The Spatial Distribution and Source Identification of Soil Heavy Metals in Croplands near Two Contaminated Sites.

Surface soil (0-20 cm) and subsurface soil (20-40 cm) samples were collected from croplands in the vicinity a battery plant (F1) and a thermal power plant (F2) in northern Zhejiang Province. The concentrations of mercury (Hg), cadmium (Cd), lead (Pb), zinc (Zn), copper (Cu), and nickel (Ni) in the soil samples were measured and the spatial distribution patterns and their pollution sources were analyzed using Geographic Information System (GIS) and geostatistics methods. Results showed that soil Cd pollution was a serious problem in the soils nearby Contaminated-site F1, with Cd concentrations in 91% of the surface soil samples exceeded the Type II limit (Cd > 0.3 mg kg-1) set by the Environmental Quality Standard for Soils (China) (GB15618-1995). Cd and Pb concentrations in the surface soil samples from F1 had similar spatial distribution patterns with the concentrations being reduced as the distance from F1 increases, and they were significantly higher than the corresponding subsurface soils. These results indicated that pollution of soil heavy metals (Cd and Pb) was mainly caused by external pollution, and F1 was most likely the pollution source. At site F2, soil Cd and Hg pollution was a serious problem. Cd concentrations in 63% of the surface soil samples, and Hg concentrations in 11% of the surface soil samples were higher than the Type II limit (Cd > 0.3 mg kg-1, Hg > 0.3 mg kg-1) set by the Environmental Quality Standard for Soils. The mean surface soil Hg concentration was significantly (p < 0.01) higher than that of the subsurface soil. These results indicated that the  heavy metal Hg pollution in the cropland soils nearby site F2 was mainly caused by external pollution from the thermal power plant. In conclusion, heavy metal pollution (Cd, Pb and Hg) in cropland soils nearby the contaminated sites F1 and F2 are caused by external factors, and the degree of pollution increases as the distance to the pollution source decreases.