The Spatial Variability Characteristics And Source Identification Of Soil Heavy Metals In Croplands Near The Contaminated Sites

Surface soil (0-20cm) and subsurface soil (20-40cm) were collected from croplands in the vicinity of a battery plant (Fl) 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. Based on the Type II limit of "Environmental Quality Standard for Soils"(GB15618-1995), the single factor method and Nemerow index method were used to assess the comprehensive pollution risks of the soil heavy metals around the contaminated sites. Inverse distance weighted interpolation (IDW) was used to simulate the pollution risk and spatial distribution and the source identification of these heavy metals pollution. The results showed that:(1) The soil around contaminated sites was polluted by Cd, Hg and Pb to various extents. The range of the concentrations of surface soil Pb was39.02-97.32mg kg-1, Cd was0.196-1.287mg kg-1, and Hg was0.066-0.306mg kg-1. There was a serioius heavy metals pollution problem in the study area, with Cd concentrations in80.77%of the surface soil samples exceeded the Type II limit (Cd>0.3mg kg-1) of Environmental Quality Standard for Soils (China)(GB15618-1995). Hg concentrations in3.85%of the surface soil samples exceeded the Type II limit. Nemerow index method assessment results showed that there were25soil samples exceeded the standard, with overproof rate of48.08%. Based on the Technical Specification for Soil Environmental Monitoring, the maximum index value was3.13, which exhibited heavy pollution.(2) Results showed that soil Cd and Pb pollution were serious problems in the soils nearby contaminated site F1, with Cd concentration in91%of the surface soil samples exceeded the Type Ⅱ limit (Cd>0.3mg kg-1) set by the Environmental Quality Standard for Soils (China)(GB15618-1995), and the maximum surface soil Cd concentration was3.3mg kg-1. The soil Pb pollution was also the potential problem even its concentration was not exceeded the Type Ⅱ limit. The maximum surface soil Pb concentration was123.67mg kg-1and the mean surface soil Pb concentration was58.69mg kg-1. While the surface soil Cd and Pb concentrations were significantly higher than the subsurface soil. Soil Cd and Hg pollution were serious problems in the soils nearby contaminated site F2. Cd concentrations in63%of the surface soil samples, and Hg concentrations in11%of the surface soil samples were higher than the Type Ⅱ limit (Cd>0.3mg kg-1, Hg>0.3mg kg-1) set by the Environmental Quality Standard for Soils. The maximum surface soil Cd concentration was0.71mg kg-1. The mean surface soil Hg concentration was0.214mg kg-1(3) Cd and Pb concentrations in the surface soil samples from Fl had similar spatial distribution patterns with the concentrations being reduced with the distance from F1, and they were significantly higher than those in the corresponding subsurface soils. These results indicated that pollution of soil heavy metal (Cd and Pb) was mainly caused by external pollution, and F1was most likely the pollution source.The battery plant can discharge Cd and Pb to environment. These results indicated that soil heavy metal Cd and Pb pollution were mainly caused by external pollution and F1was the pollution source.(4) At the contaminated site F2, some of soil samples was higher than the Type II limit (Cd>0.3mg 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 soil heavy metal Hg pollution at site F2was mainly caused by external pollution from the thermal power plant. Two higher Hg concentration areas were nearby the contaminated site F2and to the east of the contaminated site F2. These results indicated that the contaminated site F2was one of the pollution sources, and there was another source at the east of it. High soil Cd, Pb, Zn concentration area was at the east of the contaminated site F2, and its concentration increased from the contaminated site F2to east. These results indicated that the contaminated site F2was not pollution source of soil Cd, Pb, Zn. The power plant can discharge Hg and the traffic can discharge Pb, Cd, Zn, Hg. These results indicated that soil heavy metal Hg pollution was caused by the contaminated site F2and the traffic and dust emission. And soil heavy metal Cd, Pb, and Zn pollutin were caused by the traffic and dust emission.In conclusion, soil heavy metals pollution was a serious problem in the study area around the contaminated sites. The pollution of surface soil by Cd and Pb nearby site F1and pollution of surface soil by Hg and Cd nearby site F2mainly comes from external pollution sources, the increase of surface soil Cd and Pb concentrations around site Fl was most likly caused by the contaminated site F1and the increase of surface soil Hg concentration around site F2was most likly caused by the contaminated site F2and the traffic and dust emission. The increase of surface soil Pb, Cd, Zn concentrations was most likly caused by the traffic.