Surface Migration Of Heavy Metals And Risk Assessment Of Farmland Soil In Typical Black Shale Area Of Western Zhejiang Province

In recent years,the CPC Central Committee and the State Council have attached great importance to the prevention and control of soil pollution,the quality protection of cultivated land and the quality and safety of agricultural products,and the ecological risk of heavy metals in black rock series areas has become the key research field of environmental geochemistry.In this paper,the weathering profile of bedrock,surface water,soil-rice and other systems in the typical black rock series area of a county of western Zhejiang province were systematically studied according to particular line that heavy metal pollution source-heavy metal migration pattern and characteristic-farmland pollution actual-farmland ecological risk prediction in black rock system area of western Zhejiang province.The content,form,migration form and epigenetic geochemical behavior of heavy metals were emphatic analyzed.Based on this,the ecological risk prediction equation was proposed.With the deepening of weathering,the debasification,iron-aluminum enrichment and acidification intensity of the black rock series profile increase obviously.The chemical weathering of minerals in the profile mainly occurs between weak weathered layer and strong weathered layer,and the content of major elements changes obviously at the rock-soil interface.The rock fragments of the original rock in the soil layer inherit the variation law of major elements of the original rock.The contents of trace elements in the original rock fragments in each layer of soil are significantly different from those in each layer of soil,and the difference is greater with the higher degree of weathering.During the weathering process,the accumulation elements of Ni,Zn and Cd are more inclined to stay in the rocks,while Cr,Pb,Hg and Se are more inclined to enter the soil from the rocks.The heavy metal pollution in the black rock series area is mainly controlled by the outcrop location of the black rock series,the main migration route is surface water,and the diffusion range and influence degree are related to the migration distance and the volume of pollution sources.The mine wastewater in the outcroppings of black rock series is characterized by strong acidity,enrichment of various heavy metal ions,high fluorine and high sulfur,which is consistent with the elemental types of bedrock leaching in the weathering section.The water quality of the mountain spring in the non-outcrop area of the Black Rock System is clean and meets the national standards for irrigation water.In the active part of the contaminated soil,the proportion of water-soluble,ion exchange,humic acid bound and strong organic bound states is relatively high,and the trend of soil acidification is obvious.With the increase of migration distance,the proportion of water-soluble and ion exchange states decreases rapidly,while the proportion of other forms increases to different degrees.In the farmland far away from the outcropping area of the black rock series,the relative content of the carbonate bound state in the farmland soil is higher than that of the ion exchange state.In the highly acidic environment,the heavy metals in the carbonate bound state will be released again,which will form a more serious secondary pollution than the current one.The proportion of rice seed samples with ecological risk is as high as 80%,and the main safety risks were from the heavy metal Cd.The potential risk of As element accounted for 52.63%of the samples,and the overall safety risk of Hg element was few,with only 5.26%of the samples exceeding the standard of security.The contents of Cr and Pb are within the safety standards,and there is no food safety risk.The prediction equation lg(r_Cd)=0.788lg(Cd_Ca)+4.963lg(OM)-7.051 based on Cd extraction state of CaC12 and the soil management reference equation lg(r_Cd)=-0.741lg(pH)+0.009(Zn_H)+3.974 based on Zn extraction state of dilute HNO3 were established.A Cd ecological risk prediction equation based on the total amount of elements is established,lg(r_Cd)=0.801lg(s_Cd)-0.413(pH)+2.269,which is suitable for preliminary prediction.Based on the prediction of spatial ecological risk in the black rock series area of the county,the results show that farmland in the black rock series area of the county has high ecological risk.The above conclusions are of great significance for revealing the geochemical behavior and occurrence form of heavy metal elements in the supergene environment of the black rock series in western Zhejiang province,and provide a theoretical basis for the prediction of farmland ecological risk in the black rock series area,which will provide strong support for the management and regulation of heavy metal elements in the black rock series area.