Temporal and Spatial Variation of Methane Emission from Urban Lake and Its Relationship with the Hydrology-water Quality
|Keywords||urban lake methane emission temporal and spatial variation hydrology-water quality water eutrophication|
Continues rising of the atmosphere methane（CH4） aroused worldwide attention. In this paper,the methane emission flux of urban lakes were measured by using static floating chamber and gas chromatography analysis methods,which were affected seriously by anthropogenic activities.By observing the hydrologic parameter,lake water quality and meteorological parameter of the sampling sites, and combing the indoor simulated culture experiments,the hydrology-water quality impact of water on methane emissions was analyzed.The atmosphere methane concentrations were higher above the urban lake of Beijing within the Fifth Ring,and the methane concentrations of the 10 lakes were significant difference in all seasons.The methane concentrations of the 4 typical lakes present certain seasonal variation;it was highest in the July and August and in the early March when the ice was melting the methane concentration was increased slightly.The methane average emission flux was 4.105±7.927 mg m-2 h-1 in the 10 lakes.The methane emission flux present significant spatial distribution characterization.The methane emission flux was high at the long historical and small size lakes,but it was lower at the large lake and the lake that has good water sources. The methane emission flux was high also at the man-made lake or being highly renovated lake.The wind velocity,pH and Eh of the sediment were the main impact factor on the spatial distribution of the methane emission.The annual average CH4 emission flux was 2.862±5.083 mg m-2 h-1 in the four typical lakes,and the CH4 emission present a temporal characteristic.The methane emission time was from March to November,and the emission flux was higher in summer than that of at autumn and winter.The emission time was earlier in the Shicha Hai which lies in the center of city.The temperature was the main effect factor of the emission time variation.The methane annual emission amount was about 0.262-0.375 Gg in the Beijing urban lake,which accounted for 0.1%of that in the entire Beijing area.And they were high emission area among the grid distribution of the entire Beijing area.They present positive exponential correlation of the methane emission flux with air temperature,water temperature and sediment temperature separately.And the correlation index of the sediment temperature with the methane emission flux was the highest.There have weak correlation of the methane emission flux with the pH of water.And they have a relationship with the methane emission flux and TOC, BOD5.But they present the significant negative correlation of the methane emission flux and the Eh of sediment.The methane emission flux had a turning point when the Eh is lower than -150 mv.The methanogenesis ability of the sediment was inconsistent with the methane emission flux by the simulation experiment,and the Shicha Hai has the largest methanogenesis ability.The results of the orthogonal experiment showed that the eutrophication water was helpful to the product of the methane.And there have the largest promoting effect to the methanogenesis when the concentration of COD was higher and the concentration of TN was lower.The greenhouse gases which were come from the emission of the urban lake will increase with the promoting of urbanization.So we should pay more attention to it.