| Heavy metals,a kind of contaminants,are continuously harmful to the environment.They accumulate and migrate in the agricultural planting system and enter into the food chain through agricultural products,which pose a serious threat to agriculture and human health.With the extensive development of agriculture and industry in China,the contamination of heavy metals in farmland system cannot be ignored.In some areas,soil and agricultural product are seriously polluted,and the soil environmental problems around industry and mining enterprises are prominent.In this study,Dongying City(Shandong Province),Wuzhou City(Guangxi Zhuang Autonomous Region),and Baiyin District(Baiyin City of Gansu Province),China were selected as representatives of the cleaner production zone,high natural background zone,and industrial and mining contaminated zone in the agricultural planting system,respectively,depending on the degree and risk of heavy metal pollution.Various data analyses,such as correlation analysis,redundancy analysis,principal component analysis,positive matrix factorization,multiple stepwise regression,and curve fitting,were employed to analyze the distribution differences of heavy metals in the three agricultural planting systems.Collaborative assessment,accurate source appointment,calculation of contribution rate,and exploration of key prevention and control strategies for heavy metal pollution in the agricultural planting systems,and health risk assessment from the vegetable intake and re-calculation of limit standards for fresh vegetable were carried out.Multivariate nonlinear model for potential pollution sources and comprehensive pollution status,multivariate nonlinear models for predicting heavy metal contents in vegetables under internal factors and environmental effects,and modified model of heavy metal limit standards for fresh vegetables were established.These models provided theoretical basis and technical support for the prevention,remediation,and treatment of heavy metal contamination.The study of cleaner production zone(Dongying)showed that farmland in the core area of the Yellow River Delta was not contaminated by mercury,cadmium,arsenic,lead,and chromium,but the potential ecological risks brought by the gradual increase of cadmium and arsenic contents in the soil could not be ignored.The spatial distribution of mercury,cadmium,arsenic.lead,and chromium in various management zones,pollution sources,soil types.soil parent materials,and crop species were significantly different.Soil cation exchange capacity,organic matter content,clay composition,and soil acidity were four critical environmental factors closely related to the distribution of heavy metals in farmland soils.In these farmland soils,about 64.70%of mercury was formed by atmospheric deposition,66.33%of cadmium came from industrial emissions,53.23%of arsenic and 46.29%of lead originated from agricultural inputs,and 64.47%of chromium sourced from parent materials.Industrial emissions,agricultural inputs,and river sediments were the three major sources of heavy metals in farmland.Based on these results.managements and policies including eliminating the discharge of industrial pollutants,reducing the excessive inputs of pesticides and agrochemicals.stabilizing or fixing toxic elements in river sediments were put forward in order to make scientific,rational,and sustainable use of land resources and protect f'armland soil from heavy metal pollution.The collaborative assessment of soil environmental quality and agricultural product safety in high natural background zone(Wuzhou)was carried out by using the impact index of comprehensive quality.Preliminary estimates indicated that about 3.85%and 1.35%of vegetable planting systems were multiply contaminated by two to three heavy metals and more than three heavy metals,respectively.Multivariate nonlinear model based on principal component scores and comprehensive pollution indices could well fit the impact of different sources on the comprehensive pollution degree of the agricultural planting system.The nonlinear relationship was formed by the superposition,synergy,and interaction of various factors.Based on the results,human activities and natural effects were the causes of heavy metal contamination in the agricultural planting system.With the deterioration of pollution from mild to severe,the contribution rate of anthropogenic sources increased from 65.78%to 86.03%,while that of natural sources decreased from 34.22%to 13.97%.The contribution of anthropogenic sources were more significant than that of natural sources.These results had important practical significance and practical value for the prevention,control,and remediation of heavy metal pollution in vegetable planting system in areas with high natural background values.Sources of heavy metals in soils of industrial and mining contaminated zone(Baiyin)were studied by approaches of spatial variability,correlation analysis,enrichment factor,principal component analysis,geographic information system,and positive matrix factorization.Three sources of heavy metals were identified and their exact contribution rates were calculated:95.38%of arsenic came from sewage irrigation;74.64%of chromium,88.01%of manganese,59.63%of nickel,and 76.20%of vanadium sourced from natural origins such as crustal movement,rock weathering and soil parent materials;80.87%of copper,93.34%of lead,and 69.71%of zinc originated from industrial activities.Natural factors,industrial sources and sewage irrigation were the three primary sources of heavy metals in farmland soils in this area.The pollution of arsenic,copper,lead,and zinc in farmland soils was serious.Therefore,the key strategies to solve this problem were to reduce pollutant emissions at industrial zones and improve the quality of irrigation water.The uptake of heavy metals by vegetables is controlled by a variety of factors:vegetable species(internal aspects)and environmental factors(external conditions,such as the total amounts of heavy metals in soil,soil physicochemical properties and the concentrations of heavy metals in irrigation water).By stepwise regression analysis,the adjusted transfer factor representing vegetable species,the total contents of heavy metals in soil and soil pH were the three key factors determining the levels of heavy metals in edible parts of fresh vegetables.In order to accurately estimate the heavy metal contents