Acoustic Mapping of Gas Stored in Sediments of Shallow Aquatic Systems Linked to Methane Production and Ebullition Patterns

Abstract

Bubble-mediated transport is the predominant pathway of methane emissions from inland waters, which are a globally significant sources of the potent greenhouse gas to the atmosphere. High uncertainties exist in emission estimates due to high spatial and temporal variability. Acoustic methods have been applied for the spatial mapping of ebullition rates by quantification of rising gas bubbles in the water column. However, the high temporal variability of ebullition fluxes can influence estimates of mean emission rates if they are based on reduced surveys. On the other hand, echo sounding has been successfully applied to detect free gas stored in the sediment, which provide insights into the spatial variability of methane production and release. In this study, a subtropical, midsize, mesotrophic drinking water reservoir in Brazil was investigated to address the spatial and temporal variability of free gas stored in the sediment matrix. High spatial resolution maps of gas content in the sediment were estimated from echo-sounding surveys. The gas content was analyzed in relation to water depth, sediment deposition, and organic matter content (OMC) available from previous studies, to investigate its spatial variability. The analysis was further supported by measurements of potential methane production rates, porewater methane concentration, and ebullition flux. The largest gas content (above average) was found at locations with high sediment deposition, and its magnitude depended on the water depth. At shallow water depth (<10 m), high methane production rates support gas-rich sediment, and ebullition is observed to occur rather continuously. At larger water depth (>12 m), the gas stored in the sediment is released episodically during short events. An artificial neural network model was successfully trained to predict the gas content in the sediment as a function of water depth, OMC, and sediment thickness (R2 = 0.89). Largest discrepancies were observed in the regions with steep slopes and for low areal gas content (<4 L m−2). Although further improvements are proposed, we demonstrate the potential of echo-sounding for gas detection in the sediment, which combined with sediment and water body characteristics provides insights into the processes that regulate methane emissions from inland waters.

Linking Sediment Gas Storage to the Methane Dynamics in a Shallow Freshwater Reservoir

“…The range of PMP rates in the sediment of the reservoir (up to 12.8 mgCH 4 L −1 d −1 ) was lower than the range reported for other impoundments in the same region (up to 25.0 mgCH 4 L −1 d −1 in Wilkinson et al, 2015) and higher than the values reported for a subtropical reservoir in Brazil (up to 6.0 mgCH 4 L −1 d −1 in Marcon, Sotiri, et al, 2022). The highest PMP rates were measured in the top sediment layers, which had higher organic matter content (LOI).…”

“…The areal gas content in the sediment was estimated from the EA400 echo‐sounder data following Marcon, Sotiri, et al. (2022). The 200 kHz measurements were processed using the Sonar5‐Pro software (Lindem Data Acquisition, Oslo, Norway), in which the magnitude of the maximum acoustic backscatter near the sediment surface is exported together with its geographic coordinates for each ping (sound pulse).…”

“…In a follow‐up study, Anderson and Martinez (2015) proposed an empirical calibration to estimate the areal gas content based on the maximum acoustic backscatter observed in the bottom echo of high‐frequency echo‐sounders. This calibration curve was also applied to estimate gas content in another aquatic system (Marcon, Sotiri, et al., 2022).…”

Environmental management of companies in the oil and gas markets based on AI for sustainable development: An international review

“…Oil and gas are critically needed for the state (for example, to maintain the stable operation of public infrastructure and public institutions), households and businesses. The sustainability of the oil and gas markets is to fully meet the demand, but this is becoming increasingly difficult every year, as the "clean" energy industry is actively developing, and the demand for oil and gas seems to be increasingly unpredictable (Popkova and Sergi, 2021;Marcon et al, 2022).…”

Drainage ditches are year‐round greenhouse gas hotlines in temperate peat landscapes

“…The average concentration of CH 4 in the fresh bubbles was 35% (4%-65%). As inMaeck et al (2014) andMarcon et al (2022), we used this percentage to convert the volumetric ebullition fluxes to CH 4 ebullitive fluxes again using the ideal gas law. We used the CH 4 concentration of the fresh bubbles as we observed that a considerable CH 4 loss from the bubble traps occurred during deployment (75% decrease in concentration in 23 days).…”