水位变化下三峡水库典型支流库湾CH4气泡排放研究

As one of the major ways of methane (CH4) emission in reservoirs, CH4 ebullition is mainly affected by water-level fluctuations. Although the annual variation amplitude of water level of the Three Gorges Reservoir reaches 30 meters, few studies focused on its CH4 bubbles emission and mechanisms. In this project, the Xiangxi River, which is a typical tributary bay of the Three Gorges Reservoir, is chosen as the study area to carry on field campaigns. We will monitor environmental factors (physical, chemical and hydrological ones) synchronously, and the diffusion and ebullition fluxes of CH4 with high-resolution samplings. Together with incubation experiments at our laboratory, we aim to evaluate the emission level of CH4 ebullition and its spatial and temporal variabilities. We try to explore the formation, migration, transformation, and release of CH4 bubbles in the sediment-water-atmosphere continuum under different water level and its variation range, and major factors’ influences on them. Furthermore, based on indoor cultivation simulation and linear elastic fracture mechanism (LEFM), an initial physical equilibrium model will be constructed for CH4 bubbles between hydrostatic pressure, buoyancy and viscous force during the formation, growth, and ascent of CH4 bubbles, to elucidate the release mechanisms of CH4 bubbles under different types of sediment and changes in water level/amplitude. We hope that the results of this project can both contribute science knowledge of the CH4 bubble release mechanisms under dispatching and operation of the reservoir with deep water, and provide a scientific basis for accurately assessing on the CH4 emission level and for the ecological operation of the Three Gorges Reservoir.

气泡排放是水库CH4排放的主要途径之一，水位变化是其主要影响因素。三峡水库年水位变化达30 m，然而其CH4气泡排放水平及排放机制的研究目前还鲜有报道。本项目拟以三峡水库典型支流库湾—香溪河为研究对象，采用野外原位观测和室内培养模拟相结合的方法，通过对库湾水体CH4气泡通量、环境理化和水文因子的观测和分析，估算CH4气泡排放水平并分析其时空变异性及主要影响因子；探究水位变化下CH4气泡在沉积物-水-大气体系中的形成、传输、消耗和释放过程；进而结合室内培养模拟及LEFM机理模型，初步构建水位变化下典型沉积物中CH4气泡在形成、生长、上升过程中的物理模型，阐明水位及其变幅和不同沉积物类型下CH4气泡的排放机制。本项目研究成果有助于提升对深水水库调度运行下CH4气泡排放机制的认识，可为三峡水库CH4排放水平的准确评估以及水库的生态调度提供科学依据。

水库碳排放一直是科学家关注的热点问题，准确估算水库碳排可以为当前我国达到“双碳”目标、应对气候变化和制定相关政策提供理论依据。三峡水库为我国的重大水利枢纽工程之一，为有效评价其在不同运行机制下的碳排放，需要进一步厘清其碳收支平衡的各项和相关的变化。本项目以三峡水库典型支流库湾—香溪河为研究对象，采用野外原位观测和室内培养模拟相结合的方法，通过对库湾水体CH4溶存浓度、水-气界面释放通量、环境理化和水文因子的观测和分析，同时结合在不同观测断面沉积物的定点采样，探索香溪河库湾沉积物中CH4产生和消耗以及在沉积物-水界面的CH4释放通量及其时空变化特性，并阐明CH4自沉积物中产生后至沉积物-水界面和水-气界面的迁移变化规律；研究成果有助于提升对深水水库调度运行下CH4排放机制的认识，可为三峡水库CH4排放水平的准确评估以及水库的生态调度提供科学依据。结果表明，不同水位运行期除146.3m水位时期外，香溪河库湾内溶解甲烷的浓度均表现为垂向平均浓度大于表层水体平均浓度，各个时期库湾内水体有比当下时期更大的释放潜力；垂向与水平方向上浓度的差异主要来自库湾底部的高溶解浓度的积累。高水位运行期内水体平均总溶存甲烷量为2.78mg，汛前泄水期水体溶解甲烷总量为10.0(3.1-16.9)mg (库容以水位155m计算)，低水位运行期内水体溶解甲烷总量为3.7mg在汛后蓄水时期溶存甲烷总量仅有0.8 mg。在不同断面上，香溪河纵向各点位平均甲烷释放潜力为 XX01(0.54 ng g-1 d-1)<XX02(1.04 ng g-1 d-1)<XX04(1.87 ng g-1 d-1)<XX00(2.12ng g-1 d-1)<XX06(2.30 ng g-1 d-1)<XX05(4.60 ng g-1 d-1)<XX07(289.91 ng g-1 d-1)，同时各个断面上也存在着显著的空间差异，其中平均甲烷释放潜力最大值出现在上游 XX07，最小值出现在下游 XX01；而沉积物-水界面上的CH4产生通量为0.35 mmol m-2 d-1，空间上表现为从河口至香溪河库湾内逐渐上升的趋势，CH4氧化通量为0.32 mmol m-2 d-1，整体表现为出显著的空间差异，在XX07 达到最大为0.62 mmol m-2 d-1。