Long-term storage of S0 blocks poses a contamination risk to both groundwater and surface water as the oxidation of S0 to H2SO4 creates drainage waters with low pH and elevated SO42− concentrations. Because the presence of O2 in the S0 blocks is a primary control on S0 oxidation, rates of atmospheric gas migration into the blocks can be used to quantify S0 oxidation rates. Using in situ measurements and a tracer experiment combined with laboratory diffusion experiments, we determined that transport analysis of multiple gas species is required to quantify O2 fluxes into a S0 block. Advection, created by O2 consumption within the block, accounted for 21% of the total O2 influx and considerably increased H2SO4 production in the block above that attributed to O2 diffusion. While barometric pumping of the fracture porosity in the upper 8 m was probably occurring, its overall effect on O2 influx was determined to be negligible. Quantification of O2 and CO2 fluxes was necessary to explain the direction and magnitude of N2 concentration gradients that develop in the block, despite the fact that N2 was nonreactive. Our findings should be widely applicable because the construction method of S0 blocks is standard throughout the world.