Injection of CO2 into deep geological formations entails the possibility of CO2 leakage from the storage formation through wellbores, faults, and fractures. CO2 leakage may ultimately reach the near-surface environment by buoyancy and pressure driving forces where it will either flow rapidly into the atmosphere as from an open well, or be emitted over a wide area as seepage. We are investigating the processes, detection, and environmental impacts of CO2 migration in the near-surface environment. Prior simulation work has revealed fundamental behaviors for the case of small CO2 flux including (1) the tendency to fill up the vadose zone with CO2 at concentrations approaching 100%, and (2) rapid mixing of CO2 seepage as it enters the atmosphere from flat and horizontal ground surfaces provided there is wind. The effects of high CO2 flux, weak wind, topographic depressions, and back-filled trenches are being investigated. We are using a variety of approaches from scale analysis to numerical simulation to analyze near-surface migration and dispersion of dense CO2 by wind and gravity-driven flow. Test problems with a range of CO2 seepage fluxes, topography, and wind conditions on length scales of order 100 meters are being considered. Topographic depressions and back-filled trenches are capable of diminishing mixing insofar as they can be sinks for CO2 seepage and isolated from the dispersive effects of wind. As such, these features may be good places for instrumentation to detect CO2 seepage. In addition to investigating the interplay between dispersion and detection, our work contributes to the prediction of environmental impacts in the near-surface environment
