A two-dimensional numerical model was utilized to explore the flux mechanisms governing CO2 transport in the vadose zone. The simulations were set up to approximately correspond to a site of natural CO2 leakage at Mammoth Mountain, California. The mass fraction gradient driving force, responsible for diffusive and slip fluxes, was determined to lead to less plume spreading than advection alone. Density-driven flow of CO2 led to significantly greater spreading of the plume and greater storage of CO2 within the vadose zone than if density contrasts were not accounted for. Exposure assessment simulations indicate that for the conditions of interest there may be no physically realistic domain that would lead to CO2 levels below the criteria for human health impacts (sub 10%) in surface soils for the leakage rate present at Mammoth Mountain.