| The pollution of soil heavy metals?HMs?is adverse to the health of soil ecosystem and the quality safety of agricultural products,thus affecting human health,and has been attracting much attention worldwide.Previous studies have analyzed the characteristics of HMs pollution in urban and agricultural soil,and evaluated the ecological health risk of heavy metal pollution,which is conducive to improving soil environmental quality and living environment.However,Chongqing,as one of the four municipalities directly under the Central Government,has received limited systematic research on its HMs pollution level of soil and crops,eco-environment and health risk assessment.Tongnan,Hechuan,Tongliang and Dazu districts in the Northwest Chongqing are the most important regions to build the vegetable basket base for the main city and agricultural modernization.The suburban characteristic and benefit agriculture has great potential and provides Chongqing citizens with a large amount of grain,oil,pigs,aquatic products,vegetables and other major agricultural products.It is necessary to carry out systematic research on HMs in this region.To understand eco-environmental risks of soil heavy metal and impact of agricultural products on human health in the region,we collected 1695 surface soil samples,101 and 139 seed samples of rice and corn,139 samples of leafy vegetables,and their root soils.According to the relevant standard requirement,the content of Cd,Hg,Pb,As,Cr,Zn,Ni and Cu in all samples,the 7-step speciation of heavy metals in some soil samples and other related physical and chemical indexes were analyzed.Geostatistics theory,GIS technology,multiple regression analysis,pollution assessment and source apportionment,ecological health risk assessment and other methods,were performed for systematically studying the following problems:?1?characterize the concentrations and spatial distribution of HMs in soils,and?2?identify and quantify pollution sources using advanced multivariate analysis and a receptor model?APC/APCS-MLR?,and?3?clarify characteristics and safety of heavy metal accumulation in soil-crop system,establish the absorption model of excessive heavy metal content in crops,and?4?identify levels of eco-environment and health risks of heavy metals in topsoil or crops.This study aims to provide theoretical basis for local soil pollution prevention,crop safety,eco-environment and human health risk management,which has important practical significance.The main conclusions are as follows:1.Mean concentrations of As,Cd,Cr,Cu,Hg,Ni,Pb and Zn in the surface soils in northwest Chongqing were 6.21,0.33,75.49,6.99,0.077,27.9,35.24 and 87.91 mg·kg-1 respectively.Except for Cr content was slightly lower than the background value,the average values of As,Cd,Cu,Hg,Ni,Pb and Zn were all higher than the background value,Seven heavy metal elements accumulated in different degrees in topsoil.Cd was 3.01 times higher than that of the background value,with the strongest cumulative effect.The others six elements were 1.07 to 1.28 times higher than that of the background value.2.The results of spatial variation analysis showed that Cd and Pb elements were fitted as linear models,As and Ni elements were fitted as spherical models,and the other elements were fitted as exponential models.The block-basis ratios of the eight elements ranged from 0.40 to 0.71,having to a moderate degree of spatial autocorrelation,which indicated that their spatial variation was jointly affected by structural factors and random factors.The Kriging interpolation results show that,in addition to the Hg content is higher in east,Zn,Cd,Pb,As,Ni,Cu and Cr content is higher in western of the study area,the spatial distribution of soil heavy metals and stratigraphic boundary on the whole is relatively consistent.The Cd and Pb,local high value area show that soil heavy metal content in the study area obviously controlled by parent materials and soil forming process,and Cd,Hg and Pb element also affected by human activities.3.Further comparative analysis suggested that the content of Cr,Ni,Cu,Zn and As is mainly controlled by the stratum?parent rock?,which is closer to the strong spatial autocorrelation.However,Hg is more significantly affected by human activities and is close to a very weak spatial autocorrelation.Cd and Pb are influenced by both parent rock and human activities.On the whole,the parent material determines the content and spatial distribution of soil heavy metals in study area;the epigenetic geochemical dynamics process reshapes the macro trend of the distribution in surface soil heavy metals;and strong human activities?such as industrial and mining activities,agricultural production activities,etc.?disrupts the natural distribution rules of Hg,Cd,and Pb elements.4.The average values of geo-accumulation index?Igeo?in the study area were less than 1,and the descending order was Cd>Pb>As>Zn>Ni>Hg>Cu>Cr.The average value of the single factor pollution index was also less than 1,which was Cd?0.77?