沅江入湖河床沉积物重金属污染演化地球化学分析

Geochemical study on evolution of heavy-metal contamination developed in bed sediments from the Yuanjiang River inlet to Dongting Lake,China

对沅江入湖河段河床沉积物进行钻探取样,利用电感耦合等离子质谱仪(ICP-MS)分析沉积物重金属等微量元素的含量,并利用210Pb法测定沉积物沉积速率.同时,在分析沉积物重金属污染特征的基础上,探讨沉积物重金属污染的演化过程.结果表明:沉积物中Bi、Cd、Mn、Mo、Zn、Pb等重金属含量变化大、分布不均匀,而Sc、Tl、Rb、Cs、V、Co、Zr、Hf、Nb、Ta、Ga、Ge、Rb、Sr、REEs(稀土元素)等微量元素含量变化小,分布相对均一.微量元素富集特征(EF值)和主成分分析结果显示,沉积物中Cu、Sc、Tl、Rb、Cs、Th、U、V、Co、Zr、Hf、Nb、Ta、Ga、Ge、Rb、Sr、REE等微量元素主要为自然源,而Bi、Cd、Mn、Zn、Pb等重金属则既有自然源的影响,又有人为源的贡献.地累积指数(Igeo)评价结果显示,入湖段沉积物存在Cd-Mn-Zn-Pb-Bi等元素组合的重金属污染,且沉积物自下而上分布有中深部(204~192 cm)、中上部(96~64 cm)、浅表部(64~0 cm)3个重金属污染层.上述3个重金属污染层可能对应于3个期次的重金属污染事件,即早期(1913年前)的零星重金属污染、中期(建国初期至改革开放初期,1963—1979年)的中等强度重金属污染、近期(1979年至今)高强度重金属污染.且自早期到近期,沉积物重金属污染程度增强,污染元素组合趋于复杂化.沅江入湖河床沉积物的上述3期重金属污染可能对洞庭湖沉积物产生直接影响.

Bed sediments from the Yuanjiang River inlet to the Dongting Lake of China were sampled by drilling,and concentrations of heavy metals and other trace elements in the sediments were analyzed using an inductively coupled plasma mass spectrometry（ ICP-MS） machine in present study. Also,the sedimentation rate that led to the formation of the bed sediments of the river was measured by using210 Pb isotopic method on sediment core samples. Based on characterizing the heavy-metal contamination developed in the bed sediments,the study aims at understanding in detail the evolution processes of heavy-metal contamination developed in the river area. The results show that concentrations of heavy metals Bi,Cd,Mn,Mo,Zn,and Pb are significantly variable,suggesting these heavy metals are heterogeneously distributed within the sediments. However,concentrations of other trace elements Sc,Tl,Rb,Cs,V,Co,Zr,Hf,Nb,Ta,Ga,Ge,Rb,Sr,and REEs（ rare earth elements） are significantly less variable,and that suggests that these trace elements are distributed relatively homogeneously. The enrichment factor（ EF） of trace elements and the principal component analyzing applied on concentrations of trace elements in all the sediment samples suggest that trace elements Cu,Sc,Tl,Rb,Cs,Th,U,V,Co,Zr,Hf,Nb,Ta,Ga,Ge,Rb,Sr,and REEs in the sediments might be contributed from natural sources（ e.g.,source rock weathering）. While,the heavy metals Bi,Cd,Mn,Zn,and Pb in the sediments might be contributed from both natural processes and human-being activities（ e.g.,ore mining in upper river areas）. The degree of heavy-metal contamination is then assessed using the geo-accumulation index（ Igeo）,and the results show that the bed sediments of the river might be contaminated by a heavy-metal assemblage of Cd-Mn-Zn-Pb-Bi. Moreover,it is found that there are three heavy-metal contamination layers developed within the bed sediments,and they are from deep upward to the surface： the middle-lower layer（ at depth from 204 to 192 cm）,the middle-upper layer（from 96 to 64 cm）,and the sorrow surface layer（from 64 to 0 cm）. Complementing with the210 Pb isotopic dating data,it is suggested that the above three heavy-metal contamination layers correspond well with three periods of heavy-metal contamination events taking place in the river area. These three events are the early stage（ before 1913 year） of heavy metal contamination,during which the sediments were slightly heavy-metal contaminated; the middle stage（ between year 1963 and 1979） of heavy metal contamination,during which the sediments were moderately contaminated by heavy metals;and the later stage of（ from year 1979 to present） heavy metal contamination,during which the sediments were seriously contaminated by heavy metals. It is also founded that from early through moderate to later stage of heavy metal contamination,the intensity of the contamination and the number for contamination assemblage of heavy metals were increased. Such a three periods of heavy-metal contamination developed within bed sediments of the Yuanjiang River may cause a strong impact on sediments deposited in the Dongting Lake.