Mechanisms of Methane Release From Lake Sediments
Methane is a powerful greenhouse gas that can be produced in bottom sediments of lakes and reservoirs and released through ebullition and other properties. Many studies have quantified ebullition rates, however, the detailed mechanisms remain incompletely understood. This study was undertaken to better understand, through in situ and laboratory measurements, the mechanisms of gas ebullition from lake sediment. Four sites on Lake Elsinore, CA with different properties were evaluated through the use of in situ hydroacoustic measurements made using a BioSonics echosounder with a 201-kHz split-beam transducer. This transducer was rigidly mounted to a telescoping stand that allows highly precise measurements of position, rise velocity, and target strength (size) of bubbles as they are released from the sediment. Methane bubble release from cohesive sediments occurred via the viscoelastic-fracture mechanism with bursts of 2-4 bubbles released from specific locations in small increments of time, typically a few seconds. Target strength of bubbles across all sites was approximately normally distributed with a mean value of -54.6±4.4 dB, corresponding to an average bubble size of 0.073 cm3. Rise velocity was also quantified and averaged 0.21±0.2 m s–1. Properties of the sediment are recognized to regulate the production, storage, and ebullition of methane. Undrained sediment shear strength, Cᵤ, was evaluated for its role in methane storage and transport using a fall cone apparatus for sediments from 3 sites on Lake Elsinore that vary in sediment properties. Values of Cᵤ for these soft cohesive sites were found to range from 0.092-0.342 kPa. Systematic variation under controlled laboratory conditions of water content and temperature resulted in linear changes of Cᵤ. Values of Cᵤ were strongly correlated with sand content and inversely correlated with water content. Sediment shear strength was also influenced by temperature, and decreased at rates of 0.0024-0.0037 kPa/°C for fine-textured cohesive sediment. Vertical bubble pressure was quantified and found to be less than sediment shear strength. Sediment shear strength affects the amount of methane stored in sediment and the mechanism of release. This was seen in the in situ target strength distributions of bubbles released from sediments, as well as laboratory core incubations of gas release from sediments.