Southwestern China is a typical area with high background of heavy metals(HMs),where the excessive HMs in soils is closely related with natural geologic processes and thus has high natural background properties.It is worth noting that with the continuous progress of urbanization and industrialization,the soils from high geological background areas were inputted not only by long-term natural weathering,but also by the intense anthropogenic disturbances such as agricultural farming,mining activities and metal smelting,which is regarded as the typical interaction area between natural processes and anthropogenic activities.Under the superimposed influence caused by both endogenous input from the high geological background and exogenous contamination through anthropogenic activities,resulting in unidentified pollution sources of soil HMs,unclear ecological risks,and technological insufficiencies of prevention and control in superimposed pollution area.These are not beneficial for the rational utilization of land resources,the precise management of source pollution of HMs and the scientific control of ecological risks.However,there are few systematic studies on the environmental geochemical behavior of soil HMs and population health risk in the superimposed pollution areas with high geological background.Thus,it is urgent to carry out relevant studies which were conducted at the regional scale.A Indigenous zinc smelting area with high geological background in northwest Guizhou was selected as the main research object of the study,and taking“soil-crop(maize)-human health”as the principal line for the study.Through three critical process for migration,accumulation and gastrointestinal absorption,analyzing the geochemical characteristics and the spatial heterogeneity regularities of HMs(Cd,Cr,Cu,Ni,Pb and Zn)in soils,clarifying the superimposed pollution pathways of HMs in soils and their contribution ratios,expounding the characteristics of coupling and differentiation of HMs in soil-maize system and their governing factors,deducing the recommended threshold values for the risk control and environmental quality hierarchization of HMs,and deriving the population health risk levels exposed to HMs.Therefore,this study aims to investigate the pollution sources of soil HMs,develop the risk prevention and control strategies,enhance the refined management levels of cultivated land resources,and improve the population health risk assessment systems in the superimposed pollution area with high geological background in Southwestern China.The main research results obtained are as follows:(1)The concentrations of HMs in soils had the characteristics of the higher dispersion and nonuniform distribution.The mean concentrations of the tested HMs in soils were all significantly higher than their respective background values in Guizhou province.Compared with the screening values of risk control standard for soil contamination of agricultural land(GB15618-2018),the exceedance rates of Cd,Pb and Zn were 100%,97%and 62%,respectively,which exhibits an obvious cumulative phenomenon in soils.The results of chemical form analysis indicated that the investigated soil HMs existed predominantly in residual form(F7),while the bioactive components(F1+F2+F3)that are easily absorbed by plant constituted a minor percentage,indicating relatively low bioavailability of HMs in soils.However,the bioactive components of soil Cd was higher than of other HMs(an average of19.87%of the total concentrations),which showed a stronger biological activity in soils.The spatial distribution of Cd,Pb and Zn showed similar trends.Higher concentrations of these HMs were found in the southwest part of the study area,followed by the northwestern area,while the eastern area had lower concentrations.The high concentrations of soil Cu was predominantly distributed in the central-southern part.However,the spatial distribution characteristics of Cr and Ni was different from that of other HMs,high-Cr and Ni concentrations in soils were distributed in insular and streak patterns.(2)The accumulation of HMs in soils was mainly derived from four pollution superposition paths in the study area.The positive matrix factorization(PMF)model reasonably revealed the source component profiles of the investigated HMs in soils,which originated mainly from natural parent material sources,agricultural activity sources,mining activity sources,and the mixed sources of natural parent material and mining activity,accounting for 21.3%,10.8%,53.4%,and 14.5%of the total HMs accumulation,respectively.Among them,the emission source of mining activities was identified as the dominant factor affecting the accumulation of HMs in soils in the study area.The sulphide minerals in soils such as galena and sphalerite are generated by the mining activities,which could be transformed into cerussite and smithsonite with higher solubility product during the oxidation and carbonation process,resulting in the enhanced dissolution of HMs such as Pb,Zn,Cd,etc.,emerging as an important vector for the transfer process of HMs.(3)The bioaccumulation process of HMs in soil-maize grains system was affected by multiple factors.The bioactive components of Pb,along with SOM,and Mn concentrations in soils were the key factors affecting the bioaccumulation of Pb in maize grains.The bioactive components of Zn,along with CEC,and p H values in soils were the key factors affecting the bioaccumulation of Zn in maize grains.The bioactive components of Cd,along with p H,and Ca concentrations in soils were the key factors affecting the bioaccumulation of Cd in maize grains.(4)The maize plants contained multiple coordination group,while differences in the microscopic responses and the intrinsic mechanisms were observed in various tissues of maize during the bioaccumulation process of HMs.The carbohydrates,pectins and polysaccharides on the root cell walls of maize and their typical functional groups(including-OH,-COOH and-C-H)were the important binding sites that were involved in the divalent-metal cation(M2+)coordination,which generally have the role of acting as a protective barrier to alleviate soil HMs toxicity.The hydroxyl groups(-OH)provided by carbohydrates and the free carboxylic groups(-COO-)provided by pectic substances on the stems cell walls of maize,were all important sites for stems to immobilize M2+,which further inhibited the upward migration of M2+.The proteins,polysaccharides and phosphate ligands on the leaves cell walls of maize and their active groups including-C=O,-C-H,C-O-P played an important role in binding M2+,which together enhanced resistance of leaves to HMs stress.The hydroxyl groups(-OH)provided by carbohydrates,-C=O and N-H groups provided by proteins,and-C-H groups provided by polysaccharides on the grains cell walls of maize,were all easily bound to M2+leading stable compounds formation,which weakened toxicity of HMs on grains to some extent.(5)The recommended threshold values for the risk control and environmental quality hierarchization of Cd in soils were proposed based on species sensitivity distribution(SSD)in maize-producing regions from the study area.The soil Cd threshold of suitable maize-producing region was≤2.02 mg/kg,the soil Cd threshold of the restricted maize-producing region ranged between 2.02 and 8.82 mg/kg,and the soil Cd threshold of the forbidden maize-producing region was≥8.82 mg/kg.When the maize was used as feed,the suitable threshold was 14.67 mg/kg,and the forbidden threshold was 44.12 mg/kg.The area with soil Cd concentration within a range of the suitable area threshold and the forbidden area threshold was considered as restricted for the feed-producing region.The derivation thresholds were validated by the datasets,which was consistent with the set scenarios,reflecting good scientificity and rationality.(6)The oral ingestion of maize was the main exposure route for HMs in the soil-maize system to cause population health risk in the study area,and Cd,Pb and Cr were the dominant contributing factors.The calculation results of health risk assessment were calibrated by the bioaccessibility of HMs showed that the comprehensive non-carcinogenic risk index(THI)of HMs to adult males,adult females and children was 9.75E-02,1.34E-01 and 2.22E-01,respectively,indicating that the non-carcinogenic risk was not exceeded the safety threshold(THI=1),but the non-carcinogenic risk was mainly due to Cd and Pb.The contribution for different exposure routes to THI in the three populations was THI maize-dietary>THI soil-ingestion>THI soil-inhalation.The comprehensive carcinogenic risk index(TCR)of HMs to adult males,adult females and children was 2.83E-05,3.85E-05 and 1.63E-05,respectively;and 7.0%,10.1%and 2.5%of TCR was higher than 1×10-4,respectively,indicating that there was a carcinogenic risk present,and the carcinogenic risk was mainly due to Cd and Cr.The carcinogenic risk induced by maize ingestion of HMs for the three populations accounted for 97.26%,98.56%and 94.61%,respectively of the total health risks.Therefore,more attention should be paid to the dietary exposure risk in the health risk management of HMs. |