Temporal And Spatial Distribution Of Heavy Metals In Atmospheric Deposition And Their Effects On Soil And Vegetables

Cities are places where population,transportation,industry and economic activities are highly concentrated,so they have also become important places for heavy metal and other pollutants emission.Many of the heavy metals in the environment will enter the soil,and cause the pollution of crops such as vegetables.Through the transmission of the food chain,it is harmful to the health of the population or constitutes a threat to the health of the population.During the biogeochemical cycle of heavy metals,the atmospheric deposition is an important medium and link,and most of the heavy metals in the air are attached to the particles,and many particles will gradually fall to the surface system and interact with the various environmental media of the surface system.Studies have shown that atmospheric deposition was one of the main sources of heavy metal accumulation in soil.In urban areas,the main type of agricultural soil is vegetable land,the accumulation of atmospheric deposition in vegetable soil will affect the content of heavy metals in vegetables,and then affect the health of the residents.On the other hand,heavy metals in atmospheric deposition may also be directly absorbed by plant stems and leaves,thus affecting the heavy metal content of vegetables.At present,there are few reports on the accumulation of heavy metals in soil and the content of heavy metals in vegetables,and there is a lack of quantitative analysis based on long-term location monitoring in a certain region.This paper took the typical large city and the large rural area of Chongqing as the research object,carried out a year's survey and evaluation on the heavy metal content and health risk of the vegetable sold in the city for one year.It analyzed the possible relationship between the heavy metal content of vegetables and the atmospheric deposition reduction in combination with the literature report and the previous research results.On the other hand,the paper set the monitoring points in six typical functional areas,such as residential area,culture and education area,commercial area,suburb,traffic industrial area,industrial residential area and so on.This paper had carried out a monthly monitoring and analysis on the amount of atmospheric deposition and its heavy metal content for one year,in order to understand the spatial and temporal distribution of atmospheric dust in the city.It calculated the impact of heavy metal accumulation on soil system.On this basis,the paper studied the changes in the content of heavy metals in vegetables under different conditions such as the presence of atmospheric deposition and the application of different doses of atmospheric deposition,and analyzed the possible mechanisms and rules of the influence of atmospheric deposition on the content of heavy metals in vegetables.The main conclusions were as follows:(1)The results of sampling and monitoring from March 2016 to February 2017showed that the average content of Cd,Cr,Ni and Pb in the vegetables sold in the main city of Chongqing were 17.5?g/kg,196.9?g/kg,314?g/kg,and 265.6?g/kg,respectively,and the total exceeding standard rates were 2.9%,3.3%,8.6%and 63.5%,respectively.The content of these four heavy metals in autumn vegetables was relatively high.(2)The THQ of Cr,Ni,Pb and Cd in vegetables were less than one for different age groups,but the compound risk TTHQ of heavy metals in children and the middle aged were more than one,respectively,1.149 and 1.034.The result indicated that heavy metals in vegetables might cause harm to children and middle aged and elderly people.(3)From November 2016 to October 2017,the average atmospheric deposition flux in the main urban area of Chongqing was 2.94 g/(m~2·30d).The deposition in different seasons fluctuated greatly,and the deposition in autumn was relatively low.The average value of deposition in the six functional areas was from large to small in order of industrial residential area>traffic industrial area>commercial area>residential area>suburb>cultural and educational area.(4)The average contents of Cd,Cr,Ni and Pb in atmospheric deposition were:Cd2.14 mg/kg,Cr 66.63 mg/kg,Ni 27.69 mg/kg,Pb 98.33 mg/kg.In winter,the content of heavy metals was generally higher.The evaluation results of the geoaccumulation index method and potential ecological risk index method showed that the pollution level of Cd elements in deposition was serious.(5)Based on the soil background value of Chongqing,it was based on the soil background value of the soil,assuming that the input of heavy metals in the soil was all from the atmospheric deposition,and did not produce the output of the heavy metals in the soil.In accordance with the existing settlement flux,the soil Cd,Cr,Ni and Pb in the main urban areas of the city were in accordance with the existing settlement flux.The content of the four heavy metals would still be lower than the two grade standard in the soil environmental quality standard(GB15618-1995)after accumulating 100 years.Therefore,the atmospheric deposition in short time had little effect on the accumulation of heavy metals in the main urban area of Chongqing.In the four elements,the Cd concentration in the soil caused by atmospheric deposition was the fastest.(6)In the pot experiment,the contents of 5 g/(m~2·30d),10 g/(m~2·30d)and 25 g/(m~2·30d)were applied to the vegetables(asparagus lettuce and lettuce)on the ground.The contents of Cd,Cr,Ni and vegetables in vegetables were much higher than those of the control group that did not apply atmospheric deposition.It showed that the atmospheric deposition could affect the vegetables through the way of root nutrition and the way to influence the vegetables and the content of heavy metals.On the other hand,the content of heavy metals in vegetables did not increase with the raise of simulated atmospheric dust,but there was an inflection point,indicating that there may be a optimum concentration range in the absorption of heavy metals in the dust.