Underground energy storage (e.g., hydrogen) supports decarbonization efforts; however, leaks along energy infrastructure (e.g., wells, buried pipelines) can lead to stray gas migration to the shallow subsurface. Environmental risk can be mitigated using a combination of aqueous, soil gas, and surface flux measurements to detect gas leaks. As underground hydrogen storage (UHS) is deployed, existing monitoring frameworks must be adapted to account for H2 properties that differ from other gases (e.g., density, molecular weight) and multicomponent gas compositions (e.g., H2-CH4 blending, cushion gases). These UHS-specific properties are expected to impact gas migration and related monitoring signals. This study investigates the multicomponent gas transport of H2-cushion gas mixtures in the vadose zone to characterize soil gas signals relevant for leak detection. Pure gas and H2-cushion gas mixtures were injected into the bottom of a sand-filled column (60 cm) and concentration profiles were measured along the column height over time using gas chromatography. Experiments were used to validate a reactive gas transport model developed in COMSOL Multiphysics and to determine the H2-cushion gas binary diffusion coefficients. Simulations were then conducted to evaluate gas transport under variable source conditions (e.g., constant vs intermittent source) and different gas mixtures. Results showed that the magnitude and temporal changes in H2 and background gas (N2, O2) soil gas concentrations depend on the source gas composition, highlighting that the evolution of soil gas concentrations and surface fluxes will likely vary depending on the specific gases stored at UHS sites.