重庆城口黑色页岩残坡积土重金属污染特征与风险

Heavy metal pollution characteristics and risks of residual and colluvial soil from black shale in Chengkou,Chongqing

为探讨黑色页岩赋存区风化土壤的重金属地球化学特征和生态风险,选择重庆城口黑色页岩小流域采集黑色页岩风化表层土壤与深层土壤,分析土壤重金属(As、Cd、Cu、Mn、Mo、Ni、Pb、Zn和Cr)的元素组成、化学形态和重金属间相关关系,运用单因子污染指数(Pi)评价单重金属污染程度,基于污染负荷指数(PLI)和潜在生态风险指数(RI)厘清多种重金属的综合潜在生态风险程度,采用环境风险预警指数(IER)评判重金属元素的预警风险分布图.结果表明,研究区黑色页岩风化土壤的As、Cd、Cu、Mo、Ni、Zn和Cr的平均含量超过重庆土壤背景值,甚至Cd和Cu的平均含量超过风险筛选值,重金属越向山脊处越具有富集的特征,其重金属来源可分为黑色页岩风化的母质源、农业输入源和锰矿冶炼粉尘源.单因子污染指数(Pi)表明As、Cd和Mo基本处于重度污染,其重度污染面积>97%;Cu、Ni和Zn的重度污染面积达40%,而Mn、Pb和Cr以无污染和轻度污染为主,污染面积>70%.综合污染负指数(PLI)和潜在生态风险指数(RI)均表明极高生态风险(Ⅲ级)集中分布于上游近山脊和北侧坡的近山脊处,且潜在生态风险指数(RI)更适用于高~极高(Ⅱ级和Ⅲ级)生态风险区划.环境风险预警指数(IER)和重金属形态结果表明,Cu、Ni、As、Zn和Cr以轻警和预警为主,且形态以残余态为主,对环境风险较小;Mn和Pb均为无警风险,但Mn以铁锰氧化物结合态为主(34.6%~48.0%)和倾向来源于锰矿冶炼粉尘源,在环境改变时需对Mn进行关注;Cd的中警和重警面积达90%,在近山脊和山中部以可交换态(20.2%~25.2%)、碳酸盐结合态(9.6%~14.5%)和铁锰氧化物结合态(19.2%~30.4%)的形式存在,且Cd大部分超过风险管制值,需重点防控Cd对生态环境和人类的潜在风险.

In order to investigate the geochemical characteristics and ecological risk of heavy metals from weathered soil in black shale regions,a black shale catchment was selected to collect the surface soil and deep soil to analyse the heavy metals geochemistry.The elemental composition,chemical species,and correlativity of heavy metals(As,Cd,Cu,Mn,Mo,Ni,Pb,Zn,and Cr)were analysed through inductively coupled plasma mass spectrometry(ICP-MS)and inductively coupled plasma atomic emission spectrometry(ICP-AES).The single heavy metal pollution can be evaluated from single factor pollution index(Pi).Then,the potential risk of multi-heavy metals can be discriminated from pollution load index(PLI)and potential ecological risk index(RI),which can draw the spatial distribution map of potential ecological risk.The single heavy metal waring degree can be evaluated by the index of environmental risk warning(IER),which can draw warning risk distribution map of heavy metals.The results indicated that heavy metals were enriched from valley toward to sub-ridge and ridge.The average concentrations of As,Cd,Cu,Mo,Ni,Zn,and Cr exceed the Chongqing soil background values.Especially,the average concentrations of Cd and Cu exceed the risk screening values.The sources of heavy metals can be divided into the parent material sources of black shale weathering,agricultural input source,and dust source of manganese smelting.The single factor pollution index(Pi)showed that As,Cd and Mo were basically at high pollution level with the area ratio>97%.The high polluted area of Cu,Ni and Zn can reach up 40%,whereas the Mn,Pb and Cr were mainly at no pollution and lightly pollution level with area ratio>70%.The extremely high ecological risk(III level)was concentrated at the ridge,upstream area and north hillslope,which were evaluated from the PLI and RI value.The RI method was suitable for the high to extremely high(II to III level)ecological risk zoning.The Cu,Ni,As,Zn,and Cr were presented as residual factions and mainly located at early waring and low warning risk level trough IER value,which can ignore their environmental risks.Mn and Pb were both at no waring risk level,but Mn was dominated as Fe-Mn oxide faction with 34.6%~48.0%and tended to derive from dust source of manganese smelting,which should be paid attention to environmental risk of Mn when the environment changes.The Cd concentrations mainly exceed the risk screening value,resulting in medium and high waring risk level associated with area ratio reach up 90%.In addition,Cd mainly existed in exchangeable fraction(20.2%~25.2%),carbonate fraction(9.6%~14.5%),and Fe-Mn oxide fraction(19.2%~30.4%)at ridge and mid-hillslope,showing high bio-availability.Therefore,Cd in black shale soils should be focus on preventing and controlling the potential risks of ecological environment and human beings at ridge and mid-hillslope positions.