Background levels of trace elements and soil geochemistry at regional level in NE Italy

摘要

Trace elements of both lithogenic and anthropogenic origin occur in soils distributed in various solid state forms, in the organic fraction and in the soil solution, and their fate in the environment depends upon several factors. The distribution patterns of element concentration are primarily influenced by the lithology of parent material, and secondly by the soil forming processes, that modify the basic geochemical composition and redistribute the content of metals within the soil profile. Alteration processes may lead to the release of potentially toxic elements, particularly heavy metals, into the environment. In this study, our objectives were to evaluate the background level and the concentration range of heavy metals in soils of a famous environmental scenery in northern Italy, to ascertain metal distribution and to identify possible contamination of some sites, and the related environmental hazard, with special reference to the pollution of the Dolomites area, which is a unique and delicate ecosystem. A regional soil survey was carried out in recent years in forested areas of the Trentino Region, in northern Italy. The territory investigated extends over approximately 400,000 ha (70% of the whole region), mostly in mountain areas. Approximately 460 soil samples from more than 120 representative soil profiles were collected and analysed for routine analyses and total Cu, Ni, Pb, Cd, Zn, Cr, Fe, Mn contents. Soils were grouped into 5 groups according to their pedogenetic characteristics. Most soil horizons (260) have acid reaction (mean pH = 6.1), though many are developed from calcareous rocks. Soil texture varies from sandy loam to silty loam. Cation exchange capacity varies consistently with the nature of parent material, as well as base saturation. The organic matter content is generally high at surface, and decreases with depth, the richest horizons (up to 11%) being those from calcareous materials. Background levels of trace elements in the soils investigated are consistent with currently recorded trace element concentration of soils from Western Europe. A geological matrix effect may be accounted for metal release by parent material weathering. Therefore, no contamination is recorded in the whole Trentino region. Nevertheless, metal accumulation in surface horizons at some sites may be ascribed to atmospheric input from local agricultural or industrial activities. Statistical analysis of geochemical data enhanced identification of pedogenetic processes responsible for the formation of soil groups with different weathering stages, and proved a useful tool to outline pedogenetic trends.