Study On The Influences Of Atmospheric Deposition On The Heavy Metal Pollution In The Farmland Over The Bohai Sea Region And Corresponding Preventions

Soil pollution is one of the major reasons for soil degradation.The prevention and control of soil pollution is an important part of farmland soil and water conservation.As the core part of the farmland ecosystem,farmland soil is an important material basis for ensuring safety production of food.However,with the rapid development of industrialization and urbanization,heavy metal pollution in farmland soil is increasingly prominent.Heavy metals imported into farmland soils cannot degraded easily,and can affect root growth of crops and reduce crop productivity.The more serious is that heavy metal elements have adverse effects on human health through the food chain.Compared with heavy metals from other inputs,heavy metals from atmospheric deposition are more ecologically toxic.In addition,atmospheric deposition is a typical non-point source pollution with vast affected area and huge inputs,which is difficult to be controlled.Therefore,the strategies for the pollution caused by heavy metals from atmospheric deposition should be“source control and prevention first”.The study on the deposition and source apportionment of metal elements from atmospheric deposition is an important basic premise for the prevention and control of farmland soil pollution,and it is one of the important scientific foundations for studying the enrichment,transformation and fate of heavy metals in farmland ecosystems.This study verifies the applicability of using the passive sampler to evaluate the annual deposition flux of metal elements and the reliability of using positive matrix factorization（PMF）to analyze the sources of metal elements in mixed deposited samples.The field sampling were performed from March 2017 to February 2019 in 5 typical farmland sites（including urban-rural transition zone farmland,sewage irrigation area farmland,well water irrigation area farmland,saline-alkali farmland and mountain background farmland）over the Bohai Bay area,and 25metal elements,including Be（beryllium）,Na（sodium）,Mg（magnesium）,Al（aluminum）,K（potassium）,Ca（calcium）,V（vanadium）,Cr（chromium）,Mn（manganese）,Fe（Iron）,Co（cobalt）,Ni（nickel）,Cu（copper）,Zn（zinc）,As（arsenic）,Se（selenium）,Mo（molybdenum）,Cd（cadmium）,Sb（antimony）,Ba（barium）,Tl（thallium）,Pb（lead）,Th（thorium）,U（uranium）,and Ag（silver）,were analyzed.The characteristics and deposition flux of these metal elements are presented,as well as the characteristics of soil pollution in different types of farmland.The main findings in this study are as followed:（1）The dry deposition fluxes of fine,coarse and giant particles in the farmland were 892.25mg·（m²·a）^-1,1304.53 mg·（m²·a）^-1 and 10657.31 mg·（m²·a）^-1,respectively.Giant particles dominate the dry deposition of atmospheric metal elements.The mass concentration,size distribution and seasonal variation of metal elements in atmospheric particulate matter are not only affected by the emission sources,but also by atmospheric physical-chemical processes and meteorological conditions.Most of the heavy metal elements were mainly concentrated in the fine particle size,while the metal elements from crust source are mainly enriched in the coarse particle size.The smaller the atmospheric particles,the greater influences from human activities.（2）The total annual deposition fluxes of 25 metal elements in the typical farmland over the Bohai Bay area were 11747.23 mg·（m²·a）^-1,6169.82 mg·（m²·a）^-1,5474.44 mg·（m²·a）^-1,4796.68 mg·（m²·a）^-1,and 1563.71 mg·（m²·a）^-1 for urban-rural transition zone farmland,sewage irrigation area farmland,well water irrigation area farmland,saline-alkali farmland and mountain background farmland,respectively.Compared with deposition fluxes in other farmland over China,the heavy metal elements from anthropogenic sources were at moderate subsidence levels.The order of the deposition flux of each element was consistent with the order of the concentration of each element.The spatial variation of the deposition fluxes of the crust elements in there 5 representative sites was Xianghe>Wuqing>Luancheng>Yucheng>Yanqing.Due to greater influences from local and regional emission sources,the spatial distribution of anthropogenic elements was quite different.（3）The concentrations of heavy metal elements in different layered samples of two different types of farmland soils was different,and plain farmland soil was greatly affected by human activities.Heavy metals,such as As,Pb,Cr,Cu,Ni and Zn,in plain farmland and mountain farmland had no pollution to the soil.But Cd at the plain farmland and the mountain farmland showed light pollution and moderate pollution,respectively.The Nemero comprehensive pollution index analysis showed that the mountain farmland soil showed light pollution,while the plain farmland soil exhibited moderate pollution.And both types of farmland surface soil exceeded the warning line of heavy metal pollution.Heavy metal elements,such as As,Pb,Cr,Cu,Ni and Zn,in plain farmland and mountain farmland had slight ecological risks.But Cd in the corresponding farmland had moderate and strong ecological risks,respectively.The comprehensive potential ecological risk of heavy metals in both types of farmland soils was light.（4）The contributions of atmospheric heavy metals,such as Sb,Pb,Se,Cd and Tl to the heavy metals in the Xianghe plain farmland were 1.3%,20.3%,7.1%,0.9%and 2.7%,respectively.And the contributions of heavy metals,such as Sb,Se and Cd,to the heavy metals in the Yanqing mountain farmland were 1.8%,4.0%and 2.8%,respectively.The influences of atmospheric deposition on heavy metal pollution in farmland soil cannot be ignored.The impact of atmospheric deposition must be fully considered in the prevention and control of heavy metal pollution in farmland soil.（5）Soil dust and road dust contributed the most to the metal elements in the deposition samples at the agricultural sites,with the contributions of 39.1%,72.8%,55.5%,64.1%and55.9%in Yucheng,Luancheng,Xianghe,Wuqing and Yanqing stations,respectively.Due to the influences from the type and intensity of emission sources,the types and contributions of metal elements varied at a regional scale.The human activities,such as industrial sources,coal combustion,motor vehicles and biomass burning are still the main sources of harmful heavy metals in agricultural areas,and are the priority-controlled sources for the prevention and control of heavy metal pollution from atmospheric deposition in farmland.Industrial soot emissions,scattered coal combustion and the use of poor-quality fuel around the farmland are also the non-negligible sources for non-point source pollution in farmland.The results of this study can not only help to understand the impact of atmospheric heavy metal deposition on heavy metal pollution in farmland soil,but also provide theoretical supports for the control and treatment of non-point source pollution of heavy metal in farmland soil.In addition,it has very important scientific meanings for assessing the eco-environmental effects of air pollution.The innovations of this study are as follows:（1）Based on passive sampling technology,the network observation of atmospheric metal element deposition was carried out for the first time in the Bohai Bay area,and the seasonal variation and spatial pattern of metal element deposition fluxes in farmland in a regional scale were obtained.（2）For the first time in the Bohai Bay area,the contribution of atmospheric deposition to the heavy metal pollution in the different types of farmland was quantitatively analyzed.（3）The reliability of the PMF model for source apportionment of metal elements in the mixed deposition samples was verified.The source apportionment of the metal elements in the various sites was performed.It was confirmed that the heavy metal emissions from industrial and motor vehicles had potential threats to the health of farmland ecosystems.