| In recent years, with the rapid development of modern industrial and agricultural, soil pollution, especially heavy metals pollution in soil of vegetable fields, has become increasingly serious, It is a terrible threat to human’s health that accumulated in soil, heavy metals in vegetables constantly were enriched, which entered human body through food chain finally. The heavy metals pollution in soil of vegetable fields has been paid widespread attention by scholars and government departments. To improve the quality of vegetables and keep the health of residents, the Ministry of Agriculture started a " nuisance-free food plan", as a result,"nuisance-free" has become a basic requirement of soil environmental quality in vegetable fields, as a prerequisite of vegetable market access.9sample plots were selected in3towns:Fuxing, Xiema and Dongyang, about6topsoil samples collected in each sample plot,49samples in total, to analyze and assessment the soil environmental quality in vegetable fields in Beibei District. The soil physical and chemical properties, such as soil pH, SOM contents, soil texture, magnetic susceptibility, as well as total-state and available-state contents of Cd, Pb, Cu, Zn of each sample had been measured, before analyzing contents characteristics of heavy metals in soil of vegetable fields and influences of the soil physical and chemical properties and mode of operation of vegetable fields on the contents of heavy metals in soil.4exogenous sources were set in the three towns, from which soil samples were collected at different distances, in the mean time, a group of soil samples were collected from soil profiles (A,AC,C,R layer)in each town,33soil samples in total. Based on those, the sources of Cd,Pb.Cu,Zn in soil were analyzed. The status of accumulation of heavy metal in soil, pollution risk and ecological risk of soil in vegetable fields was assessed one by one, through methods like Single cumulative index, index of geo-accumulation, Nemerow pollution index, CPI synthesis pollution index, potential ecological risk index, ecological risk index, meanwhile, methods of environmental quality assessment based on the available-state contents of those four heave metals and Susceptibility concentration factor of soil were explored. The results show that: (1) The total-state average contents of Cd, Pb, Cu, Zn in soil are0.26mg/kg,20.38mg/kg,22.69mg/kg,79.58mg/kg, higher than the background value of soil in Chongqing, especially Cd reached2times higher than the background value. Available-state contents of those four heavy metals are0.05～.032mg/kg,0.14～10.51mg/kg,0.93～9.43mg/kg,0.15～13.80mg/kg, and their Coefficient of bioavailability are17.14～91.27%,0.64～26.31%,3.91～40.19%,0.25～11.22%, Cd is highest in those four heave metals. The coefficient of variation of available-state contents and the Coefficient of bioavailability of heavy metals are larger than the coefficient of variation of total-state contents of heavy metals, that means the influencing factors of available-state contents and the Coefficient of bioavailability of heavy metals are complex.(2) The total-state contents of Cd,Pb,Cu,Zn are obvious correlation,that is to say, homology probably exists between them. Endogenous source(soil parent material) make few contribution to full state contents of those four heavy metals, whose effects can be ignored in the process of ecological and environment risk assessment of heavy metals in soil. Exogenous sources have obvious effects on full state contents of those four heavy metals, apart from about30m away from the the highway of high value.From other exogenous source,such as machinery factory residential,the high value is in the close range of0-15m. Four heavy metal showed strong homology by the single impact of the highway and machinery factory.The homology showed weak by the double impact of highway,chemical factory and single impact of residential. Physical and chemical properties of soil pH has a big influence on soil Cd,Cu,Zn contents of full state.In acidic soil, the more pH in soil,the more heavy metal of the whole state will increase.In addition.different mode of operation and fertilization categories has relatively large impact on soil Cd,Cu,Zn full state contents in study area. In the vegetable base mode of operation,the inorganic fertilizers are used.In this way soil Cd,Cu,Zn full state contents is less.The traditional mode use organic fertilizer,so soil Cd,Cu,Zn full state contents is more.(3) The study vegetable area soil Cd,Pb,Cu,Zn full state contents and available contents have highly significant positive correlation.This states that four heavy metal in the available contents is master factor in study area,but changes in size of the two are not exactly same.From this,we can see the available contents is also affected by other factors.Soil pH of physical and chemical properties and organic matter (SOM) contents has a big influence on on soil Cd,Pb,Cu,Zn available contents.In the neutral and acidic soil,the more pH value, the less four heavy metal bioavailability factor. Soil organic contents and available contents of four heavy metal have significant or very significant positive correlation..Endogenous do not directly effect four heavy metal available contents. Different distance from exogenous source,the soil has no difference in Cd,Pb,Cu,Zn available contents and full state contents.Different soil distance with factory (machinery factory) have more range of high value available contents than full state contents,which can reach0-30m.In addition of different vegetable mode of operation and its fertilization categories have big influence on soil Cd,Cu,Zn available full state contents.In the vegetable base mode of operation,the inorganic fertilizers are used.In this way soil Cd,Cu,Zn full state contents is less The traditional mode use organic fertilizer,so soil Cd,Cu,Zn full state contents is more.(4) In study vegetable area,soil Cd accumulate seriously,so it contributes more rate in the overall cumulative condition of soil heavy metal. Soil Pb,Cu,Zn accumulate relatively mild condition,so the sharing rate is also lower.Though different methods of individual index studying four heavy metal full state contents in study area and evaluate environmental quality, the results show that soil Cd pollution is more serious and form a certain ecological threat to vegetable soil environment in study area.The remaining three heavy metal are basically uncontaminated state.Based on soil Cd,Pb,Cu,Zn contents of full state of the comprehensive index show vegetable soil of study area achieve pollution-free vegetable production soil environment requirements on the whole.The Fuxing town vegetable base accord with green vegetable production soil environment requirements.(5) Though the environment evaluation of soil heavy metal available contents in study vegetable soil,the results show that Cd,Pb,Cu,Zn available contents in soil are higher level.From this,there is a certain degree of pollution.Based on the four heavy metal available contents of comprehensive index show, soil heavy metal available contents in study vegetable land is in a safe level,which accord with the results of environmental quality evaluation based on full state contents.But the individual soil sample evaluation results are big different with the evaluation results of full state contents. This illustrates that the environmental quality evaluation of soil heavy metal full state can be judged from the overall regional soil heavy metal pollution,but its evaluation criteria only consider the impact of soil pH on heavy metal ecological validity.which cannot directly reflect the evaluation ecological risk and hazards of soil heavy metal in the soil-plant system.At present,Soil Environmental Quality Evaluation of soil heavy metal in contents-based research in China is still in its infancy, so the evaluation methods are lacking and have not developed the evaluation criteria for the available contents of heavy metal.In other words,soil heavy metal in contents-based of environmental quality evaluation research is very necessary and urgent.Soil susceptibility,soil heavy metal contents and its evaluation of the whole state single,composite index have a good correlation,but using concentration factor of soil magnetic susceptibility to evaluate the vegetable soil environmental quality of the study area,the results are not accurate enough. |
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