[Source and enrichment of heavy metals in sewage-irrigated area soil of Dagu Sewage Discharge Channel].

Concentrations of metals, total nitrogen, total phosphorus and organic matter were determined in topsoil (0-25 cm) and subsoil (25-50 cm) samples collected from 16 selected blocks in the sewage-irrigated area of Dagu Sewage Discharge Channel in Tianjin, China. Contamination factors (CF), relative enrichment factors (REF) and cluster analysis (CA) were taken to analyze the source and enrichment of heavy metals. The results show that the concentrations of Hg, Cd, Pb and Zn are above the 1st limit value of Environmental Quality Standard for Soils (China), while those of As, Ni and Cr are below the 1st limit value of the standard. The concentrations of Hg, Cd, Pb, Zn, As, Ni and Cr in topsoil are 0.323, 0.341, 44.7, 108.29, 11.2, 29.86 and 66.6 mg/kg, respectively. CF and REF of Hg in topsoil are 10.8 and 7.2, respectively, which indicates very high contamination and moderately severe enrichment. Both factors of Cd are 3.8 and 4.5, and indicate considerable contamination and moderate enrichment. Those of Pb, Zn and As are all less than three, which indicates moderate contamination and minor enrichment. As for Ni and Cr, their contamination is low and no enrichment occurs. REF for Hg, Pb and Zn in topsoil is relatively higher than that in subsoil while As in subsoil is higher. When compared with data measured in 1985, the concentrations of Cd and As increase while those of Hg, Pb and Zn decrease. Furthermore, measures, including CA and in situ investigation, are taken to identify the main sources of heavy metal contamination, and it shows that Hg and As contamination result from anthropogenic effect as well as deposition of coal combustion. Cd, Zn and Pb contamination are mainly from sewage irrigation, and atmosphere deposition also makes certain contribution. Ni and Cr contamination are attributed to natural sources.