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Zhao, Kai

University of British Columbia

Methane ebullition (bubbling) from lake sediments is an important source of atmospheric methane. Rising bubbles can also negatively impact water quality. However, most ebullition surveys measure ebullition at intervals that are much greater than the duration of ebullition events (typically 2-4 days), introducing uncertainties in our understanding of temporal variations in ebullition. In addition, most lakes are seasonally ice-covered, but this period is often ignored. In this dissertation, I provide high-frequency (minute to hourly) ebullition data from Base Mine Lake, a boreal pit lake in Alberta, Canada. The high-frequency data reveal that during ice cover ebullition almost exclusively occurs when atmospheric pressure decreases below a threshold, which is approximately the average pressure. During open-water season, ebullition is still regulated by pressure variations. Over the 4-month open-water period, 24 ebullition peaks were observed. 22 of them occurred when atmospheric pressure was at its local trough. Semi-empirical equations are provided that can reproduce the time-series of ebullition events well. A physics-based ebullition model is also developed. This model only utilizes a minimum number of physical principles; yet its results compare well with field observations. The analyses suggest that there exist three different regimes, namely the saturation-controlled regime, the pressure-controlled regime and the undersaturation regime. Inside the pressure-controlled regime, ebullition is regulated by pressure variations and changes in atmospheric pressure can trigger and stop ebullition events. Due to the opaque nature of sediments, the rise of bubbles inside sediments is not readily observed. In laboratory experiments, transparent Carbopol is used as a surrogate for sediments. A layer of Carbopol is capped with a layer of water and air bubbles are injected at the base of the Carbopol layer to mimic ebullition. Many novel behaviors are observed. For example, a gas-venting conduit develops inside the Carbopol layer. Inside this conduit, rising bubbles resemble Taylor bubbles, but have velocities that are 5-7 times that of the standard Taylor bubble. Rising bubbles also expel water out of the conduit and overlying water flows back after the bubbles escape. This process can enhance the exchange of heat and contamination between a lake and its sediments.

Text

2023-02-08

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/83811

Civil Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/