通江湖泊水沙节律性波动环境下重金属铜的迁移机制

Heavy metal pollution in lakes is an important problem of water environment. Different from the general independent lake, river-connected lakes are always affected by external rivers, which results in frequently fluctuated water flow, suspended sediment and other environmental factors. Heavy metal migration mechanisms in a river-connected lake are more complicated. Here, Poyang Lake, the most typical river-connected lake in China, is taken as an example to study the migration mechanism of Cu under the rhythmic fluctuation of water currents and suspended sediment. Based on the annular flume, the vertical sedimentation mechanisms of Cu under the different conditions of sediment particle size (fine silt, medium silt, and coarse silt), sediment concentration and water disturbance intensity will be explored, and the vertical release mechanisms of Cu from the wet and dry-wet altering deposited sediments of Poyang Lake will be studied. Based on the rectangle flume, the influence mechanisms of sediment particle size, sediment concentration and water disturbance intensity on the horizontal suspension of Cu in overlying water will be revealed, and the horizontal transfer mechanisms of Cu on the surfaces of two different deposited sediments, wet and dry-wet altering, will be investigated. Based on the experiment results, the complete migration parameters of Cu in both vertical and horizontal directions will be refined for varied flow-sediment conditions. An improved heavy metal model for Poyang Lake will be developed, and by numerical simulation, the migration characteristics of Cu in vertical and horizontal directions will be calculated under different flow conditions, Gravity-style, Jacking-style and Backflow-style. Finally, the driving weights of vertical release and horizontal transport on Cu load of three important lake areas will be decoupled.

湖泊重金属污染是一个重要水环境问题。与一般独立性湖泊不同，通江湖泊受外部江河影响，水流、泥沙等环境要素波动频繁，重金属迁移转换机制更为复杂。本项目以我国最典型的通江湖泊鄱阳湖为例，综合运用野外监测、室内实验、数值模拟，研究水沙节律性波动环境下重金属铜的迁移机制。基于环形水槽装置研究同泥沙颗粒粒径(细粉砂、中粉砂、粗粉砂)、不同泥沙浓度、不同水流扰动强度下铜的垂向沉降机制以及鄱阳湖常浸水、干湿交替湖区两种沉积物铜的垂向释放机制；基于直形水槽装置揭示泥沙颗粒粒径、泥沙浓度、水流扰动强度对铜在上覆水中水平悬移影响机制以及铜在常浸水与干湿交替两种沉积物床面的水平推移的机制。基于实验结果，凝练不同水流、泥沙条件下，铜在垂直-水平两向完整迁移参数；优化开发鄱阳湖重金属模型，揭示鄱阳湖重力型、顶托型、倒灌型三种流态下，铜在垂直、水平两向迁移特征，并对重点湖区铜负荷驱动因子进行解耦。

通江湖泊，指与外部江河相连通的湖泊。特殊的地理位置及复杂的江湖相互关系导致了通江湖泊与一般独立湖泊生态环境特征有所不同。本研究以鄱阳湖作为典型通江湖泊，基于野外长期观测数据积累以及补充观测，获得鄱阳湖相关特征数据，系统分析鄱阳湖水沙节律性波动特征及铜负荷时空分布特征。其中，基于环形水槽与直形水槽实验，研究不同水流扰动、泥沙颗粒尺度、泥沙浓度对铜的垂向沉降及水平迁移影响机制。凝练不同水沙条件下，铜在垂直-水平两向迁移扩散参数，优化开发鄱阳湖重金属数学模型并运用实测数据率定、验证；通过数值模拟，研究鄱阳湖重力型、顶托型、倒灌型三种流态下，铜在垂直-水平两向迁移规律并进行通量计算。结果表明：（1）沉积物颗粒大小对表层水和上覆水之间的铜交换通量有重要影响。在底部剪切力小于0.05 N/m2的情况下，重金属Cu的迁移特点为地表沉积物大量沉积。底部剪应力超过0.08 N/m2时，表面沉积物层被侵蚀，重金属Cu被逆向交换。（2）随着水流流速的增大，水体重金属浓度将增加，水体流速达到65 cm/s时，中下层水体重金属铜总含量相比上层水体增大较为明显，越靠近沉积物处Cu总含量增加幅度越大，但此时重金属在垂向的迁移及扩散作用不明显。（3）自2003年以来，鄱阳湖表层沉积物中重金属Cu呈现逐步减少的趋势。2015年全湖的平均表层沉积物中重金属Cu浓度为54.26 mg/kg，比2003年下降了约30.01%，表面沉积物中重金属Cu浓度随空间位置以及年份的变化而变化。（4）鄱阳湖在重力型流态下，长江下游处重金属运动速率较快，运动速率为2.111 km/d。各保护区受重金属Cu的影响状况不同：鄱阳湖长江江豚自然保护区>鄱阳湖南矶湿地国家级自然保护区>鄱阳湖国家级自然保护区，其影响指数分别为37.83，13.88，6.28。本项目提供了鄱阳湖上覆水流对重金属铜的垂向沉降的驱动影响和水流节律性波动对重金属铜的垂向分布的影响；重金属模型数值模拟研究应用的具体方法研究，对于通江湖泊的重金属修复及水环境保护工作具有重要参考价值进行解耦。