Total and exchangeable concentrations of heavy metals in soils near Bytom, an area of Pb/Zn mining and smelting in Upper Silesia, Poland

摘要

High concentrations of several heavy metals were suspected in soils in an area of some contemporary and extensive historical mining and smelting of Pb and Zn near the town of Bytom. In order to investigate the spatial distribution of heavy metals, 152 soil samples were taken at high sampling density in an area of 14 km~2 on a regular grid as well as along an 11 km transect. The samples were analysed for total Pb, Zn and Cd content by ICP-AES; a selection of samples were also analysed for total As content. Significant levels of contamination were found. Median topsoil concentrations (0-10 cm) for Pb, Cd, Zn and As were 430 µg g~-1, 13 µg g~-1, 1245 µg g~-1 and 35 µg g~-1, respectively. The detected levels of Pb, Zn and Cd were mostly in reasonable agreement with findings from a previous low-density study, but As concentrations were up to 6 times higher than had previously been reported for the area. Additional zones of particularly high concentrations could be identified for all 4 elements by this higher-density survey. Contaminant concentrations were generally found to decrease substantially with increasing depth, on average by a factor of 3.5 for Cd, 3.0 for Zn and 2.6 for Pb. However, significant subsoil contamination (40-50 cm) was also detected, in particular for Zn, Pb and As, which appeared to be enriched at depth in certain locations. To assess the potential availability of the metals to plants, the exchangeable fraction (0.5 M MgCl_2) was estimated for Pb, Zn and Cd for 84 samples. Levels were strongly influenced by soil pH and were generally low for Pb (less than 1% of total, max 15.6%), moderate for Zn (less than 10% of total, max 32.4%), and high for Cd (mean 35% of total, max 59.8%). For Zn and Pb, there seemed to be a threshold pH value of about 6, below which a significant increase in the exchangeable fraction was observed. No such threshold value appeared to exist for Cd, which was found to be relatively labile even in slightly alkaline soils (mean of 27.6% exchangeable Cd in pH range 7-8). The detected levels of total metal contamination exceed various national and international threshold, indicating the need for further investigation and an assessment of the suitability of the land for agricultural use, particularly in view of the high levels of exchangeable Cd. The pattern of spatial variation of the metals in the topsoil indicates that a variety of sources might be responsible for the contamination, historical mining and smelting probably being the most important.