Spatial Distribution Characteristics And Source Apportionment Of Heavy Metals In The Main Wheat Regions Of Shaanxi Province

Soil environmental quality is related to national economy and people's livelihood.According to the ‘Bulletin of National Survey on Soil Pollution' jointly released by the Minstry of Environmental Protection and the Ministry of Land and Resources in 2014,the rate of over-standard points of arable land soil is 19.4%,of which the percentages of slight,light,moderate and severe over-standard points are 13.7%,2.8%,1.8% and 1.1%.Wheat is the third largest crop in China and the staple food of northerners.Therefore,it is important to determine the source and pollution level of HMs in soil.Shaanxi Province is one of the main cradle of agriculture in China.For this reason,the object of this study was the soil in main wheat regions of Shaanxi Province.There are 2 areas(Weibei Loess Plateau,Guanzhong Irrigation Zone),32 counties,and 521 samples included in this research.Totally eight HMs including Cd,Cu,Pb,Zn,Cr,Ni,Hg and As were involved in this research.The Nemerow pollution index,potential ecological risk index and geoaccumulation index were selected for pollution assessment.The principal component analysis and correlation analysis were selected for source apportionment.It is meaningful to provide a reference for different land use of the main wheat regions of Shaanxi Province.The main conclusions of this research as followed:1.The average content of 8 HMs in 521 soil samples from the main wheat regions of Shaanxi Province are Cd(0.19 mg/kg),Cu(28.73 mg/kg),Pb(32.22 mg/kg),Zn(83.50 mg/kg),Cr(62.67 mg/kg),Ni(43.29 mg/kg),Hg(0.12 mg/kg)and As(15.52 mg/kg)are lower than the ‘Soil Environmental Quality-Management Standards for Soil Pollution Risk of Agricultural Land'(GB 15618-2018).There was no pollution currently,but compared with the background value of Guanzhong the enrichment rates are 61.02%,22.26%,97.67%,26.90%,0.32%,41.93%,39.54% and 22.21%,of which Cd and Pb are the most obvious elements of enrichment.The HMs in the main wheat regions of Shaanxi are distributed in the northeast and south-central regions and in the west are lower.The points which are higher focus on Yanliang District,Gaoling District,Huyi District,Lantian County,Chang'an District and Jingyang County.2.Heavy metal pollution presents a long incubation period and is obviously affected by human activities.Therefore,this study comprehensively evaluates the status quo,impact and future risks.According to the Nemerow comprehensive pollution index,6.33% of the soil samples(a total of 33)in the main wheat regions of Shaanxi Province present a slight pollution level.Cd and Pb are the heavy metal elements with the highest single pollution index.According to the geoaccumulation index,the pollution rates ofPb,Cd,Ni,Hg,Zn,Cu and As in the soil samples of major wheat regions of Shaanxi Province are 80.61%,63.34%,32.44%,25.73%,5.38%,0.58%,and 0.39%,respectively.According to the potential ecological hazard index,20.92% of the main wheat regions of Shaanxi Province(a total of 109)presents moderately ecologically hazardous.Comparing the three evaluation methods,the analysis shows that the potential ecological hazard index is the most suitable method for pollution assessment of heavy metals in soil in the main wheat regions of Shaanxi Province.3.Combined with the local agricultural production and industrial emissions data,cluster analysis and principal component analysis showed that the HMs in the soil in the main wheat regions of Shaanxi Province mainly came from five aspects: Chemical fertilizer abuse,organic fertilizer pollution,pesticide pollution,irrigation water pollution and atmospheric deposition.The northeastern part of the area is an important coal mine production area in Shaanxi Province,HMs released during mining will enter the soil with dust.The central part of this area is an important agricultural production area in Shaanxi Province,the main reason for the heavy metals in the soil in these areas was the abuse of fertilizers and pesticide pollution caused by farmers in pursuit of high interests.At the same time,it is located in the geographic location of a transportation hub,and there were also traffic pollution sources such as tire wear and exhaust emissions.The southern part of the district is mineral-riched as an important industrial zone in Shaanxi Province.In these areas,the accumulation of heavy metals in the soil was related to atmospheric deposition caused by industrial emissions.In summary,there was currently no pollution in the soil of major wheat regions of Shaanxi Province,but there were potential risks.Cd and Pb are the most obvious elements of enrichment.Chemical fertilizer abuse,organic fertilizer pollution,pesticide pollution,irrigation water pollution and atmospheric deposition were the main sources of heavy metals in the soil in major wheat regions of Shaanxi Province.It was recommended to take into account the management of soil environmental quality while developing production.