Spatiotemporal Variability and Source Apportionment of Soil Heavy Metals in a Industrially Developed City

Abstract

In the present study, a total of 1130 soil samples were collected in 2003 and 2013 in the Ningbo city district, and the content of eight typical heavy metals were measured. The degree of heavy metal pollution in the soil in the study area was evaluated using the pollution index evaluation method. The temporal and spatial variation in the pollution were analyzed by the geostatistics method, and principal component analysis (PCA) and the UNMIX model were used to analyze the heavy metal pollution sources. The results showed that:① excluding As, the mean values of the other seven heavy metals in both periods exceeded the soil background values in Zhejiang province and national background values, which indicated that heavy metal pollution is common in the soil from the study area. From 2003 to 2013, the mean content of all elements increased to some degree, indicating that the heavy metals pollution was generally aggravated in 10 a; the increase in the mean Hg content was the greatest. ② The evaluation results based on the single-factor pollution index showed that the pollution degree was high for Hg; moderate for Cd, Cu, and Pb; slight for Zn, Cr, and Ni; and unpolluted for As. The evaluation results based on Nemero's comprehensive pollution index showed that the pollution index for all samples exceeded the precaution value, and the number of soil samples that were characterized as heavily polluted with a comprehensive pollution index over 3 had increased significantly. ③ The spatial distribution of the same heavy metal in different periods presented similar characteristics. Generally speaking, the spatial distribution of Pb was similar to that of Hg, and the spatial distributions of Cr, As, Cu, Zn, and Ni were similar to each other; high value areas were mainly concentrated in the four areas of Haishu, Jiangbei, Jiangdong, and Zhenhai districts, where the polluted enterprises were densely distributed and human activities were frequent. ④ Based on PCA and the UNMIX receptor model, the source analysis results of soil heavy metals in 2013 were essentially the same, i.e., the source of heavy metal pollution in the study area included industrial sources and living sources. Industrial sources of heavy metal pollution had the highest contribution rate and derived from industrial wastewater, waste gas, and waste residue discharged by plasticizing, mining, and metallurgical enterprises; chemical and mechanical manufacturing enterprises; etc., causing the accumulation of Cr, As, Cu, Zn, and Ni in the soil. The living sources had the second highest contribution rate and originated from the pollution of particulates and dust caused by domestic waste, automobile exhaust, and road wear. Additionally, natural factors had small contribution rates, mainly due to the spread of pollutants by prevailing winds and atmospheric settlement.