Geologic CO2 sequestration is being considered as a way to offset fossil fuel–related CO2 emissions to reduce the rate of increase of atmospheric CO2 concentrations. The accumulation of vast quantities of injected CO2 in geologic sequestration sites may entail health and environmental risks from potential leakage and seepage of CO2 into the near-surface environment. We are developing and applying a coupled subsurface and atmospheric surface-layer modeling capability built within the framework of the integral finite difference reservoir simulator TOUGH2. The overall purpose of the modeling studies is to predict CO2 concentration distributions under a variety of seepage scenarios and geologic, hydrologic, and atmospheric conditions. These concentration distributions will provide the basis for determining aboveground and near-surface instrumentation needs for CO2 sequestration monitoring and verification, as well as for assessing health, safety, and environmental risks. A key feature of CO2 is its large density (ρ = 1.8 kg m−3) relative to air (ρ = 1.2 kg m−3), a property that may allow small leaks to cause concentrations in air above the occupational exposure limit of 4% in low-lying and enclosed areas such as valleys and basements where dilution rates are low. The approach we take to coupled modeling involves development of T2CA, a TOUGH2 module for modeling the multicomponent transport of water, brine, CO2, gas tracer, and air in the subsurface. For the atmospheric surface-layer advection and dispersion, we use a logarithmic vertical velocity profile to specify constant time-averaged ambient winds, and atmospheric dispersion approaches to model mixing due to eddies and turbulence. Initial simulations with the coupled model suggest that atmospheric dispersion quickly dilutes diffuse CO2 seepage fluxes to negligible concentrations, and that rainfall infiltration can cause CO2 to return to the subsurface as a dissolved component in infiltrating rainwater.