水库退役后库区沉积物内源磷的迁移转化特征

A large number of dams have been removaled in foreign countries due to aging, ecological restoration, etc.. Reservoir degrading and decommissioning in China is also processed in an orderly way. The change of hydrodynamic conditions after the dam removal will lead to a large amount of sediment transporting to the downstream of the reservoir and a large amount of desorption of sediment pollutants, which may pose a threat to the health of aquatic ecosystems. A large number of researches on the environmental influence of dam removal have been carried out internationally, but it has not yet formed a theoretical system. At the same time, those researches focus on sediment transport and ecological effects, but not on the influence of pollutants migration and transformation. We have carried out the ecological environment monitoring in the Liaohe River basin before the removal of two ready-to-decommission reservoirs. Based on existing research, this application proposed to further focus on phosphorus, we will complementarily monitor index of phosphorus in sediment and water, comprehensively use field Monitoring, sediment phosphorus release experiment and mathematical modeling, compare and analyze occurrence characteristics of phosphorus in water and riverbed before and after the reservoir decommissioning, systematically study the relationship of "dam removal-sediment disturbance- phosphorus migration and transformation" and "pressure - state - response", identify characteristics and mechanism of temporal and spatial response of sediment phosphorus after the reservoir decommissioning. The purpose of this study is to provide scientific basis for the decision making of high nutrient salt background dam removal, for the retirement plan formulation and for the environmental management in the downstream area of the reservoirs in china.

由于坝体老化、生态修复等原因，国外已有大量水库退役，我国水库降等报废工作也在有序推进。水库退役后水动力等条件的改变将导致库区沉积物向下游大量输送及沉积物中污染物的大量解吸，进而可能对水生态系统健康造成损害。国外已开展了大量水库退役环境影响研究，但尚未形成系统的理论体系；侧重于对泥沙输运和生态影响的研究，而对污染物迁移转化影响的研究较少。申请人已在辽河流域进行了1座水库退役前后和2座拟退役水库退役前的生态环境监测。本申请拟在现有研究基础上，补充监测沉积磷及水体磷指标，综合采用现场监测、沉积磷释放实验和数学模型方法，比较分析水库退役前后磷在库区及下游河道水体及底床的赋存特征，系统研究“水库退役扰动-沉积物再悬浮-内源磷迁移转化”的“压力-状态-响应”关系，识别水库退役后库区沉积磷的再分布及时空响应特征。为我国高营养盐背景值水库退役的科学决策、退役方案制定及退役后库区及下游环境管理提供科学依据。

由于安全、经济、生态、重建等原因，水库退役已成为全球范围内普遍存在的现象。国内除常规水库降等报废工作外，近年来相继开展或正在开展长江经济带小水清理整改、赤水河流域小水电清理整改、黄河流域小水电清理整改等水库退役相关工作。水库退役后库区及上下游河段的水文情势、泥沙、地貌、水质及生态系统均受到影响。国外已开展了大量水库退役环境影响研究，但尚未形成完备的理论体系，且对污染物迁移转化影响的研究相对较少；国内鲜见基于监测资料研究水库退役环境影响的文献。本研究以正在开展的小水电清理整改工作为契机，综合采用现场监测、实验室分析和数学模型方法，获取了国内水库退役后库区水体及沉积物第一手资料，系统研究了我国典型水库退役后，因施工扰动、水动力等环境条件改变导致的库区水质及沉积物污染物的迁移转化及再分布特征，研究“水库退役扰动-沉积物再悬浮-内源磷迁移转化”的“压力-状态-响应”关系。研究内容为：水库退役前后库区沉积物内源磷迁移转化驱动因子变化研究；水库退役前后沉积物磷释放特性及水体中磷分布状况研究；水库退役后库区沉积物内源磷的再分布及时空响应特征研究；库区沉积物内源磷对水库退役的时空响应机理探讨。结果表明：（1）拆坝后库区河段发生溯源侵蚀，出现泥沙输运现象，河道地貌发生改变；（2）库区河段的pH值、溶解氧、总氮、总磷浓度均发生改变；（3）拆坝后，整个库区水体中总磷浓度显著上升，沉积物总磷向下游流失并向水体中释放，导致库区上游表层沉积物总磷浓度下降；库区中间位置及坝前受上游输送、向下游流失、原库区污染程度及释放速率等综合影响，沉积物总磷的浓度整体上升；（4）在所有电站中，化学需氧量与氮磷污染存在同步性；且库容越大，水体总磷浓度升高越少。本研究结论可以为完善水库退役环境影响的理论体系和库区下游水环境管理提供科学依据；为有关水库退役工作提供决策依据，为制定水库退役方案及施工流程提供参考。