衡水湖西湖区土壤重金属污染特征及退田还湖的生态风险初探

Pollution Characteristics of Heavy Metals in Soils and Ecological Risk with Returning Farmland to Lake in West Lake Area of Hengshui Lake

衡水湖西湖区作为衡水湖重要的生态系统服务供给区,在开展生态补水和退田还湖的过程中,现有西湖区大部分露滩在淹水后土壤中赋存的重金属释放会产生一定的生态风险。本文从土壤重金属赋存特征、生态风险评价及淹水释放通量风险估算等方面综合评估西湖区土壤生态安全。在调查区域内布设9断面46个样方,以0~60 cm土壤为研究对象,7种管控重金属作为研究要素。采用地累积指数法和潜在生态危害指数法分析西湖区重金属空间分布特征和生态风险等级,并通过室内柱样模拟的方法开展土壤淹水后重金属释放风险评估。西湖区土壤中Zn和Cd的平均含量显著高于河北省土壤背景值,其中重金属Cd累积特征比较明显,中度污染水平以上的区域占97.83%,各重金属含量在空间上呈现南北高中间低的分布规律,西湖区土壤总体处于低风险等级。7种重金属元素的生态风险贡献率依次为:Cd>As>Pb>Cu>Ni>Cr>Zn,其中37.71%的生态风险来自Cd的贡献,其次是As(18.81%)和Pb(4.94%)。淹水后土壤中重金属的释放特征随土地利用类型变化差异显著,其中菜地释放通量最高,果园次之,麦田最低,不同土地利用方式造成的有机质累积可能是土壤重金属释放通量差异的主要原因。此外,衡水湖周边主要以石灰性土壤为主(土壤表层浸出液pH≈9;Ca^(2+)含量为1.135~1.143 mg/g),这些以碳酸盐结合态为主要赋存形式的重金属相对稳定,淹水之后,经跟踪监测发现各重金属水溶态浸出浓度均未超标。因此,西湖区退田还湖后整体的水生态安全风险为低风险。

The West Lake area of Hengshui Lake is an important ecosystem service supply area,part of the soil would be flooded in the process of ecological water replenishment and returning farmland to the lake.This paper scientifically evaluated the ecological risk of soils in the West Lake area from the following three aspects:the distribution characteristics of soil heavy metals,the assessment of ecological risk,and the risk estimation of flood release flux,in which 46 quadrats of 9 sections were arranged,0–60 cm soils were sampled,and 7 types of controlled heavy metals were determined,the spatial distribution characteristics of heavy metals and ecological risk levels were assessed using the methods of geo-accumulation index and potential ecological hazard index,the risk of heavy metal release of flooded soils were evaluated by indoor simulation method.The results showed that the average contents of Zn and Cd in the soil of the West Lake area was significantly higher than the background values in Hebei Province,Cd accumulation was relatively obvious,with 97.83%of the area above the moderate pollution level.The contents of all heavy metals were higher in the south and north but lower in the middle,the soil was at a low risk level as a whole.The contribution rates of heavy metals to ecological risk were as follows:Cd>As>Pb>Cu>Ni>Cr>Zn,37.71%of the ecological risk was from Cd,followed by As(18.81%)and Pb(4.94%).The release characteristics of heavy metals in flooded soil varied significantly with land use types,in which the vegetable fields had the highest release flux,followed by the orchards,while the wheat fields were the lowest.The accumulation of organic matter caused by different land use types may be the main reason for the difference in heavy metal release flux in soils.The surrounding area of Hengshui Lake is dominated by calcareous soil(soil surface leaching solution pH≈9;Ca^(2+)content was 1.135-1.143 mg/g),these heavy metals mainly occurred in carbonate binding state are relatively stable,the leaching concentrations of all heavy metals in water soluble state did not exceed the standard after being flooded.Thus,the overall risk of water ecological security is low after returning farmland to lake in the West Lake area of Hengshui Lake.