Variation Characteristics And Influence Factors Of Heavy Metal In Cultivated Land In Shenfu Irrigation District

Shenfu irrigation district is one of the largest wastewater irrigation in China.Historical research indicates that in nearly 30 years after the cessation of irrigation,the content of heavy metal in soil and crops still exceeds national standards,and the content of heavy metal changing under the influence of historical factors such as historical farming,transportation,urban life and industrial activities.Article 16 of the law of the People’s Republic of China on prevention and control of soil contamination stipulates that “the agricultural land with the history of sewage irrigation and the agricultural products shall be tested which the content of pollutants exceeding the standard.”Through historical data collection and field surveys,the study area was identified.Referring to the previous results of our study group,27 grids were placed by the intervals of 1500 meters,and considering the vertical variation of heavy metals,81 soil samples were collected at 3 layers of 0-20 cm,20-40 cm,40-60 cm in the soil,and 54 plant samples such as stem,leaf and seed were collected.The content of heavy metals of Cd,Pb,Ni,Cr,Cu,Zn in soil was determined by experiments,and the chemical forms of heavy metal elements of Cd and Pb were measured,as well the content of heavy metals in stem,straw and seed of crop.Based on the survey data,experimental data and historical data in the past 20 years,the temporal and spatial variation characteristics of heavy metals in cultivated soil are studied.The chemical forms of heavy metals in Cd and Pb in soil and the variation of content in crop stems and straw are analyzed.The main factors affecting the variation of Cd and Pb in soil were analyzed.For providing scientific basis and decision-making reference for farmland utilization and management of heavy metal contaminated cultivated land,and for soil remediation as well.（1）Through the investigation of historical data and experimental analysis,Cd and Pb were the main elements causing the contamination.The average content of Cd in the 3 soil layers were 1.07 mg·kg^-1,0.96 mg·kg^-1,0.95 mg·kg^-1,respectively,which were more than 5 times higher than the environmental background,which was significantly higher than the current soil environmental quality standard.The risk screening value was seriously affected by external contamination,except Cd,other heavy metals have not exceeded the standard.The spatial distribution of Cd in the 0²0 cm layer was relatively uniform,and existing high value areas nearby the villages and irrigation canals in the layer of 20⁴0 cm.According to the content of each layer and the soil environment background,the average value of the leaching ratio between soil layers is 0.46 and 0.49,respectively,which was more than twice higher than other elements except Zn.The average content of Pb was 28.38 mg·kg^-1,22.89 mg·kg^-1,19.93 mg·kg^-1,respectively.The content of Pb was significantly higher than the other layers at 0²0 cm,which was affected by traffic.The area was distributed in the vicinity of the suburban road.The value of the leaching ratio between the layers was relatively low,only 0.18 and 0.07 respectively.The vertical direction did not affect the other two layers of soil.（2）Through the comparison of the historical data of the 4 periods of 2001,2004,2012 and 2014,the annual variation rate ranges from 17% to 85% by years of the corresponding points of Cd,with an average of 36%.Cd was increasing at a rate of 0.10 mg·kg^-1·a^-1,and generally affected by human activities.Except Cd,the content of Pb changed significantly,and the annual variation rate ranged from -9% to 6%,with an average of -3%.Except for the increase in the content of urbanization and traffic,Pb was decreasing at a rate of -1.88 mg·kg^-1·a^-1.The average annual rate of variation of other heavy metals was between-1% and 1%,and the variation is not obvious.（3）The average content of Cd and Pb in corn seed was 0.11 mg·kg^-1 and 0.54 mg·kg^-1,respectively,and the over-standard rate was 66% and 100%.The uptake of Cd in crop seeds and stems and straw were 0.87 g·hm^-2·a^-1,7.25 g·hm^-2·a^-1,respectively,and Pb content 4.29 g·hm^-2·a^-1,31.65 g·hm^-2·a^-1,respectively.A large amount of heavy metal elements in crop stems and straw,39% of Cd and 6% Pb in the content of straw exceeded the standard.Therefore,the straw resources should be disposed of reasonably to avoid heavy metals uptaked by plants returning to soil,and it is still necessary to consider the hazards arising from other uses.（4）The exchangeable content of Cd in each layers are 0.15 mg·kg^-1,0.11 mg·kg^-1,0.13 mg·kg^-1,respectively.The content was only lower than the residual state.The migration coefficient of each layer was 17%,12%,14%,respectively.The exchangeable content of Pb was 1.16 mg·kg^-1,0.73 mg·kg^-1,and 0.92 mg·kg^-1,respectively.The content of Pb was the lowest in all forms,and the migration coefficients are 4%,3% and 4%,respectively,only 1/3 of Cd,the mobility is relatively low.The bioavailability coefficients of Cd and Pb in 0²0 cm of soil were 27% and 16%,respectively,which were significantly higher than the next two layers,and they all had significant influence on crop content.Correlation analysis showed that the deep Cd exchangeable state and organic binding state still have a significant effect on the content of Cd in crop seeds,while the Pb in crops effected by the exchangeable state of 0-20 cm.The soil p H in the study area decreased significantly in the past 10 years,the decrease of soil p H led to the decrease of the critical value of soil contamination determination and the increase of the exchangeable content of heavy metals,which increased the risk of contamination.（5）By analysis the factors affecting the variation of content of heavy metals Cd and Pb shows that the variation of Cd content was affected by distance factors（distance to the suburban road,distance to the factory）,farmland management（time of stop irrigation,fertilizer application）,and natural conditions（soil p H and elevation）.By counting on the distance to the suburban road（0¹ km,1² km,>2 km）,distance to the factory（0² km,2⁴ km,>4 km）,stop irrigation time（0⁵ a,5¹0 a,>10 a）,fertilizer application rate(269.6 kg·hm^-2·a^-1),soil pH（5.5）and elevation（82.46 m）as boundary division,the different of Cd in soil was significant.The contribution of the three groups to the content of Cd is 7%,13%,and 41%,respectively.The interaction between distance factor and natural factor was the highest,reaching 35%.The factor of Pb was single,and mainly reflects in 0²0 cm,affecting by the distance of the suburban road（0¹ km,1² km,>2 km）.