西南河道型水库对河流关键微界面磷再生及迁移的影响机制

Development of cascaded hydro-power is the key strategy for national energy security and international climate treaty. The southwest China is the origination of several large rivers, and is therefore called the water tank of the earth. Exploitation of the hydro-energy in the region will largely improve national energy structure and diplomatic circumstance. However, the sediment movement processes and hydrodynamics condition are altered by large reservoir, which will directly affect the biogeochemical cycling of nutrients in river system, such as phosphorus and nitrogen. Phosphorus element in river is a significant component of the nutrient cycling. Regarding on phosphorus cycling influenced by large reservoirs, much more attention has been paid to the physical retention of longitudinal transport, while scarce focus on the interaction mechanism between micro-interface vertical exchange processes and longitudinal transport of phosphorus in river system. This is the most probable reason for the uncertainties of longitudinal phosphorus transport estimate and modeling calculation, and regeneration ability and interface release of phosphorus play a pivotal role in this process. In the present study, the large river-type reservoir Xinuodu in Jinshajiang river is served as study area, (1) to comprehensively analyze the effects of sediment processes and hydrodynamics on physico-chemical and bacterial properties alteration of sediment-water interface and suspended particle solids-water interface, (2) to reveal the migration pattern of phosphorus across the key micro-interfaces and to quantify the regeneration ability of phosphorus, and (3) to preliminarily illuminate the coupled mechanism of phosphorus vertical interface exchange and its longitudinal transport processes in river system, through the field investigation, laboratory simulation, in-situ trial and model’s establishment and solution. The ultimate objective of this study is to improve the beneficial cycle of phosphorus in river systems.

流域水电基地化建设是我国能源环境和国际履约的重要战略。西南地处我国主要江河的源头地区，是世界水塔，该区水电开发对改善我国能源结构和外交战略具有重要意义。水库将人为改变河流原有的泥沙过程及水动力条件，直接影响生源要素在河流中的迁移转化过程。河流磷素(P)是生源物质生物地化循环的重要组成部分，目前在水库影响下河流P循环研究中注重P纵向输移的物理滞留效应，而忽略了P在微界面垂向交换过程对纵向输移的作用机理研究，是河流P纵向输移估算和模拟结果存在不确定性的重要原因之一。磷的再生能力和界面交换在这一过程中具关键作用。本项目以金沙江河道型水库—溪洛渡为研究对象，基于野外跟踪、室内模拟、原位试验和模型构建求解，分析水库泥沙及水动力对河流不同区关键微界面物化和生物性质的影响，揭示磷在河流关键微界面的迁移规律、量化磷的再生能力，初步阐明河流磷的界面垂向交换过程及其纵向输移的耦合机制，促进河流磷健康循环过程。

磷是生态系统初级生产力的重要生源要素，也是生物圈物质循环和能量流动的重要元素。水库蓄水运行将人为改变了河流自然的径流过程和水动力条件，直接影响生源要素在河流中的迁移转化过程。本研究依西南托澜沧江梯级水库和三峡消落带，通过现场监测、原位实验、室内模拟和数值模型，分析了水库运行影响下磷的输移、沉积物—水界面的迁移转化过程，揭示了沿程沉积物磷的运移分布规律。取得的主要结果如下：从上游河流向水库下游，沉积物总磷的滞留率为61-92%，但沉积物和悬浮颗粒物生物可获得性磷含量逐渐增加，占总磷比例增幅分别为15-86%和7-40%。从上游急流环境向水库深水缓流条件过渡，沉积物岩性从以钙质为主向铁为主转化，沉积物溶解氧逐渐降低、有机质逐渐增加，这将促进水柱中生物可获得性磷更容易迁移到水柱中悬浮颗粒物中并向水库下游输移并进行排放。沉积物内源磷释放为悬浮物在氧化还原多介点条件下吸附溶解态活性磷提供物质来源，悬浮颗粒物对磷的吸附过程在氧化还原多变条件下是可逆的，富含活性磷的细颗粒在水流作用下逐渐从河相—湖相过渡区向深水湖泊区进行输移，并在库首深水区出现累积，最后随水体或颗粒进人下游。因此，梯级水库将不会减少沉积物生物活性磷向下游湄公河进行输送。