962, supplied copper to the Venice Republic for coinage and armaments. Ore exploitation was clearly expanding since the opening of the mine: metallic copper production was 15 tonnes in 1574, 62 tonnes in 1669, and 120tonnes in 1788, equivalent to approximately half of the necessity of the Republic, and about 200 tonnes in 19th century. Afterwards, since the 1868, the decline of international price determined the abandonment of exploitation for copper, and pyrite ore was exploited for sulphuric acid production, until final closure. The Imperina creek valley is located in the mountain district of Belluno (North-East Italy), with an altitude ranging between 543m a.s.l. and 990m a.s.l., and oriented in the SW-NE direction. The geological substrate consists of dolomite rocks (Upper Triassic) on the right side and the predominantly metamorphic basement (Pre-Permian) on the left side, while at the bottom the calcareous-arenaceous complex of Werfen (Upper Permian - Lower Triassic) is outcropping (Bini et al., 2010; Fontana et al., 2010). Even if no human settlement could be found presently in the area, many buildings and tunnel outlets still bear witness to the past mining activity (Figure 12). Part of the area (right side and a portion of the bottom) lies within the National Park of the Belluno Dolomites. The Imperina stream crosses the valley, along a tectonic contact between the metamorphic basement and the Mesozoic dolomite rocks. The mineralized area of Valle Imperina, which is located along the above contact, is a deposit of mixed sulphides, composed primarily of cupriferous pyrite, pyrite and chalcopyrite, with minor amounts of other metallic minerals. Its exploitation has continued almost uninterruptedly from the XV century until the year 1962; copper and sulphur were the main products extracted. Until the beginning of the XX century, copper was extracted and processed directly in situ through roasting, a method with a severe impact on the area due to acid rains formation and intensive wood cutting. Yet, in early ‘900 vegetation in the area was lacking, due to cuttings and acid rains derived from SO2 production after metal roasting; presently, the whole area is naturally vegetated with mixed forest. Climate is humid temperate, with maT = 13°C and maP = 1250mm. A preliminary soil survey was carried out in the area in the last years by Bini et al. (2004) and Bini and Zilioli (2010), with the aim of characterizing soil genesis and evolution in the alpine environment. More than 70 soil profiles were described and analyzed; of these, eight profiles were selected for specific environmental analyses by Fontana et al. (2010). In general, the soils encountered in the survey are shallow (30 cm to a maximum of 100 cm) and undeveloped, with little presence of diagnostic subsurface horizons, and this applies particularly to those located in the areas affected by mining and metal processing, while those sampled as control are more developed. The pH varies from about 4.0 to nearly 8.0, depending on the nature of the substrate; the highly acidic pH values found in some soils are probably due to the alteration process of iron sulphides (pyrite and chalcopyrite) in the soil and substrate (Delgado et al., 2009). The texture is typically loamy, sandy-loamy or silty-loamy. The structure is usually weak and in some cases soils tend to be structureless and loose. The cation exchange capacity is low for all the soil samples, except for the profile on dolomite. The abundance of the soil skeleton is variable; in some profiles (1 and 4) it consists of waste from processing of ferrous minerals and coal from roasting, which show a clear anthropogenic influence. The concentrations of heavy metals in soil samples are shown in Table 11. Comparing the values found with those of control levels, according to the Italian legislation (D.L. 152/2006) and the world averages (Angelone and Bini, (1992), the area seems not contaminated with Ni, Cr and Mn, while there is a contamination by Zn and a high contamination by Cu, Pb and Fe, which are present in high concentrations, particularly in the soil nearby the areas affected by mining and ore processing (profiles 1, 2, 4, 6). Soils sampled in the immediate vicinity of the stream Imperina (profiles 3, 5) are less polluted than others in the valley, even though they are included in the area of greatest influence of human activities. This is probably due to the leaching of water and the establishment of periodic reducing conditions that increase the mobility of most elements considered, favouring the removal from soil and amplifying the risk of water contamination (Adriano, 2001).
The distribution of selected heavy metals (Cu, Fe, Pb and Zn) along the soil profile shows a general tendency to metal accumulation at surface. This is particularly true of Pb, and is consistent with pedogenetic processes occurring in the area. Significant variations of metal concentration with depth mark some discontinuities recorded in the profile morphology as well. Table 11. Concentration of metals in soils of Imperina Valley, average values of reference (Angelone and Bini, 1992) and limit values in the Italian legislation (D.M. 152/2006)
Bold values significant to P <0.05. Linear correlation coefficient between the concentrations of heavy metals in each horizon was calculated in order to assess the presence of any common behaviour, as listed in Table 12. The concentrations of iron, lead, zinc and copper are significantly correlated, according to their calcophilous behaviour, since they all tend to form compounds with sulphur. In this case, these elements are all present in the mineralized ore body of Valle Imperina in the form of pyrite (FeS2), chalcopyrite (CuFeS2), sphalerite (ZnS) and galena (PbS). Most of iron in soils of the study area derives from the alteration of pyrite and chalcopyrite, and this fact explains its low correlation with manganese, even if usually Mn tends to accompany Fe, due to their similar geochemical behaviour. The same combination of elements present in the mineralization of Imperina Valley is found in mine soils. These elements in the soil tend to have similar behaviour, and have a limited mobility, especially in oxidizing conditions. Accordingly, no element of the mineralization has been removed in a preferential way, and this is due to the fact that the agents of pedogenesis have acted for a short time (some decades) in the areas affected by processing the ore material, and thus the chemical characteristics of soil are still tied to that of the substrate. Recent studies by Bini et al. (2011), however, show that several factors may contribute to trace elements concentration in the A horizon. Yet, surface horizons are generally enriched in organic matter, which could have a role in adsorbing trace elements. On the other hand, it is possible that heavy metals be accumulated at surface since the past century, when mining activity was operating in the area. Exploitation, grinding and roasting of minerals could have generated solid particulate added to soil. A third possibility is that metals could have a partial natural (geogenic) origin, and a partial anthropic origin, and the observed stratification could be a result of these two forms of diffused contamination. Moreover, it is likely that higher concentration of trace elements could be related to the migration of species released by the mineralised area, via riverine transport, in the extreme parts of the valley. Chapter 5 Yüklə 0,52 Mb. Dostları ilə paylaş:
|