CONCENTRATIONS AND POOLS OF HEAVY METALS IN URBAN SOILS IN STOCKHOLM, SWEDEN
The concentrations of heavy metals (Cd, Cr, Cu, Hg, Ni, Pb and Zn) and arsenic (As) were surveyed and the metal pools estimated in soils in Stockholm Municipality. The sampling sites were distributed all over the entire municipality with a higher sampling density in the city centre. Soils were sampled to a maximum depth of 25 to 60 cm. Soil texture, total-C content, electrical conductivity and pH were analysed. Heavy metal concentrations were determined after wet digestion with boiling 7 M HNO3.
The results showed a wide range in heavy metal concentrations, as well as in other soil properties. The city centre soils constituted a rather homogeneous group whereas outside this area no geographical zones could be distinguished. These soils were grouped based on present land use, i.e. undisturbed soils, public parks, wasteland (mainly former industrial areas), and roadside soils. The city centre and wasteland soils generally had enhanced heavy metal concentrations to at least 30 cm depth compared to park soils outside the city centre and rural (arable) soils in the region, which were used to estimate background levels. For example, the mean Hg concentration was 0.9 (max 3.3) mg kg−1 soil at 0–5 cm and 1.0 (max 2.9) at 30 cm depth in the city centre soils, while the background level was 0,04 mg kg−1. Corresponding values for Pb were 104 (max 444) and 135 (max 339) mg kg−1, at 0–5 and 30 cm, respectively, while the background level was 17 mg kg−1.
The average soil pools (0–30 cm depth) of Cu, Pb and Zn were 21, 38 and 58 g m−2 respectively, which for Pb was 3–4 times higher and for Cu and Zn 1.5–2 times higher than the background level. The total amount of accumulated metals (down to 30 cm) in the city centre soils (4.5*10 6 m2 public gardens and green areas) was estimated at 80, 1.1, 120 and 40 t for Cu, Hg, Pb and Zn, respectively. The study showed (1) that from a metal contamination point of view, more homogeneous soil groups were obtained based on present land use than on geographic distance to the city centre, (2) the importance of establishing a background level in order to quantify the degree of contamination, and (3) soil samples has to be taken below the surface layer (and deeper than 30 cm) in order to quantify the accumulated metal pools in urban soils.