Investigating the heavy-metal concentrations in soils from rainwater-harvesting green spaces in Beijing

The aim of this study was to determine whether the heavy-metal levels in the soils collected from five types of rainwater-harvesting green spaces in Beijing posed ecological risks. The heavy-metal levels (Cd, Pb, Cu, Cr, Zn, Ni) of the collected soils were determined. The mean concentrations of Cd, Pb, Cr, Cu, Zn, and Ni were 0.22, 35.61, 59.34, 32.87, 87.30, and 25.15 mg/kg, respectively, and, apart from the Ni mean concentration, were higher than the background values in Beijing. The Zn and Ni concentrations were significantly higher in the soils from the commercial green space than in the other soil types, while the Zn and Cr concentrations were highest in the soils from the park green space and the traffic green space, respectively. The Nemerow integrated indices for soils from all the green-space types were between 1 and 2, indicating light pollution. The indices of the potential ecological risk, at less than 150 for all the soils, indicated a slight ecological risk. The results from wheat-seed (Triticum aestivum L.) germination tests to determine the possible phytotoxicity of the soils showed that plant growth in the soils from the rainwater-harvesting green spaces might be adversely affected. Assessments of the leaching toxicity showed that the heavy metal concentrations in all the leachates were below the allowable national standard. The results demonstrate that the heavy metals were present in the soils from rain-harvesting green spaces in Beijing at levels that posed a minimal ecological risk.