Risk Assessment Of Heavy Metals In Soils And Agro-products Along The Yangtze River Of Northern Nanjing

Safety of agricultural producing areas has become an extensive concern.Heavy metal pollution of agricultural soils not only brings about adverse effects on the soil ecosystem,but also is closely related to food safety and potential health hazard.How to assess the risk of heavy metal pollution of soils and agro-products is of some practical and far-reaching signif-icance.Therefore,it is an urgent task to investigate the status and assess the risk of heavy metals in soils and agro-products with a variety of assessment methods.The findings in this study may help decision-makers select appropriate assessment methods for rational assess-ment and zoning of the agricultural producing area and remedy contaminated soils scientifi-cally.Taking the agricultural areas along the Yangtze river in northern Nanjing as an example,this study focused on the five heavy metal elements,i.e.As,Cd,Cr,Hg and Pb,which are relatively more harmful than the others to the human body.A total of 527 soil samples and the editable parts of 284 rice(Oryza sativa L.),52 maize(Zea mays L.),57 bok choy(Bras-sica chinensis L.),26 napa cabbage(Brassica pekinensis Lour.Rupr.),34 seleng wormwood(Artemisia selengensis Turcz.ex Bess.),53 radish(Raphanus sativus L.)and 21 sweet potato(Ipomoea batatas Lam.)samples were collected from agricultural areas along the Yangtze river in northern Nanjing.The concentrations of 5 heavy metals were mensurated separately to estimate the pollution and spatial distribution of metals.Using and innovating different evaluation models of heavy metals,we analyzed and evaluated the risk of heavy metals in local soils and agro-products.By comparing the differences of evaluation results,we ex-plored the appropriate risk evaluation models of heavy metals applied to the study area.This paper further explored the relationship of the heavy metals content between soils and agro-products,and the enrichment ability of different crops to heavy metals.Based on GIS tech-nology,we analyzed the distribution of heavy metals in soils and agro-products in the region and the comprehensive risk grading maps of evaluated indexes.The main conclusions of this paper were as follows:1.The average concentrations of As,Cd,Cr,Hg and Pb were 9.3,0.30,84.0,0.09 and 31.3 mg kg-1,respectively.The average concentrations of Cd,Cr and Pb were beyond the soil background values in all farming types of soils,especially the exceeding rates of Cd were 2.16,1.58 and 1.61 folds for maize soil,seleng wormwood soil and radish soil.The study used the Traditional evaluation model(T-EM),Improved geo-accumulation index evaluation model(I-EM)and Agricultural production area safety evaluation model(A-EM)to assess the risk of heavy metals in the study area and compared the 3 methods for differences.The use of A-EM found that over 95%of arable land in study area was risk-free and elements Cd and Pb were the main factors that posed a low risk.Risk areas scattered across the south parts of the study area.The use of T-EM displayed that only Cd exceeded the standard.The use of I-EM revealed that the risks of Cd,Cr and Pb were relatively high due to human activities,while the use of the I-EM disclosed that the study area as a whole was good in ecology.T-EM and A-EM can work together to identify risk-free region,but A-EM can better evaluate and divide risk areas.Therefore,it is recommended to use A-EM in conjunction with I-EM to assess risk levels,spatial distribution and ecological condition of the heavy metals in the agricultural soils of the region.2.The seleng wormwood samples absorbed these 5 heavy metals easily and the Cd and Pb concentrations of about 30%seleng wormwood samples had exceeded the criteria(GB 2762-2012).The cereals(rice and maize)tended to absorb element As,Cr,Hg and leafy vegetables(bok choy and napa cabbage)absorbed Cd and Pb easily and root vegetables(rad-ish and sweet potato)rarely absorbed these 5 heavy metals.Because different types of agro-products have different thresholds,the absorptive capacity doesn’t represent the risk degree.The orders of average concentration of heavy metals in edible parts of seven agro-prod-ucts were:Rice:Cr>As>Pb>Cd>Hg;Maize:Cr>Pb>As>Cd>Hg;Bok choy:Pb>Cr>Cd>As>Hg;Napa cabbage:Pb>Cr>Cd>As>Hg;Seleng wormwood:Pb>Cd>Cr>As>Hg;Radish:Cr>Pb>Cd>As>Hg;Sweet potato:Cr>Pb>Cd>As>Hg.The orders of absorption capacity of the elements in different agro-products were:As:rice>seleng wormwood>maize>napa cabbage>sweet potato>bok choy>radish;Cd:seleng wormwood>rice>bok choy>sweet potato>napa cabbage>maize>radish;Cr:rice>seleng wormwood>maize>sweet potato>napa cabbage>bok choy>radish;Hg:seleng wormwood>rice>maize>bok choy>napa cabbage>sweet potato>radish;Pb:seleng wormwood>bok choy>napa cabbage>rice>maize>sweet potato>radish.The bioaccumulation of elements Cd and Hg was stronger than other elements.Because of element Pb,the order of the average bioaccumulation factors(BAFs)of heavy metals showed:BAFs of cereals:Cd>Hg>As>Cr>Pb;BAFs of vegetables:Cd>Hg>Pb>As>Cr.3.This study used the Crop risk index evaluation model(C-EM)and human health risk evaluation(H-EM)model to analyze the risk degree of heavy metals in agro-products.The order of crop risk index(CRI)was:seleng wormwood>radish>sweet potato>rice>bok choy>napa cabbage>maize.It showed that the excessive risk of heavy metals was the highest in seleng wormwood.On the overall,agro-products were safe in the study region(Total Target hazard quotient,TTHQ<1).The order of health risk of heavy metals in different agro-products was:seleng wormwood>rice>maize>bok choy>napa cabbage>sweet potato>radish.There should be greater monitoring of the heavy metals of local seleng worm-sood crops and rice crops(TTHQ>1),especially for children’s health risks.It was suggested that the two evaluation models(C-EM and H-EM)should be used in combination to evaluate the risk levels of heavy metals in agro-products from the excessive degree and toxicity to human body of heavy metals.4.This study used the Maximum single factor index combination evaluation model(MSFI-EM)and Influence index of comprehensive quality evaluation model(IICQ-EM)to assess the comprehensive risk.The result showed that above 85%samples were safe.The order of soils and agro-products comprehensive risk index(IICQ)was seleng wormwood origin>radish origin>maize origin>bok choy origin>napa cabbage origin>rice origin>sweet potato origin.The elements Cd,Hg and Pb in seleng wormwood origin should be mon-itored strictly and it shouled be prevented from multiple pollution sources for Cd and Pb.The results of the two models were similar,while IICQ-EM was more scientific.It was suggested that the classification standards of MSFI-EM should be improved and then applied to a wide range of evaluation works.Based on geostatistics of GIS technology,we found that the con-centrations of heavy metals in soils and agro-products showed significant spatial variation because of the distribution differences of pollution sources and cultivation types.Considering both soils and agro-products,the risk regions of heavy metals were mainly located in the southern study area.The medium-risk and high-risk regions in southwest of study area,which was similar with risk distribution of soil heavy metals,mainly cultivating seleng wormwood,rice and radish.This phenomenon may be associated with the distribution of local factories and geographical environment factors,should be monitored strictly and carried on pollution control.