in vegetables,the binary nonlinear equation models transformed from stepwise linear regression were established.Generally,the absorption and accumulation of heavy metals in different vegetable species and cultivars were significantly different.Therefore,combined with the rational agronomic measures and mature soil remediation technology,planting vegetable varieties with low accumulation of heavy metals is one of the effective strategies for the cleaner agricultural production and food safety in high natural background zones.Appraising the quality and safety of vegetables according to the maximum standards is an effective way to ensure human health.The maximum standards of heavy metals in fresh vegetables can be calculated by the reference dose.From the point of view of health risk,the calculation method was not only based on toxicological dose,but also on individual indices,such as body weight and daily intake of vegetables.The correction coefficients including dietary partition coefficient(p),processing compensation coefficient(c)and uncertainty(u)were further introduced into the calculation model.This method could correct the unreasonable limits in vegetable quality and safety assessment,make up for the deficiencies of some pollutant indicators in the current limit standards,and provide new theoretical guidance for the formulation of vegetable safety standards for various populations.The results showed that the re-calibrated maximum limits for the populations in the study area were different from the current standards:the limits of arsenic,cobalt,chromium.copper,manganese,and vanadium were more stringent;the limits of cadmium,nickel,and zinc were similar;and the limits of mercury and lead were more relaxed.Referring to the limits of current standards,about 14.29%,2.86,2.86,1.43,and 0.71%of vegetables were contaminated by lead,cadmium,mercury,zinc,and chromium,respectively,with the most polluted element of lead.There were no limit standards for cobalt,manganese,and vanadium.According to the re-calibrated maximum limits,about 60.00%and 20.71%of vegetables were contaminated by arsenic and chromium for the children's standard,while 38.57%and 4.29%of vegetables were contaminated by arsenic and chromium for the adults' standard,respectively.Approximately two or three samples exceeded the new limits of cadmium,cobalt,manganese,and zinc,with the most dengerous elements of arsenic and chromium.The study also found that with the increase of age,the health risk of vegetable intake gradually decreased,and children's exposure to vegetable consumption was the most serious.Therefore,stricter food safety standards for children should be established and implemented.The scientific problems solved in this study are:(1)the method of collaborative assessment of soil environmental quality and agricultural product safety by using impact index of comprehensive quality is more comprehensive and reasonable than the previous one-sided assessment method;(2)the combined application of positive matrix factorization,geographic in formation system,and principal component analysis can conduct heavy metals source research more accurately,practically and effectively;(3)from cleaner production area to high natural background area and then to working condition pollution area,with the aggravation of pollution degree,the accumulation of heavy metals brought by human activities is far higher than that of natural factors,and the metal-related industrial production and sewage irrigation especially are the most important source of pollutants.The innovations of this study are as follows:firstly,based on the impact index of comprehensive quality,the method of collaborative assessment was proposed and applied,which effectively combined soil environmental quality and agricultural product safety,and comprehensively evaluated the pollution status of heavy metals in the agricultural planting system.This method is more scientific and reasonable than the previous evaluation methods.Secondly,based on the linear model,multivariate nonlinear model for potential pollution sources and comprehensive pollution status,multivariate non-linear models for predicting heavy metal contents in vegetables under internal factors and environmental effects,and modified model of heavy metal limit standards for f'resh vegetables were established and optimized by using correlation analysis.redundancy analysis,principal component analysis,positive matrix factorization,multivariate stepwise regression,curve fitting and other data analysis methods.This study needs to be improved in the following aspects:First,the source appointment results were based on the analyses of heavy metal contents and the identification of spatial maps in the study,and have not been verified in the actual field.Therefore,it is the next step to verify the source appointment results of data analysis under specific environmental conditions.Secondly.based on the soil and agricultural product samples of large-scale farmland,this study has obtained more reasonable results.However,for small-scale contaminated land or farmland with known pollution sources,whether this source appointment method is applicable and how to adjust it need to be carried out more in-depth.Thirdly,the corresponding prevention and control strategies based on the contribution rates of different sources are the theoretical basis for clean and safe production in farmland and sustainable development of agriculture.However,this study involved less effective remediation measures.such as chemical and physical immobilization and stabilization,bioremediation with hyperaccumulating plants,and agricultural remediation with low accumulative crops and reasonable agronomic measures,which would be a critical direction for future research.Collaborative assessment,accurate source appointment,calculation of contribution rate and exploration of key prevention and control strategies for heavy metal pollution in different agricultural planting systems,as well as health risk assessment for vegetable intake and recalculation of limit standards in fresh vegetables,are imperative for formulating prevention and control measures for heavy metal pollution in farmland,maintaining sustainable utilization of land resources,ensuring clean and safe agricultural production and human health.This study has vital theoretical and practical significance. |
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