High-frequency measurements of gas ebullition in a Brazilian subtropical reservoir-identification of relevant triggers and seasonal patterns

摘要

Water bodies, either natural or constructed impoundments, are sources of methane to the atmosphere, in which ebullition is frequently mentioned to be the dominant pathway. Ebullition is a complex process that is spatially dependent on factors acting over large distances (atmospheric pressure changes, wind) and factors acting locally (sediment characteristics, gas production) and is temporally variable due to the parameters' oscillation with time. Its quantification through measurements is still limited, as is the identification of production processes and triggers for ebullition. This research focused on obtaining high temporal resolution measurements of gas ebullition from a water supply reservoir located in Brazil, to compare its temporal variability with changes in reservoir conditions, and obtain insights on its spatial patterns. Three automated bubble traps were deployed in the reservoir and measured gas flux from February 2017 to March 2018. The time series data showed a large temporal variability in ebullition. Less intense fluxes occurred with higher frequency, and short-duration events made a larger contribution to the total amount of gas emitted. A strong seasonal variation was observed, in which the mean flux recorded during periods when the reservoir was stratified was 2-16 fold the bubbling rates recorded during colder months and mixed water column. In addition, high flux events were correlated with decreasing atmospheric pressure and increased wind intensities. Lastly, we show that the mean gas emission flux tends to be underestimated during short sampling periods (probability 41% for sampling periods shorter than 10days).