Numerical modelling on rate-limited dissolution mass transfer of entrapped petroleum hydrocarbons in a saturated sub-surface system

Petroleum hydrocarbons generally referred to as light non-aqueous phase liquid (LNAPL) entrapped within a saturated subsurface geological formation can result in longer duration of contamination due to mass transfer limitations associated with heterogeneity of the system. Considering a part of the total pore volume as immobile is significant in capturing the behaviour of extended tailing by solute breakthrough profiles. In this context, the present numerical model investigates dissolution characteristics of residual petroleum hydrocarbons (LNAPL) resulting from both mobile and immobile domains under varying conditions of rate-limited diffusive mass transfer, longitudinal dispersivity and mean mobile fluid velocity in a saturated groundwater system. The numerical results suggest that the influence of residual LNAPL saturation on dissolution mass transfer coefficient is critical in comparison with mobile flow velocity and dispersivity. In addition, the rate-limited effect becomes predominant with significant increase in advective velocity and reduction in residual LNAPL saturation for structured porous media. It is concluded from this study that the transport of dissolved contaminant is dominated by advection at the mobile pores, while the diffusive equilibrium at the immobile volume is significant in characterizing the extended tailing and subsequently determines the overall mass transfer resistance.