杭州市河道底泥重金属污染评价与环保疏浚深度研究

重金属是河道底泥疏浚要控制的重要污染物,而疏浚深度是生态疏浚工程中需要确定的关键参数.本文以杭州经济技术开发区河道为研究对象,共采集18处底泥柱状样,采用土壤背景值、土壤环境质量二级标准值、变异系数和相关性分析方法对底泥中Cu、Zn、Pb、Cd、Ni、As的污染状况进行了评价,同时分析重金属总量和有效态含量在垂向上的变化特征,并引入地累积指数法对各深度底泥中的重金属进行累积性评估,利用本文提出的临界累积深度方法来确定合理的环保疏浚深度.结果表明,河道表层底泥中Cu、Zn、Pb、Cd、Ni、As的平均含量分别为165.16、342.40、55.34、3.61、34.06、36.70 mg·kg-1,Cd、Cu、As、Zn的有效态含量分别为0.007、3.37、0.095、20.76 mg·kg-1,重金属总量均超过了杭嘉湖平原土壤背景值,其中Cd污染最为严重.各重金属元素在底泥中的变异系数为0.29~2.39,空间分布并不均匀.重金属总量和有效态含量随底泥深度的增大整体呈下降趋势,但受不同时期人类活动污染及河道整治影响而呈不规则变化.各重金属元素的富集程度为CdAsZnPbNiCu,其中,Cd污染等级为2~6级,污染程度为中度污染至极强污染.利用临界累积深度方法推算得到研究区域河道底泥环保疏浚深度为0~0.9 m.

Assessment of heavy metal status in sediments and environmental dredging depth in Hangzhou

Sediment contamination caused by heavy metals is the most detrimental impact in river sediments dredging. Dredging depth is a key parameter in environmental dredging engineering. In this study, we collected 18 columnar sediment samples from rivers in Hangzhou Economic & Technological Development Area (HEDA). The pollution status, spatial and vertical distribution of six heavy metals (Cu, Zn, Pb, Cd, Ni and As) are evaluated and analyzed. In order to get a reasonable environmental dredging depth, a method named critical accumulation depth is proposed. The results show that the average contents of Cu, Zn, Pb, Cd, Ni and As in surface sediments are 165.16, 342.40, 55.34, 3.61, 34.06 and 36.70 mg·kg^-1, respectively, and the effective contents of Cd, Cu, As and Zn are 0.007, 3.37, 0.095 and 20.76 mg·kg^-1, respectively. The variation coefficients range between 0.29 and 2.39, which means a non-uniform spatial distribution of heavy metals pollution. Because of the accumulation effect of heavy metals in sediments, the contents of heavy metals tend to decrease along sediments depth direction, but this trend may vary as a result of human activities and river dredging in different periods. The geoaccumulation degrees of heavy metals are in the order Cd>As>Zn>Pb>Ni>Cu. Finally, the recommended environmental dredging depths for removing heavy metal contamination in sediment are obtained based on the method of critical accumulation depth proposed by this paper.