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SPECIAL SECTION: RESEARCH ADVANCES IN VADOSE ZONE HYDROLOGY THROUGH SIMULATIONS WITH THE TOUGH CODES

Coupled Vadose Zone and Atmospheric Surface-Layer Transport of Carbon Dioxide from Geologic Carbon Sequestration Sites

^a Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
^b Earth Sciences Department, University of Waterloo, Waterloo, ON, Canada N2L 3G1
^* Corresponding author (cmoldenburg{at}lbl.gov)

Received 9 April 2004.

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 ( = 1.8 kg m^–3) relative to air ( = 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.