Numerical investigation of NAPL Source Zone Architecture in Two-Dimensional and Three- Dimensional Unsaturated Porous Media

The effects of the spatial distribution of soil permeability and water saturation, the NAPL spill scenario, water infiltration events, and vapor transport on NAPL distribution in two-dimensional (2-D) and three-dimensional (3-D) unsaturated porous media were investigated. 3-D homogeneous and heterogeneous fields were considered and a 2-D vertical cross-section along the center of the 3-D field was used for the 2-D simulations. The same NAPL and water infiltration rates over a source zone at the center of the top boundary were used in 2-D and 3-D simulations. The NAPL distribution was most strongly influenced by NAPL evaporation to the atmosphere and NAPL and water infiltration rates. The fraction of total NAPL mass that reached the groundwater table was higher in 2-D than in 3-D. The difference between 2-D and 3-D simulations can be primarily attributed to the following factors. First, water saturation was higher in 2-D than in 3-D because the water plume spread out more evenly due to the additional horizontal direction in the 3D case. Hence, NAPL can migrate vertically faster in 2-D than in 3-D due to the higher NAPL relative permeability in the former. Second, the effect of vapor transport in 3-D was more significant than in 2-D, mainly due to the presence of the additional horizontal direction for vapor transport in 3-D. Hence, more NAPL mass moved out of the NAPL source zone in the vadose zone in the 3-D simulation, resulting in a lower fraction of the total NAPL mass in groundwater. These simulations indicate that the 2-D simulation for organic compounds with high vapor pressure needs to be compared with the 3-D simulation in both homogeneous and heterogeneous unsaturated porous media. The effect of variability in the permeability field and quantitative analysis of dimensionality on NAPL distribution will be further explored through stochastic modeling.