>Cr?0.37?>Cu?0.34?=Zn?0.34?>Ni?0.29?>As?0.28?>Pb?0.23?>Hg?0.11?,and the average value of the comprehensive pollution index was 0.6.Enrichment factor?EF?descending by Cd?3.03?>Hg?1.30?>As?1.26?>Ni?1.1?=Zn?1.1?>Pb?1.09?>Cu?1.04?>Cr?0.95?.Although the evaluation results of the three methods are slightly different,the overall results are the same.Soil pollution is relatively low,mainly pollution-free and slight pollution,with moderate to severe pollution to a certain extent.That is,there are a certain number of soil sites with high indexes of HMs such as Cd,Hg and As,which shows that the pollution of these HMs has existed in the study area;especially Cd pollution is the most prominent.5.Correlation analysis,principal component analysis/absolute principal component score and multiple-linear regression receptor model?PCA/APCS-MLR?analysis showed that the main sources of HMs in soil were natural sources,industrial and agricultural sources,and atmospheric dust sources.Among them,Soil Cu,Cr,Ni,Zn and As are mainly derived from natural sources,and the contribution rates to the five heavy metals are 85.51%,84.75%,86.78%,71.14%and 83.95% respectively,which are controlled by geological background?parent material?obviously.Cd mainly comes from industrial,agricultural activities and natural sources,with the contribution rates of 56.49%and 43.51%respectively.The input of Cd caused by industrial,mining and agricultural activities in the study area is obvious,and its ecological effect should be paid attention to.The main sources of Pb were industrial and agricultural activities,with a contribution rate of 55.2%.Meanwhile,natural sources?parent material?were also one of the sources of Pb.While Hg,a relatively isolated element,is mainly sourced from anthropogenic atmospheric dust,which contribution rate of 86.9%.Controlling the accumulation of major polluting elements in farmland soil from the source can be conducive to reduceing the risk of heavy metal accumulation in agricultural products.Comprehensive measures should be taken to control soil Cd pollution in study area,such as preventing soil acidification,reducing emissions from industrial activities and agricultural fertilization input.The control measures of soil Pb pollution are mainly to control the discharge of industrial activities,While controlling air pollution from coal combustion is one of the important measures to prevent Hg pollution in soil.6.The over-standard rates of Cd and Ni in the root soil of rice,corn and leaf vegetables were25.5%and 20.6%,27.3%and 30.2%,45.5%and 15.9%respectively.Others such as As,Cr,Cu and Pb also have over-standard soils.Generally,vegetable soil>corn soil>paddy field.For the corresponding crops,only rice seed Cd and maize seed Cd exceeded the standard limit,and the over-standard rate was 9.90%and 8.63%respectively.The bioenrichment coefficient of Zn and Cd were higher.Although the Hg coefficient was the highest in vegetables,the Hg content in vegetables did not exceed the standard limit.This indicates that the bioavailability of heavy metals Cd and Zn is relatively strong,while the bioavailability of other HMs is relatively weak,which is also one of the main reasons why crops exceed the standard of Cd.7.The corresponding relationship between rice,maize,leaf vegetables and their root soil that the Cd content in crops under acidic conditions will have a higher misjudgment,that is,the heavy metal content in crops and the heavy metal content in soil are not simply linear,other key soil factors should be introduced for further study.Using the bioenrichment coefficient of Cd and introducing other factors in soil for multiple regression analysis to construct crop absorption model,the Cd content in rice seeds,corn seeds and leafy vegetables were negatively correlated with soil p H,and soil acidity would promote the absorption and accumulation of Cd by crops.Meanwhile,Ca O in soil can inhibit the accumulation of Cd in rice seeds,and K2O and S in soil have an inhibitory effect on Cd accumulation in corn seeds and leaf vegetables,while Si O2 in soil can promote the accumulation of Cd in rice seeds.In practice,the biological availability of heavy metals can be slowed down by controlling the content of soil Ph and related factors,and then improve the quality of agricultural products in study area.8.The topsoil is at a medium potential ecological risk level,and the harm degree of heavy metals is Cd>Hg>As>Pb>Cu>Ni>Cr>Zn,and the average values from high to low is Dazhu?184.9±57.6,mean±S.D.??Tongliang?182.0±90.8?