Resolving Spatial Heterogeneities of Methane Ebullition Flux from a Brazilian Reservoir by Combining Hydro-acoustic Measurements with Methane Production Potential

Greenhouse gas fluxes from the sediment to the atmosphere are a common byproduct of the anthropogenic disruption of rivers by damming. Recent studies have reported that reservoir surfaces contribute to around 1.5% of the global anthropogenic GHG emissions in a CO₂-equivalent base and over a 100-year time span. However, quantifying the influence of greenhouse gases from artificial water bodies on the global climate is still challenging. Major uncertainties arise from the extrapolation of scarce point measurements over space and time. This study aims at the assessment of potential methane fluxes using combined field measurement approaches. Firstly, hydro-acoustic surveys combined with sediment coring technologies provided high-spatial-resolution data of the reservoir sediment gas content. Secondly, long-term ebullition measurements with automated bubble traps (ABT) provided long-term site-specific gas ebullition fluxes with high temporal resolution. Thirdly, the sediment methane production potential was analyzed using sediment incubation experiments. We deployed three ABTs recording continuous data in a time interval of 15 seconds. The most continuous data series has a total of 102.4 days of data without gaps. The hydro-acoustic data was collected in two campaigns in 2016 and 2017 in the subtropical drinking water reservoir of Passaúna (southern Brazil) and consists of single-point measurements and driven profiles. Extended sediment analysis (granulometry, LOI, total C and methane production potential) was performed at three locations below the ABTs. Significant correlations could be observed between acoustic sediment backscatter and methane production potential as well as ebullition rates. Regression analysis was performed for defining a mathematical relation between these parameters at three different positions. Spatial backscatter patterns were used to estimate the distribution of sediment methane production potential and related ebullition. The results of this study help to identify emission patterns and overall fluxes in a reservoir with small water level fluctuations and homogeneous sediment distributions.