Trace metal speciation and bioavailability in urban soils

Urban soils often contain concentrations of trace metals that exceed regula tory levels. However, the threat posed by trace metals to human health and the environment is thought to be dependent on their speciation in the soil solution rather than the total concentration. Three inactive railway yards in Montreal, Quebec, were sampled to investigate the speciation and bioavai lability of Cd, Cu. Ni. Pb and Zn. Soil solutions were obtained by centrifu ging saturated soil pastes. In the soil solutions, up to 59% of the dissolv ed Cd was in its free ionic form. For Cu, Pb and Zn, organic complexes were the predominant species. Over 40% of Ni was present as inorganic complexes if the solution pH exceeded 8.1. Multiple regression analyses showed that pH and total metals in soil were significantly correlated with the activiti es of free metal ions: except for Cd2+, which only had a weak correlation w ith soil pH. Free, dissolved and total soil metals were tested for their ab ility to predict metal uptake by plants in the field. However, none of thes e metal pools were satisfactory predictors. The results indicated that in t hese urban soils, trace metals were mainly in stable forms and bioavailabil ity was extremely low. (C) 1999 Elsevier Science Ltd. All rights reserved.