Low metal bioavailability in a contaminated urban site

Bioavailability of Cd, Cu, Ni, Pb, and Zn in a metal-enriched railway yard in Montréal, Québec, Canada was assessed using metal speciation, plant uptake, and microbial assays. Metal speciation of extracted soil solutions was estimated using the Windmere Humic Aqueous Model. In soil solutions, free Cd, Ni, and Zn ions represented as much as 80%, 72%, and 62%, respectively, of the total dissolved metals. Copper and Pb were strongly bound by dissolved organic matter, and metal-fulvic acid complexes represented as much as 99% of the total dissolved metals. Three field-collected plant species (dandelion, bladder campion, and chicory) varied in their tendency to accumulate metals in either their leaves or roots. Chicory grown in the greenhouse had significantly higher metal bioconcentration factors than wild chicory. Although the site studied is considered to be contaminated, no metal pool, such as free ions or dissolved or total soil metals, consistently predicted metal uptake by potted chicory. Regression analysis revealed that soil total metal concentrations could adequately predict tissue accumulations of Cd, Ni, Pb, and Zn in bladder campion but only Cd and Zn in dandelion. Data from microbial assays also showed that the soil respiration was not affected by the metal contamination, but that nitrification was inhibited for the most contaminated soils. These results indicate that the metal bioavailability in the railway yard is low, but they also suggest that nitrogen cycling may be affected.