Soil contamination by organic and inorganic pollutants at the regional scale: the case of Piedmont, Italy

Diffuse soil contamination has often been neglected in scientific literature, as most studies focus on contaminants from point-sources (either of industrial or agricultural origin). However, soil pollution from diffuse sources is recognized as one of the major soil threats by the EU Soil Thematic Strategy. In fact, some pollutants are nowadays ubiquitarious in the soil system, and they have to be considered for the implementation of environmental legislation, the definition of clean-up values in remediation activities and, more generally, for a sustainable management of rural areas. In the literature large scale studies on diffuse contamination are few and scattered and often do not consider a wide range of contaminants, the effect of land use, the vertical variability and the potential natural contribution. Aim of this work was to provide an overview of the diffuse soil contamination on a regional scale for a large set of contaminants. Soil inorganic (Cd, Cr, Ni, Pb, Cu, Zn) and organic contaminants (polychlorinated dibenzo-p-dioxins(PCDD), polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs) as well as other soil general parameters were investigated on a 18 x 18 km grid covering a whole Italian region heavily industrialized and intensively cultivated. Soils were sampled at different depths both for natural-undisturbed soil and for agricultural-plow soil. Differences in the trends of investigated contaminants, as a consequence of land use, vertical variability, natural or geogenic origin, relationships among contaminants, and with main soil properties, were explored by means of enrichment factors, bi- and multi-variate statistics. The sampling scheme of this study is based on a systematic 18 x 18 km grid covering the whole region. Overall, 43 monitoring sites located at the center of each cell were sampled. At each site, five individual core samples within a 10 x 10 m area were taken at different depths for natural-undisturbed soil (topsoil 0-10 cm, subsoil 10-30 cm) and for agricultural-plow soil (topsoil Ap horizon, subsoil 20 cm below the Ap lower limit). Samples were processed for general soil properties as well as for organic (PAHs, PCBs, PCDDs/dibenzofurans (DFs)) and inorganic (Cd, Cr, Ni, Pb, Cu, Zn) contaminants analyses (aqua regia digestions). Contaminants were detected after extraction by means of ICP-MS and GC-MS. Statistical analysis was conducted using the software SPSS 13 (SPSS) and Minitab 15 (MINITAB). Data were geographically managed and processed with the Arcview 3.2 (ESRI, CA, USA) GIS software. Organic contaminants such as PCDDs were found to accumulate in natural areas while inorganics mostly concentrate in agricultural soils, confirming the presence of important phenomena of long-range diffuse contamination for the first and short-range for the latter. Soil use was also confirmed to be a major parameter in influencing the type, degree, and distribution of contaminants. Vertical variability was found to be high for organic contaminants and for Pb, suggesting their main anthropogenic origin, while other elements such as Cr and Ni appeared to be more related to the natural background. Data were compared with those from soils of a large industrial city present in the area, confirming the strong enrichment of the urban environment with respect to some contaminants such as metals, PCBs, and PAHs. Other contaminants such as PCDD/DFs showed lower differences, confirming their diffuse and almost ubiquitarious pollution. Even if natural soils in this study are mostly located far from major sources of contamination, notable differences appeared when compared to agricultural areas. In particular, the enrichment in the concentrations of organic contaminants such as PCDDs in natural areas and of inorganics in agricultural soils confirm the presence of important phenomena of long-range diffuse contamination for the first and short-range for the latter, which appeared also to be related to agricultural activities. Contaminants like PCDDs, PCBs, PAHs, and Pb presented high vertical variability, confirming their anthropogenic origin and strong affinity to soil organic matter, while others appear to be more related to the natural background. The comparison of data with those from soils within a large-industrial city present in the study area confirms the strong enrichment of the urban environment with respect to some contaminants such as metals, PCBs, and PAHs. Other contaminants such as PCDD/DFs showed lower differences confirming their diffuse and ubiquitarious pollution. Soil diffuse contamination revealed to be an important source for some contaminants that appear to distribute over a large scale. The type of land use strongly influences the distribution of pollutants in soils, by diluting them in depth (plowing in agricultural areas) or concentrating contaminants in the surface (natural areas or meadows). Data provided in this study constitute an important dataset of the soil environmental quality on a large scale that can be used for the development of guidelines for soil management, the definition of local clean-up values, and the implementation of risk assessment procedures.