Factors Affecting Multiphase Benzene Breakthrough into Drainage Concrete Pipe in the Unsaturated Subsurface Profile

Analyses presented herein explore transport aspects related to benzene breakthrough into gasketed subsurface concrete pipe embedded in an unsaturated soil profile. A series of three-dimensional numerical analyses were conducted using the T2VOC code. The simulations yielded results on multiphase multicomponent transport providing insights onto the effect of pipe material and gasket conditions, subsurface soil saturated and unsaturated properties, volatilization to the atmosphere, and the thickness of initial contamination at the source on rate and magnitude of the transported mass. Results showed that the use of lower pipe concrete material quality led to increasing mass transport into the pipe by more than 60%. The use of a damaged gasket also allowed for a considerable breakthrough of the contaminant into the pipe with two orders of magnitude higher mass compared to the use of an intact gasket. While the main mechanism of contaminant migration in the vadose zone is vapor diffusion, benzene transport in the aqueous phase was the main phase breaking through the pipe walls, given the initial matric suction of the concrete pipe wall that leads to the near saturation of the pipe walls once water flow commences. The thickness of the nonaqueous phase liquid (NAPL) at source has a relatively secondary effect on the mass transfer into the pipe. Considering coarse-type soil material versus the sandy clay as the native soil where the pipe is located led to an increase in the concentrations in both aqueous and gas phases inside the pipe by a factor of 6 after 1 year.