Assessment of trace metal and PAH concentrations in soils from peri-industrial regions: lessons from a survey in the Walloon region (Belgium)

Soils from peri-industrial areas are usually disturbed by urbanization, mixed with technogenic material and affected by smoke and dust fallout. A precise knowledge of the local baseline content of pollutants in soil is needed by the government agencies in order to implement the environmental protection policies. However, in peri-industrial areas, pollutant concentrations often exhibit a high spatial variability which requires an appropriate investigation methodology. Here, we outline the key results obtained from a survey on baseline soil concentrations of trace metal elements (TME) and polycyclic aromatic hydrocarbons (PAH) in peri-industrial soils from the Walloon region (Belgium). For the survey, 8 areas were selected in order to represent the range of current and historical industrial activities of the region. These include chemical plant, steelmaking blast furnace, collieries, coal agglomeration and coke production plants and nonferrous metals smelting plants (Pb, Zn). The sampling points (70 to 200 in each area) were spatially distributed and were representative of the various land uses that occur in peri-industrial area: vegetable gardens, urban parks, farmland and forest. At each sampling point, we collected a sample of the topsoil and, when possible, a sample from a deeper horizon (between 60cm and 1m depth). Concentrations were measured for 5 major elements, 14 TME and 16 PAH (EPA). This sampling and analyzing scheme made it possible to distinguish the effects of the four main factors that affected the concentrations of TME and PAH in the topsoil: (1) the pedogeological soil type (2) the predominant industrial activities responsible for the atmospheric fallout of pollutants (3) the amount and the nature of the technogenic materials and (4) the type of land use. In the peri-industrial areas of Wallonia, we found that the presence of As, Co, Cr, Ni, Be, Ni and Mo was predominantly linked with the pedogeological soil type and was not related to atmospheric fallout of pollutants. Two different kinds of industrial activities were shown to have a clear footprint in terms of TME and PAH contents in topsoil: the non-ferrous metallurgy smelting plants were associated with high values in Cd, Pb and Zn, and the steel and coal industrial activities were associated with high PAH values and moderately high Zn values. The presence of technogenic material resulted in elevated Ba, Cu, Hg, Ni, Pb and PAH values. An influence of land use on pollutant contents in topsoil was also observed: compared to other land uses, vegetable garden soils showed the highest values of Ba, Cu, Zn and PAH. Vegetable garden and forest soils had the highest Hg, Pb, Sb and Sn values. In conclusion, the results of this survey show that our sampling and analyzing scheme is well-designed to identify which types of soils potentially constituted a risk for the environment and for human health. These results provide the valuable input data needed to draw meaningful and reliable maps of baseline contents of pollutants in soil