Occurrence and distribution of key potentially toxic elements (PTEs) in agricultural soils: a paradigmatic case study in an area affected by illegal landfills

摘要

A study was carried out in the Volturno River's lower basin (Campania, south Italy), an area of significant agronomic economic, and cultural value but which is characterised by extensive environmental concerns. The area is a part of the "Litorale Domizio-Flegreo e Agro Aversano" Site of Regional Interest (SRI), due to the presence of several "potentially polluted" sites and illegal landfills, some containing toxic/harmful pellet foundry slag. Taking into account the relevance of potential environmental risks, linked to dangerous waste dispersion, and their serious socio-economic implications, research began with the general aim to assess whether or not the pollution hazard really had jeopardised the agricultural soils of Volturno River's lower basin. Specifically, this study was concerned with the distribution and concentration of selected potentially toxic elements (Cd, Cu, Pb, and Zn), in soil samples and representative profiles, with reference to different pedological features and physical-chemical characteristics. The main aim was to clarify the natural and anthropic factors influencing their occurrence. Hence, a detailed survey was carried out by collecting 64 core samples at fixed depths (0-20, 20-40 cm) and examining seven different soil profiles through the identification of 39 genetic horizons. Univariate and multivariate statistics were used for the interpretation of the data sets. As consequence of the intrinsic heterogeneity of the investigated Pedons, which are influenced by the variable flooding regime, PTEs distribution usually fails to show a sharp, linear concentration gradient with depth. On the opposite, evident fluctuations along profiles were systematically observed. For those concerned core drill samples, the raw data sets (median ± 2MAD) for Cu (12.93-48.33 mg kg~(-1)), Pb (1.70-50.10 mg kg~(-1)), and Zn (32.13-129.13 mg kg~(-1)) appear to conform to the background population. On the opposite, an element specific situation appears to exist for Cd (0.01-11.89 mg kg~(-1)) characterised by values exceeding the geogenic background concentration (0.43 mg kg~(-1)) as well as the threshold values (2 mg kg~(-1)) ruled by the Italian legislation. However, according to current laws, the investigated area cannot be defined as "contaminated site" sensu strictu, since some anomalous PTEs concentration is erratic and inconsistent with others. Principal factor analysis (PFA) shows that soil organic matter (SOM) is the most important sink for the investigated PTEs, suggesting that their occurrence and distribution have a mainly pedogenic control. From this point of view, SOM acts as a sink of PTE by secondary accumulation processes, most probably favoured by the formation of organo-metal complexes. This suggests that PTEs linked to SOM can substantially arise from the weathering of the geogenic PTE forms, as well as unintentional or illegal "anthropogenic" factors may influence the whole soil PTEs pool. Indeed, such findings do not exclude that some PTE accumulation in the investigated soils may also have been due to anthropogenic sources. As a matter of fact, given the long-standing human impact on the investigated area, an anthropogenic influence on PTE background concentrations cannot be a priori excluded. From this point of view, the agricultural soils of Volturno River's lower basin require strict control and protection from the illegal deposition of waste materials, in order to prevent the widespread diffusion of pollution.