>Tongnan?165.6±36.9??Hechuan?165.4±71.3?.Pb,Cu,Ni,Cr and Zn all have a slight ecological risk.As is basically at a slight ecological risk,while Cd and Hg are mainly at a medium ecological risk.The possible ecological hazards caused by the two should be taken seriously.9.Points of 81%soil HMs have no or negligible environmental risk?priority protection?,the rest of points?19%?where environmental risks may exist but controllable?safe utilization?,and no obvious environmental pollution risk area?strict control?points in the study area.The priority protection category is mainly distributed in the west and northeast parts of the study area,with a continuous distribution around the safe use of soil as a whole.Safe use classes are mainly distributed in the east and south of the study,area scattered in the central and western regions,mainly in irregular patches distributed.The results of the risk assessment code method showed that,except Cd,other heavy metal elements mainly exist in the form of residues,with no environmental risk or low risk,while soil Cd is in a high-risk state,with biologically effective components reaching 39.67%.Compared with other regions,the biological activity of heavy metal Cd in the non-geological high background area is significantly higher than that in the geological high background area.Therefore,the soil environmental risk in the study area was mainly caused by Cd element and its high bioavailability.10.There may be non-carcinogenic health risks caused by heavy metals in the study area.In addition to dietary intake of heavy metals,adults have higher carcinogenic risks than children do.Children were more susceptible to the potential health risk irrespective of the carcinogenic or non–carcinogenic risk,regardless.Soil Ni has the largest contribution rate to non-carcinogenic health risk,and children's single non-carcinogenic health risk index is greater than 1;soil Cr has the largest contribution rate to carcinogenic risk;As in three crops contributes the largest non-carcinogenic contribution rate,while Cd contributes to carcinogenicity maximum.The non-carcinogenic risk coefficients of the eight heavy metals in the soil-crop systems for adults and children were 0.397 and2.17 respectively.There were no significant carcinogenic and non–carcinogenic risks for adults,while the comprehensive non-carcinogenic risk index for children was greater than 1,showed significant non–carcinogenic effect,which is mainly caused by the non-carcinogenic risk generated by Ni element through soil skin contact.In conclusion,based on the the pollution assessment,ecological environment and health risk assessment section above,the Cd,Hg and Ni are selected as priority control heavy metals.However,the control of As,Pb and Cu should not be neglected as either their concentrations are gerater than risk screening values for soil envitonmental quality or higher indexes?Igeo,EF and RI?for some soil sites.Therefore,it should be paid more attention to the safe use of Cd in study area.It is necessary to take safety measures such as agronomic regulation or screening of low-accumulation varieties for alternative planting to reduce the current situation of agricultural products exceeding the standard.Meanwhile,reduce the discharge of Cd and Hg from industrial and mining activities,and the input of Cd into soil by agricultural production activities?such as the application of high Cd phosphate fertilizer?,and to avoid excessive exposure of children to soil in order to eliminate children's non-carcinogenic health risks.Our first systematical assessment provided important information for making policy by local government,and its quantitative evidence demonstrating the critical need for strengthened soil pollution control law in order to protect residents from heavy metal discharges into environment. |
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