Influence of environmental dredging on the water quality and studies on sensitive parameters of a typical deep reservoir (Tongjiqiao Reservoir)

Abstract

国内对于作为饮用水水源地并具有水温分层特征的深水型水库环保疏浚，缺乏长期实测评价工程前后水质变化规律以及易于发生变化的水质参数的实例.因此，对于这种类型的环保疏浚，如何进行效果评价并在工程实施中控制敏感参数成为一个没有解决的问题.本文以典型的饮用水水源地、深水型的通济桥水库为对象，通过长期水质监测，研究了环保疏浚的影响.结果发现环保疏浚对总氮（TN）削减起到较好的效果，但是疏浚期间总磷（TP）浓度明显上升，完工半年后TP浓度仍略高于往年同期水平.其中，坝前库区TN浓度降低至疏浚前多年同期均值的56%~87%，而TP浓度却为疏浚前的1.87倍以上.并发现浊度、TN、TP和锰（Mn）对于环保疏浚敏感响应.其中，TP波动与底泥扰动引起的浊度变化有密切的关系；坝前库区疏浚新生底泥Mn含量较高（1251.25 mg/kg），夏季水温分层加剧了库底水体的厌氧状态，是Mn出现异常的主要原因.以通济桥水库为鉴，建议深水型环保疏浚重点考虑底泥扰动及后续季节性水温分层对水质控制效果产生的影响.;The effect of environmental dredging projects in China has received wide attention with many case studies. However, little care was paid for dredging in stratified water sources. How to evaluate the effects and then control sensitive parameters is still an unsolved problem. Tongjiqiao Reservoir, a typical deep reservoir as well as a drinking water source, was chosen as a long-term case study. Result shows that Tongjiqiao Reservoir mainly reached water quality standard Ⅲ. Within half a year after the project, environmental dredging partially reduced near-dam total nitrogen (TN) concentration to 56%-87% of the past level, but 1.87-5.12 times as high as before in total phosphorus (TP) concentration for near-dam area. Turbidity, TN, TP and manganese (Mn) concentrations were sensitive parameters. The change of TP was closely related to turbidity caused by sediment disturbance. Besides, exorbitant Mn concentration at the bottom of the reservoir was related to both the exorbitant Mn in after-dredging surface sediments, where the highest content located in the near-dam area, reaching 1251.25 mg/kg. The seasonal stratification caused anaerobic conditions in deep water area. Therefore, environmental dredging of deep source ought to consider the impact of both sediment disturbance during dredging and thermocline after dredging.