Methane gas in lake bottom sediments quantified using acoustic backscatter strength

PURPOSE: Freshwater lakes and reservoirs can produce substantial quantities of methane (CH₄) within bottom sediments. Due to its low solubility in water, CH₄forms bubbles that accumulate within bottom sediments and are released from sediments when buoyant forces exceed viscous, frictional, and inertial forces. Methane ebullition can enhance internal loading of nutrients and hasten release of other sediment-associated contaminants while also exporting a significant amount of C out of the lake basin and delivering a potent greenhouse gas to the atmosphere. The purpose of this study was to evaluate use of hydroacoustic measurements to remotely sense the CH₄content of sediments in lakes. MATERIALS AND METHODS: The acoustic signatures of bottom sediments measured using a BioSonics DT-X echosounder at 38, 201, and 430 kHz were compared with gas volumes of surficial sediments determined using a displacement method. Measurements were made on three lakes in southern California, USA that varied in depth, productivity, and other features. RESULTS AND DISCUSSION: The maximum volume backscatter strength of bottom echoes at 201 kHz was highly correlated with gas content (r² = 0.93) and, with suitable ground-truthing, can quantify the concentration and spatial distribution of CH₄gas in soft cohesive sediments of lakes and reservoirs. As an application of this technique, the spatial distribution of CH₄within bottom sediments of Lake Elsinore was determined from a hydroacoustic survey conducted in June 2010. Results are also presented from a transect on Lake Hodges demonstrating high levels of CH₄in this eutrophic lake. CONCLUSIONS: Hydroacoustic measurements of the maximum volume backscatter strength of soft organic sediments at 201 kHz provide a way to remotely estimate the volume of methane and other gases present in bottom sediments of lakes and reservoirs.