浙江姚江平原特大洪水滞留沉积物中重金属特征及磁诊断探讨

Characteristics and magnetic diagnosis of heavy metals in deposits by flooding slackwater in the Yaojiang River Valley, Zhejiang Province, China

为探讨洪水滞留沉积物中重金属元素的含量、分布特征及其污染磁诊断有效性,以浙江姚江现代特大洪水滞留沉积物为例,洪水退去后及时沿平原段河岸自上游入城段到下游出城段系统采集24件表层沉积物样,开展了重金属元素实验分析和磁性参数测量。结果表明,8种重金属元素Cu、zn、Cr、Cd、Pb、Ni、Mn和Fe平均含量分别为175.23、400.74、145.32、0.83、43.71、32.12、564.35 mg·kg^(-1)和4.37%,除Fe不与当地河流底泥沉积物重金属背景值进行比较外,其余各元素均不同程度超背景值,其中Cu、Cd和Zn最严重,分别是背景值的10.88、6.75、6.32倍;重金属元素含量沿入城段-城区段-出城段呈递增趋势,在城区汉流段含量最高接近严重污染,指示了重金属元素受城市工业化的深刻影响。上述重金属元素含量的空间变化在沉积物磁性参数上也有良好体现,其中频率磁化率(χ_(fd))和饱和等温剩磁的矫顽参数(S_(-100mT))与Cu、Zn、Cr、Cd、Pb、Ni含量具有显著正相关关系,表明磁学方法同样可用来指示洪水滞留沉积物的重金属污染,但洪水期间复杂的沉积环境以及人类污染物质输移等因素影响,增加了利用磁学手段定量诊断重金属污染的复杂性。

Floodwater often leaves lots of soil in the lower area when it recedes. It is of significance to reveal the content, spatial distribution of heavy metals in the flood slack water deposits（SWD）.Twenty-four deposits by an extra-ordinary floodwater were collected from the upper to the lower reach of the Yaojiang River in Yuyao city, Zhejiang province. The feasibility of magnetic diagnosis for heavy metal pollution was explored. Results indicated that the average content of Cu, Zn, Cr, Cd, Pb, Ni, Mn, and Fe in the SWD was 175.23, 400.74, 145.32, 0.83, 43.71, 32.12, 564.35 mg·kg^-1 and 4.37%, respectively, all higher than the local background values（Fe not considered pollutant here）. Copper was the most serious pollutant with 10.88 times the background value, followed by Cd and Zn, which were 6.75 and 6.32 times the background values, respectively. In spatial distribution, heavy metals displayed increasing trend from the upper to the lower reach, with the highest values occurring in the tributaries flowing through the urban areas, implying industrial impact. The spatial distribution of heavy metals was also reflected in magnetic properties of the deposits. Frequency-dependent magnetic susceptibility（χfd） and S-100mT displayed significant positive correlations with content of Cu, Zn, Cr, Cd, Ni and Pb. It suggested that magnetic parameters might be used as an indicator of heavy metal pollution in flood SWD. However, due to the complicated hydrodynamic environment and pollutant transfer during flood events, it is necessary to further study the use of magnetic properties as quantitative indicators of heavy metal pollution.