Capillary Heterogeneity Linked to Methane Lateral Migration in Shallow Unconfined Aquifers

We investigate mechanisms that enhance lateral methane (CH4) plume migration in shallow aquifers that exhibit complex and multiscale sedimentary architecture. We show how heterogeneity in capillary pressure characteristics related to the sedimentary architecture causes gaseous CH4to spread over larger areas by retarding, deviating, or blocking upward buoyancy‐driven CH4migration. Simplifying or ignoring capillary pressure heterogeneity thus leads to overestimation of leaked CH4to the atmosphere, and underestimation of mobile gaseous CH4in aquifer. We show, both qualitatively and quantitatively, that meter‐scale sedimentary stratification contributes more to CH4plume migration than the millimeter‐ and centimeter‐scale strata comprising them. Results indicate that the extent of gaseous CH4leakage, and its associated impacts on groundwater quality and global warming, cannot be accurately assessed unless the sedimentary architecture and resulting heterogeneity in capillary pressure are represented. Increasing levels of methane (CH4) in shallow aquifers from both natural and anthropogenic sources have become a serious concern worldwide. Leaked CH4not only contaminates drinking water resources, resulting in long‐term adverse health effects, but may also enter the atmosphere, accelerating global climate change. To assess the potential risks associated with CH4leakage and design effective mitigation strategies, it is crucial to understand how geological features at different spatial scales affect the behavior of CH4in groundwater. Field experiments have shown that horizontal migration of CH4is beyond that expected due to groundwater flow alone, but the mechanism driving these phenomena is unknown. In this work, we show that this unexpected behavior may be caused by geological heterogeneity and the resulting heterogeneity in capillary pressure, which is the pressure difference at the interface between gaseous CH4and groundwater. Capillary heterogeneity in relatively homogenous and shallow aquifers places leading‐order controls on lateral CH4plume spreadingNeglecting capillary heterogeneity leads to overestimation of atmospheric CH4and underestimation of mobile gaseous CH4in aquiferMeter‐scale sedimentary stratification contributes more to CH4plume migration than millimeter‐ and centimeter‐scale features Capillary heterogeneity in relatively homogenous and shallow aquifers places leading‐order controls on lateral CH4plume spreading Neglecting capillary heterogeneity leads to overestimation of atmospheric CH4and underestimation of mobile gaseous CH4in aquifer Meter‐scale sedimentary stratification contributes more to CH4plume migration than millimeter‐ and centimeter‐scale features