长江中游近岸表层沉积物重金属污染特征分析及风险评估

为了探究长江中游近岸沉积物中重金属污染情况，2020年6月对长江中游14个采样断面的沉积物进行样品采集并测定沉积物中汞（Hg）、镉（Cd）、砷（As）、铜（Cu）、铅（Pb）、铬（Cr）和锌（Zn）等7种重金属元素的含量.首先分析了长江中游近岸表层沉积物重金属含量的空间分布特征，然后采用相关性分析方法（CA）、主成分分析法（PCA）和正定矩阵因子分解法（PMF）相结合的途径分析表层沉积物中重金属的来源，最后采用地累积指数法（I_geo）、潜在生态风险指数法（RI）和沉积物质量基准法（SQG）对重金属进行了风险评价.结果显示，Hg、Cd、As、Cu、Pb、Cr和Zn的平均含量分别为0.13、0.77、11.20、36.45、36.40、83.99和124.21 mg·kg^-1，其中Cd和Pb的平均含量超过背景值的1.72和1.35倍；PCA提取了前3个主成分（累积贡献率85.16%），结合CA结果显示重金属Cd、As、Cu、Pb和Zn来源一致，Hg和Cr来源一致；PMF模型将7种重金属元素的污染源分成3个因子并得到因子的贡献率，并且工业和生活废水、煤炭燃烧、采矿业3个因子的综合贡献率为41.96%、32.48%和25.55%；地累积指数法（I_geo）评价结果显示，Cd是主要重金属污染物，处于轻度污染程度等级，Hg、As、Cu、Pb、Cr和Zn等6种重金属元素处于无污染等级；潜在生态风险指数法评价结果显示，Hg的最高风险等级为中等生态风险等级，位于城陵矶和新厂采样点，Cd的最高风险等级为强生态风险等级，位于牯牛沙水道和武汉上采样点，As、Cu、Pb、Cr和Zn在14个采样断面均属于低生态风险等级.综合潜在生态风险指数（RI）为62.59~138.59，其中处于低微和中度风险等级的采样点分别占总采样点的71.43%（10个采样点）和28.57%（4个采样点），整体上长江中游干流污染不严重；沉积物质量基准法（SQG）评价结果显示，长江中游沉积物等级为Ⅰ级，定性评价为优，显示长江中游14个采样断面的沉积物对底栖生物没有毒性作用.综合以上结果，长江中游重金属污染不严重，Cd为重点防治的重金属元素.

Pollution characteristics of the heavy metals and their potential ecological risk assessment in nearshore sediments of the middle reaches of the Yangtze River

To investigate the heavy metal pollution in sediments of the middle reaches of the Yangtze River, the contents of Hg, Cd, As, Cu, Pb, Cr and Zn from 14 sampling sections (52 samples) were detected in June 2020. The correlation analysis (CA), principal component analysis (PCA) and positive definite matrix factor model (PMF) were applied to identify the possible pollution sources of heavy metals in nearshore surface sediments, and the potential ecological risk of heavy metals were evaluated by the methods of geoaccumulation index (I_geo), potential ecological risk assessment index (RI) and environmental quality standard for sediments (SQG). Results showed that the average concentrations of Hg, Cd, As, Cu, Pb, Cr and Zn were 0.13, 0.77, 11.20, 36.45, 36.40, 83.99 and 124.21 mg·kg^-1, respectively. Among the seven heavy metal elements, Cd and Pb exceeded as 1.72 and 1.35 times as background value of Yangtze River basin. Combining with correlation analysis, the cumulative contribution rate of the three principal components extracted by PCA was 85.16%, and the pollution sources of Cd, Cu, Zn, Pb and As were the same while Cr and Hg originated from the same pollution sources. The PMF model divided the pollution sources of seven heavy metals into three factors and obtained the contribution rate of the three factors. The comprehensive contribution rate of industrial and domestic wastewater, coal combustion and mining were 41.96%, 32.48% and 25.55%, respectively. The evaluation of geoaccumulation index (I_geo) suggested that Cd was the main pollution element in the sediments of the middle reaches of the Yangtze River, which was characterized as slight pollution grade, while Zn, Cr, Pb, Cu, As and Hg presented non-pollution grade. The potential ecological index of Cd in the sampling sites of Chenglingji and Xinchang were in serious ecological risk grade, and the potential ecological index of Hg in the sampling sites of Guniusha and Wuhanshang were in moderate ecological risk grade, while Cr, Pb, Cu, Zn and As in all sampling sites were in low ecological risk grade. RI was ranged from 62.59~138.59, and 71.43% of the sampling sites were in low ecological risk grade while 28.57% of the sampling sites were in moderate ecological risk grade. The evaluation results of sediment quality standard method (SQG) showed that the sediments quality in the 14 sampling sections in middle reaches of the Yangtze River were all in grade I, and the qualitative evaluation was excellent, indicating that the sediments of the 14 sampling sections in the middle reaches of the Yangtze River had no toxic effect on zoobenthos. Based on the above results, the heavy metal pollution in the middle reaches of the Yangtze River is not serious, and Cd is the prior heavy metal element to prevent and control.