CH<sub>4</sub> variation and main influencing factors of bottom water column in the middle section of Three Gorges Reservoir

Abstract

甲烷（CH₄）是一种重要的温室气体，对全球气候变化有不可忽视的影响.三峡水库是中国最大的水库，其潜在的CH₄释放近年来备受关注.然而，此地区现存研究主要集中于水-气界面通量观测，对库底沉积物同底层水体CH₄浓度变化之间关系的认知仍然欠缺.为探究三峡水库泥沙主要沉降淤积的中段区域库底水体CH₄浓度变化及其主要影响因素，本研究于2017年8月-2018年11月在涪陵南沱镇、忠县石宝寨、万州小周镇采集库底上覆水体和底泥样本，并结合三峡水库调度运行特征进行分析.结果表明，三峡水库中段库底上覆水CH₄浓度范围为0.02~0.91 μmol/L，二氧化碳（CO₂）浓度范围为0.006~0.105 mmol/L，沉积物有机碳含量范围为7~90 g/kg，总氮含量范围为0.27~45.6 g/kg.另外，三峡水库低水位运行时期（5—9月），上游及陆源输入大量异源性有机碳是该时期三峡库中段底部CH₄积累的充分条件.在水库高水位运行时期（10月-次年4月），水位与径流变化对三峡水库中段底部CH₄的影响并不明显，库底自源性有机质相对比重有所增加，温度是该时期影响水库底层水体CH₄浓度分布的主要水环境因素.;Methane (CH₄) is an important greenhouse gas contributing to global climate change. The Three Gorges Reservoir (TGR) is the largest reservoir in China and has been receiving concerns for its potential CH₄ emissions during the past decades. However, the current research in the TGR mainly focuses on the observation of water-air interface fluxes, the knowledge of relationship between bottom sediments in the reservoir and CH₄ concentrations variation of the bottom water column is still lacking. To explore CH₄ concentrations variation and main influencing factors of the bottom water column in the middle section of TGR, this study collects overlying water and sediment of the bottom respectively at Nanling Town of Fuling, Shibaozhai of Zhongxian and Xiaozhou Town of Wanzhou in the middle section of the TGR during August 2017 and November 2018, and analyzing the data combined with reservoir operation characteristics. The results indicate that CH₄ concentrations in the overlying water of the bottom ranges from 0.02 μmol/L to 0.91 μmol/L, carbon dioxide (CO₂) ranges from 0.006 mmol/L to 0.105 mmol/L, the organic carbon (OC) content in the sediment ranges from 7 g/kg to 90 g/kg, and total nitrogen ranges from 0.27 g/kg to 45.6 g/kg. During the low water level period of the TGR (May to September), a large amount of allochthonous OC input from upstream and terrestrial sources is a sufficient condition for CH₄ accumulation of the bottom in the middle section of the TGR. During the high water level period (October to April of the following year), water level and runoff variations have no obvious effect on CH₄ of the bottom in the middle section of the TGR, the relative proportion of autochthonous organic matter of the bottom in the reservoir is increased, and temperature is the main aquatic environmental factor that affected the CH₄ distribution.