Study On Speciation Characteristics Of Heavy Metals In Farmland Soil And Their Spatial Correspondence With Heavy Metals Of Wheat In An Industrial Park In Northwest China

Cultivated soil is one of the most precious natural resources on the earth,providing an indispensable material foundation for the development of human society.With the rapid development of industrialization in recent years,heavy metal pollution of farmland soil has become one of the global environmental pollution problems that have attracted much attention.Soil pollution by heavy metals not only directly affects the normal growth of crops,reduces their quality,but can even endanger human health through the food chain.Scientific assessment of soil heavy metal pollution is an important foundation for remediation of pollution,reduction of agricultural production risks,and rational use of soil resources.However,in the past regional scale soil pollution evaluation studies,the total amount of heavy metals was used to compare with the threshold value,while relatively little attention was paid to its speciation characteristics,and the research on the spatial correspondence with crop heavy metals was even rarer.Therefore,this study took the farmland soil around an industrial park in northwest China as the research object,collected 62 pairs of topsoil and wheat grain samples,and determined five different occurrences of soil heavy metals(Cd,Cu,Ni,Pb,Zn)and used Indicators such as ratio of secondary to primary phases,risk assessment code,and bioavailable metal index assess the pollution status of soil heavy metals and environmental risks;on the basis of analyzing the accumulation and pollution characteristics of heavy metals in wheat grains,geo-detector is used to reveal the effects of heavy metal accumulation in wheat,the main factors and contributions are researched on their spatial correspondences through geographically weighted regression.The main results obtained are as follows:(1)There are obvious differences in the occurrence of heavy metals in farmland soil.Cd is mainly exchangeable and carbonate combined state;Cu,Zn and Ni are the most in residue state,and their exchangeable state content is the lowest;The content of Pb in the residue state is the highest,followed by the iron-manganese oxidation combined state in the non-residue state.In all soil samples,more than 90%of Cd is heavily contaminated,about 40%of Pb samples are contaminated,and the remaining elements(Cu,Ni,and Zn)are basically uncontaminated.Among all samples,69.53%of soil Cd showed extremely high environmental risk,Pb and Zn in 1.61%of samples are at a medium risk level,and Cu and Ni are only in a mild risk range.Overall,heavy metals in farmland soils in the study area have potential environmental risks,especially Cd risks.(2)Heavy metals in wheat grains are polluted to varying degrees.Although the average concentration of Cd and Pb did not exceed the national food pollution threshold,22.8%and48.39%of the wheat samples exceeded the standard,and the concentrations of Ni in wheat grains did not reach the pollution level.The single-factor pollution evaluation results show that the order of heavy metal pollution in wheat grains is:Pb>Cd>Ni,4.84%of wheat grains are heavily polluted by Pb,59.68%are slightly polluted by Cd,and Ni is generally at a safe level.It can be seen from the pollution risk probability map that the high-risk areas of Cd and Pb pollution are concentrated in the intersections of industrial and mining enterprises,traffic arteries,and low-altitude areas.(3)The results of the geo-detector analysis show that,except for wheat Cd,the other four heavy metals have no significant relationship with the environmental factors studied.The Cd content of wheat grains is affected by both natural and man-made factors.The significant natural factors are mainly soil secondary phase Cd content,elevation and soil type;man-made factors include distance from lead-zinc smelter,distance from railway and distance from thermal power plant.(4)Using geographic weighted regression(GWR)to reveal the spatial correspondence between wheat grain Cd and the main influencing factors.The regression results show that the corrected R~2 is 0.70,and the fitting effect is better.Among the main influencing factors,the average regression coefficient of the secondary soil Cd is the largest,followed by the distance from the lead-zinc smelter.The spatial correlation strength of each influencing factor and wheat Cd is different,and the correlation of human factors on wheat Cd accumulation is slightly stronger than natural factors.The regression coefficients of different factors are also different.The non-residual soil Cd content and the distance from the thermal power plant are positive values,and the elevation and distance from the railway are both negative,while the soil type and the distance from the lead-zinc plant are positive.There is a negative.The results of this study revealed the accumulation status,pollution degree,potential environmental risks of heavy metals in soil and wheat in the study area,as well as the main factors affecting the accumulation of heavy metals in wheat grains and their spatial correspondence.Soils with relatively high pollution characteristics and heavy metals from crops with complex sources have already threatened the safety of local agricultural production and should be prevented early.