Assessment on heavy metal pollution status in paddy soils in the northern Chengdu Plain and their potential ecological risk

Abstract

Soil heavy metal pollution has received wide concern due to its widespread and highly toxic nature in environment. Accumulation of heavy metals in agricultural soils tends to elevate crop uptake and finally harm human health through food chains. In order to determine heavy metal pollution status in paddy soils in Chengdu Plain and to assess their potential ecological risk, 158 topsoil samples were collected from the study area, a county typical of paddy rice production with potential pollution of soil heavy metals, to analyze concentrations of seven heavy metals (Cd, Hg, Ni, Cu, Pb, Cr and As) and pH in the soils. The background values of the soil heavy metals in Chengdu Plain measured in 1980's were used as references and the Hakanson potential ecological risk index method was adopted in the assessment. The results showed that the average concentrations of Cd, Hg, Ni, Cu, Pb, Cr and As were measured as 0.709, 0.187, 32.08, 34.12, 31.52, 82.13 and 7.25 mg/ kg, respectively. Compared to level II of the China Soil Environment Quality Standard (GB 15618- 1995), about 87.34%, 8.23%, 3.80% and 3.80% of the total samples were polluted by Cd, Ni, Cu and Hg, respectively. The average concentration of each heavy metal exceeded its soil background values, reflecting an increase in soil heavy metal pollution in Chengdu Plain in recent years. Compared to its background values, Cd had the highest accumulation by 312.2% while Hg accumulated the least. Accumulation of the other metals ranged from 5.0% to 49.03%. The range of variation coefficient was 18.35% to 49.03% for the seven heavy metals and variation coefficient followed an order of Cd>Hg >Cu>As>Ni> Cr>Pb. Cadmium, Hg and Ni showed a logarithmic normal distribution, while Cu, Pb, Cr and As gave a normal distribution. The Person correlation analysis revealed that significant correlations (P<0.01) were observed among Cd, Cu, Hg and Ni as well as among Pb, Cr and As. The results further suggested that the pollution of Cd, Cu and Ni in the soils might be highly associated with industrial activities, while Pb, Cr and As in the soils could be mainly originated from the soil parent materials. Unlike the other heavy metals, the pollution of Hg in soils could be most likely due to both the soil parent materials and human activities including industry emissions and loadings from agricultural activities. The ranges of single ecological risk index (Er) were 28.62-305.8 for Cd, 2.22-11.31 for Cu, 4.58-9.92 for Pb, 1.50- 3.59 for Cr, 16.57-83.54 for Hg, 3.60-22.38 for As and 3.97-9.27 for Ni. About 75.32% of the total samples reached medium or strong level of the potential ecological risk of the heavy metals, but the overall potential ecological risk remained medium (the average RI = 198.65). It was found that Cd and Hg contributed most to the ecological risk, accounting for 62.27% and 20.78%, respectively, and the risks of As, Pb, Cu, Ni and Cr were relatively minor. Risk probability map demonstrated that areas around the city and along the Mianyuan River were highly risky. Thus, some counter-measures should be taken to control the pollution of Cd and Hg to secure the safe agricultural production in Chengdu Plain.