| Heavy metal contamination has been an environmental problem of widespread concern to scholars at home and abroad.The acceleration of industrial production and human living processes,as well as the historical legacy of industrial activities,has caused an increasingly serious problem of soil heavy metal contamination,especially in agricultural soils,which may be taken up by crops and enriched in the food chain,thus posing a serious potential threat to human health and the environment.Lanzhou City was one of the major industrial cities in China.Although Lanzhou City is undergoing industrial green transformation in recent years,there are still more soil heavy metal pollution problems due to the easy accumulation of heavy metal,heavy metal pollution in farmland cannot be ignored yet because of the long-term sewage irrigation in some areas of Lanzhou City.In addition,the special topographic conditions of Lanzhou City（a river sandwiched by two mountains）restrict the distribution of its farmland,and the continuous development of urbanization and industrialization makes the area of farmland decreasing,so the safety of farmland becomes especially important.In addition,the soil in Lanzhou City is mostly loess as the parent material,and there are some saline-alkali soils,which integrate the four major geomorphological units:loess hilly area,loess beam area,bedrock mountain area and Yellow River valley area.In view of its special soil nature and topographic features,the heavy metal pollution condition may be different from other areas.Therefore,a systematic study on the concentration,distribution,pollution,sources and risks of heavy metals can provide data support for the control of heavy metal pollution in farmland soils and crops in Lanzhou City.In this paper,we investigated the concentrations of heavy metals（five toxic metals:As,Cd,Cr,Hg and Pb）in farmland soils and the edible fraction of crops in Lanzhou City,and identified the spatial distribution characteristics of heavy metals in soils from the perspective of districts,counties,functional areas and the whole Lanzhou City using the kriging interpolation method;explored the main influencing factors of the soil heavy metals spatial distribution through multiple linear regression analysis;indicated the sources of soil heavy metals using correlation analysis,principal component analysis and APCS-MLR analysis;analyzed the pollution,ecological risk and health risk characteristics of soil heavy metals using the geo-accumulation index,ecological risk index and health risk index;in addition,the transfer of soil heavy metals to the edible part of crops were elucidated and its influencing factors were explored by correlation analysis,and the pollution and health risk of heavy metals in the edible part of crops were analyzed by the health risk index.The main findings are as follows.（1）The accumulation of Hg and Cd mainly existed in the surface soil of all counties in Lanzhou,among which Xigu District had accumulated all other heavy metal elements to different degrees except Cr,and the concentrations of Hg and Cd were the highest in the industrial area.In addition,different counties and functional areas showed the largest variation coefficient in the concentration of Hg,followed by Cd,indicating that the concentrations of Hg and Cd were mainly influenced by human activities,among which the influence of human activities on Hg was greater than that on Cd,and industrial activities were the main factors causing the accumulation of Hg and Cd.（2）The spatial dependence among all neighboring sample points of surface soil in Lanzhou city was above moderate,and all of them had positive spatial correlation and were characterized by clustered distribution,but the spatial autocorrelation in Honggu and Qilihe district was weak and the clustered distribution was not obvious.Based on the above analysis,spatial interpolation was performed to describe the spatial distribution of heavy metals in each county as well as the whole Lanzhou city,and the effects of p H,cation exchange capacity（CEC）,soil organic matter（SOM）,nutrients（total N,available K and available P）and topographic factors（elevation,slope and aspect）on the spatial distribution of heavy metals were analyzed.The results showed that p H and available P showed significant negative correlation with heavy metal As,and total N,slope and elevation showed significant positive correlation with heavy metal As;elevation showed significant negative correlation with Cd,and total N,available P,available K and SOM showed significant positive correlation with Cd;p H,available K,available P and elevation showed significant negative correlation with Cr,and CEC and SOM showed significant positive correlation with As;total N and elevation showed significant negative correlation with Hg,and SOM and available P showed significant positive correlation with Hg;p H,available P and elevation showed significant negative correlation with Pb,and CEC and SOM showed significant positive correlation with Pb.（3）Anthropogenic and natural sources were the main sources of heavy metals in Chengguan,Xigu and Yongdeng District;heavy metals in Honggu District were influenced by industrial and coal-fired-domestic waste sources;heavy metals in Qilihe and Yuzhong District were both influenced by industrial-domestic waste sources,livestock-agricultural sources and natural sources;and heavy metals in Gaolan District were influenced by natural,industrial and coal-fired-domestic waste sources.In general,heavy metals in agricultural areas were mainly influenced by industrial-domestic waste sources,natural sources and livestock-agricultural sources;industrial areas were influenced by anthropogenic sources and natural sources.Combining the heavy metals in the whole Lanzhou City,it was found that Pb and Cd were mainly from industrial sources,As and Cr were mainly from natural sources,and Hg was influenced by both industrial and coal-fired-domestic waste sources.（4）The overall exceedance rate of heavy metals in the soil of Lanzhou City was low.Only Cd in Gaolan County and As and Cd in Yuzhong County exceeded the risk screening value,and the proportion of overstandard was 1.68%,0.69%and 0.69%,respectively.but compared with the background value,there was still enrichment pollution of heavy metals,among which Xigu District had the highest pollution rate of heavy metal（38.08%）,followed by Qilihe District（35.27%）,and the overall pollution rate was industrial area（32.11%）>agricultural area（29.34%）>trade and logistics area（24.07%）.For ecological risk,the average value of the ecological risk index in Lanzhou was 180.72,which was at a medium risk level.The ecological risk index of Xigu District and Qilihe District is relatively high,255.03 and 208.39 respectively,presenting strong ecological risk,while all other counties presented a medium ecological risk.In general,the proportion of"moderate or above ecological risks"was industrial area（87.81%）>agricultural area（87.67%）>trade and logistics area（70.37%）,while both pollution and ecological risk were caused by the enrichment of Hg and Cd.For health risk,the degree of non-carcinogenic risk was As>Cr>Pb>Hg>Cd,the degree of carcinogenic risk was Cr>As>Pb>Cd,the health risk of different exposure routes was oral intake>dermal contact>respiratory intake,overall the health risk of agricultural area was the largest,followed by industrial area,and the smallest in trade and logistics area,but the health risk caused by heavy metals in farmland soil in Lanzhou City was all below the limit value.The risk was negligible.（5）The concentration of heavy metals in cereals in Lanzhou City was Yuzhong(0.091mg·kg-1)>Gaolan(0.084 mg·kg-1)>Qilihe(0.076 mg·kg-1)>Honggu(0.051 mg·kg-1)>Chengguan(0.044 mg·kg-1);the concentration of heavy metals in vegetables was Qilihe(0.084mg·kg-1)>Yuzhong(0.081 mg·kg-1)>Xigu(0.025 mg·kg-1)>Chengguan(0.024 mg·kg-1)>Honggu(0.019 mg·kg-1);in general,the concentration of heavy metals in crops showed industrial area>agricultural area>trade and logistics area;the concentration of heavy metals in different types of crops in Lanzhou City showed cereals(0.067 mg·kg-1)>leafy vegetables(0.033 mg·kg-1)>root/tubers(0.029 mg·kg-1)>fruits(0.022 mg·kg-1).The bioconcentration of Hg and Cd in crops was high,and the enrichment of heavy metals in different types of crops was characterized as root/tuber>fruits>cereals>leafy vegetables,and was mainly related to available K,CEC,SOM and available P in soil.In addition,crops in Lanzhou city showed overall health risks caused by Cr and As,among which crops in industrial areas mainly showed health risks caused by Cr,trade and logistics areas mainly showed health risks caused by As,and crops in agricultural areas did not cause risks to human health. |
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