Spatio-temporal variation and source changes of heavy metals in cultivated soils in industrial developed urban areas

Abstract: Soil samples were collected from cultivated soil in Dongguan, a representative fast-growing economic area of the Pearl River Delta, China. Variation characteristics of mercury (Hg), cadmium (Cd), chromium (Cr), lead (Pb), arsenic (As), copper (Cu), zinc (Zn) and nickel (Ni) over the last decade were described by multivariate statistics. Spatio-temporal variation and source change of 4 pollution elements (Hg, Cd, Cu, Ni) were further analyzed by geostatistics and principal component analysis/absolute principal component scores (PCA/APCS) model. The results indicated that the concentrations of Cd and Zn increased by 71.4% and 15.9% respectively from 2002 to 2012, whereas those of Cr, As, Cu, and Ni remained relatively stable, Hg and Pb decreased respectively by 40.0% and 25.7%. The use of unleaded petrol in China since 2000 was the significant cause of the decrease in Pb concentration, while the decrease of Hg was attributed to strict emission controls implemented by the government, harvesting, Hg evaporation from farmland topsoil and runoff. These showed that the pollution of heavy metals was strongly impacted by industrial and agricultural activities, and the accumulation of heavy metals could be prevented if human activities were properly constrained. Hg, Cd, Cu and Ni were the pollution elements, which exceeded the second level of the Standards for Soil Environmental Quality in some soil samples. Cd, Cu and Ni among the pollution elements showed an aggravated contamination from 2002 to 2012. The ratios of nugget to sill of Hg, Cd and Cu were 43.4%, 31.9% and 19.4% respectively in 2002, and 49.9%, 54.7% and 25.3% respectively in 2012. This showed that these elements were interfered more seriously by anthropogenic factors in 2012 than in 2002. The results of PCA/APCS model showed that heavy metals were mainly from 3 types of sources in 2002: water area and parent material, electroplating, chemical industry and railway, and electronic industry, electroplating, leather making and highway traffic; there were two main types of sources in 2012: water area and parent material, and electronic industry, electroplating and domestic waste. Hg (69.9%) mainly came from electronic industry, electroplating, leather making and highway traffic, and Cd (68.0%) mainly came from electroplating, chemical industry and railway in 2002. Hg (57.8%) and Cd (52.3%) were both mainly from electronic industry, electroplating and domestic waste in 2012. The reasons of pollution source change were the strengthening of government pollution control, improvements in environmental protection technology of transportation, and the increasing of domestic waste caused by the rapid development of urbanization. Water area and parent material were main sources of Ni both in 2002 (63.8%) and in 2012 (82.0%). Cu mainly came from water area and parent material (46.9%) and electroplating, chemical industry and railway (32.5%) in 2002, while mainly came from water area and parent material (64.4%) in 2012. Industrial upgrading and emission control led to the decreasing of contribution rates of electroplating, chemical industry on copper pollution in 2012. Considering the contribution rate of parent material was stable in short term, the water quality deterioration of river may be the important reason for the rise of contribution rates of water area and parent material. Understanding the law of spatio-temporal variation and source change of heavy metals in the process of social and economic development is a basis for taking appropriate action to control heavy metal pollution and protect environmental quality of soil.