Numerical simulation has been applied in support of the U.S. Department of Energy's efforts to characterize the nature and distribution of CCl4 contamination in the deep vadose zone at the Hanford site, near Richland, WA. Three-dimensional simulations were executed using layered and heterogeneous distributions of soil properties to investigate the vertical and lateral distribution of CCl4 beneath its release point (216-Z-9 trench) and the effects of soil vapor extraction. The complexity of the modeled physical processes—namely, the nonlinearities associated with multifluid subsurface flow, including phase transitions and hysteresis in the relative permeability–saturation–capillary pressure functions—limits the grid resolution when executed using single-processor computers. To achieve higher grid resolutions and acceptable detail in the simulation results for subsurface distribution and remediation of CCl4, execution on multiple processors was required. We have developed a scalable implementation of a multifluid subsurface flow and transport simulator, STOMP, with capabilities for volatile organic compounds. Developing scientific software for execution on parallel computers has unique challenges. The guiding objectives for developing this scalable code were to keep the source coding readable and modifiable by subsurface scientists, to allow for both sequential and scalable processing, and to depend on domain scientists for code parallelization and scalable linear system solvers.