Understanding the Relationship Between Soil Processes and Atmospheric Methane Concentrations

Abstract:

As vehicle-based atmospheric surveying becomes more commonplace, its natural evolution will see an increased movement towards detection of multiple gases and geochemical approaches for discriminating leaks of different origin. While multi-gas surveys are already feasible, the factor limiting our ability to interpret them is the understanding of gas source-sink dynamics, particularly at the soil level. This study aims to understand the relationship between soil processes and atmospheric methane concentrations. Using source regions of approximately 100 km², extensive soil gas surveys were completed, measuring CH_4, δ¹³CH₄ and CO₂. We compared this to daytime and nighttime vehicle-based surveys where we acquired data for the same gases to see which of these individual gases, or ratios thereof, could be detected in the lower atmosphere. These surveys were done in two contrasting regions, which were also expected to have different source/sink processes. Results showed that atmospheric CH₄ concentration, its isotopic signature, and the CO₂/CH₄ ratio of above-background concentrations showed the highest level of correspondence with the soil CH₄ values. Anomalies in CH₄ concentrations in the first study area appeared to be from predominantly biological sources (δ¹³CH₄ values near −60‰) rather than from a fossil source (underlying coal beds). However, the study area also showed anomalous values of δ¹³CH₄, which may have been due to a soil CH₄ sink. In both regions, nighttime atmospheric studies generally yield stronger signals and correlations because decreased night winds contributed to pooling of gases and higher atmospheric concentrations. This study helps advance our understanding of the relationship between soil processes and atmospheric methane, which is essential for improving vehicle-based surveys for use in detecting environmental side-effects of energy and geosequestration projects in regions of complex surface gas dynamics.