滇东喀斯特镉砷高背景值区耕地土壤重金属污染现状及潜在生态风险评估

Assessment of Heavy Metal Pollution and Potential Ecological Risks of the Soils in Karst Farmland with High Background Levels of Cadmium and Arsenic in Eastern Yunnan

为了解滇东喀斯特区镉砷高背景值情况下耕地重金属污染状况及潜在生态风险,采集研究区耕地土壤表层(0~20 cm)样品451件、农产品样品324件,分析土壤和农作物样品中As、Cd、Cr、Pb含量,运用单因子指数法、内梅罗综合指数法、潜在生态危害指数法开展土壤重金属潜在生态风险评估。结果表明,研究区土壤重金属超标较为严重,As、Cd、Cr、Pb筛选值超标率分别为60.98%、88.25%、28.82%、24.17%,土壤As、Cd管制值超标率分别为1.77%、12.42%;对比区域背景值,Cd、As平均值超过背景值,Cr、Pb平均值低于背景值,土壤As与Cd之间相关关系不显著,其他重金属间存在显著相关关系,研究区土壤重金属元素主要来源于区域高地质背景和成土作用,存在Cd、As外源污染。Pb、Cr、As、Cd单指标符合单因子指数法清洁标准的样点占比分别为75.83%、71.18%、38.80%、11.75%,Cd、As污染较为严重,污染比例分别为88.25%、61.20%;按照内梅罗综合指数法评价,清洁、尚清洁、轻度污染、中度污染、重度污染的样点数占比分别为19.07%、5.32%、15.96%、12.20%、47.45%,接近1/2样点为重度污染;多金属潜在生态风险指数(R_(I))显示,研究区中等及以上风险等级样点数占比达到23.95%,存在一定重金属潜在生态风险,潜在生态危害程度最大的元素是Cd,其次为As,Cr、Pb的潜在生态危害程度较低;研究区农产品均未出现As、Cr、Pb超标情况,35个样品出现Cd超标,种类主要为辣椒、小麦、大麦、豆类蔬菜,超标率分别为53.66%、22.58%、10.53%、12.50%;农产品重金属含量与对应土壤重金属全量间无明显相关关系,农产品Cd、Cr、Pb与土壤pH存在显著相关关系;当地需对重金属Cd进行重点关注,科学管控,以保障区域农产品安全。

A total of 451 topsoil samples(from 0-20 cm soil layer)and 324 plant samples were collected from Karst farmland with high background levels of cadmium and arsenic in eastern Yunnan to analyze the contents of As,Cd,Cr and Pb in the samples.Single factor index method,Nemerow composite index method and Hankanson potential ecological hazard index method were adopted to carry out soil heavy metal pollution status and potential ecological risk assessment in this area.The results showed that the situation of heavy metal pollution was serious in the study area.The over standard rates of screening values of As,Cd,Cr and Pb were 60.98%,88.25%,28.82%,and 24.17%,respectively,and the over standard rates of control value of As and Cd were 1.77%and 12.42%,respectively.Compared with the regional background value,the average values of As and Cd exceeded the background values,Cr and Pb were lower than the background values,the correlation between As and Cd was not significant,but there existed significant correlations between other heavy metals.The soil heavy metal elements in the study area were mainly from the regional high geological background and pedogenesis,and there existed exogenous pollution sources of As and Cd.The results of single factor index method indicated that the proportions of the cleaning standards of Pb,Cr,As,Cd were 75.83%,71.18%,38.80%and 11.75%,respectively.The pollutions of Cd and As were more severe,with the pollution proportions of 88.25%and 61.20%,respectively.According to the Nemerow comprehensive index method,the proportions of still clean,clean yet,light pollution,moderate pollution and heavy pollution samples were 19.07%,5.32%,15.96%,12.20%,and 47.45%,respectively,and nearly half of the samples were over the heavy pollution standard.The potential ecological risk analysis showed that the proportion of samples with intermediate or above risk level reached 23.95%.So there existed certain potential ecological risks of heavy metals.The most potential ecological risk was from Cd,followed by As,Cr and Pb.Furthermore,As,Cr and Pb in agricultural products in the study area did not exceed the standard,and Cd content in 35 samples exceeded the standard,mainly including pepper,wheat,barley and legume vegetables,with the exceeding rates of 53.66%,22.58%,10.53%and 12.50%,respectively.There was no significant correlation between the content of heavy metals in agricultural products and the total amount of heavy metals in soil,but there were significant correlations between Cd,Cr and Pb in agricultural products and soil pH.Therefore,the local government should focus on scientific control of Cd,to ensure the safety of regional agricultural products.