Study On Geochemical Baseline Values And Pollution Risk Assessment Of Heavy Metals In Bole Farmland Soil

The accumulation of heavy metals in the soil to a certain extent can lead to soil contamination.In addition,heavy metals are very toxic and difficult to biodegrade.Heavy metal contamination in agricultural soils can lead to excessive accumulation of heavy metal concentrations in crops.If these crops are accidentally consumed,they can threaten the health and safety of humans and other organisms.This has therefore become a social problem and an important environmental issue.The city of Bole,located in north-western China,is not only an important base on the ancient Silk Road,but also one of the typical oases of the north-western arid zone and an important base for food production.Therefore,in order to clarify the contamination of local farmland soils with heavy metals in a deeper way,116 surface soil samples were collected from farmlands around Bole City in this study,and the standardized method and the relative cumulative frequency method were applied to determine the geochemical baseline values in the study area.The Single factor pollution index method,the Nemero composite pollution index method,the contamination load index method and the geoaccumulation index method were used as well as conducting potential risk assessment to evaluate the contamination risk of agricultural soils in the study area.Correlation analysis and principal component analysis were used to analyse the sources of contamination and to analyse the spatial distribution of the content of the eight heavy metals.The main findings are as follows.（1）The average contents of eight heavy metal elements Cr,Cu,Ni,Zn,Hg,As,Cd and Pb in the surface soil of farmland around Bole City were 52.59,29.34,23.74,86.82,0.62,9.03,0.14 and 32.21 mg.kg-1 respectively.The results showed that none of these elements exceeded the "Soil Environment of Farmland Soil Pollution Risk Quality Control Standards（Test）"（GB15618-2018）for the national risk of soil contamination on agricultural land,and the environmental quality of the surface soil in the exploration area was generally good.（2）The baseline geochemical values for the eight heavy metals calculated using the two methods are similar and the average can be used to determine the geochemical baseline values for the whole study area.The baseline values for Cu and Ni were slightly lower than the background values for irrigated soils in Xinjiang,while those for Cr,Zn,Hg,As,Cd and Pb were all higher than the background values for irrigated soils in Xinjiang,indicating that anthropogenic activities have contributed to the accumulation of heavy metals in the study area to some extent.（3）The spatial distribution characteristics of the eight heavy metals were analysed through GIS,and the results show that the enrichment of Zn,Hg and Cd is mainly in the central part,the content of Pb is higher in the central and north-western part,and the content of Cr is mainly concentrated in the north-eastern part.Overall,the distribution pattern tends to decrease gradually from the northwest to the southeast,with the high value areas all occurring in the central farmlands and township centres.This indicates that pollutants emitted from agricultural activities and pollution from human production activities cause the accumulation and contamination of heavy metals in soil cultivated land under certain conditions.（4）The pollution evaluation indicators and risk assessment show that the pollution level of elemental mercury in agricultural soils is relatively serious and the main source of pollution is human agricultural production activities.The potential ecological risk and ecological warning generated by elemental mercury is the highest,therefore controlling the emission of elemental mercury should be the focus of heavy metal pollution management in agricultural soils in Bole.In the human health risk model the carcinogenic risk index for As is higher than that for Cd,both within manageable limits.Of the three exposure pathways,the hand-oral intake pathway is greater than the other two exposure pathways.Overall,the level of heavy metal contamination of land and health risks from farmland in Bole are within tolerable levels,and none of these sources have significant impacts on the atmosphere or human health.In order to evaluate the risk levels of heavy metals in agricultural soils in Bole in a more scientific and rational manner,the focus is on risk control of heavy metals at high contamination levels.