Contaminated Characteristics And Source Apportionment Of Heavy Metal In Farmland Soil-Wheat System In Typical Industrial Area

Heavy metal contamination in farmland soil threatens food security and human health.Most previous studies have ignored the relationship between soil and crop pollution and lack of comprehensive assessment of soil-crop heavy metal pollution as a whole,which undoubtedly hurts scientific pollution prevention and control.In addition,the relationship between soil heavy metals and its bioavailability and biological effects is complex,and its biotoxicity depends more on the availability of heavy metals.It is difficult to accurately reveal the accumulation mechanism and main influencing factors of heavy metals in crops only based on the total content index.Therefore,it is necessary to include the bioavailability of heavy metals.In this study,a typical industrial park in the northern part of Baoji Province was selected as the research area.Taking the farmland soil-wheat system around three large-scale energy-consuming enterprises(lead-zinc smelters,coal-fired power plants and coal-to-methanol plants)in the region as the research object.And 62 pairs of soil(0-20 cm)and wheat grain samples were collected to determine total and available contents of soil heavy metals,and content of grain heavy metals,including Cd,Cu,Ni,Pb and Zn.The influence index of comprehensive quality(IICQ),bioaccumulation factor(BCF),stepwise multiple linear regression(SMLR)and absolute principal component scores-multivariate linear regression(APCS-MLR)were used to comprehensively evaluate the heavy metal pollution and influencing factors in soil-crop system.And the main pollution sources and contributions of heavy metals were revealed.The results are as follows:(1)The concentration of heavy metals in farmland soil was different.The mean values of total content in the five soil heavy metals studied were all higher than the local background values,and the highest was Cd,with an average value of 7 times the background value.In all samples,about 37% of the soil samples exceeded the national threshold for Cd.(2)The pollution of heavy metals in soil and wheat grain is not consistent,and its comprehensive pollution effect cannot be ignored.The results of soil heavy metal pollution and ecological risk assessment show that among the five kinds of soil heavy metals studied,Cd pollution and potential ecological risk assessment are the most prominent.But the evaluation results of heavy metal pollution in wheat grains showed that the main pollutants were Cd and Pb,and the over-standard samples were 22.58% and 45.16% respectively.Comprehensive assessment of heavy metal pollution in soil-wheat system,the results of IICQ showed that about 53.23% of soil-wheat samples in this area were seriously and extremely polluted,mainly distributed around Pb-Zn smelters and coal-fired methanol plants.(3)It is significant to reveal the bioaccumulation of heavy metals and its influencing factors for understanding crop pollution.For the total content of heavy metals in soil,the BCF index was less than 1,but for the available content,the BCF value was more than 1(except Pb),which showed higher bioaccumulation potential.Among the factors affecting the accumulation of heavy metals in wheat grains,the main factor affecting the content of Cd in grains(64.9%)was the available content of soil Cd.For other elements,the explanation degree of each soil parameter to the accumulation of heavy metals in grain was lower,ranging from 18.1% to 34.4%.The total content of heavy metals in soil,pH and organic matter are not the main factors affecting the mechanism of heavy metal accumulation in grains.According to the correlation analysis,they indirectly affect the bioavailability by affecting the available content of heavy metals in soil.(4)The results of source apportionment of heavy metals in soils implied that Pb-Zn smelting and traffic emission sources contributed more to Cd,Pb and Zn,and the contributions of natural sources to Cu and Ni were more than 79%.Moreover,coal combustion is also one of the potential sources of Cu and Ni.(5)According to the comprehensive pollution effect of soil-crop heavy metals,the study area was divided into remediation area,protection area and prevention area,and the corresponding prevention and control countermeasures were put forward according to the characteristics of pollution sources.The results of this study indicated that non-essential metal Cd has high bioavailability and potential ecological risk,and its source is mainly from human activities such as Pb-Zn smelting,which has already threatened the local soil environment and crop production,and should be paid attention to it.