Spatial-Temporal Trends and Pollution Source Analysis for Heavy Metal Contamination of Cultivated Soils in Five Major Grain Producing Regions of China.

Heavy metal pollution of farmland soils in China has been identified as a threat to ecosystem safety and human health. A total of 3006 soil samples were analyzed from arable lands in five grain producing regions of China, which included data from published studies from 2000 up to now. An additional 656 historical samples were derived from the 1980s by a digitizing grained point sites map (Cd, Pb, As, Ni, Cu, Zn, Cr, and Hg) from the PRC Atlas of Soil Environmental Background Values. A GIS-based approach and single factor index method were employed to identify the current status and spatial distribution of heavy metal (Cd, Pb, As, Ni, Cu, Zn, Cr, and Hg) contamination in agricultural soils, and these were then compared with historical data to explore contamination trends over time. Then, based on the Igeo method and the effects of the surrounding environment on contamination rates, pollution sources were analyzed. Results showed that 21.49% of the agricultural soil samples exceeded the environmental quality standard set by the Ministry of Environmental Protection. The proportions of slight, moderate, and severe pollution were 13.97%, 2.50%, and 5.02%, respectively. Pollution is more extensive in the south compared with the north. Exceedance percentages in the Sichuan Basin (SC), Yangtze River Middle Plain and Jianghuai Plain (CJ), Huang-Huai-Hai Plain (HHH), Songnen Plain (SN), and Sanjiang Plain (SJ), were 43.55%, 30.64%, 12.22%, 9.35%, and 1.67%, respectively. The main pollutants were Cd, Ni, Cu, Zn, and Hg, with exceedance percentages of 17.39%, 8.41%, 4.04%, 2.84%, and 2.56%, respectively. Since the 1980s, heavy metal pollution has increased by 14.91%. The proportion of Cd, Ni, Cu, Zn, and Hg increased by 16.07%, 4.56%, 3.68%, 2.24%, and 1.96%, respectively. Except for SJ, exceedance percentages in cultivated land increased significantly, while the exceedance percentages of Cd, Ni, and Cu in the southern areas were higher than for the northern areas-although the growth rate of Hg and Cr in the south was lower than that in the north. The main sources of Cd and Hg were anthropogenic pollution, while the other six heavy metals were from predominantly natural sources. However, about 20.00% of Pb, Zn, and Cu were affected by anthropogenic activities. Mining, industry, and sewage water were the main sources of pollution. In addition to the larger impact of sewage irrigation in the north, other sources of pollution showed greater influence in the south. Mining mainly caused pollution by Cd, Hg, Ni, Cr, and Cu, while excessive levels of Cd, Ni, Cu, Zn, and Hg was the signature of industrial pollution. Irrigation with sewage effluent causes excessive Cd, Ni, and Zn. Results from this study provide valuable information for agricultural soil management and food safety in China.