A parametric study of the transport of CO2 in the near-surface

Previous simulations of the behavior of a CO2 plume from a discrete line source near the ground surface have generally assumed that the water-table, soil layering, and land surface were all horizontal. As might be expected, these conditions produce plumes with a high degree of symmetry. The three dimensional TOUGH2 simulations reported here begin to consider several complications: degree of soil heterogeneity, presence of a capillary barrier, water table depth, CO2 leakage rate, sloping topography, and regional groundwater flow (sloping water-table). The results show that the ground surface CO2 flux distribution can be significantly different due to variation in the aforementioned factors, e.g. presence of a capillary barrier. The plume affected by these factors can assume an irregular shape characterized by multiple pathways to the surface. These pathways make the detection of a CO2 leak by soil gas or surface flux measurements conditionally possible at tens of meters away from the initial leak location. Such phenomena should be considered when designing CO2 detection network design.

► Water table tilt can conditionally lead the CO2 plume to migrate in unexpected directions.
► Maximum local flux and plume width show different behaviors upon changes in the CO2 leakage rate.
► The CO2 seepage flux values decrease strongly with an increase in vadose zone thickness.
► Capillary barriers divert the leaking gas laterally, hence higher possibility of detection.