湘西河流表层沉积物重金属污染特征及其潜在生态毒性风险

花垣河和峒河是湘西地区受到锰矿和铅锌矿生产影响严重的两条河流。通过表层沉积物采样分析了Cd、Pb、Cu、Ni、Cr、Zn和Mn的总量,根据BCR连续提取程序分析沉积物样品中重金属的地球化学赋存形态,采用内梅罗指数法和地积累指数法评价了沉积物重金属污染特征,根据重金属的富集程度探讨了重金属污染来源,采用淡水生态系统沉积物质量基准(SQGs,TEL/PEL)和毒性单位评价了花垣河和峒河沉积物中重金属元素的生态毒性风险。结果表明,花垣河和峒河绝大多数位点的表层沉积物中Cd、Pb、Cu、Ni、Cr、Zn和Mn的总量高于参照点,形成严重的复合污染,花垣河沉积物中重金属的污染水平明显高于峒河,但沿程变化规律不明显,而峒河沉积物中重金属的沿程变化较有规律,即上游含量低,中下游含量较高。两条河流表层沉积物中富集程度居前列的均为Cd、Pb、Zn和Mn。花垣河和峒河沉积物重金属污染主要来源于矿业生产所产生废渣和废水的点排放。在花垣河和峒河的大多数位点,Cd、Pb和Mn的形态具有共同特征,其生物可利用态均较大程度地超过生物不可利用态,而且Mn和Cd的生物可直接利用态所占比例远高于其它重金属,而Cu和Cr的生物可直接利用态所占比例很低。花垣河沉积物中Cd、Pb和Zn在所有位点极大地超过PEL,在峒河中下游,Cd、Pb、Ni和Zn超过PEL,具有较大的潜在生物毒性。除上游S1位点外,花垣河的其余各位点都具有明显的急性毒性,峒河中下游各位点具有明显的急性毒性,这些河段需要重点治理。

Pollution characteristics and potential ecotoxicity risk of heavy metals in surface river sediments of western Hunan

Huayuan River and Dong River are two important freshwater rivers, in western Hunan, and strongly affected by Mn and Pb-Zn mining and processing activities. The assessment for potential ecotoxicity risk of surface sediments of two rivers had never been conducted in a comprehensive way. The surface sediments were sampled and total contents of 7 heavy metals including Cd, Pb, Cu, Ni, Cr, Zn, and Mn were measured. The geo-chemical fractions of heavy metals were determined by BCR sequential extraction procedure. The pollution characteristics of heavy metals were evaluated according to Nemerow index and geo-accumulation index, and pollution sources were discussed. The heavy metal ecotoxicity risks were assessed with the application of Sediment Quality Guideline (SQG) (threshold effect level/probable effect level, TEL/PEL) and toxic units approach. The results showed that the total contents of Cd, Pb, Cu, Ni, Cr, Zn, and Mn in the surface sediments from most sampling sites of two rivers were greatly higher than that of the reference site, and thus constituted serious combined pollutions. The heavy metal pollution of surface sediments in Huayuan River was clearly heavier than that in Dong River. In Huayuan River, no clear pollution trend was found, the maximal levels of Cd, Pb, and Zn occurred at S6 site, the maximal levels of Cu, Ni, Cr, and Mn occurred at S4 site, and the degree of heavy metal pollution at different sites indicated by Nemerow index was S6 > S4 > S2 > S7 >S3 > S5 >S1. In Dong River, the relatively low heavy metal levels existed in the upstream and the high levels existed in the middle/downstream, the maximal levels of Cd, Pb, Cu, Ni, Cr, and Zn occurred at L6 site, the maximal levels of Mn occurred at L5 site, and the degree of heavy metal pollution at different sites was L4 > L5 > L3 > L1 > L2. In two rivers, the heavy metals with top accumulation were Cd, Pb, Zn, and Mn. The heavy metal levels at different sites were related to the distributions of mining or processing factories, and thus heavy metal pollution might mainly originate from point discharge of mining dregs and mining wastewaters. In most of the sampling sites of Huayuan River, Cd, Pb, and Mn showed a similar geo-chemical fraction characteristic, that is, those bioavailable fractions were greatly higher than the residual fraction, moreover, the percentages of the exchangeable and weak acid soluble fraction of Mn and Cd were much higher than other heavy metals, however, the percentages of the exchangeable and weak acid soluble fraction of Cu and Cr were relatively low. The levels of Cd, Pb, and Zn in Huayuan River surface sediments were greatly above PEL, and the levels of Cd, Pb, Ni, and Zn in the middle/downstream of Dong River were also above PEL, these heavy metals might show a high potential biological toxicity. In Huayuan River, except for S1 site, the surface sediments from other sampling sites showed clear acute toxicity, and the surface sediments from middle/downstream of Dong River also showed clear acute toxicity, therefore, these river stretches should be controlled with great efforts.