Evaluation of the heavy metal pollution ecological risk in topsoil: a case study from Nanjing, China

We examined the topsoil heavy metal concentration of four different urban functional areas (UFAs) in Nanjing, China: a heavy traffic area (HA), an industrial area (IA), a leisure area (LA), and a farmland area (FA). The purpose of the study is as follows: (i) to determine which heavy metal(s) were the primary pollutant(s) in the study area and the contamination status of five heavy metals (Cr, Ni, Cu, Zn and Pb) in the urban topsoil of Nanjing city, (ii) to assess the comprehensive pollution risk of heavy metals in UFAs using two methods: the ecological risk index and the fuzzy comprehensive evaluation based on Analytical Hierarchy Process (AHP). X-ray fluorescence (PXRF) was used to measure the soil heavy metal content, the potential ecological risk method was implemented to evaluate the risk level of each UFA, and the fuzzy comprehensive evaluation method was applied to assess the pollution degree of heavy metals in UFAs. PXRF analysis showed that Cr was the primary metal pollutant in the study area. There were also significant enrichments of Cu, Zn and Pb in collected soil samples. The four UFAs were ranked IA > HA > LA > FA in terms of the potential heavy metal ecological risk; the distribution pattern determined by the fuzzy comprehensive evaluation model followed the same order. Overall, 84.70%, 12.98% and 2.32% of the 647 sites were classified as class I (clean), class II (lightly polluted), and class III (heavily polluted), respectively, suggesting that most soil in Nanjing was clean. Based on the calculated weight matrices of the potential ecological risk results, the major pollutants were Cr and Zn. This investigation highlighted that most of the soil in the study area is clean, but certain sites are polluted to some degree, especially in the IA. The study area showed an overall moderate ecological risk for heavy metal pollutants, with Cu and Pb posing the greatest risk.