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 CH4 to spread over larger areas by retarding, deviating, or blocking upward buoyancy‐driven CH4 migration. Simplifying or ignoring capillary pressure heterogeneity thus leads to overestimation of leaked CH4 to the atmosphere, and underestimation of mobile gaseous CH4 in aquifer. We show, both qualitatively and quantitatively, that meter‐scale sedimentary stratification contributes more to CH4 plume migration than the millimeter‐ and centimeter‐scale strata comprising them. Results indicate that the extent of gaseous CH4 leakage, 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.