Abstract

Urban areas can be considered risk areas as regards trace metals and will continue to be so for a long time, according to predictions. The present work started as a sub-project in the urban part of the multifaceted research project Metals in Urban and Forest Environments.The overall aim of the work was to gain systematic knowledge about the amounts and behaviour of trace metals in urban soils, with Stockholm (the capital of Sweden) as the study area. The concentrations of trace 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 results showed a wide range in trace metal concentrations, as well as in other soil properties. The city centre soils constituted a rather homogeneous group, whereas outside this area no soil geographical zones could be distinguished. These soils were instead 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 Cu, Hg, Pb and Zn concentrations compared to the other soil groups and rural soils in the region. The Guideline values for sensitive land use (set by SEPA) are exceeded by the mean concentrations of Hg and Pb and maximum concentrations of Cd, Cu and Zn in the city centre soils. The average soil pools (0-30 cm depth) of Cu, Pb and Zn in city centre soils were 21, 38 and 58 g m-2 respectively, which for Pb was 3-4 times larger and for Cu and Zn 1.5-2 times larger than that in park soils outside the city centre. The total amounts accumulated in city centre soils were calculated at 40, 1.1, 120 and 80 t for Cu, Hg, Pb and Zn respectively. Solubility and mobility of the trace metals were studied with sequential extraction and column leaching extraction. In the sequential extraction, four steps (1M NH4NO3, 0.1 M NaCl, 1M NH4Ac pH 4,8 and 0.5 MH4Ac+0.02 M EDTA) were used. Four different treatments aimed at imitating the effect of specific urban activities were used in the leaching extraction (i.e. rain, acid rain, salt and bark). The sequential extractions were not found to be useful for quantitative prediction of metal mobility. The column extraction experiment showed that contaminated soils might be one possible explanation for the enhanced concentrations of Cd, Cu, Pb and Zn in Stockholm groundwater. Leachate solutions from all treatments showed that most Cd, Ni and Zn was mobilised in the salt treatment and that the concentrations of Cu, Cr, and Hg were strongly positively correlated to the DOC concentration. The positive correlation between Pb and DOC and the negative correlation between Cd and pH were weak. The computer model SHM was used to evaluate the leaching results. For Cd and Zn simulated values corresponded rather well with measured values, but for Cu and Pb the results were more ontradictory.